Introduction to Hydraulic Components 221

accumulator

A fluid system component that stores pressurized hydraulic fluid from the pump as a source of fluid power. Accumulators release hydraulic fluid as necessary at the required system pressure.

active components

Any fluid system component characterized by being a source of activity. Active components, such as hydraulic pumps, perform an explicit action on hydraulic fluid.

actuator

An active fluid system component at the end of a hydraulic circuit. Actuators, such as cylinders or hydraulic motors, convert hydraulic energy into mechanical energy.

additives

A substance added to another substance or material to improve its chemical and physical properties. Additives in crude oils can protect against rust and improve flow.

air-cooled heat exchanger

A component in a hydraulic system that uses cold air to relieve excess heat. Air-cooled heat exchangers dissipate heat from the hydraulic fluid.

axial piston pump

A hydraulic pump that has pistons arranged in a circle and parallel to the drive shaft centerline. An axial piston pump uses an angled swashplate that maintains contact with the piston shoes to reciprocate pistons.

backflow

The movement of pressurized fluid opposite its intended or desired direction. Backflow causes fluid to return to its source.

balanced vane pump

A hydraulic pump that consists of extendable blades attached to a rotor that is centered in an elliptical cam ring. A balanced vane pump is a fixed-displacement pump.

bearings

A device that minimizes friction between moving parts. Bearings allow for linear or rotational movement, and may also prevent movement by controlling forces that act on moving parts.

bladder

An expandable flexible bag that can be filled with air or liquid. The bladder in a bladder accumulator separates compressed gas from pressurized liquid.

bladder accumulator

A hydropneumatic accumulator that uses a flexible elastic bag to separate the hydraulic fluid from the compressed gas. A bladder accumulator transfers power from the compressed gas to the hydraulic fluid.

boosters

A component of a hydraulic system that converts low-pressure fluid power into high-pressure fluid power. Boosters, also referred to as hydraulic intensifiers, typically receive pressure from oil.

cam ring

An external rotor casing. The cam ring in a vane pump remains stationary as the rotor turns.

check valves

A directional control valve that allows fluid flow in one direction and blocks flow in the reverses direction. Check valves prevent backflow.

conductors

A passive fluid system component that conveys fluid through a hydraulic system. Conductors include pipes, tubes, and hoses.

contamination

The presence of any unwanted substances in a fluid system. Contamination is the primary cause of problems in a fluid system.

control valves

An active fluid system component that controls fluid flow rate, direction, and pressure as fluid moves through the system. Control valves include pressure relief valves and needle valves.

corrosion resistance

A fluid's ability to stop the formation of rust within the components of a hydraulic system. Corrosion resistance in a hydraulic fluid prevents fluid deterioration.

crescent pumps

An internal gear pump consisting of an internal gear that is separated from an external gear by a metal seal. Crescent pumps are fixed-displacement pumps.

crescent seal

The stationary component in a crescent pump that separates the internal gear from the external gear. A crescent seal is machined into the pump where there is maximum clearance between the gears.

crude oils

Unrefined petroleum oil. Crude oil is often refined and mixed with additives for use as hydraulic fluid.

cylinder block

A solid unit that houses the pistons in a piston pump. The cylinder block in an axial piston pump rotates and the pistons maintain contact with the angled swashplate in order to reciprocate.

cylinders

A hollow bore that houses a movable piston attached to a piston rod. A cylinder applies force in a straight line.

demulsibility

A fluid's ability to release water. Demulsibility in a fluid means that the fluid will separate from water.

diaphragm accumulator

A hydropneumatic accumulator that uses a metal partition to store compressed gas and pressurized liquid. Diaphragm accumulators are similar to bladder accumulators.

directional control valves

A fluid system component that determines the path hydraulic fluid takes in a hydraulic system. Directional control valves start, stop, and control the direction of fluid flow.

double-acting cylinder

A hydraulic linear actuator that applies force in both directions as the piston reciprocates. Double-acting cylinders use fluid pressure to extend and retract the piston.

drive gear

A gear that transmits power. A drive gear is attached to a drive shaft, which is driven by the prime mover.

drive shaft

A driving mechanism that rotates to transfer motion. A drive shaft is powered by a prime mover, such as an electric motor.

driving mechanism

A component that rotates to transfer motion. The driving mechanism in a positive-displacement pump is typically a drive shaft.

efficiency

The amount of power output a hydraulic pump produces compared with the amount of power input. Efficiency ratings are used to determine the condition of a pump.

electric motor

A machine that converts electricity into mechanical energy or motion. An electric motor is a type of prime mover for a hydraulic system.

external gear pump

A fixed-displacement hydraulic pump that moves fluid using two meshing gears that rotate. An external gear pump is available in single or multiple configurations.

filter

A device that separates and retains insoluble contaminants from a fluid. A filter for a hydraulic system has a maximum flow rate and pressure rating.

fire-resistant fluids

A hydraulic fluid that is very difficult to ignite. Fire-resistant fluids do not support combustion once the ignition source is removed.

fittings

A fluid component that connects, redirects, extends, or terminates the run of a fluid conductor. Fittings that are installed improperly can result in external leakage.

fixed displacement

A hydraulic pump with a fluid displacement value that stays the same from cycle to cycle. Fixed-displacement pumps are not as common in industrial settings as variable-displacement pumps.

flammability

The ability or tendency to ignite or burn when exposed to an open flame or ignition source. The flammability of petroleum-based oils can present a fire hazard in hydraulic systems used near high temperatures or open flames.

flow capacity

The total volume of fluid that a pump moves into the hydraulic system in a given period of time. Flow capacity is also known as flow rate and is measured in gallons or liters per minute.

flow control valves

A fluid system component that controls the rate of fluid flow through a hydraulic system. Flow control valves make it possible to control other system components, such as the speed of an actuator.

flow rate

The total volume of fluid that a pump moves into the hydraulic system in a given period of time. Flow rate is also known as flow capacity and is measured in gallons or liters per minute.

fluid displacement

The total volume of fluid a hydraulic pump moves in one reciprocating or rotating cycle. Fluid displacement is measured in cubic inches or cubic centimeters per revolution.

fluid flow

The motion of a fluid, usually a liquid or a gas, subjected to unbalanced forces or stresses. Fluid flow in a hydraulic system is subjected to resistance, which allows pressure to develop.

force

The push or pull that gives energy to an object. Force results from an object's interaction with another object.

force multiplication

The increase in available power associated with power transmission systems. Force multiplication allows a hydraulic system to produce an output force that is larger than the input force if the same pressure is applied to differently sized areas.

friction

A force that resists motion between two moving objects that touch. Friction between the moving components in a fluid system can lead to excess wear.

gallons per minute

gpm. A unit of English measurement that is used to measure the flow capacity or flow rate of a hydraulic pump. Gallons per minute is a unit of volumetric flow.

gear motor

A hydraulic rotary actuator with two or more meshing gears. A gear motor has a fixed displacement.

gear pumps

A fixed-displacement hydraulic pump that uses meshing gears that rotate to move fluid. Gear pumps are relatively inexpensive and compact.

glycol

An organic compound belonging to the alcohol family. A glycol and water solution forms a fire-resistant hydraulic fluid.

gpm

gallons per minute. A unit of English measurement that is used to measure the flow capacity or flow rate of a hydraulic pump. Gpm is a unit of volumetric flow.

heat exchanger

A hydraulic system component that helps relieve excess heat in the hydraulic fluid. Heat exchangers may use water or air to cool a system.

heat transfer rating

The rate at which thermal energy is exchanged between physical systems. The heat transfer rating of a heat exchanger describes its effectiveness in transferring thermal energy.

horsepower

hp. A unit of power. One horsepower is equivalent to 33,000 foot-pounds per minute.

hoses

A type of flexible fluid conductor that joins components of a hydraulic fluid system, usually in a nonlinear fashion. Hoses are made from rubber and often include wire braiding for reinforcement.

hydraulic actuator

A hydraulic system component that converts hydraulic energy from the pump into mechanical energy. Hydraulic actuators can direct linear or rotary force.

hydraulic fluid

A liquid, typically oil, that transfers the power in a hydraulic system. Hydraulic fluid generally has high viscosity.

hydraulic intensifiers

A component of a hydraulic system that converts low-pressure fluid power into high-pressure fluid power. Hydraulic intensifiers, also referred to as boosters, typically receive pressure from oil.

hydraulic motors

A device that converts hydraulic energy into mechanical energy. A hydraulic motor is a rotary actuator that can rotate continuously.

hydraulic pump

A mechanical power source that creates fluid flow and moves fluid through a hydraulic system. Hydraulic pumps receive energy from a prime mover, such as a motor or engine.

hydraulic system

A type of power transmission system that uses oil or another liquid under pressure to transmit mechanical power. A hydraulic system can transmit large amounts of power using a small input force.

hydropneumatic accumulator

A type of accumulator that uses compressible gas to keep stored hydraulic fluid pressurized. Hydropneumatic accumulators use a device, such as a piston, bladder, or diaphragm, to separate the hydraulic fluid from the compressed gas.

idler gear

A gear that is driven by the drive gear in a gear pump as the two gears mesh. The idler gear in an internal gear pump is typically smaller than the drive gear.

ignition source

Any process or event capable of causing a fire or explosion. Ignition sources include an open flame, hot metal, or sparks.

incompressible

Relatively incapable of losing volume in response to pressure. Liquids are incompressible, while gases are compressible.

inlet port

An opening on the side of a hydraulic pump through which fluid enters the pump. Multiple inlet ports can be included in one pump.

in-line check valve

A directional control valve with the inlet and outlet port located directly opposite each other. An in-line check valve can completely allow flow in one direction while blocking flow in the reverse direction.

internal combustion engine

A type of engine that internally produces heat energy and mechanical energy. Internal combustion engines used for hydraulic systems commonly run on gasoline or diesel.

internal gear pump

A fixed-displacement hydraulic pump with an internal gear that is smaller than the external gear. An internal gear pump is available in single or multiple configurations.

kilopascals

kPa. An SI unit used to measure pressure. Kilopascals may be used to express pressure requirements for a hydraulic system.

leakage

The movement of fluid out of a hydraulic system or from one hydraulic system component to another. Leakage decreases efficiency and can lead to system failure.

linear actuators

An actuator that directs force in a straight line. Linear actuators include double- and single-acting cylinders.

liters per minute

lpm. A unit of metric measurement that is used to measure the flow capacity or flow rate of a hydraulic pump. Liters per minute is a unit of volumetric flow.

loads

The amount of force or pressure placed on a hydraulic system. The load exerted on a system must be compatible with the hydraulic pump and other system components.

lpm

liters per minute. A unit of metric measurement that is used to measure the flow capacity or flow rate of a hydraulic pump. Lpm is a unit of volumetric flow.

lubricant

A substance used to reduce friction between moving components. Lubricants often include oil and grease.

lubricity

The ability of a fluid to reduce friction and provide lubrication. High lubricity in a hydraulic fluid indicates an effective lubricant.

mechanical properties

The collection of properties that describe a liquid's ability to lubricate and flow. Mechanical properties of a hydraulic fluid include viscosity, lubricity, and demulsibility.

metal-forming presses

A machine that uses hydraulic power to generate compressive forces. Metal-forming presses often use hydraulic intensifiers to produce increased pressure.

needle valve

A flow control valve that adjusts fluid flow rate by changing the size of the outlet port. A needle valve consists of a sharp conical obstruction that extends or retracts to enable variable flow.

normally closed

A valve position in which the valving element is unactuated and covering a port. A normally closed valve opens when a specific pressure is reached.

normally open

A valve position in which the valving element is unactuated and not covering a port. A normally open valve closes when it actuates.

outlet port

An opening on the side of a hydraulic pump through which fluid is expelled into the hydraulic system. Multiple outlet ports can be included in one pump.

outlet pressure

The pressure of the fluid at the hydraulic pump's outlet port. Outlet pressure is typically measured in pounds per square inch (psi) or kilopascals (kPa).

oxidation

A material's chemical reaction with oxygen. Oxidation causes rust to form on metal surfaces and can lead to deterioration.

passive components

Any fluid system component characterized by inactivity. Passive components, such as conductors, do not perform an explicit action on fluids, but they allow fluids to act upon them.

petroleum-based oil

A thick hydraulic fluid developed from petroleum. Petroleum-based oils provide good lubrication and are compatible with most seal materials.

pipes

A fluid conductor that joins components of a fluid system. Pipes typically convey fluid in a straight line and on a permanent basis.

piston accumulator

A hydropneumatic accumulator with a liquid section and a nitrogen-charged gas section. A piston accumulator has a piston separating the liquid chamber from the gas chamber.

piston motor

A rotary actuator that uses the reciprocating motion of pistons to produce torque. A piston motor can have a fixed or variable displacement.

piston pumps

A hydraulic pump that uses the reciprocal motion of pistons to move fluid through the pump. Piston pumps can have a fixed or variable displacement.

piston rod

A movable, cylindrical component attached perpendicularly to a piston. A piston rod is housed in a cylinder.

piston shoes

The swiveling end portion of a piston that contains ball and socket joints. The piston shoes in an axial piston pump maintain contact with the angled swashplate to reciprocate the pistons.

poppet

A conical valving element that opens and closes in response to variations in pressure. Poppets are used in some directional control valves.

port

An opening on a valve or cylinder through which fluid can flow. A port allows for inlet or outlet flow.

positive-displacement pumps

A hydraulic pump with a given fluid displacement per cycle or revolution. Positive-displacement pumps are used with a pressure relief valve on the outlet side.

pounds per square inch

psi. A unit used to measure pressure. Pounds per square inch units are used to measure pressure requirements for the hydraulic system and for system components.

power source

A device that generates the energy needed to power a tool or system. The power source in a hydraulic system is the hydraulic pump.

power transmission system

A grouping of components that work together to move energy from a source to an output device in order to perform work. Power transmission systems include hydraulic and pneumatic systems.

pressure

The exertion of force on a confined liquid. Pressure is a function of resistance to fluid flow.

pressure compensator

A device used with variable-displacement pumps. Pressure compensators automatically control a pump's displacement.

pressure control valves

A type of control valve that regulates pressure in a hydraulic system. Pressure control valves are active elements of a hydraulic system.

pressure intensification ratio

A specification that indicates how much pressure a hydraulic intensifier can deliver to the hydraulic system. The pressure intensification ratio is inversely proportioned to the area ratio of the cylinders.

pressure rating

The amount of pressure a hydraulic system can withstand without increased leakage or damage. Pressure ratings are typically expressed in pounds per square inch (psi) or kilopascals (kPa).

prime mover

The component in a hydraulic system that provides mechanical energy to power the hydraulic pump. Prime movers for hydraulic systems include electric motors and internal combustion engines.

prime mover

The component in a hydraulic system that provides mechanical energy to the hydraulic pump. Prime movers for hydraulic systems include electric motors and internal combustion engines.

properties

The physical and mechanical characteristics of a material that distinguish it from other materials. Properties determine how a material will perform in different environments.

psi

pounds per square inch. A unit of English measurement used to measure pressure. Psi units are used to measure pressure requirements for the hydraulic system and for system components.

pumping chambers

A compartment in a hydraulic pump between gear teeth or vanes. Pumping chambers trap fluid and push it to the outlet port.

radial piston pump

A type of piston pump with a radial arrangement of pistons within a cylinder block. In a radial piston pump, the pistons are perpendicular to the drive shaft centerline.

radially

Having parts arranged in straight lines coming from the center of a circle. Radial arrangements of pump components are found in vane pumps and some piston pumps.

ratio of areas

A physical law stating that a given pressure exerted in a larger cylinder can be intensified when transferred to a smaller cylinder. The ratio of areas in a hydraulic intensifier is the area of one piston compared with the area of a second piston.

reciprocating

The repeated back-and-forth movement of a component. Reciprocating mechanisms in some hydraulic pumps push fluid through the pump.

relief valve

A pressure control valve that allows pressurized fluid to drain back into the reservoir if the hydraulic system reaches a critical pressure. A relief valve is typically the first control valve after the hydraulic pump.

reservoir

The main storage container for fluid in a hydraulic system. A reservoir may be pressurized or nonpressurized.

resistance

The opposition to force. Resistance in a hydraulic system results in pressure.

revolutions per minute

rpm. A unit of measurement that indicates the number of complete rotations a cylindrical component makes in one minute. Revolutions per minute is a measurement of speed.

right-angle check valve

A directional control valve with an inlet port and an outlet port located at right angles to each other. A right-angle check valve can divert the direction of fluid flow.

rotary actuator

An actuator that directs force in a circular motion. Rotary actuators include hydraulic motors and limited-rotation devices.

rotating

Spinning or turning movement that takes place around an axis, without a change in linear position. Rotating motion is a type of mechanical energy.

rotor

A rotating part of an electrical or mechanical device. A rotor is the driven component of most hydraulic pumps.

rpm

Revolutions per minute. A unit of measurement that indicates the number of complete rotations a cylindrical component makes in one minute. Rpm is a measurement of speed.

sealant

A material or liquid that provides an airtight or watertight barrier. Sealants protect moving components in a hydraulic system from leaks and contamination.

seals

A device in a fluid power system that prevents fluid leakage and blocks contaminants from entering the system. A seal that is in contact with hydraulic fluid must be compatible with the fluid.

sequence valves

A pressure control valve that allows hydraulic fluid to flow into a secondary part of the circuit after a critical pressure is reached in the primary part of the circuit. Sequence valves can control two operations performed in sequence.

single-acting cylinder

A linear actuator that directs force in one direction as the piston extends. Single-acting cylinders use an external force to retract the piston back into the cylinder.

sludge

A form of contamination. Sludge is gummy liquid matter that forms in the reservoir when contaminated hydraulic fluid mixes with water.

spill-containment

An area where chemical, oil, or sewage spills are closed off with barriers or by a drainage system. Spill-containment areas can present potential hazards, particularly if petroleum-based oils are used.

spool

A cylindrical valving element that alternately allows and blocks flow depending on its linear position. A spool slides to block and open valve ports.

steel

A metal consisting of iron and carbon, usually with small amounts of other elements. Steel is a common metal in manufacturing.

stroke

The length of travel of a piston when it moves into or out of a cylinder. The stroke of a piston helps to determine a piston pump's displacement.

swashplate

A flat, angled component of an axial piston pump that maintains contact with the piston shoes to cause the pistons to reciprocate. The swashplate angle determines an axial piston pump's displacement.

synthetic oils

A hydraulic fluid developed without petroleum. Synthetic oils are manufactured using chemicals and additives not found in nature.

thin wall steel

A strong steel material with high thermal properties. Thin wall steel tubing is a conductor that can bend or flex.

throttle

A control used to vary the speed of a mechanical device. A throttle on an internal combustion engine allows operators to adjust the speed of the hydraulic pump.

torque

A force that produces rotation. Torque is measured in foot-pounds in the English system and newton-meters in the metric system.

tubing

A fluid conductor that joins components of a fluid system, usually in a nonlinear fashion. Tubing is easily bent and eliminates the need for fittings and connection points.

unbalanced vane pump

A hydraulic pump that consists of extendable vanes and a rotor mounted off-center in a circular cam ring. An unbalanced vane pump can have fixed or variable displacement.

vacuum

The state of negative pressure. A vacuum that forms in the inlet port of a hydraulic pump allows fluid to enter the pump.

valving mechanism

The component of a valve that covers a port. Valving mechanisms include spools and poppets that cover or open a port to block or allow flow.

vane motor

A rotary actuator with extendable blades attached radially to a slotted rotor. Vane motors used in hydraulic systems are typically balanced with a fixed displacement.

vane pumps

A hydraulic pump that uses a series of flat protrusions that extend within a rotor to trap and move fluid. Vane pumps are efficient at fast operating speeds.

vanes

A retractable mechanism that extends radially from the center of a rotor in a vane pump. Vanes rotate to move fluid through the pump.

variable displacement

A hydraulic pump that can be adjusted to change the amount of fluid that is moved in one pump cycle. Variable-displacement pumps are generally more expensive than fixed-displacement pumps.

viscosity

A fluid's ability to flow at a given temperature. The viscosity of a fluid decreases as temperature increases.

volume

The amount of space that a substance or object occupies within a container. Volume is measured in cubic inches or cubic centimeters.

water glycol solutions

A fire-resistant hydraulic fluid consisting of 40% water and 60% glycol. Water glycol solutions are compatible with many synthetic seal materials.

water-cooled heat exchanger

A component in a hydraulic system that consists of a cylinder with pipes containing cold water. Water-cooled heat exchangers remove heat from hot hydraulic fluid by passing the fluid over the pipes.

water-in-oil emulsions

A fire-resistant hydraulic fluid consisting of 40% water dispersed in an oil base. Water-in-oil emulsions are characterized by small droplets of water completely surrounded by oil.