Common Hydraulics Terms

2022-04-15

common hydraulics terms

Accumulator - A container which stores fluids under pressure as a source of hydraulic power. It may also be used as a shock absorber.

Actuator - A device which converts hydraulic power into mechanical force and motion. (Examples: hydraulic cylinders and motors.)

Aeration - The presence of dispersed air bubbles in the system's hydraulic fluid. Aeration can result in severe erosion of pump components when the bubbles collapse as they suddenly encounter high pressure when entering the discharge area of the pump.

ANSI - American National Standards Institute

ASAE - American Society of Agricultural Engineers (sets standards for many hydraulic components for agricultural use)

Back Pressure - The pressure encountered on the return side of a system.

Ball Valves - These valves are manual shut-off valves. They differ from needle valves in that they cannot meter the flow of oil. These valves can be used to shut off the flow of oil while repairs are made to the system or for shutting off the flow in one particular line of a circuit, etc.

Base plate - The end of the cylinder opposite the rod end. Some device, such as a cross tube or a clevis, is welded directly onto the base plate and used to secure the cylinder.

Bleed - The process by which air is removed from a hydraulic system.

Bore - The inside diameter of the cylinder tube.

Breakout Pressure - The minimum pressure which starts moving an actuator.

Breather Plug - A vent installed into a port on a hydraulic cylinder to allow a double-acting cylinder to be used in a single-acting application.

Bypass - A secondary passage for fluid flow.

Case drain - An external port used to drain off the small amount of oil that collects in the seal and bearing pockets of a hydraulic motor or pump. This oil has slipped through clearances between the gear sides and the housing. If the oil is not drained, the pressure inside the housing would blow out the shaft seal. This becomes a problem only when two or more hydraulic motors are connected in a series.

Cast Iron Ring - This is a style of piston seal that is a non-positive seal. It is made from cast iron and is like the piston rings on and engine piston.

Cavitation - A phenomenon which occurs when the pressure at a point in a hydraulic system is lowered below the vapor pressure of the oil in the system. This allows bubbles of oil vapor to form in the oil. If this occurs at the pump inlet, the quick pressure rise inside the pump forces these bubbles to collapse violently. This can cause erosion of metal parts, noise and vibration.

Check Valve - These valves are most often a ball, or poppet, and spring design. Oil is allowed to flow unrestricted in one direction by pushing the poppet off its seat. Oil is blocked in the other direction by forcing the poppet onto its seat and closing off the flow path. Applications include load holding, directing flow into other accessory valves, and any other application where backflow is unwanted.

Circuit - A series of component parts connected to each other by fluid lines or passages. Usually part of a "system".

Clevis - A "U" shaped bracket used for mounting a cylinder to an application. A clevis is either located at the rod end or on the end cap or both. The clevis is two narrow tangs that have a machined hole to receive a common pivot pin.

Closed Center System - A hydraulic system in which the control valves are closed during neutral, stopping oil flow. Flow in this system is varied, but pressure remains constant.

Closed Center Valve - A valve in which inlet and outlet ports are closed in the neutral position, stopping flow from pump.

Closed loop circuit - Once the fluid has been circulated, much like an open loop system, the oil is recirculated at low pressure back to the pump inlet, rather than being returned to the reservoir. This type circuit is more adaptable to hydraulic motor applications.

Cooler (Oil) - A heat exchanger which removes heat from a fluid. (See "Heat Exchanger.")

Corrosion Inhibitor - A compound or material deposited as a film on a metal surface that either provides physical protection against corrosive attack.

Coupler - A device to connect two hoses or lines, or to connect hoses to valve receptacles.

Cushion Valve - Cushion Valves (or crossover relief valve). These valves absorb pressure spikes caused if a motor or cylinder is brought to an abrupt stop while moving a heavy load. Both the inlet line and the outlet line are connected to each other inside the valve body by two relief valves, each facing opposite the other. If a spike occurs, the valve can discharge the flow to the opposite line. There are four ports on the cushion valve - two are connected to the cylinder or motor, and two are connected to the control valve, and slows down the piston.

Cracking Pressure - The pressure at which a relief valve, etc., begins to open and pass fluid.

Cross tube - Small pieces of pipe welded to the base plate or the rod of a cylinder which serve as a means to attach a cylinder to an application.

Cycle - A single complete operation of a component which begins and ends in a neutral position.

Cylinder - A device for converting fluid power into linear or circular motion. Referred to as an "actuator". Customers also refer to these as "pistons" "rams" and "pushrods."

Dead head - A situation that occurs when pressure within a system is stopped or blocked with no place to relief. Pump flow continues to build pressure until something gives way within the system, and thus resulting in damage.

Detent - A device which maintains a preselected spool position (either "in" or "out") on a directional control valve.

Differential Pressure - The difference in pressure between any two points in a system or a component. (Also called a "pressure drop.")

Direct-Acting Solenoid Valves - The armature is directly linked to the spool and provides the physical force for lifting.

Directional Control Valve - A valve which directs oil through selected passages. (Usually a spool or rotary valve design.)

Displacement - The volume of oil displaced by one complete stroke or revolution (of a pump, motor, or cylinder). Usually expressed as CIR (cubic inches per revolution). For example, if a motor rated is at 2.2 CIR, this indicates that for every revolution, the motor displaces or expels 2.2 cubic inches of fluid.

Double-Acting Cylinder - A cylinder in which fluid power can be exerted on both sides of the piston. Sometimes referred to as "power up, power down". This cylinder can be controlled in both the extend and retract phases.

Double pump - Two pumps in one housing. Two separate inlets and outlets are utilized. One shaft drives both pumps. There is a pump at the shaft end and one at the cover end.

Drift - Motion of a cylinder or motor due to internal leakage past components in the hydraulic system.

Dust Cap - A removable device that protects the male tip half when disconnected from the female coupling half. Excludes contamination.

Dry Armature Coils - The coils operate in a dry environment, protected by solenoid housing.

Dynamic - Capable of movement or motion

Dynamic Seal - This is a seal that seals a moving surface like a rod or a barrel.

Eccentric - Uses a locking ring that fits onto the outside of the bearing, when the ring is rotated up to 180 degrees, the shaft is secured to the bearing. The letters HC indicate eccentric collar.

Electro-Hydraulic / Solenoid Valve - A valve that is opened and closed by a solenoid.

Extended length - The length of a cylinder measured from the center of the mounting hole on the rod end to the center of the mounting hole on the base end when the piston and rod are in the "out" position. The longest overall length of a cylinder.

Filter (OIL) - A device which removes solids from a fluid.

Fitting - An adapter designed to connect different types of hose, tubing or pipe together.

Fixed Displacement Pump - A pump in which the output per cycle cannot be varied.

Float - This valve connects the "A" and "B" work ports to the tank port in a detented fourth position. This allows oil to flow from both ends of a double-acting cylinder which, in turn, allows the rod to extend or retract depending on the force applied to the rod. For example, a bulldozer blade or a snowplow blade.

Flow Control Valve - A valve which controls the rate of flow

Flow Divider Valve - A valve which divides the flow from one source into two or more branches. (Includes "priority" and "proportional" types.)

Flow Meter - A testing device which gauges either flow rate, total flow, or both.

Flow Rate - The volume of fluid passing a point in a given time.

Fluid Power - Energy transmitted and controlled through use of a pressurized fluid.

Force - A push or pull acting upon a body. In a hydraulic cylinder, it is the product of the pressure on the fluid, multiplied by the effective area of the cylinder piston. It is measured in pounds or tons.

Four-Way, Four Position - This valve is identical to four-way, three-position with the fourth position as "float" or "motor" position. Power up, gravity down.

Four-Way, Three-Position - Controls double-acting cylinders. Four-way denotes the flow pattern: (1) inlet, (2) "A" work port, (3) "B" work port, and (4) outlet. Three-position denotes the handle position: (1) forward or up, (2) neutral or middle, and (3) back or down. Power up, gravity down.

Friction - The resistance to fluid flow in a hydraulic system. (An energy loss in terms of power output.)

Full-Flow Pressure - The pressure at which a valve is wide open and passes its full flow.

Gauge (Pressure) - A device for measuring the pressure of a gas or liquid.

Gear Pump - Uses two gears. The "drive" gear is keyed to the shaft and meshes with the "driven" gear. The oil flows around the outside diameter of each gear as they revolve. A suction is formed on the inlet side by the oil being carried away in the cavities formed beneath the teeth of the gears. The oil is then transported around and discharged into the cavity of the outlet port. The meshing of the teeth in the middle seals the inlet from the outlet. These are fixed displacement pumps. The output flow can be varied by changing drive speed.

Gerotor - This type of motor has two elements -- the inner being keyed to the shaft, which rotate and mesh together inside a housing. At top dead center the two elements come into full mesh. At bottom dead center, the two elements are completely out of mesh. The center element has one less tooth than the outer element, which causes cavities to form as each tooth of the inner element moves from one cavity to the next.

Gerole - This type of hydraulic motor also has two elements. As the inner gear rotates, rollers which form the displacement chambers provide support by a rolling action which minimizes friction. As with the gerotor type motor, the inner gear has one less tooth than the outer element formed by the rollers.

Gland - The end cap through which the rod extends from the cylinder tube. The gland has seals for the rod and for the tube of the cylinder as well.

GPM - Gallons per minute - common flow rate term in hydraulics

Heat Exchanger - A device which transfers heat through a conducting wall from one fluid to another.

Horsepower - The work produced per unit of time.

Hydraulics - The engineering science of liquid pressure and flow.

Hydraulic Amplifier / Intensifier - A hydraulic intensifier is a hydraulic machine for transforming hydraulic power at low pressure into a reduced volume at higher pressure

ID - inside diameter (as of a hose or tube)

In-Line Flow Control Valves - Has one inlet and one outlet. Has complete shut-off and variable speed control. Since there is no excess flow port, this valve is used to meter the flow of oil only.

ISO - International Standards Organization

Load Check - Prevents a load from dropping while the control valve spool is being shifted from one direction over center to the opposite direction.

Loaded U-Cup - Also called a Poly-Pak, a loaded u-cup has an O-ring installed in the u-cup to help it seal at cold temperatures and low pressures.

Lock Valves - These valves are designed to lock a cylinder, or part of a circuit, without leakage while a control valve is in a neutral position. Essentially, these are pilot-operated check valves, allowing flow to an actuator and blocking reverse flow until pilot pressure is applied to "unlock" the circuit. Lock valves can be used for safety devices. They prevent movement of the load if the control valve is accidentally operated (while the pressure source is inactive) and if a line rupture occurs. They are designed for applications where leakage through the control valve could adversely affect the performance of the system such as clamps, outriggers, and work platforms. Both lines to the cylinder from the control valve must be connected to the lock valve, in this way pilot pressure from both the extend side and the retract side of the cylinder can be detected. If pressure drops on one side, the valve "locks" until the pressure is equalized on both sides.

Manifold - A fluid conductor which provides many ports. Commonly used with solenoid valves. Available in parallel circuit or series circuit

Metering (Feathering) - Regulating the flow of oil through the valve by gradually moving the spool in one direction or the other.

Monoblock Valves - One or more spools contained in a single housing

Motor (Hydraulic) - A device for converting fluid energy into mechanical force and motion - usually rotary motion. Basic design types include gear, vane, and piston units.

Motor Spool - Similar to the float spool but intended to allow a hydraulic motor to freewheel. Also prevents "dead stop" in a hydraulic motor by allowing gradual slow down when the valve is moved to neutral.

Needle Valve - A valve with an adjustable tapered point which regulates the rate of flow. Can be used as either a flow control or a shut-off valve. A small, tapered needle restricts the flow in both directions. When the needle is firmly seated, the flow is completely stopped. By adjusting the orifice on the needle valve, the operator can control the speed of the cylinder.

Nitriding - (Nitro Rod) - A case hardening process that depends on the absorption of nitrogen into the steel. All machining, stress relieving, as well as hardening and tempering are normally carried out before nitriding. The parts are heated in a special container through which ammonia gas is allowed to pass. The ammonia splits into hydrogen and nitrogen and the nitrogen reacts with the steel penetrating the surface to form nitrides. Nitriding steels offer many advantages: a much higher surface hardness is obtainable when compared with case-hardening steels; they are extremely resistant to abrasion and have high fatigue strength.

OD - Outside diameter (as of a hose or tube)

Operating Pressure - The maximum pressure at which a system is operated.

Open Center System - A hydraulic system in which the control valves are open to continuous oil flow, even in neutral. Pressure in this system is varied, but flow remains constant.

Open Center Valve - a valve, in which the inlet and outlet ports are open in the neutral position, allowing a continuous flow of oil from pump.

Open loop circuit - A system where the oil is drawn from a reservoir at atmospheric pressure, circulated by a pump under pressure, through valves, into an actuator and then returned to the reservoir at near atmospheric pressure. Most hydraulic systems are of this design.

Operating Pressure - The pressure at which a system is normally operated.

Orifice - A restricted passage in a hydraulic circuit. Usually a small drilled hole to limit flow or to create a pressure differential in a circuit.

O-Ring - A static and/or dynamic seal for curved or circular mating surfaces.

O-Ring Boss (SAE) Port - This is a port that has straight or parallel threads as well as an O-Ring to seal the threads.

Packing Kit - All the necessary seals to rebuild a hydraulic cylinder or component.

Parallel Circuit - The most commonly used in mobile equipment. Oil is available to all ports. However, if two or more spools are fully shifted at the same time, the oil will follow the path of least resistance and the cylinder or motor with the lightest load will begin to function first. The oil can be divided so that it will flow to two or more functions by metering the spools.

Pascals Law - states that "pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio (initial difference) remains the same. Force applied to a confined fluid acts with equal pressure in all directions.

Pilot-Operated Solenoid Valves - The spool is shifted by fluid pressure tapped from an inlet port or an outside pilot line. The solenoid opens and closes miniature orifices directing fluid to the end of the spool. The pilot valve usually sits atop the slave valve.

Pilot Pressure - Auxiliary pressure used to actuate or control a component.

Pilot Valve - A valve used to operate another valve or control.

Pipe Port (NPT) - Pipe threads are tapered threads that count on interference between the male and female threads to do the sealing.

Piston - A cylindrical part which moves or reciprocates in a cylinder and transmits or receives motion to do work. The disc-shaped element within a cylinder connected to the rod. The surface area of the piston dictates the cylinder’s force capabilities. The hydraulic fluid acts on the surface of the piston opposite the rod. Seals are used on the piston to prevent leakage.

Poppet Valve - A valve design in which the seating element pops open to obtain free flow in one direction and immediately reseats when flow reverses.

Port - The open end of a fluid passage. May be within or at the surface of a component.

Positive Seal - A positive seal is a term that refers to a seal that has the capability to seal with no leakage.

Power Beyond - An optional power port on open center valves for directing flow to a downstream valve.

Proportional Flow Dividers - These valves will divide the flow of one pump into two equal flows regardless of variations in load. Most of these valves are 50/50 ratio; however, other ratios can be ordered from the manufacturer.

Pressure - Force of a fluid per unit area, usually expressed in pounds per square inch (psi).

Pressure Compensated Flow Control Valves - These valves are designed to control the speed of hydraulic cylinders or motors, eliminating the variations in speed caused by changes in load. These valves have an inlet and a "controlled flow" (CF) port as well as an "excess flow" (EF) port. Adjustable flow control valves allow the operator to adjust the flow of the "CF" by varying the size of the orifice of that port. Once the "CF" is set, it will remain nearly constant with variations in pressure on either the "CF" or the "EF" port. Any remaining flow is bypassed to the excess flow port which can be used to supply another circuit or can be directed to tank.

Pressure Relief Valve - These are "safety valves". They serve to set a limit to the rise in pressure in a line or circuit. There are two categories; direct-acting and pilot-operated relief valves. The direct-acting is one in which the poppet is half closed by direct force of a mechanical spring. Any increase in pressure past the "cracking pressure" will cause the poppet to be unseated and allow a small part of the oil to escape. Advantages of the direct-acting relief valve are that they are less expensive than the pilot-operated type and they have a faster response time. Pilot-operated reliefs hold the poppet on its seat by adjustable pilot pressure. The pilot pressure can be supplied internally or externally and is most often from the pump line. When the pump line pressure rises higher than the adjustment set on the relief, the poppet becomes unseated and oil is directed back to tank. When pump line pressure drops below the control setting, the poppet can re-seat and close the valve. The pilot-operated reliefs can be more accurately adjusted and are used more often as the main relief in hydraulic systems.

PSI - Pounds per square inch (of pressure)

PTO Pump - This pump is operated by the power take-off shaft of a tractor or other piece of equipment. As opposed to the standard gear pump, which has a shaft of its own that couples to drive a motor or engine.

Pump - A device which converts mechanical force into hydraulic fluid power. Basic design types are gear, vane, and piston units.

Rated Pressure - The operating pressure which is recommended for a component or a system by the manufacturer.

Regenerative Circuit - A circuit in which pressure fluid discharged from a component is returned to the system to reduce flow input requirements. Often used to speed up the action of a cylinder by directing discharged oil from the rod end to the piston end.

Regulator - Hydraulic regulators maintain the output pressure of a hydraulic system at a set value, minimizing fluctuations in a pressurized line. A hydraulic regulator is typically made of steel, coated steel, or stainless steel and comes with a variety of connections. Hydraulic regulators are designed to hold a tight seal even as the pressure in the line rises. Hydraulic regulators may be used in land-based applications or may be specially designed to work underwater.

Relief Valve - A valve which limits the pressure in a system, usually by releasing excess oil.

Remote - A hydraulic function such as a cylinder which is separate from its supply source. Usually connected to the source by flexible hoses.

Rephasing Cylinder - Rephasing cylinders are two or more cylinders plumbed in series or parallel, with the bores and rods sized such that all rods extend and/or retract equally when flow is directed to the first, or last, cylinder within the system.

Reservoir - A container for keeping a supply of working fluid in a hydraulic system.

Restriction - A reduced cross-sectional area in a line or passage which normally causes a pressure drop. (Examples: pinched lines or clogged passages, or an orifice designed into a system.)

Retainer ring - A ring that screws down over the outside diameter of a cylinder tube and holds the gland in place on a welded cylinder.

Retracted length - The length of a cylinder measured from the center of the mounting hole on the rod end to the center of the mounting hole on the base end when the piston and rod are in the "in" position. The shortest overall length of a cylinder.

Rod - The chromed shaft, attached to the piston inside the cylinder tube, the end of which extends out of the gland of the cylinder and is attached to an application by a clevis, cross tube, or a hole bored directly through the rod. The rod also serves and dynamic sealing surface for the rod seal.

RPM - Revolutions per minute

Rubber shields - A bearing number is followed by the suffix 2RS indicates that the bearing is sealed on both sides with two rubber shields.

SAE - Society of Automotive Engineers (sets standards for many hydraulic components)

Seal Kit - All the necessary seals to rebuild or repair a hydraulic cylinder or component.

Sectional Control Valves - Consists of an inlet, an outlet, and work sections. Can be from one to 10 valves. Provides the customer the ability to customize a bank of valves. Work sections can be mixed and matched to provide several functions.

Selector Valve - A valve which selects one of two or more circuits in which to direct oil, usually operated manually. The single selector valve permits the flow of one pump to be diverted to one of two hydraulic lines. The double selector valve permits the flow of one pump to be diverted to two separate circuits. The double selector valve has one inlet and one outlet and two sets of work ports, which allow one four-way valve to control two double-acting cylinders. The single selector can control two single-acting cylinders.

Sensor - a device that responds to a physical stimulus (as heat, light, sound, pressure, magnetism, or a particular motion) and transmits a resulting impulse (as for measurement or operating a control)

Series Circuit - The entire flow of hydraulic oil is available to each work section in sequence from inlet to outlet port. As oil is directed from the inlet port to the work port of the first spool which is shifted, the returning oil to that section is directed back to the open center passage and not the tank port as in parallel valve circuit. The returning oil is then available for any downstream section.

Set screw - Uses two recessed set screws. The letters UC indicate set screws.

Shuttle Valve - Used primarily to control pilot flows. This valve has two inlets and one outlet. The flow with the highest pressure is accepted and the other is blocked. Both flows are kept separate. The floating poppet is free to move back and forth, closing the inlet with the lowest pressure. Used with very low-pressure systems.

Single-Acting Cylinder - A cylinder will have the capacity to be utilized in only one direction. Commonly referred to as "power up, gravity down". The extend or push phase is controlled by a valve, but the retract or return phase is achieved by the weight of the application. A dump body is a prime example.

Solenoid - An electro-magnetic device which positions a hydraulic valve.

Spherical Bearing - A spherical bearing is a bearing that accommodates some misalignment of one or both pivot pins. It is a bearing that is pressed in to a tang and is located at the rod end, base end or both.

Spring Return to Neutral - The handle springs back to the neutral position when released.

Starvation - A lack of oil in vital areas of a system. Often caused by plugged filters, etc.

Static - Stationary or fixed

Static Pressure - The pressure in a fluid at rest. (A form of "potential energy.")

Static Seal - This is a seal that does not move. It’s not dynamic, it’s static.

Steel Shields - A bearing number is followed by the suffix ZZ indicates that the bearing is sealed on both sides by steel shields.

Strainer - A coarse filter.

Stroke - The distance which the piston travels through the cylinder tube between the gland and the base plate. Since the rod is attached to the piston, this is the usable travel or movement of the cylinder.

Stroke Controls - These controls are used to limit the stroke of a hydraulic cylinder. There are several variations of this idea. Stroke control segments can be added to the rod to limit the return stroke. Some stroke limiters are installed inside the cylinder tube to control the extension or the outward stroke.

Sub-plate - A method for mounting a single solenoid valve.

Suction Pressure - The absolute pressure of the fluid at the inlet side of the pump.

Surge - A momentary rise of pressure in a hydraulic circuit.

Surge Pressure - The pressure changes caused in a circuit from a rapidly accelerated column of oil. The "surge" includes the span of these changes, from high to low.

Switch - a mechanical, electrical, electronic, or optical device for opening or closing a circuit or for diverting energy from one part of a circuit to another. Such as a remote control or solenoid switch.

System - One or more series of component parts connected to each other. Often made up of two or more "circuits".

System Pressure - The pressure which overcomes the total resistances in a system. It includes all losses as well as useful work.

Tandem Center Valve - A valve, in which the inlet and outlet ports or connected in the neutral position, allowing flow back to tank, while the work ports are blocked.

Tang - A single protrusion from the base plate and/or the rod of a cylinder, drilled and centered to allow for mounting on an application. This male end fits inside a clevis-type mounting and is pinned.

Telescopic Cylinder - Consist of two or more "stages". Each stage extends the cylinder to its maximum and then another stage extends and so on until the complete cylinder is extended. Telescopic cylinders can either be single or double acting. Telescopic cylinders are used in applications where the retracted length needs to be shorter than half of the extended length.

Terminal Expansion - Expansion of the fluid volume due to heat.

Three-Way, Three-Position - Controls single-acting cylinders. Three-way denotes the flow pattern. Oil flows through three individual ports: (1) inlet, (2) "A" work port, and (3) "B" work port. Three-position denotes the handle position: (1) forward or up, (2) neutral or middle, and (3) back or down. Power up, gravity down.

Tie Rod Cylinder - A hydraulic cylinder held together by 4 tie bolts that hold the rod end gland and the base plate onto the cylinder tube. The advantage of the tie-rod cylinder is that they are easier to repair in the field and are less expensive.

Torque - The turning effort of a hydraulic motor or rotary cylinder. Usually given in inch-pounds (in-Ibs) or foot-pounds (ft-lbs). The twisting motion produced on the shaft of a motor which provides it the capacity to do work.

Trunnion - The trunnion mount is a mounting that is typically housed around the barrel and has two male pins located 180 degrees from one another.

Two-stage Pump - Two pump sections contained in one housing, one with a large gear set and one with a small gear set. Both sections utilize a common inlet and outlet. A high-volume flow, at low pressure, is provided as the gear sets work together, resulting in a fast-forward movement of a hydraulic cylinder. An internal sequence valve isolates the small gear set when a preset pressure is reached. This enables the small gear to set to generate several times greater operating pressure. But, because the discharge flow is correspondingly less, the increase in force doesn’t require any more engine horsepower. First stage: low-pressure, high-volume flow. Second stage: high-pressure, low-volume flow.

U-Cup - This is a very common seal that is usually made from rubber or urethane. They are commonly used for dynamic sealing for either a piston or rod seal.

Unloading Valve - A valve which allows a pump to operate at minimum load by dumping the pump's excess oil at a low pressure.

Valve - A device which controls either 1) pressure of fluid, 2) direction of fluid flow, or 3) rate of flow.

Vane Pump - A pump having multiple radial vanes within a supporting rotor. The rotor is keyed to the shaft and is offset in relation to the cam ring. As the rotor turns, the vanes extend and retract depending on the point of contact with the cam ring. Oil is trapped between the vanes during one-half of the evolution on the inlet side, and the expelled during the other half of the revolution on the outlet side. The vanes extend and retract by either a spring or centrifugal force. These can be variable diplacement pumps.

Variable Displacement Pump - A pump in which the output per cycle can be varied.

Velocity - The distance which a fluid travels per unit time. Usually given as feet per second.

Vent - An air breathing device in a fluid reservoir.

Viscosity - The measure of resistance of a fluid to flow.

Volume - The amount of fluid flow per unit time. Usually given as gallons per minute (gpm).

Welded Cylinder - A hydraulic cylinder which has its base end welded into place. The gland is usually held in place by a retainer ring or snap ring. These cylinders are usually rated for higher pressures.

Working Pressure - The pressure which overcomes the resistance of the working device.

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