This glossary defines common hydraulic terms used in component identification, sourcing, replacement and system troubleshooting. Use it alongside model-code guides and technical checklists.
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Pumps
| Term | Definition |
|---|---|
| Axial piston pump | A pump where pistons move parallel to the drive shaft. Available in variable-displacement (swashplate) and fixed-displacement (bent-axis) configurations. Common for high-pressure industrial and mobile applications. |
| Vane pump | A pump using sliding vanes in a rotor to move fluid. Available in single, double and triple configurations. Typically used at medium pressure in industrial systems. |
| Gear pump | A fixed-displacement pump using meshing gears. Simple construction, tolerant of contamination, commonly used for low-to-medium pressure applications and charge circuits. |
| Displacement | The theoretical volume of fluid a pump delivers per revolution, expressed in cm³/rev or in³/rev. Actual flow is displacement × speed × volumetric efficiency. |
| Pressure compensator | A pump control that adjusts displacement to maintain a set pressure. When system pressure reaches the compensator setting, the pump de-strokes to near-zero flow. |
| Load sensing | A pump control that adjusts displacement to maintain a set pressure margin above the highest load pressure. Improves efficiency in multi-function systems. |
| Through-drive | A rear mounting pad and coupling on a pump that accepts an auxiliary pump. The auxiliary pump is driven by the main pump shaft. |
| Case drain | A dedicated return line from the pump or motor housing to the reservoir. Carries internal leakage and provides cooling and lubrication. Must never be plugged. |
Valves
| Term | Definition |
|---|---|
| Directional control valve | A valve that directs fluid flow to a selected path. Classified by number of ways (ports) and positions. Spool type determines flow paths in each position. |
| Spool | The moving element inside a directional valve. The spool lands and grooves determine which ports are connected or blocked in each position. The spool symbol is critical for correct circuit function. |
| Center condition | The flow-path configuration when a directional valve is in its spring-centered (de-energized) position. Options include all ports closed, pump to tank (open center), A and B to tank (float), etc. |
| Solenoid valve | A valve shifted by an electromagnetic solenoid. Available in AC and DC voltages. Wet-armature (wet-pin) solenoids are immersed in hydraulic fluid; dry types are separated. |
| Proportional valve | A valve where spool position is proportional to an electrical input signal. Used when variable flow or pressure control is needed. Requires compatible electronics/amplifier. |
| Modular (sandwich) valve | A valve that mounts between a directional valve and its subplate or manifold. Common types: pressure relief, pressure reducing, flow control, pilot-operated check. |
| Pressure relief valve | A normally closed valve that opens at a set pressure to limit maximum system pressure. Direct-acting for low flow; pilot-operated for higher flow and stability. |
| Pressure reducing valve | A normally open valve that reduces outlet pressure to a set level below inlet pressure. Used when a branch circuit needs lower pressure than the main system. |
| Check valve | A valve that allows flow in one direction and blocks reverse flow. A pilot-operated check valve can be opened by an external pilot signal. |
| Mounting pattern / interface | The standardised bolt pattern and port layout for mounting a valve. Common standards: ISO 4401 / CETOP (NG6, NG10, NG16), NFPA D03/D05/D07. |
Cylinders
| Term | Definition |
|---|---|
| Bore | The internal diameter of a cylinder barrel. Together with pressure, it determines the extending force (Force = Pressure × Bore area). |
| Rod diameter | The diameter of the piston rod. Together with bore, it determines the retracting force via the annular area. Rod diameter affects buckling resistance. |
| Stroke | The distance the piston travels from fully retracted to fully extended. Stroke plus retracted length gives the extended pin-to-pin dimension. |
| Mounting style | How the cylinder is attached to the machine. Common types: clevis, flange, trunnion, foot, eye. Mounting affects force transmission and alignment tolerance. |
| Cushioning | A device at the cylinder end that restricts flow near stroke-end to decelerate the piston. Reduces impact and noise. May be fixed or adjustable. |
Motors
| Term | Definition |
|---|---|
| Orbital (gerotor) motor | A low-speed high-torque motor using an orbiting gear set. Simple, robust, widely used in mobile equipment for wheel drives, augers and conveyors. |
| Axial piston motor | A motor where pistons are arranged parallel to the drive shaft. Available in fixed and variable displacement. Used where high efficiency and speed range are needed. |
| Radial piston motor | A motor where pistons are arranged radially around the crankshaft. Capable of very high torque at low speed. Used in winches, slewing drives and heavy machinery. |
| Flushing valve | A valve in a closed-circuit hydrostatic transmission that exchanges a portion of the loop fluid to control temperature and contamination. |
Systems & Performance
| Term | Definition |
|---|---|
| Working pressure | The pressure at which a system or component normally operates. Components should be rated for working pressure plus a safety margin for peaks. |
| Flow rate | The volume of fluid passing a point per unit time, typically L/min or GPM. Pump flow is displacement × speed; actuator flow is area × velocity. |
| Cavitation | The formation and collapse of vapour bubbles in hydraulic fluid when inlet pressure falls below vapour pressure. Causes noise, erosion and component damage. |
| Filtration | The removal of solid contaminants from hydraulic fluid. Typically measured in microns (µm). Return-line, pressure-line and off-line filtration protect components. |
| ISO VG | ISO Viscosity Grade — a standard classification for hydraulic oil viscosity at 40°C. Common grades: ISO VG 32, 46, 68. Selection depends on pump type, temperature and duty. |
| NBR / FKM | Nitrile (NBR) is the standard hydraulic seal material. FKM (Viton) is used for higher temperatures or aggressive fluids. Seal compatibility must match the actual fluid and operating temperature. |
| Open circuit | A hydraulic circuit where pump inlet is connected to the reservoir and return flow goes back to the reservoir. Most industrial systems are open-circuit. |
| Closed circuit | A hydrostatic circuit where pump outlet connects directly to motor inlet and motor return connects back to pump inlet. Uses a charge pump to compensate for leakage. Common in mobile propel drives. |
Related Product, Series & Technical Pages
Read a valve model code
Apply these terms when decoding Rexroth, Vickers and Parker valve codes.
Open related page →Identify a hydraulic pump
Use terminology to confirm nameplate, shaft, flange and port details.
Open related page →Why series names mislead
See how suffixes encode the functional differences defined in this glossary.
Open related page →