Eagle Pro Hydraulics

Glossary of Terms


ACCUMULATOR A chamber in which a fluid/ gas can be stored under pressure, and can be taken on system demand
ACTUATOR Converts hydraulic energy into mechanical energy. A motor or cylinder.
AERATION Air in the hydraulic fluid. Aeration causes the fluid to appear milky and components to operate erratically because of the air trapped in the fluid.
ANNULAR AREA A ring shape area. Often refers to the effective area of the rod side of a cylinder piston.


BACK PRESSURE Pressure existing on the discharge side of a load. It adds to the pressure required to move the load.
BLEED-OFF To reduce a portion of pump delivery to the reservoir.
BYPASS Alternative passage for fluid flow.


CAVITATION Where fluid does not completely fill the existing space. Commonly associated with the pump inlet.
CAPACITY The amount of fluid required to fill the device at its maximum usage point.
CHARGE (SUPERCHARGE)1. To fill a hydraulic system above atmospheric pressure. 2. To fill an accumulator with fluid under pressure.
CHARGE PRESSURE The pressure that fluid is forced into the hydraulic system.
CHECK VALVE A valve that allows flow in one direction only.
CLOSED CIRCUIT A system designed so that when the fluid has been used in the system and Hydraulic components, bypasses the reservoir and returns directly to the pump inlet
COMPENSATION CONTROL A displacement control for variable pumps and motors which alters the displacement in response to pressure changes in the system.
CONTROL An item that can change the function of a unit.
COUPLING Consists of two parts Male (Probe) and female, Can be disconnected and connected quickly and contain check valves to prevent leakage of fluid when not fitted together.
COUNTERBALANCE VALVE A valve which maintains resistance to flow in one direction but permits free flow in the other. Usually connected to the outlet of a double-acting cylinder to support weight or prevent uncontrolled failure.
CRACKING PRESSURE The pressure at which a valve begins to pass fluid.
CUSHION Built into the ends of a cylinder which limits the hydraulic flow to the outlet port, in turn decreasing the speed of the piston.
CYLINDER A cylinder barrel with a piston rod for pushing and pulling using pressurized fluid


DECOMPRESSION The release of pressurized fluid to steadily reduce the pressure of the fluid in the system.
DELIVERY The amount of fluid expelled by the pump in a set time. Commonly expressed as litres per second (L/s).
DIFFERENTIAL CYLINDER Any cylinder in which the two opposing piston areas are not equal.
DIRECTIONAL CONTROL VALVE A valve that controls the direction of fluid flow into predetermined paths.
DISPLACEMENT A measurable amount of fluid that passes from the inlet port to the outlet port of an actuator in a single revolution.
DOUBLE-ACTING CYLINDER Hydraulic force can be applied in either direction. (Extend or retract)
DRAIN Used in a hydraulic system to direct fluid back to reservoir.


FILTER For the removal of particle contamination from a fluid by using internal materials like paper or mesh screen.
FLOW CONTROL VALVE A valve which controls the rate of flow of the fluid.
FLOW RATE Units of volume of fluid passing any given point in one period of time. For example - litres per second (L/s).
FORCE A force is often defined as a push or a pull or movement that can modify motion. The basic unit of force is a Newton, which is force required to accelerate ONE kilogram mass by ONE meter per second for each second it is applied. In hydraulics, the total force is shown as the product of the pressure (force per unit area) and the area on which the pressure acts. F = p x A.
FOUR WAY VALVE A directional control valve with four flow paths (ports).
FRONT CONNECTED A condition where the connections are on the exposed surfaces of components.


GAUGE An instrument used to display the pressure reading of an area or device in the system often displayed in BAR, Psi, KPA or Mpa.


HEAD The force exerted by a column of fluid due to its height and weight (density).
HEAT EXCHANGER A device in which heat is added or removed from fluids to control a set temperature.
HYDRAULIC BALANCE Equal and opposed hydraulic forces acting on a part in a hydraulic component.
HYDRAULIC CONTROL A method of control that is actuated by hydraulically induced forces.
HOSE A flexible tube used to transmit fluid / hydraulic energy to a desired location or device in the system.


LINE A tube, pipe which acts as a conductor of fluids and transmits the fluid energy.
LINEAR ACTUATOR A device for converting fluid/ gas power energy into a straight line motion. For example- A cylinder or ram.


MANUAL CONTROL ( From the Latin ‘ Manus’ the hand) A control actuated by the operator, through e.g. lever, foot pedal or button etc.
MANIFOLD A chamber that has several outlets through fluid or gas is distributed or gathered to or from different parts of the system.
MECHANICAL CONTROL Operated by machine linkages, gears, screws, cams or other mechanical elements.
METER To regulate the rate of fluid flow.
METER-IN To regulate the amount of fluid flowing into an actuator or system.
METER-OUT To regulate the amount of fluid flowing out of an actuator or system.
MOTOR A device which converts other energy forms into rotary mechanical energy For example - Rotary actuator


ORIFICE A small opening found in various devices and components throughout hydraulic systems. Used to control and restrict flow for many applications.


PASSAGE A way or means for fluid to flow in a system.
PISTON Cylindrical shaped part that fits inside a cylinder or ram barrel and transmits or receives motion.
PLUNGER A part which is used to transmit thrust.
POPPET A part within valves which prevents flow when it seals against a seat.
PORT The open end of an internal passage. Usually within or at the surface of a components exterior.
PILOT PRESSURE System pressure used to control components. May be at a lower pressure.
POWER PACK A stand alone hydraulic system containing a pump, reservoir, relief valve and drive unit. To produce its own power, often compact.
PRECHARGE PRESSURE The pressure reading of gas (nitrogen) in an accumulator prior to the addition of fluid.
PRESSURE The exertion of force upon a surface by an object, Pressure is created by resistance to flow due to increased load or friction. For example - one Newton per meter square equals ONE Pascal. 1 Nrn = 1 Pa. In fluid power.
PRESSURE DROP The reduction in pressure between two points in a line or device caused by the increase in resistance. May be induced by fitting an orifice in the system.
PRESSURE OVER-RIDE The difference between the cracking pressure of a valve and the pressure reached when the valve is passing full flow.
PRESSURE REDUCING VALVE A valve that limits the maximum set pressure at its outlet regardless of the extreme inlet pressure.
PRESSURE SWITCH A switch operated by increase or decrease in fluid pressure
PROBE The male section of a coupling that is inserted in to female coupling half join the two sections together.
PUMP A device to convert mechanical energy into fluid energy.


RELIEF VALVE A pressure operated valve which by-passes pump flow to the reservoir, limiting system pressure to a preset level.
REPLENISH To add fluid to a hydraulic system
REMOTE To control the operation or performance of a device or apparatus from a distance or mobile position.
RESTRICTION A reduced area in a line/hose or passage which creates a pressure drop at the other side of the system.
ROTARY ACTUATOR Used for converting hydraulic/pneumatic energy into mechanical energy. For example - Hydraulic motor


SEQUENCE VALVE Pressure operated valve which diverts flow to a secondary circuit once the primary circuit goes above a preset pressure.
SINGLE ACTING CYLINDER A cylinder in which fluid energy produces thrust or motion in one direction only. Commonly spring or gravity returned.
SPOOL A term applied to any moving cylindrical shaped component which moves to change fluid flow through the component.
STRAINER A component that removes solids from fluid commonly by a wire mesh.
STROKE The length of travel of a piston or plunger.* Also used to explain the changing of the displacement of a variable pump or motor. For example - To de-stroke the pump.
SURGE A sudden rise in pressure. Can be associated with a sudden shock or blockage to flow.


THROTTLE Allow the passing of a set amount of fluid flow. May control flow or create a pressure drop.
TORQUE Rotary energy or force of a turning shaft like a hydraulic motor. Measured in Newton Meters (Nm).
TWO WAY VALVE A directional control (DCV) valve with two flow paths or ports.


UNLOAD To release flow to the reservoir to prevent increased pressure being exerted on the system or components.
UNLOADING VALVE A valve that directs flow to reservoir when a set pressure is maintained on its pilot port.


VALVE For controlling flow rate, flow direction or pressure of a fluid. Can be manually, electrically, mechanically or pilot controlled.
VENT To allow a non pressurized reservoir to breath to atmosphere.
VISCOSITY The measure of a fluid’s resistance to flow. The greater the resistance to flow, the higher the viscosity number.
VISCOSITY INDEX An arbitrary scale for lubricating oils that indicates the extent of variation in viscosity with variation of temperature.
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