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Hydraulic Glossary

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Absolute Micron Rating

An indication of the largest opening in a filter element. This rating indicates the diameter of the largest hard, spherical particle that can pass through an element. This is not a true rating of what size contaminants may pass; for example, a long, thin sliver of contaminant could still float through. (Also see Nominal Micron Rating).


A container in which fluid is stored under pressure; most commonly with a gas pre-charge above the fluid. Used to obtain high flow from a small pump, intermittently, provide a holding pressure to a clamping cylinder, absorb hydraulic shock, or to provide an emergency power source (one-time use, then recharge).

Bladder Type – The most common type of hydraulic accumulator; made up of a steel outer shell, an inner bladder of synthetic rubber, a poppet valve, and a charging valve. The bladder is pre-charged with an inert gas (usually nitrogen) to the desired pressure. Oil must be forced into the shell of the accumulator at a higher pressure. If charged or discharged rapidly (adiabatic operation), the usable volume will decrease due to gas heating and expansion.

Piston Type – A type of gas-charged accumulator that utilizes a free-floating piston to separate the gas charge from the fluid. As seals wear, leakage occurs. Do not mount piston accumulators horizontally.

Adapter Fittings

Hydraulic hose, tube, or pipe fittings that are used where a change in diameter, a change in conductor type, or a change in thread style is needed. Nearly limitless combinations are available.

Adiabatic Operation

Rapid expansion of the accumulator pre-charge gas during oil discharge does not allow enough time for the accumulator walls to absorb and radiate heat, resulting in an ever-changing gas volume. More heat = more gas expansion = less oil volume. Generally, a cycle time of under 1 minute is considered adiabatic operation.

Adjustable Throttle (Needle Valve)

A hydraulic volume control valve that consists of an orifice with a tapered needle and seat to adjust the flow. It is infinitely variable within its flow range. When the valve is fully closed (no flow), the needle is screwed completely down, blocking flow through the orifice. When the needle is fully backed out, full flow is available.


The useful piston area on the rod end of a hydraulic cylinder. Equal to the cross-sectional area of the piston minus the cross-sectional area of the rod. Also known as the ‘rod end’ of the cylinder.


American National Standards Institute. Maintains standards for schematic symbols, drawing conventions, etc.


A square inch measurement of the surface of an object, arrived at by the formula:

π x radius² = Area   or   d² x 0.7854 = Area

Use Area to calculate pressure or force.

Atmospheric Inlet

A pump inlet/reservoir configuration where the pump inlet is above the reservoir. Atmospheric pressure (14.7 PSI at sea level) pushes the oil up through the inlet line. As the pump rotates, there is a relatively low-pressure area created at the inlet. This effectively decreases resistance to the atmospheric pressure that is constantly being applied to the column of oil. This is the most common type of hydraulic pump inlet.

Atmospheric Pressure (PSIA)

The weight of a column of air one inch square and as high as the atmosphere. This equals the pressure exerted by the atmosphere at a specific location. PA = 14.7 PSIA at sea level. Atmospheric pressure moves the oil to the pump inlet due to vacuum created in the pumping chamber. Most fluid power gauges consider atmospheric pressure to be 0 PSI. Generally measured in inches of mercury (Hg).


Back-Ported Housing

A pump casing where the inlet and outlet ports go through the back of the unit, opposite the shaft.

Backup Ring (Non-Extrusion Ring)

A flat, circular ring placed on the low-pressure side (or both sides) of the o-ring for the purpose of adding mechanical stability. Under high temperature/pressure, o-rings tend to extrude into long fingers in the direction of force. This quickly destroys them in dynamic sealing applications such as on a piston/barrel seal. The backup ring prevents this extrusion.

Baffle Plate

A piece of steel plate that runs lengthwise throughout the reservoir, essentially dividing the inside volume in half. There is a small notch in the plate on the bottom side, to allow oil to pass from the reservoir inlet (system return) side to the reservoir outlet (pump inlet) side. The baffle plate helps cool the oil, provides a low velocity through the reservoir allowing sediment and debris to fall out of the oil.

Balanced Spool

In a compound relief valve, a spool-poppet valve spool that has one large land with an orifice in it to equalize pressure on either side of the spool (hydraulically balanced). The only pressure needed to shift the valve is a light spring pressure (usually about 20 PSI). The pump may be unloaded to tank at low pressure using a balanced spool valve.

Balanced Vane

A vane pump vane which has the same oil pressure underneath it, holding it outward against the cam ring, as it has trying to push it back down into the rotor (hydraulically balanced).

Balanced Vane Pump

A high-performance version of the vane pump. It has two outlets and two inlets opposite each other. This balances the side loading forces on the bearings, allowing for longer life. Usually fixed displacement; most fixed displacement vane pumps are hydraulically balanced.

Barrel/Cylinder Block

The part on a bent-axis or inline piston pump that serves to house the cylinder bores. The barrel rotates with the pistons.


A type of hydraulic pump that has an offset angle shaft. The angle is changed to alter displacement. The offset varies between 0° and 30°. These are the most accurate and reliable hydraulic pumps available if the fluid is kept clean.

Bernoulli’s Principle

If the flow rate is constant, the sums of the kinetic (velocity) energy and the pressure energy must be constant.

Beta Filtration Ratio

A rating of filtration performance. The beta ratio (β) is the number of particles of a given size, or greater, entering a filter, divided by the number of particles of the same size or greater leaving the filter. For example, a ratio of 75:1 means that 74 particles are stopped for every one that gets through. 100 – (100/75) = 98.67% efficiency.

Biodegradeable Hydraulic Fluid

For environmentally sensitive applications, it is common to use hydraulic fluids with a vegetable oil base so that the risk of spill damage is minimzed.

Bleed-Off (Bypass Flow Control)

A type of flow control circuit in which the flow control is plumbed parallel to the actuators it controls. Due to this arrangement, the ΔP across the orifice is not noticed by the actuator. This method of speed control is the most efficient, but is less accurate than Meter-In or Meter-Out. Used with resistive loads only.


Same as Bleed-Off, above.

Braided Wire Hose

A type of hydraulic pressure hose that utilizes one or two reinforcement plies of braided steel wire (SAE100R1 or SAE100R2).

Brake Valve

A pressure control valve usually used in hydraulic motor circuits to stop an overrunning load when pump delivery stops. At this point, the motor has turned into a pump itself, causing the outlet pressure to soar towards infinity, and blowing the side off the motor. The brake valve senses pressure in both the inlet and outlet lines. When outlet pressure rises sharply, the brake valve opens just enough to relieve the sudden buildup of pressure. Plumbed parallel to the motor, between ‘A’ and ‘B’ lines.

Buna-N (Nitrile)

This is the most common hydraulic seal material. Nitrile is tough, easily molded, wears well, is inexpensive, has a wide temperature range (-40° F to 230° F), and retains its shape in petroleum fluids. Black o-rings with no colored dots are Buna-N.


Cam Ring

An eccentric ring in which the moving members of a vane pump are housed. As the vanes trace the inside perimeter of the cam ring, oil enters, is pushed into a shrinking chamber, and expelled at the point of minimum displacement. The cam ring is elliptical on balanced vane pumps. Width of cam ring and eccentricity determine displacement.

Cartridge Mounting Style

A valve mounting style wherein the valve fits into a standardized chamber in a manifold. Advantages include greater design flexibility, reduced leakage, faster cycle time, and smaller package size.


The formation and immediate implosion of tiny cavities (bubbles) in a liquid. These implosions are quite powerful, exploding away fragments of metal from nearby components.

Clean-Out Cover

A metal plate on one or both ends of a reservoir. Must be removable to facilitate cleaning. On some reservoirs, the entire top/lid is a large removable cleanout cover.

Closed Circuit

A circuit in which oil flows from the actuator directly back to the pump inlet. The circuit must be pre-filled. It uses a small gear pump to supercharge the inlet of the larger (and much more expensive) piston pump. A hydrostatic transmission is a common example of this.

Conservation of Energy

A theory stating that energy can be neither created nor destroyed but may be transformed. In a hydraulic system, energy which does not do work will be transformed into heat.

Contamination Control

Contaminated oil is the cause of more than 75% of all hydraulic system failure. Use the proper filters in the proper locations! Contamination destroys a system by interrupting one or more of the four key functions of hydraulic fluid.

Counterbalance Valve

A valve used with hydraulic cylinders to keep a suspended load from free-falling, when the directional valve is shifted in either direction, due to pressure intensification in the cylinder. To lower the load, a pressure greater than load pressure must be applied to the pilot port.

Cracking Pressure

The pressure at which a simple relief valve just starts to pass oil. Cracking pressure must be accounted for in circuits where a precise oil flow is required.

Crimp-On Hose Fitting

Three general types of hose end couplings/interfaces. These fittings are attached to the hose ends by crimping (evenly smashing) the hose inside the fitting with powered machinery. Lower cost than reusable or field-make hose fittings. It is essential to use the proper dies and spacers for the hose size, fitting style, and manufacturer. The two types are one-piece crimp-on, and two-piece crimp-on (both available in skive or no-skive varieties).

Cup Seal

A positive cup shaped seal used on cylinder pistons. When pressurized, the outer lip of the cup pushes against the cylinder barrel creating a pressure actuated seal. Requires the use of backup rings. Useful for very high-pressure applications.


A mechanism in a cylinder, either on the blind, end or rod end, consisting of an adjustable needle valve in the cap, and a plug attached to the piston. Cushions serve as protection against hard or abrupt stops with high velocity and/or heavy loads. Properly setting the cushions can aid in retaining the end caps of a cylinder.

Cylinder Displacement

The volume within a hydraulic cylinder which must be filled with fluid to cause motion. Usually measured in cubic inches (in³).

Extend Volume: The volume of oil needed to fill the blind end of a cylinder to fully extend the rod.

Blind End Area x Stroke = Extend Volume.

Retract Volume: The volume of oil needed to fill the rod end of the cylinder to fully retract the rod.

Annulus Area x Stroke = Retract Volume.

Cycle Volume: The amount of oil needed to complete one full cylinder cycle (extend and retract).

Extend Volume + Retract Volume = Cycle Volume.

Cylinder Mounting Styles

The means of attaching the cylinder to the machine; trunnion, clevis, foot flange, side-lug, side tapping, tie rod, bolt.

Cylinder, JIC Type

A list of standard tube sizes, rod sizes, and mounting styles set by the Joint Industry Congress. Allows for interchangeability between machines, lower prices, and helps to ensure consistent quality.

Cylinder Rod

A machined rod attached to the piston of a cylinder. When fluid enters the cylinder, the piston and rod move together. With the load attached to the rod or the cylinder tube, the load will move as well. When sizing a cylinder rod, give special consideration to the types and magnitudes of forces to which it will be subjected (columnar strength, tensile strength, side loading).

Cylinder Rod

A machined rod attached to the piston of a cylinder. When fluid enters the cylinder, the piston and rod move together. With the load attached to the rod or the cylinder tube, the load will move as well. When sizing a cylinder rod, give special consideration to the types and magnitudes of forces to which it will be subjected (columnar strength, tensile strength, side loading).


Deceleration Valve

A flow control valve which gradually reduces the flow rate to provide deceleration. Usually roller cam operated using either a tapered spool, or adjustable orifice design.


On circuits with very large bore cylinders, it is necessary to decompress the cylinder before shifting the directional valve to avoid the hydraulic shock of suddenly releasing a large quantity of high-pressure oil.

ΔP (Delta Pee, Pressure Differential)

Pressure Drop. There must be a difference in potential energy for fluid to flow. As fluid is pumped through a decreased diameter orifice, it gives up some of its energy in the form of heat as a result of passing from high potential to low potential. Pressure and flow are inversely proportional; the more the pressure decreases, the more the flow increases.


The ability of a hydraulic fluid to separate the water from the fluid base. Too much water in the oil (beyond its demulsibility) promotes rust, buildup of contaminants, sticky valves, and accelerated wear.


A pre-determined position maintained by a holding device acting on the flow directing elements of a directional control valve.

Detergent and Dispersant

Chemical additives used primarily in motor oil to keep dirt and combustion particles in suspension. Very basically speaking, these chemicals are suspended in the oil and have a chemical attraction to solid contaminants, keeping contaminants in suspension, and causing them to repel each other.

Differential Cylinder

A double-acting cylinder with a single rod wherein the ratio of the full piston area to the piston’s annular area on the rod side is exploited.

• The flow rate out of a single rod, double-acting cylinder will always be
different than the flow into it.

• Less pressure is required to extend than to retract a load.


Direct Drive

Refers to how the prime mover drives the pump. Direct drive connects the pump and prime mover drive shafts together end-to-end using a coupling device with a shock-absorbing, ‘rubber’ element.

Directional Control Valve

A directional control valve routes hydraulic fluid to and from the circuit.

Direct-Operated Relief Valve

A simple version of a pressure control that works by offsetting a spool, ball, or poppet by a spring. When oil pressure reaches a certain limit, the valve element is pushed off its seat, allowing oil to pass to tank. When using direct-operated pressure controls, be aware of pressure override; these valves will begin to open somewhat, slightly before the valve pressure setting is reached.


The amount of fluid which will pass through a hydraulic motor during one revolution. Measured in cubic inches per revolution. 231 cubic inches = 1 gallon. Displacement = in.3/min. ÷ RPM.

Double-Acting Cylinder, Single Rod

A cylinder that has only a single rod extending from one end, which extends or retracts with pump flow. Due to the area the rod occupies, the effective working area on each end is different. When retracting, flow out of the blind end will exceed pump flow into the rod end. This type of cylinder is also known as a differential cylinder.

Double-Acting Cylinder, Double Rod

A cylinder which has a rod extending from each end and may be extended or retracted with pump flow. Work may be done on both ends at the same time. Due to the rod occupying both sides of the piston, the effective working area on each end is the same (usually the rods are the same diameter). Extend and retract will be at the same rate with both rod diameters being equal.

Double Pump

A pump design allowing for a single source to power independent circuits, or additional flow in the same circuit. Avoids using two separate prime movers and two separate pumps.

Drain Oil Flow Path

The route taken by oil that has leaked past a spool or has flowed through the orifice on the land of a balanced spool. It is important to know enough about the valve’s operation to properly connect the drain line, establishing this drain path.

Dual Vane

A vane pump design using two chisel-edge vanes facing each other so as to form a ‘V’ between the upper tips of the vanes. This ‘V’ area holds a pocket of oil at the same pressure that is pushing up from the bottom edge of the vanes, and lubricates the ring and vanes. This means more accurate vane tracking due to 95% less unbalanced oil pressure on the vane edge and the sliding motion between vane pairs.


An instrument that measures the hardness (resistance to indentation) of elastomers. Also, elastomers are rated on the durometer scale which assigns a numerical value to the degree of hardness. The higher the number, the harder the seal.

Duty Cycle

The length of time a flow control valve is actually metering oil. Generally, this is directly proportional to the cylinder stroke length. Flow controls are not useful in applications requiring long duty cycles because restricting pressurized oil produces much excessive heat (wastes money).

Dwell Time

The period required for the pump to refill the accumulator. If a machine needs to cycle very rapidly, a larger pump may be necessary to shorten the dwell time.

Dynamic Seal

A seal made between a moving and a normally stationary part. Seals used in dynamic applications are subject to considerably more wear than those in static applications. The softness of silicone makes it a very poor dynamic seal.



Any natural or synthetic material which has enough resilience (memory) to return to its original state after being distorted.

Ethylene Propylene

An elastomer seal with a wide temperature range (-65° F to 300° F). works very well with phosphate ester fluids but not petroleum fluids. Makes a good dynamic seal.

Expanded Hydraulic Symbol

A fully represented hydraulic symbol showing all the internal and external passageways. Much more graphic than standard hydraulic symbols.

External Case Drain

An external connection port facilitating the draining of a valve’s spring chamber, or the case of a hydraulic pump or motor, to tank.

External Drain

In a solenoid-controlled, pilot-operated directional valve, the external connection on the manifold labeled as ‘Y’. external draining is useful for a system with high shock pressure spikes in the tank line; such as when a large cylinder extends in another part of the system, a very high load pressure may be felt in the tank line.

External Pilot

In a solenoid-controlled, pilot-operated directional valve, an oil pressure applied to the smaller, solenoid-controlled pilot valve from a remote pressure source, external to the valve assembly. Used to reduce the force with which the spool is moved in a high pressure system, or to smooth out radical system pressure differences without the energy waste and side effects of a flow control. Indicated as the ‘X’ port.



The component of a bite-type (flareless) tube fitting which is slipped onto the tube end and ‘set’ so that it ‘bites’ into the tubing, becoming an integral part of it. The outside of the ferrule is beveled to mate with the nut, providing the sealing surface. Bite-type fittings/components usually have a black oxide finish.


The oil reservoir cap assembly, usually. Consists of a strainer basket and a cap with an integral 10µ filter. The primary purpose is to vent the reservoir, as the modern method of filling involves using an oil filter cart connected through a quick-disconnect which pumps clean, filtered oil into the reservoir.

Filter Bypass Check Valve

A check valve built into a filter mounting body. If the filter plugs up and stops flow, the flow path diverts around the filter element, through the check valve. The spring value on the check valve is usually about 40 PSI.


Having only two positions, fully open or fully closed. An example is an unloading valve with an external pilot. When pilot pressure exceeds the preset, the spool/poppet fully opens at once. When pilot pressure drops, it is immediately forced closed by the offset spring.

Fixed Displacement

Refers to a pump or motor that has a single displacement volume that cannot easily be varied without modification.

Fixed Orifice

A hydraulic volume control valve consisting of a decreased diameter of a given size (non-adjustable) which the fluid must flow through due to the difference in pressure across the orifice. The input pressure must be higher than the output pressure. If the load changes, the pressure drop across the orifice changes correspondingly. The larger the ΔP, the more oil flows through. Fixed orifices are an excellent source of excessive heat.

Fixed Side Plate

A gear pump style that basically consists of two gears and a housing. The side plates contact the gear faces to provide an oil film seal; when the side plates begin to wear, efficiency declines. These are the lowest cost hydraulic pumps and are very rugged.

Flange Head Hose Fitting

A hydraulic hose fitting which has an end designed to interface with a two-bolt or four-bolt SAE flange.

Float Switch

An electrical input device that is wired in series with the electric motor starter coil. When the fluid level gets too low, the motor shuts down until more fluid is added to the reservoir.

Flooded Suction

A pump inlet/reservoir configuration where the reservoir is above the pump inlet. This guarantees a constant inlet pressure, even with the poor suction capabilities of a piston pump. If the inlet sucks air, the pump cavitates.

Flow Control

A flow control valve regulates the flow of a fluid. Flow controls vary speed by restricting or bleeding-off flow.

Flow Path

The route to the lowest pressure potential coming from any higher-pressure area of a circuit. Fluid always takes the path of least resistance. Oil does not flow if there is no pressure difference (ΔP) between two areas.

Fluorocarbon (Viton)

A seal material that is synthesized by combining fluorine with an elastomer. Viton is the most well-known brand of fluorocarbon seal. Exceptionally high heat resistance. Compatible with most fluids.

Foaming/Aeration Resistance

The two main causes of foaming and oil aeration are excessive return flow velocity and pump inlet leaks. Often, foaming can be eliminated by proper design and maintenance. In some cases, it becomes necessary to use oil containing an anti-foaming agent. Care must be taken when selecting an oil as some anti-foam chemicals actually entrain more air in the oil as a means of keeping foaming to a minimum. Anti-foam additives increase a fluids aeration resistance.


The measured amount of pressure applied to a given area. A pushing or pulling measured in units of weight. The product of pressure and the surface area upon which it acts. F = P x A.

Forged Steel Pipe Fitting

A high-pressure (up to 3000 PSI) hydraulic pipe fitting made of forged steel. Two different styles: plumbing style, machined from solid steel, and is black in color, and the ‘hydraulic’ style, machined from hexagonal bar stock and are zinc plated. Available in threaded or socket-weld varieties.

Four-Bolt SAE Flange

A mounting device consisting of a plate of given dimensions that has four holes, through which, the mounting bolts pass. When the bolts are tightened, an o-ring is compressed, and the seal is made.

Full Cartridge Pump

A hydraulic vane pump design featuring a quickly changed cartridge including rotor, rings, vanes, wear plate, pressure plate, and aligning dowels, all held together with machine screws.

Full Flow Filter

A term signifying that all of the fluid that flows into the filter inlet, goes through the filter element, and is not diverted over the bypass check valve.

Full Flow Filter

A term signifying that all of the fluid that flows into the filter inlet, goes through the filter element, and is not diverted over the bypass check valve.


Gear Pump

A category of hydraulic pumps typified by the meshing of two gears such that rotating the shaft causes the gears to turn, creating high and low pressure areas (on either side of the enmeshed gear teeth). Oil flows from the inlet around the circumference of the gears to the outlet.


Head Pressure

Pressure, in pounds per square inch, created by the weight of a column of oil. The pressure felt at the pump inlet caused by the weight of the oil above the inlet. Equal to 0.4 PSI per foot of height with hydraulic oil.

Heat Exchanger

A fluid conditioning device used on a hydraulic system. Usually refers to fluid cooling devices, although tank heaters are included in this category. The two main types of heat exchangers are oil to air, and oil to water. 140° F to 160° F is the ideal temperature range for hydraulic oil operating temperature.

High Pressure Filter

A type of filter designed to be installed in the pressure line. Used to isolate a branch with dirt-sensitive components in it, such as proportional valves. Pressure spikes/surges can cause captured particles to be knocked loose and migrate downstream. Also, protects the system in case of catastrophic pump failure.

High Pressure Hose

A broad classification of hydraulic hose that is capable of handling high pressure. The two general types are braided wire and spiral wire. SAE100R! and SAE100R2 have one and two wire braids, respectively. SAE100R9, SAE100R10, SAE100R11, and SAE100R12 have four, four, six, and four wire spirals, respectively.

High Venting

To remote control a pilot-operated relief valve, you drain, or vent, the spring chamber of the pilot valve to tank. This leaves about 20 PSI spring pressure to hold the balanced spool seated. ‘High venting’ involves replacing the spring with a stiffer value, about 80 PSI.


A measure of the energy required to move 550 lbs. 1 foot in 1 second, or 33,000 lbs. 1 foot in 1 minute, equal to 746 watts. The amount of work produced over an interval of time.

Hydraulic Cylinder

A hydraulic cylinder is a mechanical actuator used to provide a directional force through a stroke distance.

Hydraulic Fitting

A type of forged steel, high-pressure pipe fitting, machined out of hexagonal bar stock. Hydraulic fittings are zinc plated and always threaded. Highly recommended in pipe installations.

Hydraulic Horsepower

Equal to the product of pressure and flow. 1 Hydraulic Horsepower = 1 GPM @ 1500 PSI (with 87% efficiency).

Hydraulic Hose

A fluid conductor which is very expensive, short-lived (even when unused), absorbs mechanical and hydraulic shocks, and is easy to design, make-up, route, and connect. Made up of an inner tube (usually an elastomer), one or more reinforcement plies of braided wire, wire spiral, or textile braids, and an outer cover to protect the reinforcement plies. Hydraulic hose is sized by true inside diameter. Hydraulic pressure hose has one or two wire braid, or four, six, or eight wire spirals. Hydraulic suction hose is usually one-wire spiral (SAE100R4) hose; similar in construction to automotive radiator hose, except with a very heavy reinforcement wire.

Hydraulic Motor

A mechanical actuator that converts hydraulic energy into torque and rotation.

Hydraulic Motor, HSLT

High Speed, Low Torque. Relies on speed (RPM) to do work. Small shaft. Often used to replace an electric motor. Can be identified by the embarrassingly small shaft.

Hydraulic Motor, LSHT

Low Speed, High Torque. Relies on torque to do work. Very large, robust shaft. Often used to replace an electric motor and gearbox combination.

Hydraulic Principles

Industrial hydraulics relies on two foundational principles: Pressure and Flow.

Pressure is created by resistance to flow. Pressure applied to a confined liquid is transmitted equally and undiminished in all directions. Pressure determines force.

A pressure drop (ΔP) must exist for oil to flow. Fluid takes the path of least resistance. Flow determines speed.

Hydraulic Pump

A hydraulic pump is a device, driven by a prime mover, that converts mechanical power into hydraulic energy.

Hydraulic Schematic

A graphical representation of a hydraulic circuit. Essentially, it is a map of the circuit. Utilizes hydraulic symbols. A schematic is usually used in conjunction with a bill of materials, if available. These two documents are extremely crucial for accurate, efficient troubleshooting.

Hydraulic Steel Pipe

High strength steel pipe used in high-pressure applications. Standard plumbing pipe cannot handle high pressure, and will burst. When ordering, it is important to specify ‘pickled, oiled, and plugged’. This means that all the mill scale has been washed out with an acid bath, then oiled to protect against rust and corrosion, and finally, it is capped on both ends to keep out dirt, moisture, cigarette butts, and other various items.

Hydraulic Symbols

Clear, concise, graphical building blocks representing various hydraulic components.

Hydraulic Training

Intensive courses in industrial hydraulics designed to impart maximum useful knowledge in a relatively short time period, such as HTNW’s Essential Hydraulics, Part 1, or More Essential Hydraulics, Part 2.


A principle utilizing the flow velocity of liquids to do work, such as in an automatic transmission of a car. Transmission of power by impact or kinetic energy of the liquid.


The spring-loaded poppet in a pressure-compensated flow control. Biased by a set spring force (20 to 100 PSI). it feels load pressure on the spring end, and system pressure on the other end. When either the inlet or outlet pressure changes, the hydrostat moves to maintain a constant ΔP across the valve (equal to the spring value).

Hydrostat Hunting

The hydrostat poppet oscillates between fully open and fully closed for about 3-4 seconds at startup, until the system stabilizes. This causes the actuator to move accordingly. Using a lunge control can greatly reduce the hydrostat travel so that full open is inaccessible.


A principle utilizing the properties of liquids at rest to do work. Energy transmission through the application of force applied to a confined fluid.

Hydrostatic Drive (Transmission)

A hydrostatic drive (transmission) is a closed loop circuit consisting, essentially, of a bi-directional pump driving a bi-directional motor. Oil flows from the pump to the motor and back around to the other port of the pump. Oil is not directly returned to the reservoir.

Hydrostatic Piston Pump

Usually an axial-piston pump used in a closed loop circuit to drive a matching piston motor. Use as a hydrostatic pump also requires use of a supercharge pump, a small pump used to refill the main pump’s inlet and to provide oil to make up for losses. Also requires a relief and two make-up check valves. The over-center swashplate adjustment feature is used to reverse oil flow to the motor.


In a relief valve, the difference between cracking pressure and fully open.



Inside diameter. Hydraulic hose is measured by actual I.D.

Inches of Mercury

A unit of measurement indicating negative pressure (vacuum). The pressure exerted on a pan of mercury by the atmosphere will cause a column of mercury to rise 29.92 inches (indicated as 29.92 inHg).

Indirect Drive

Driving a pump indirectly with a prime mover using a v-belt, timing belt, timing chain, roller chain, drive shaft, or jackshaft to transfer the power. These are not a good choice for most vane pumps due to their smaller shaft bearings.

Industrial Hydraulic Shock Absorber

An energy absorbing device used for the deceleration of a heavy overrunning load. There are two main types: Regenerative and Non-Regenerative (most common). The regenerative type is basically a large collapsible spring whose energy is slowly released after impact. The non-regenerative type decelerates the load by converting kinetic energy to heat. This is accomplished by orificing the oil flowing out of the piston area of the shock absorber. A brief dwell time for oil cooling is necessary between impacts unless used with a larger, external reservoir. The return mechanism is usually air pressure or a mechanical spring. Construction is similar to a hydraulic cylinder with knife-edge orifices along the stroke. Not to be confused with a hydraulic cylinder cushion.


Having an infinite number of positions from fully open to fully closed. An example would be a sliding spool directional valve.

Inline Mounting Style

A valve mounting style wherein the valves mount directly in the fluid lines. This makes it easy to install new valves in an existing system. May leak more than other mounting styles due to hydraulic shock in the lines.

Inline/Axial Piston Pump

A pump having multiple pistons that all lie axially, or in line, with each other. The most popular piston pump style, especially for hydrostatic transmissions. Operates on the swashplate principle. The pistons are connected to shoes that ride on a thin film of oil on the swashplate; transforming swashplate angle into pump displacement as a function of swashplate angle.


A hydraulically actuated device that exploits the difference in areas between the actuating and the actuated pistons to increase oil pressure. The two basic classes are oil pressure intensifier and oil pressure booster.

Internal Case Drain

Draining the spring chamber of a pump, motor, or valve through an internal connection port. On a hydrostatic transmission circuit, the hydraulic motor’s case drains through the pump case to tank.

Internal Drain

The path of the oil being drained out of the pilot valve when one side or the other is pressurized. This oil is drained to the tank line of the main valve.

Internal Pilot

A hydraulic actuating pressure taken off the main pressure line used to operate the solenoid-controlled, pilot-operated directional valve. In the center position, the pilot valve connects both ends of the main spool to the same pressure (~65 PSI). When the pilot valve is shifted, one end of the main spool is exposed to system pressure, shifting the spool hydraulically.


A special type of vane pump vane incorporating a vane within a vane. The oil is ported so that the vane feels hydraulically balanced except for just enough pressure to keep a seal with the cam ring as the vanes oscillate in and out. Because of special porting in the rotor, the vanes do not have to ‘fight’ the oil pressure underneath to retract. This means longer life and less vane skipping.

ISO Cleanliness Code

The ISO Cleanliness Code is made up of three-digits, such as 17/15/12. Each digit value refers to the number of particulates found in each of three different size categories. Used to communicate particulate contamination in hydraulic fluid.

ISO Number

A rating of an oil’s viscosity assigned by the International Standards Organization. Measured in centistokes at 40° C. The most common ISO viscosity grades for hydraulic oil are 32, 46, and 68; approximately equal to the SSU or SUS divided by five.

Isothermal Operation

Slow expansion of the accumulator pre-charge gas during oil discharge, which allows the accumulator walls to absorb and radiate the heat, maintaining a constant gas temperature, and thus, volume.


JIC Swivel

A swiveling female connector joint that mates-up with JIC 37° flare fittings. A female JIC swivel on each hose end is ideal as the entire hose length does not have to be turned to attach the fitting to a manifold or another line, and it allows an infinite number of re-makes.


Kinetic Energy

The energy of a substance or body due to its mass and velocity.


Laminar Flow

A condition where the fluid particles move in continuous parallel paths. Allows maximum transfer of energy due to minimum friction between particles. Opposite of Turbulent Flow.

Linear Deceleration

The slowing of a load evenly throughout the stroke of a hydraulic shock absorber. For example, as a rubber cushion is compressed, the force required to compress it further is more than the force required to compress it thus far. This results in an exponential energy absorption curve. The non-regenerative hydraulic shock slows the load evenly throughout its stroke, allowing for smooth, steady deceleration.

Load Holding Valve

Same as a counterbalance valve. Naming varies by manufacturer.

Load Pressure

The amount of pressure required to move a loaded actuator. This is always lower than system pressure. Load pressure is the result of a load on an actuator, as opposed to the ‘false load’ of undersized lines or an orifice.

Load Sense Circuit

A load sense circuit is useful for saving energy in systems that use less than full pump flow. A load sense control adjusts the output flow of the pump to maintain a constant pressure drop (ΔP) across an orifice.


The ability of an oil or grease to form a durable fluid film between contacting surfaces. The key characteristics of lubricity are the fluids natural film thickness, and its ability to cling to, or ‘wet’ a surface. Lubricity of a fluid is improved by adding anti-wear agents. Synthetic oils have excellent lubricity due to their natural affinity to metal.

Lunge Control (Over-speed Control, No-Jump Feed)

A mechanical means of adjusting the stroke of a hydrostat. This provides a preset flow rate that is closer to the actual operating flow of the system. Hydrostat hunting is greatly reduced, allowing the compensator to gain control more quickly and smoothly.



A manifold is a hydraulic integrated circuit; a pre-drilled mounting block to which cartridge valves, subplate mount valves, and other components are attached.


The term used to describe resilience in a seal material. Memory is the capacity of an elastomer to rebound completely to the pre-distorted shape. Opposite of compression set.


A type of volume control circuit in which a flow control valve (usually with a reverse free-flow check) is plumbed in series with the actuator. Oil is pumped through the valve before entering the cylinder. The ΔP across the orifice is additive to the load pressure. More accurate than a bleed-off circuit, but not as efficient. Pump flow in excess of the valve setting goes over the relief valve at system pressure. May be built into a cylinder or directional valve port. Used with resistive loads only.


A type of volume control circuit in which a flow control valve (usually with a reverse free-flow check) is plumbed in series with the actuator. Fluid must be pushed through the orifice after leaving the actuator and before returning to tank. The ΔP of the restriction adds to the load pressure. More accurate than a bleed-off circuit, but not as efficient due to the fact that the full pump flow must go somewhere, and if some of it cannot be used for work, then that portion will be dumped to tank at system pressure. May be built into a cylinder or directional valve port. Used with resistive or tractive (overrunning) loads.


Suction strainers are measured by mesh number or standard sieve number. The higher the mesh, or sieve number, the finer the screen. The mesh number refers to the number of vertical and horizontal wires in one square inch of material. Sizes 60 and 100 are the most common for use in suction strainers. 100 mesh will stop particles of 150µ and larger.


A measurement of particle size equivalent to one-millionth of a meter, or 39-millionths of an inch. The limit of visibility is about 40 microns (40µ). Contamination much smaller than this can severely damage hydraulic systems.

Mill Scale

A carbon compound that results from hot-working steel. The mill scale must be removed from the inside of the pipe using an acid bath to keep the mill scale from flaking off and going through the system. This is not good! Mill scale is second only to diamond, for hardness. Buy clean pipe – keep it clean.

Motor Spool

A directional valve center condition where the ‘P’ port is blocked, and ‘A’, ‘B’, ‘T’ ports are connected together. When placed in a motor circuit, it equalizes pressure between the motor ports when pump delivery stops, allowing freewheeling without damage. Often, the spool has orifices built in which slow the flow, thus smoothing out the valve response.

Motor Spool

A directional valve center condition where the ‘P’ port is blocked, and ‘A’, ‘B’, ‘T’ ports are connected together. When placed in a motor circuit, it equalizes pressure between the motor ports when pump delivery stops, allowing freewheeling without damage. Often, the spool has orifices built in which slow the flow, thus smoothing out the valve response.


Needle Roller Bearing

A type of roller bearing that uses long, thin, needlelike rollers mounted in a bearing cage. Used as shaft support bearings on fixed side plate gear pumps because their large surface area allows using an indirect drive.

Needle Valve

A type of flow control valve that consists of a decreased diameter orifice which is infinitely variable within its range by an adjustable needle and a tapered seat. A pressure differential across the orifice must exist for flow to occur. May be used to adjust load speed, isolate a pressure gauge, or simply as a small, expensive heater.

Nitrogen Pre-Charge

Nitrogen is a dry, inert (stable, non-combustible) gas. Used almost exclusively as the pre-charge in hydro-pneumatic accumulators because of these characteristics. Further, nitrogen is heavier than air.

Nominal Inside Diameter

The way in which pipe is measured. Refers to the smallest inside diameter of the pipe. The outside diameter of pipe is constant to facilitate using the same fittings with all pipe schedule numbers. The heavier the pipe (thicker walls, higher schedule number), the smaller the nominal inside diameter.

Nominal Micron Rating

Since all the holes in a filter element are not the exact same size, a nominal size rating is assigned to it. This number is the smallest average-size hole which particles can pass through. (Also see Absolute Micron Rating).

Non-Compensated Flow Control

A broad category of flow controls that includes orifices (adjustable or not) which do not automatically compensate for changes in load or system pressure. The greater the pressure drop across an orifice, the more fluid goes through.

Non-Compensated Flow Control

A broad category of flow controls that includes orifices (adjustable or not) which do not automatically compensate for changes in load or system pressure. The greater the pressure drop across an orifice, the more fluid goes through.



Outside diameter. Hydraulic pipe is measured by nominal O.D. Hydraulic steel tubing is measured by actual, measured O.D.

Oil Flow

The movement of oil in a circuit or system for the purpose of doing work. Ten gallons of oil passing a given point in one-minute equals 10 gallons per minute (10 GPM). Fluid always takes the path of least resistance.

Oil Pressure Booster

Very similar to an oil pressure intensifier, only with a small piston on each side of a large piston. This allows for a very fast cycle rate as each pressure stroke is also a return stroke. Delivers a continuous high-pressure oil flow as a result of the reciprocating piston.

Oil Pressure Intensifier

This intensifier type is a ‘one-shot’ device (intermittent high pressure oil flow) which must be returned after the pressure stroke. Very similar to a double-piston cylinder with two different piston/bore diameters. May also be actuated by air pressure. Intensifiers are available as either double-acting or single-acting. The single-acting type must be returned mechanically by spring, or other external means. Double-acting intensifiers are hydraulically returned.

Oil To Air

A type of oil cooling heat exchanger used when a large supply of cooling water is not readily available. The oil is pumped through a network of finned tubes which transfer the heat to the air. Usually, a hydraulically driven fan blows cooling air across the radiator fins, similar to automotive radiator operation.

Oil To Water

A type of oil cooling heat exchanger where the oil flows around many small copper tubes which have cold water flowing through them in the opposite direction. The heat transfers from the oil to the tubes to the water. This is the most efficient of the main heat exchanger types.

Oil Velocity

The speed (distance per unit of time) of oil flowing in a hydraulic system. Usually measured in feet per second (fps).

One Side of Center

The condition of a variable volume piston pump when the selector lever is fully one way or another. Moving the lever moves the yoke, which moves the swashplate. Changing swashplate angle changes the maximum piston stroke, altering displacement.

Open Center Spool

A directional valve with a center condition of ‘P’, ‘T’, ‘A’, and ‘B’, all interconnected. When used with a fixed displacement pump, it allows the pump to unload at very low pressure (100 to 200 PSI) and does not affect cylinder rod position when not stroking cylinder.

Open Circuit

A hydraulic system where oil passes through the reservoir before returning to the pump inlet. Most hydraulic circuits are open circuit.


The most common seal shape used in hydraulics. An o-ring is an ‘O’ shaped, molded seal with a circular cross-section. Made of a synthetic elastomer (Buna-N is the most common). The sealing is accomplished by the deformation of its profile. Mounts in an annular groove. May be used in static and dynamic sealing applications. May extrude into long fingers under extreme pressure.


A restriction, the length of which is small in respect to its cross-sectional area. An orifice size can be fixed or adjustable.

Over Center

A swashplate style pump is operating over center when the lever is thrown back fully in the opposite direction. This means the swashplate has been shifted through about 30° to 40°. Oil then flows in the opposite direction.

Over-Center Valve

Essentially, two load holding valves in one package. Used to stop a load from overrunning when it goes over center and becomes mechanically unbalanced.

Overall Efficiency

The general efficiency rating of a pump expressed as a percentage. It is the ratio of total power out to total power in. Efficiency = PowerOUT ÷ PowerIN

Overhung Load

An unguided pulling force on the rod of a cylinder that may cause rod or bushing damage due to a lack of support against the side forces. The force applied to a pump either by an external drive or by internal pressures. The further the load is from the bearing, the more overhung load there is. Overhung loads shorten component life.

Overrunning Load

A load which, because of its inertia, tries to move faster than pump flow is moving the actuator. Also called a runaway load.

Oxidation Resistance

When oxygen contacts most compounds, oxidation occurs. This happens on the surface of oil in a reservoir. This interaction of oil and oxygen produces acids and other chemical byproducts that attack metal surfaces and further increase oxidation. Another cause of oxidation in hydraulic systems occurs as entrained air bubbles are compressed and collapsed at the pump outlet. This compression causes extreme localized temperatures that literally fry the oil. Chemical stabilizers will not help this kind of oxidation. Oxidized oil feels, and acts like water and will have a characteristic pungent odor if it has been oxidized at the pump outlet. Oxidation resistance is a fluids resistance to breakdown by oxidation.


Parallel Flow

A circuit that offers more than one path for oil to flow, oil will take the path of least resistance. The pressure is equal in each connected, unrestricted branch. Closing one branch will not necessarily stop system flow.

Pascal’s Law

Pressure applied on a confined fluid transmits undiminished in all directions, and acts with equal force on all equal areas, and perpendicular to them.

Petroleum Based Fluid

A complex chemical compound whose properties are determined by the type of crude oil used, the degree and method of refining, and the additives used. Petroleum based fluid is the most widely used, and least expensive hydraulic fluid. Excellent lubricity characteristics.

Pilot Choke

On a pilot-operated directional valve, a pilot choke is a needle valve placed between the pilot valve and the main-stage valve for metering oil into the pilot valve from the system main pressure line. It is useful to equalize widely varying load pressures as seen by the pilot valve; the valve will not slam open one time and not shift another time. May produce a large pressure drop (ΔP).

Pilot Oil Flow Path

The route taken by oil shunted around the main valve for the purpose of hydraulically balancing, or overriding a valve spool or poppet. An externally piloted valve senses a remote valve actuating pressure (such as from a pre-fill pump line). An internally piloted valve senses pressure either from its own inlet (relief valve), or from its own outlet (pressure reducing valve).

Pilot-Operated Check Valve

A type of directional control that consists of a poppet, spring, pilot piston, sleeve, and housing. They are of two types: Pilot-To-Open and Pilot-To-Close. Pilot-to-open is the most common; and external pilot signal is required to pass oil. Used as a load-holding valve by connecting the inlet to the rod end, the outlet to the directional valve ‘B’ port and the pilot line to the directional valve ‘A’ port. The pilot-to-close version may be used to isolate a section of the circuit when certain pressure conditions are met and transmitted to the valve via the pilot line. When pilot pressure is felt in the valve, it closes.

Pilot-Operated Relief Valve

A pressure control valve that is actually made up of two different valves: a small stand-alone direct-operated relief is the pilot valve, and a large dependent pilot-operated main valve. When oil pressure reaches the valve setting, the pilot valve unseats and passes oil to tank. This creates a hydraulic imbalance with high pressure below the piston and low pressure above it. When fluid pressure is enough to overcome spring pressure (about 20 PSI), the main spool unseats and passes the main flow to tank.

Pilot Valve (Pressure Control)

A small capacity (~2 GPM), direct-operated relief valve that is put into the circuit with its ‘P’ line connected to the large valves pilot line. When this valve unseats, it causes an imbalance in pressure causing the main spool to shift. See Pilot-Operated Relief Valve.


The lateral pivot points on the yoke of a piston pump are pintles. Usually the pintle attaches to some form of control (manual, electrical, electronic, hydraulic, pneumatic, etc.).

Pipe Thread

Pipe is threaded with specially tapered threads that seal by an interference fit, wherein the threads are wedged together until they deform into each other. Only one ‘make’ allowable in high-pressure systems, as each remake distorts the threads further. Take care not to overtighten tapered pipe threads, especially on cast component housings with female pipe thread ports, or else the casting may crack.


A cylindrical cast part inside the cylinder, against which pressure is applied to create movement. It is attached to the rod.

Piston Pump

A type of hydraulic pump that provides positive displacement of oil using pistons reciprocating in barrels or cylinders. There are three general types of piston pump; axial, radial, bent axis. High precision, high pressure, high flow, high price.

Piston Shoe

This piece of a piston pump rides on the swashplate and is connected on the other end to a reciprocating piston. A very small orifice runs through the piston, ball joint and shoe to provide a thin lubricating film between the swashplate and the piston shoe.


A high flow, zero leakage, seating-valve design in which the seating element pops open to obtain free-flow in one direction. The valve automatically reseats when the flow direction changes.

Positive Displacement

A characteristic of a hydraulic pump or motor that features the pump outlet sealed from the pump inlet. Output is constant regardless of pressure except for differences due to inefficiency.

Pour Point

The lowest temperature at which a fluid will flow. Pour point is important in very low temperature operation. Generally, the pour point should be 20° F below the lowest expected temperature.


The measure of the rate at which work is done over time. Usually measured in horsepower or Watts.


A measure of force on a unit of area. Measured in pounds per square inch (PSI), bar, or kilopascal (kPa). Equal to the force of the load divided by the piston area. Pressure = Force ÷ Area (P = F ÷ A).

Pressure Compensator

A device that alters the displacement of a pump to continuously maintain the pressure and flow requirements of a hydraulic circuit. These pumps are of the vane or piston variety and referred to as ‘variable volume, pressure-compensated’.

Pressure-Compensated Flow Control

A category of flow controls which includes all orifices through which flow is automatically increased or decreased to maintain a constant ΔP, regardless of changes in load or system pressure. This is accomplished with a device called a hydrostat. The pressure drop across the orifice is always equal to the bias spring value.

Pressure Control Valve

A fluid power device used to hold system pressure safely below an upper limit and/or to maintain a set pressure in a part of a circuit. Types include relief, reducing, sequence, counterbalance, and unloading valves.

Pressure Drop (ΔP)

The difference in pressure between two points in a system. Pressure is always lower downstream. You must have a pressure drop to have flow.

Pressure-Loaded Side Plate

Style of gear pump which, in addition to the basic parts and operation of fixed side plate gear pumps, also contains a mechanism by which oil outlet pressure is used to push the side plates against the gear faces. This results in a well-sealed, positive displacement pump boasting a very high volumetric efficiency (95%+). They are also fairly rugged and very precise.

Pressure Override

The section of a valve response curve of a direct acting relief valve which begins at cracking pressure and goes to full flow. Cracking may be only 30% of relief valve setting, and may cause problems in a flow sensitive machine.

Pressure Plate

In a vane pump, the side plate that fits against the rotor on the pressure side of the pump. The opposite side, on the inlet, is the wear plate.

Pressure Reducing Valve

A pressure control valve designed to maintain a constant outlet pressure regardless of inlet pressure (as long as inlet pressure exceeds the valve setting).

Prime Mover

The source of power in a hydraulic system. The electric, gas, or diesel engine which drives the hydraulic pump.

Proportional Valve

A directional valve that controls and varies flow, pressure, direction, acceleration, and deceleration from a remote position. Electrically controlled by proportional solenoids. Output flow is proportional to input signal, providing accurate control of a circuit. Proportional valves are used extensively on mobile hydraulic equipment; they are very tolerant to dirt.


Pounds per Square Inch Absolute, including atmospheric pressure. A pressure measurement that starts at a perfect vacuum (0 PSI). Atmospheric pressure is equal to 14.7 PSIA.


Pounds per Square Inch Gauge, not including atmospheric pressure. A pressure measurement which starts at a relative zero (0 PSIG = 14.7 PSIA).

PT = PS + PV

Total Pressure = Static Pressure + Velocity Pressure. If the flow rate is constant, the sums of pressure energy (static pressure) and kinetic energy (velocity pressure) at various points in a system must be constant.

Push Pin

A small rod in a dry-type armature which passes from the solenoid (wet) area into the piston (wet) area. Sealed with a dynamic seal (o-ring) between them. Subject to leakage, over time. Wet armature types need no push pin, therefore will not leak.



A specially designed seal profile. Similar to an o-ring, but has a cross-section shaped like a concave sided square. The advantage being that there are two points of contact on each surface, instead of just one. Also, the concave area holds oil for lubrication. Used primarily in dynamic applications.

Quick Disconnect

A two-piece mechanical device that may be coupled, or uncoupled, to connect, or to isolate two fluid passages. Typically, they may be coupled or uncoupled by hand.


Radial Piston Pump

A subcategory of piston pumps. Radial piston pumps have multiple pistons disposed radially, and actuated by an eccentric element (cam). Radial piston pumps have the highest pressure rating of all hydraulic pumps.

Ram Type Cylinder

A hydraulic cylinder in which the moveable element has the same cross-sectional area as the piston rod. Commonly used on hydraulic floor jacks.

Relief Valve

A type of safety valve used to control/limit the pressure in a hydraulic system. You must protect all hydraulic systems against over-pressurization!

Regenerative Circuit

Regenerative circuits take advantage of the differential areas of a double-acting, single rod cylinder. Regeneration works by applying an equal pressure to both sides of a differential cylinder. Pressure against unequal areas generates unequal forces. The largest area prevails and the cylinder extends. The oil forced out of the rod end adds to the pump flow entering the blind end.

Remote Pressure Control Valve

Remote pressure modulation may be accomplished by using a pilot-operated valve, connecting its external drain line (vent from pilot valve) to a remotely mounted, direct-operated relief valve. Any number of remote valves may be used, each with different pressure settings. Remote pressure settings must be lower than the system relief valve setting as they are in parallel.

Resistive Load

A load that resists the movement of the actuator is a resistive load.

Return Line Filter

A full-flow type filter installed in the return line of a hydraulic system. Traps fine particles and sediment. Especially needful in mobile equipment and other applications requiring a smaller reservoir, and it is essential when using a high-performance pump.

Rocker Spring

A type of spring used under the vanes in a vane motor. Spring loading of the vanes is necessary to keep a positive seal around the inner circumference of the cam ring.

Rod Bushing

The support bearing for the cylinder rod, located on the rod end of the cylinder. The rod bushing contains the rod seals and wiper. The cylinder piston can force the rod bushing out of the end of the cylinder if excessive side loading persists without installing a stop tube.

Rod Cartridge

A type of cylinder rod gland that contains the rod seal and wiper in a single cartridge design.

Rod Seal

The dynamic seal between the cylinder rod and the rod bushing. Usually made from rubber-like materials, or more recently, polymers.

Rod Side Load

A perpendicular force acting on a cylinder rod, usually due to the cylinder working at a tremendous physical disadvantage. The cylinder side load, as well as columnar strength, and/or tensile strength, are the primary considerations when sizing hydraulic cylinders.

Rod Wiper

A conical-profiled rod seal element made of a material compatible, not only with the hydraulic fluid, but also the external environment (ice, dirt, chemicals, etc.). The purpose is to scrape dirt & other contaminants off the rod as it retracts into the cylinder.

Rotary Actuator

A hydraulic device that transforms hydraulic power to rotary power. Usually refers to a particular type of actuator, which, through a movable vane(s), chain and sprocket, or rack and pinion, rotate a shaft approximately 100° to 280°. The term ‘rotary actuator’ also includes most fluid motors. A circular chamber with a shaft running through it, supported by bearings. One or two vanes are attached to this shaft. Each vane has a u-cup type seal on all three edges.

A single-vane type can rotate through 280° before the vane hits the internal stop (shoe) and breaks it off. This often happens when someone adjusts the external stops to get a little more movement out of a machine. The double-vane type can only rotate through 100° but has 2X the torque output of the single-vane variety.

A rack and pinion type is essentially made up of a double-chambered hydraulic piston or cylinder assembly, in which the ‘rod’ is a rack, which meshes with a spur gear keyed to the shaft.


The moving member of a vane pump that houses the vanes. The rotor is often designed and ported to equalize most of the pressure felt on the vane. The rotor is attached to the input shaft.

Rust Inhibitor

An additive in hydraulic oil which ‘plates’ the metal to protect it against rust and corrosive acids caused by oil oxidation and moisture. Rust is the chemical interaction of oxygen and iron.


SAE Hose Identification Number

A hose designation number used to specify reinforcement, dimensional, and performance requirements. SAE100R1 and SAE100R2 are both steel braid reinforced. SAE100R4 is single-wire spiral. SAE100R9, SAE100R10, SAE100R12 are all four-wire spiral reinforced. SAE100R11 is six-wire spiral reinforced.

SAE Oil Grade

An oil viscosity rating standard established by the Society of Automotive Engineers (SAE). It specifies viscosity ranges of oils at specific temperatures. A ‘W’ suffix in the designation indicates a winter or low test-temperature rating. For example, 10W-40 motor oil has a viscosity of 10 in cold temperatures, and a viscosity of 40 in warm temperatures. The lower the number, the more stable the oil, the better for your car.

Sandwich Valve Mounting Style

A style of mounting valves between the subplate or manifold and other valves. Conserves space and allows for the addition of other valves to modify circuit behavior later.

Saybolt Universal Seconds (SUS, SSU, Saybolt Seconds Universal)

An oil viscosity rating that indicates how long it takes for a given quantity of fluid to flow through an orifice of a given size, at a given temperature. Industrial hydraulic oil is usually 150 to 200 second oil.

Schedule Number

The pipe designation that indicates wall thickness (pressure capability). All pipe of a given size has the same outside diameter to allow for standardization of fittings. Schedule 80 is closest to true inside diameter. The larger the schedule number, the thicker the walls, but also smaller inside diameter.

Seal Compatibility

There are several different hydraulic fluids in common use, some of which are harmful to Buna-N seals. Likewise, petroleum oil breaks down ethylene propylene seal materials. It is imperative that the seals used in a hydraulic system are compatible with the fluid used.

Sequence Valve

A pressure control valve that is connected into a circuit in parallel between two actuators from which you want sequential operation, such as a clamp and shear circuit. The clamp cylinder extends first to clamp the part with a pressure equal to the sequence valve setting. When pre-set load pressure (+20 PSI) is reached, the valve opens, passing oil to the shearing cylinder.

Series Flow

A circuit where oil has only one path to flow through or follow through components. If this path is closed anywhere, flow stops and pressure builds. Pressures in series are additive.

Servo Valve

A valve that modulates output flow as a function of an input command. They are electronically controlled, forced-solenoid positioned, and very, very accurate. Extremely susceptible to contamination.

Shelf Life

Any natural or synthetic elastomer has a definite useful life; whether in service, or just sitting on a shelf in stock. Oxygen and/or ultraviolet radiation breaks down most elastomers continuously. Check with the manufacturer for the shelf life of particular compounds.

Side-Ported Housing

A pump casing where the inlet and outlet ports go through the side of the unit.

Sight Glass

A clear tube mounted on the side of a reservoir that indicates the level of oil in the reservoir. Often includes a built-in thermometer.

Silicon Rubber

A type of seal material that has the highest temperature ratings of all hydraulic seal materials. It also works well at low temperatures. Compatible with most hydraulic fluids, however, at high temperature, silicone absorbs petroleum fluid. Other characteristics of silicone included a very low abrasion resistance (low durometer), and very expensive.

Single-Acting Cylinder

A type of hydraulic cylinder that has only one port, located on the blind, or cap-end of the cylinder. The cylinder is powered to extend, but retracts by gravity, spring, or other external force.

Single/Double Bypass

Most heat exchangers may be connected for the oil to make either a single pass, or a double pass through the heat exchanger. Double pass allows more heat to be dissipated, but does not allow the higher flow rates of the single pass type.


One of three component pieces, which together, make a 37° JIC fitting. The sleeve is the piece that slides over the tube and up against the flare. The other side of the sleeve has a face that mates with a face on the inside of the nut. Remember to install the nut and sleeve over the tubing before flaring. 37° JIC fitting parts are shiny, zinc plated.

Sleeve Bearing

A bronze bushing used in pressure-loaded side plate gear pumps as part of the hydrostatically centered bearing system. There is actually no metal-to-metal contact on the bearing surface. The gears ride on a cushion of oil that also serves to center the gears, cool, and lubricate.

Slide Block/Pivot Point

A mechanism in a variable-volume, pressure-compensated vane pump that is the fulcrum of the cam ring. When the pump begins compensating, it shifts the ring to change displacement. As the ring shifts, it hinges on the slide block/pivot point.

Socket-Weld Pipe Fitting

A type of high pressure, steel pipe fitting that connects to the pipe via unthreaded sockets in the fitting body. Socket weld is best in sizes ¾” and up. It is very difficult to weld sizes smaller than ¾”.

Solenoid Armature

The acted upon member of a solenoid assembly.

Dry Type – an electromagnet made up of a t-plunger, coil, and c-frame. The manual override controls tend to leak due to the o-ring static seal on the push pin.

Wet Type – The coil and frame are encapsulated in plastic, which allows for the coil to be immersed in oil. This eliminates the need for a static seal on the manual override push pin, allowing use in food processing facilities.

Solenoid Coil

A coil of fine wire wrapped around a hollow core. When energized, the coil becomes an electromagnet with the strength of the magnetic field varying proportionally with the number of turns. The armature is pulled into, and centers up, inside the coil.

Solenoid Assembly

The complete solenoid, including the coil, armature, frame, and pin. Attaches to a directional valve either as the valve actuator for a direct-acting valve, or as the pilot valve actuator in a pilot-operated valve.

Spherical Washer

A bell shaped washer, which attached to the yoke, allows it to pivot forward and backward on the shaft.

Spiral Hose

A type of hydraulic hose that utilizes heavy spiral wires as wall reinforcement. Single wire spiral (SAE100R4) is usually chosen for hydraulic suction lines as the single heavy wire spiral provides resistance to wall collapse under a vacuum. Multiple wire spirals are utilized in very high-pressure hoses. Generally, pressure hoses are four-wire spiral, but six-wire, and even eight-wire spiral hose is available. Larger diameter six and eight-wire spiral hose is very difficult to work with.

Split-Bolt Connector (Flange)

Essentially consists of a four-bolt SAE flange that has been split in a straight line between the pairs of bolt holes, leaving two halves, that together make up the four-bolt flange. Uses an o-ring as a sealing surface. This design allows attaching a fluid line to the SAE four-bolt flange pattern on a hydraulic pump or other component. This is the type of flange head found on a hose-end fitting.

Spring Centered

The type of centering mechanism used on most directional valves. A coil spring on each end of the spool housing pushes against a washer. When the valve is shifted, one end spring is compressed while the other remains uncompressed. Low cost, reliable. Under very high velocity-pressure, spring tension may be overridden.

Spool Crossover

The center envelope in a two-position valve. This is not a valve position, but, rather, a condition existing when shifting between two envelopes of the valve. Somewhat analogous to a make-before-break, or a break-before-make switch for electricity.

Spool Lands

The large diameter areas on a valve spool. The area between the lands provides the moving sidewalls of the flow-shifting chambers.

Spool Poppet

A hybrid valve element sporting features of both spools and poppets. Notably, spool poppets have the no leak, high flow characteristics of a poppet, and the hydraulically balanced, infinite positioning of a spool. Has the appearance of a poppet with a large land on it. This land has an orifice drilled through it. There is often an orifice drilled through the poppet section. These orifices serve primarily as pressure equalizers, and restrictors.

Spring-Loaded Vane

A type of hydraulic motor vane that uses coil springs, or rocker springs, to keep the vanes sealed snugly against the inside eccentric of the cam ring. In high performance balanced vane motors, coil springs are used most often.

Square-Cut Seal

An elastomer seal used in static sealing applications. The cross-sectional profile is square or rectangular. This type of seal is replacing gaskets in many static sealing applications. Used on automotive (spin-on) oil filters.

Start-Up Valve (Air Bleed Valve)

A type of pilot-operated directional valve used primarily with fixed displacement hydraulic pumps. It allows most of the pump flow to go to tank, slowly closing off the flow to tank proportionally, until the pump is up to full flow. Pilot signal causes this valve to shift.

Static Seal

A seal made between two stationary parts (some motion is always present due to expansion and contraction of surfaces, and vibration). Static sealing is accomplished through the compression of the seal between mating faces.

Steel Hydraulic Tubing

Alloy steel tubing which has been cold-worked to avoid mill scale. There are two types available, welded and seamless. Both are equally strong, provided the welded type is of good quality. Seamless tubing costs more, and is not as readily available (it is seldom specified). Steel hydraulic tubing is specified by outside diameter and called out in dash numbers (each dash number equals 1/16 of an inch). True ID = OD – (2 x Wall Thickness). Should be ordered oiled and capped.

Stop Tube

A metal collar which fits over the rod next to the piston. The purpose is to keep the piston and the rod bushing from touching where heavy side loads on the rod would tend to force the piston out through the bushing sideways. Not necessary, except with heavy side loads, or when using an extra-long rod.

Straight Thread O-Ring

A fluid line mounting thread style developed to be a leak-free seal which can be remade an unlimited number of times. One advantage is that the fitting may be aligned in any direction, and then tightened without the fitting rotating. Excellent for high-pressure use as the seal improves as the pressure increases.

Subplate Mounting Style

A style of valve mount where the valve is bolted to a subplate. This reduces line losses, provides quicker response time, and makes it easy to change out valves. Subplates have straight, through ports.

Suction Hose

Hydraulic hose which has a single, heavy wire spiral for wall reinforcement to withstand vacuum conditions. 100R4 is the SAE hose designation. Similar in construction to automotive radiator hose.

Suction Strainer

A coarse filtering element located in the pump suction line. Usually consists of a corrugated wire mesh basket (mesh numbers 60 and 100 are most common). It is important to clean the suction strainer regularly. If the suction strainer plugs, the pump will cavitate. In most cases, suction strainers are not recommended. They tend to be ‘out of sight, out of mind’ until a pump is destroyed.

Swage Hose Fitting

A type of hose fitting used with thermoplastic hydraulic hose. Swage fittings squeeze on, not crimp on. This leaves the body smooth (no crimp marks) and gives them a very large bulge at the top. Making does not crush the hose walls.


A stationary canted plate in an axial piston pump that causes the pistons to reciprocate as the cylinder barrel rotates.

Synthetic Fluid

Laboratory synthesized chemicals which are less flammable and more expensive than other hydraulic fluids. Not compatible with many seal materials. Some synthetics, due to their high specific gravity, require a strong suction at the pump inlet, or a flooded suction inlet.

System Pressure

Total combined pressure seen by the pump when the system is operating. Static Pressure + Velocity Pressure = System Pressure.


Tandem Center Spool

A directional control valve spool center condition where ‘P’ and ‘T’ ports are connected, and the ‘A’ and ‘B’ ports are blocked. Allows idling of a fixed displacement pump without serious energy losses.

Tank Drain

A ¾” tank flange and plug located in a lower end corner. Serves the purpose of draining oil out of the tank for service/maintenance. May also have a quick-disconnect through which oil is pumped into, or out of, the reservoir.


A non-elastomeric seal material made of a rigid tetra-fluorocarbon resin. Possesses extremely low friction characteristics as well as an extremely wide temperature range (-320° F to 500° F). Makes a good non-stick pan coating (do not use a metal spatula or exceed 500°). The trade name Teflon is owned by the E.I. DuPont Company.

Telescoping Cylinder

A single-acting (usually) hydraulic cylinder incorporating a nesting cylinder sleeve design. As the cylinder extends, the smallest diameter sleeve rises first. Next is the second smallest, and so on. Commonly used in hydraulic jacks and some tilting truck beds.

Temperature-Compensated Flow Control

A type of flow control that automatically compensates for changes in temperature (ΔT). As oil heats up, its viscosity decreases, and there is less resistance to flow. Maintaining accurate control of the rate of oil being metered through an orifice requires either adjusting orifice size (bi-metallic rod type), or using an extremely short orifice, which due to its minimal length, is practically immune to changes in viscosity due to temperature change (knife-edge orifice). Most often used in conjunction with a pressure compensator to provide very accurate flow control regardless of changing temperature and pressure.


A very handy high-pressure pipe fitting. If you are installing fluid lines, and later realize you forgot to put a tee in a line, you can simply drill a hole in the pipe, and weld on a Thread-O-Let. Then simply thread in the new line. Also available in a socket-weld style.

Three-Port Flow Control (Priority Flow Divider)

A type of pressure-compensated flow control valve that provides priority flow to a main circuit, and only when priority flow requirements are satisfied, passes excessive pump flow through the third port. Secondary flow may be sent either to a secondary circuit or to tank in a bleed-off circuit.

Tie Rod

A long grade-8 bolt that runs the length of the cylinder and through both end caps for holding the end caps onto the cylinder. There are four of these on a typical heavy-duty industrial hydraulic cylinder. Nuts thread onto the ends of the tie rods to hold the cylinder together. The tie rods may be extended on one end, or both, to facilitate cylinder mounting.


A rotary thrust. The turning power of a fluid motor usually expressed in inch-pounds (lbs.-in). Torque is a function of the pressure applied and the area to which it is applied. A measurable amount of twisting force created by a motor.

Torque Motor

An electro-mechanical transducer that is infinitely adjustable by varying input current. When used on a three-port pressure-compensated flow control valve, the metering spool is connected to the torque motor. As current is varied, the metering spool is moved proportionately. This allows for remote adjustment with a potentiometer, programmable logic controller (PLC), personal computer, or other electrical means.

Tube Bending

When attempting to install hydraulic steel tubing, occasionally you will come to an obstruction blocking the path of the tubing, or you need to route it in a different direction than it has been going. At times like these, a tube bender is very useful. The tubing may be bent any number of degrees, but the minimum bend radius of the tubing size must be observed, and the bend gain must be taken into account. Small diameter tubing is usually bent with a hand bender, and large diameter tubing must be bent with a mechanical or hydraulic bending machine.

Tube Fitting (Bite-Type)

One of two major techniques used to attach fittings to tubing. A bite-type fitting consists of three component parts: ferrule, nut, body. The ferrule is slipped over the end of the tube and set with a ferrule setting tool. In an emergency, a ferrule may be ‘field-set’ by tightening down the nut against the ferrule (this requires seemingly extreme force). Do not forget to add the nut before setting the ferrule. Bite-type fittings provide a leak-free seal. It takes approximately the same amount of time to properly set the ferrule as it does to flare a tube. Bite-type fittings sport a sleek, black oxide finish. Allows unlimited remakes if the ferrule is properly set (a ferrule is properly set when it has become a part of the tubing; it cannot rotate at all.

Tube Fitting (37° JIC Type, Cone Fitting)

The most popular fitting for tubing which can be flared. The seal is made by tightening the nut, which pulls the 37° mating cone (on the body) into the 37° flare. Three parts make up a 37° JIC fitting: sleeve, nut, and body. Do not forget to slip the nut and sleeve onto the tubing before bending and flaring. The sleeve also helps to support the tubing. 37° JIC fittings always have a shiny, zinc-plated finish. Care must be taken to not over-flare or under-flare the tubing. If over-flared, the nut won’t fit over the sleeve. If  it is under-flared, a positive seal may not be established.

Tube Flaring

The ends of a length of tube must be flared to 37° if JIC fittings are to be used. The flare provides a sealing surface with the mating nose cone. Care must be taken to carefully ream and deburr after cutting tubing. Burrs can become weak points in the flare. Also, longitudinal scratches on the flare face will probably leak. A properly flared tube end will be the same diameter as the sleeve, which mates up against the back of the flare.

Tubing Wall Thickness

Steel hydraulic tubing is called out by outside diameter as a dash number (sixteenths of an inch), and by wall thickness. Inside Diameter – (2 x Wall Thickness) = True Inside Diameter. The thicker the tube diameter, the thicker the walls must be to maintain the same pressure rating. This is due to F = P x A. as area increases (by squares), the force pushing outward on, and at right angles to the tubing increases proportionately.

Turbulent Flow

Turbulent flow is a condition wherein fluid particles move chaotically, in random paths. This creates heat and wastes energy due to the high fluid friction. However, inside a heat exchanger, turbulent flow is necessary for efficient transfer of heat energy to the cooling medium. Laminar flow tends to create an outer insulating layer in the oil flow.

Two-Piece Reusable Hose Fitting

This is one of the more popular hose fitting types with farmers, loggers, and sheep herders because it can be made up in the field with no special equipment. Consists of a nipple and a sleeve. The nipple screws into the inner tube of the hose with left-hand threads. Then the sleeve screws on over the outer cover (with right-hand threads). These threads are very sharp and deep to facilitate cutting through the tough outer cover and biting into the reinforcement wire. More expensive and larger than crimp-on or swage-type hose fittings. The sleeve has a hexagonal cross-section.

Two-Port Flow Control

Any volume control device having only two ports; inlet and outlet.



A type of piston seal with a ‘U’ shaped cross-section. Requires using a three-piece piston. Provides two contact points (sealing surfaces) and an oil retention area.

Unbalanced Vane

A vane pump design that uses centrifugal force to throw the vanes out against the ring, holding them, and sealing the chambers.

Unbalanced Vane Pump

This type of vane pump has only one pressure chamber and one exhaust chamber. They are mostly used in variable-volume, pressure-compensated style pumps. Unbalanced vane pumps are limited by their design to have relatively low pressure-ratings due to the large pressure difference between each successive 180° rotation.

Universal Link

An articulating joint in the shaft of a bent-axis piston pump. This joint transfers the power of the prime mover through an approximately 120° angle. Increasing this link angle increases displacement.

Unloading Valve

A pressure control valve whose primary function is to permit a pump or compressor to operate at minimum load. An unloading valve unloads to tank with a low pressure drop (ΔP = 20 PSI)

Unloading-Relief Valve

A pressure control valve used mostly in accumulator circuits to unload the pump when the accumulator reaches the pressure setting. Serves as an unloading valve and a relief valve in one package.



A pressure less than atmospheric pressure (0 PSIG) usually expressed in inches of mercury (inHg) relative to the existing atmospheric pressure.

Valve Poppet

A valve design utilizing a round steel rod with taper or bevel on one end that seals against a similarly ground mating surface or seat. Controls oil flow between two ports, using a spring to close or hold the starting position. Finite positioning – open or closed, no in-between.

Valve Spool

A cylindrical object with sections measuring different diameters, which moves through the fluid path within the valve body to allow or block fluid flow. Infinite positioning – it can be open, closed, or somewhere in-between.

Valve Spool Positioning

Using spring tension, oil pressure, or other physical force to move the valve spool to the desired location.


A small rectangle made of steel, the purpose of which, is to provide a closed, sealed chamber to fill with oil, pressurize, and expel it. The vanes are held in the rotor and rotated within the cam ring. Vanes may have special designs to ensure positive vane tracking (see Intra-Vane).

Variable Volume

A pump or hydraulic motor feature which permits modulating the displacement, usually by hydro-mechanical means. A common application is maintaining flow and pressure requirements.

Vent Port

The port on a pilot-operated pressure control through which you can externally exhaust the pilot valve spring-chamber, causing the main valve to shift.


Externally exhausting the pilot valve spring-chamber, causing the main valve to shift. Venting is controlled remotely by a knob on a remote-mounted direct relief; or by solenoid, or by manually shifting a directional valve. A remote pressure control valve incorporates a directional control and a pilot-operated relief in a single, subplate-mount housing.


A short tube with a constricting passage that increases velocity and lowers the pressure of a fluid flowing through it. Used to measure fluid flow, or draw fluid into the air stream.


The measure of a fluid’s resistance to flow. Hydraulic fluid viscosity is rated many ways. Most notable are the SUS, and the ISO viscosity grades. Viscosity is rated based on a quantity of fluid flowing through a certain orifice in a certain time. Viscosity is the single most important characteristic of hydraulic fluids.

Viscosity Index (VI)

An arbitrary measure of a fluid’s resistance to viscosity change, despite temperature changes. The oil is tested with a specific orifice size, and a specific quantity of fluid under varying temperature conditions. An oil which is thick when cold, and thin when hot, would have a low viscosity index.

Volume Control (Flow Control)

A hydraulic valve through which the volume of oil allowed to pass is regulated. Available in many sizes and types; pressure-compensated, temperature-compensated, adjustable, type of operator (oil, manual, electric), and two-port or three-port priority type.

Volumetric Efficiency

The measure of how much of the rated oil flow of a pump is lost through internal leakage and lubrication. Expressed as a percentage. EfficiencyVOLUMETRIC = GPMACTUAL ÷ GPMRATED

‘V’ or ‘W’ Packing

A type of piston or rod seal with either a ‘V’ or ‘W’ shaped cross-section. Several rings are nested together with backup rings on both ends of the stack. There are as many sealing surfaces as there are rings. As the packing wears, the rings can be further compressed, regaining the original seal integrity.


Water-Glycol Fluid

A type of hydraulic fluid made up of about 40% water, and 60% glycol (very similar to automotive anti-freeze). Added thickener improves viscosity. Water-glycol is fire resistant and works well in cold weather. Also includes other additives to improve lubricity, inhibit rust, etc.

Water-Oil Emulsion

A type of hydraulic fluid that takes advantage of the lubricity of oil, as well as the cooling ability and fire resistance of water. Made up of tiny droplets of water which are dispersed in petroleum oil (~40% water). An emulsifier may be added to keep the water from falling out of solution.

Wear Plate

In a vane pump, the side plate that fits against the rotor on the inlet side of the pump. The opposite side plate, on the pressure side, is the pressure plate. A thin film of oil separates the wear and pressure plates from the rotor.


Same as a Thread-O-Let, except the pipe connection is socket-weld rather than pipe thread. See Thread-O-Let.

Wiper Seal

The style of seal used on the rod of hydraulic cylinders for the purpose of scraping the dirt off of the rod before it retracts through the packing gland. The rod seal is a major entry point for contamination to enter a system. Replace rod wipers periodically.

Wire Braid

The reinforcement used on a SAE100R1 has a single wire braid, and SAE100R2 has two wire braids. In a cross-sectional view, wire braid reinforcement appears to be uneven around the circumference of the hose. for non-conductive applications, SAE100R7 (synthetic braid reinforcement) is used.


A force exerted over a distance, usually measured in foot-pounds or ton-inches. Time is not a factor.


‘X’ Port

The external pilot pressure port of a pilot-operated directional control valve. Connects to the ‘P’ port of the pilot valve. When using this port, do not neglect installing the internal plug to isolate pilot pressure from system pressure.


‘Y’ Port

The external drain port of a pilot-operated directional control valve. Connects to the ‘T’ port of the pilot valve. When using this port, the internal passage between the ‘T’ ports of both valves must be plugged to isolate the pilot valve from the main tank line, avoiding excessive backpressure conditions caused by one or more actuators exhausting at once.


A yoke is the framework inside an axial piston pump that holds the swashplate. As the yoke rotates on the pintles, the swashplate angle (displacement) changes.

Yoke Cylinder

The hydraulic control actuator in the compensator of an axial piston pump. As this small internal cylinder strokes, it pushes against the yoke, and thus the swashplate angle shifts, changing displacement.



A grease fitting facilitating the lubrication of an enclosed bearing surface. Utilizes a small check valve to allow the grease to enter, but keeps the dirt out.