Hyrdraulic Power Glossary of Terms

Our hydraulic services are underpinned by continual investment in product development, and a deep understanding of our Industry. We know our language can sometimes seem a little technical and have decided to counter that by providing a resource for our customers and website visitors in the form of this online Glossary of terms relating to Hydraulics, Hydraulic Power and the packs, pumps valves and units we produce. This is quite a large undertaking and we'll be building this Glossary over the coming weeks. We'll do it alphabetically and here we go:

ABRASION: External damage to a hose outer braiding by friction.

ABSOLUTE VISCOSITY: The ratio between the stress to shear rate or the hydraulic fluid’s resistance to internal movement.

ACCUMULATOR - A device that stores oil under pressure as a source of energy, to supply flow or absorb shock.

ACTUATOR - A hydraulic component which provides mechanical force and motion using hydraulic power. (eg: hydraulic cylinders and motors.)

ADDITIVE: A substance added to a hydraulic fluid to change or enhance it properties.

AERATION: Air entrained within the hydraulic oil. Excessive aeration causes emulsified oil and can lead to erratic systems performance. See our article on air contamination in hydraulic systems.

AIR BREATHER: A breather allows air movement between the atmosphere and the hydraulic system in which it is installed, commonly contains filtration. Hydraulic power packs have these fitted to the reservoir.

AIR/OIL HEAT EXCHANGER: A hydraulic matrix whose surface area is designed to cool the oil, by transducer to the surrounding air.

AIR/OIL INTENSIFIER: A hydraulic device that uses compressed air increase the pressure in the hydraulic oil.

ANNULAR AREA: A ring shaped net effective area of the rod side of a cylinder piston.

ANTI-FOAM AGENT: One of two types of additives used to reduce foaming in hydraulic oils.

Atmospheric pressure: is the force per unit area exerted on a surface by the weight of air above that surface in the atmosphere of Earth (or that of another planet).

AUTO RECIPROCATING VALVE: A valve that automatically changes the internal direction of flow, commonly used to cycle a hydraulic cylinder.


BACK PRESSURE: The pressure on the downstream or tank side of a device.

BACK-UP RING: A seal used with an o-ring or other gasket to prevent it from extruding under pressure.

BAFFLE: A plate or series of plates in a reservoir or tank to slow fluid flow in specific direction(s).

BALL VALVE: A hydraulic valve using a ball shaped obstruction to stop and start hydraulic flow or divert the flow of fluid in a passage.

BAR: The measure of pressure equal to 14.5 psi

BLADDER: A separator or diaphragm, usually found in an accumulator to separation of two fluids.

BLEED VALVE: A valve for removal of fluid or air from a system.

BULK MODULUS: The measure of a substance's resistance to uniform compression.

BURST PRESSURE: The pressure at which a device fails or ruptures.

BYPASS: A flow path other than the main path.

CAM-LOBE MOTOR: A radial piston hydraulic motor that generates a rotational force by the pushing lobes out under hydraulic pressure against a cam.

CARTRIDGE VALVE: A valve that screws into a manifold body to form a hydraulic system.

CASE DRAIN: A low pressure drain line that allows the “leakage” flow to return to tank, ensuring the case of a motor or pump is drained.

CAVITATION: This happens when pressure in a part of a hydraulic system is dropped below the oil vaporisation pressure. This makes oil vapour to form in the oil. If this happens in the pump inlet, the pressure increase inside the pump pushes the bubbles to collapse explosively. This causes erosion of metal parts, noise and vibration.

CENTRIFUGAL FORCE: A force directed away from the centroid.

CHECK VALVE: A hydraulic control valve in which fluid can flow in one direction only.

CIRCUIT- A collection of hydraulic parts linked together, by either hydraulic lines or by manifold assemblies.

CIRCUIT, OPEN: A hydraulic circuit in which downstream flow is not directed to the control valves. 

CIRCUIT, REGENERATIVE: The low pressure hydraulic flow that would ordinarily be returned to reservoir is re-used within a circuit.

CLOSED CENTRE SYSTEM:  A hydraulic system where the neutral condition of the control valves is closed to flow. 

CLOSED LOOP: In a control system that utilises measurement of out to modify the condition of the input.

COEFFICIENT OF FRICTION: A relative measure of the surface condition, or reduction in friction of a lubricant.

COMPENSATOR CONTROL: A controller for varying the displacement of pumps and motors depending on pressure.

COMPRESSIBILITY: The amount of volume change of a hydraulic fluid when subjected to a variation of pressure.

CONTROL VALVES: A collective term for the hydraulic valves that are used throughout a hydraulic system to control the pressure, flow and direction of the hydraulic power.

COOLER: A cooler is used to transfer heat from a hydraulic oil to air or another fluid. Two main types are Air blast or heat exchanger (shell-tube or plate types).

CORROSION INHIBITOR: Additive to prevent the corrosion of metal surface by water or other contaminants.

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.

COUPLING: A term used to describe a device that joins mechanical objects such a hydraulic hose or a pump and motor of a hydraulic power pack.

CRACKING PRESSURE: The pressure at which a hydraulic valve begins to open.

CUSHION, CYLINDER: A hydraulic cylinder with a built in cushion to slow the movement at the end of the stroke.

CYLINDER: A hydraulic actuator with a piston inside a cylindrical body. The piston or plunger operates using pressurized fluid and converts it into linear motion.

CYLINDER; DOUBLE ACTING: A hydraulic cylinder which can apply force and motion in both directions.

CYLINDER; SINGLE-ACTING: A hydraulic cylinder which can apply force in only one direction.

CYLINDER, TELESCOPIC ACTING: A hydraulic cylinder with multiple moving shafts  inside one another. Application of pressure extends each section in-turn.

CYLINDER THRUST: The push force produced by the hydraulic cylinder.

DETENT: A spring operated device is configured to retain its set position until it is reset. Lever control valves are commonly detent.

DECOMPRESSION The gradual and controlled reduction of hydraulic pressure from parts of hydraulic system, such as a hydraulic cylinder.

DIFFERENTIAL PRESSURE: The value of pressure measured as the absolute difference between the inlet pressure and outlet pressure.

DIRECTIONAL VALVE: A valve that controls the direction of fluid flow into different flow lines.

DISPLACEMENT: A measured volume of fluid that moves from inlet port to outlet port of a pump or ram in a single action.

DRAIN: A low pressure line that stops the build-up of unwanted pressure by returning to the reservoir.