What is Nabtesco Glossary?
Technical terms related to hydraulic equipment are listed here. Terms are listed in alphabetical order. Please click the corresponding rows below.

Terms Explanations
ABT (ABR) connection A type of flow connecting port A, B and T but connection to port P is cut off.
Accumulator A pressurised storage container for application of fluid as the energy source.
Actuator Machinery using pressure from hydraulic oil/ other fluids to accomplish mechanical tasks.
Anti-cavitation valve A check valve for preventing cavitation; Also called anti-void valve.
Aperture property Relationship between spool stroke and cross-section area of oil passage.
Terms Explanations
Back pressure Pressure exerted on the back of pressure acting surface, discharge side or return side of the passage.
Back pressure valve An adjustment valve for keeping the primary pressure at a preset level by discharging fluid according to changes in primary pressure.
Bleed-off circuit A circuit located at the oil supply side of the actuator for varying oil flow velocity by regulating oil flow in the bypass passage.
Bridge circuit A circuit for keeping the output pressure at the same level with the input pressure.
Bypass valve A 3-port flow adjustment valve in which the passage of the pressure compensation valve branches in front of the variable restrictor so that the remaining fluid is directed to the oil tank or secondary supply passage.
Terms Explanations
Cap side The side of the cylinder opposite to that on which the piston rods protrude outwards ;As opposite to "Rod side".
Cavitation A phenomenon involving formation of vapour cavities due to a partial decrease in fluid pressure.
Chattering A self-induced vibration which, when hitting the base of the relief valve, gives a relatively high-pitch sound.
Check valve Valve for ensuring one-way flow of fluid; for prevention of fluid backflow.
Check valve/ Non-return valve A valve only allowing fluid flow in one single direction and forbiding backflow of fluid.
Choke A choke with a length longer than its diameter.
Closed centre A closed oil passage in control valve connecting the pump input port to the tank port when the spool stays at the central position.
Contaminant Unwanted particles mixed in hydraulic fluid.
Contamination A condition involving intrusion of unwanted particles into hydraulic fluid.
Control valve Device for regulating fluid flow direction, presure and velocity.
Counter-balance valve Pressure regulating valve for maintaining the back pressure to prevent the load from falling.
Cracking pressure Pressure level at which, due to a pressure rise in relief valve, the oil passage opens up and allows oil flow.
Cylinder Actuator conducting straight-line motion.
Terms Explanations
Detent A mechanism aiming to keep the valve at the designated position by mechanical resistance.
Diversion valve A type of valve in which, when the pressure source is diverted into 2 or more streams, the flow at each exit is maintained at a preset level regardless of the pressure at the other passage.
Drain Return of fluid from passages in the machine to the tank/manifold.
Terms Explanations
Electromagnetic valve A valve operated with an electromagnet.
Emergency valve Valve for fully opening/closing of oil circuits in case of emergency.
Erosion Wearing at the surface due to collsions with or ejections of fluid or suspended particles.
(Electrohydraulic) servo valve Valve for regulating oil flow or pressue, with electricity as the input signal.
Terms Explanations
Floating position The position of the valve body when the entry port is blocked while all exits are connected to the return port or discharge port.
Flow (rate) Volume of fluid passing through a certain point of the passage surface per unit of time.
Flow control valve Control valve for maintaining a a constant flow by the principle of pressure compensation regardless of any change in back pressure/ pressure at the oil entry.
Fluid force Force exerted on the passage wall surface by passing fluid.
Flushing A system operation for washing internal passages and cavities at low pressure with special flushing fluid.
Full efficiency Ratio between fluid force in hydrualic motor/ pump and axial force; Product of displacement efficiency and mechanical efficiency.
Terms Explanations
Gasket A type of seal clamped and bolted in place between mating surfaces (like flange surfaces in valves) for prevention of fluid leakage.
Gear pump A pump operating with 2 or more gears meshing together inside the casing for sucking in and pushing out of fluid.
Terms Explanations
Heat shock Impact due to a sudden change in temperature.
Hydraulic fluid Fluid used in hydraulic devices/systems.
Hydraulic lock/ Hydrolock/ Hydrostatic lock A condition when the piston/ spool stops its motion due to an uneven distribution of pressure.
Hysteresis Maximum difference in output flow arises from the same input signal in one complete signal cycle.
Terms Explanations
Leakage Oil escape at an unfavourable position/position where oil flow should have been stopped.
Load-sensing An operation principle involving detection of the system's maximum loading pressure and, by controlling the pressure compensation valve, diversion of hydrualic fluid to system with high pressure for flow control in the pump.
Lock characteristic The extent to which a hydraulic motor rotates due to the load exerted onto the output shaft when the discharge side of the motor is blocked.
Lock valve; Load-holding valve; Anti-drift valve (Caterpillar) Check valve for preventing free fall due to the weight of the cylinder.
Logic valve A two-port valve constructed with cover, sleeve, poppet and springs; Installed inside the block which opens and closes the poppet upon receiving pressure signals from the pilot line.
Terms Explanations
Manifold A block installing to two or more devices and acting as pipes by completing internal oil passages.
Mechanical efficiency Ratio of theoretical torque and actually output torque in hydraulic motors and pumps.
Meter-in circuit A circuit for regulating the oil flow velocity on the oil inflow side of the actuator.
Meter-out circuit A circuit for regulating the oil flow velocity on the oil outflow side of the actuator.
Motor An actuator providing rotary motions.
Terms Explanations
Negative control A type of control in which the pump's oil flow decreases porportionally according to the signal pressure from the control valve.
Normal position Valve's position when operating power/ control signal is suspended.
Terms Explanations
Open centre An opened oil passage in control valve connecting the pump input port to the tank port when the spool stays at the centre.
Operating position The position of the valve's end during operation.
Orifice A choke for limiting oil flow, adjustment of reaction time and prevention of hunting phenomenon.
Oscillation Periodic fluctuation in pressure level during regular movement.
Terms Explanations
Pilot pressure Pressure exerted to the pilot circuit.
Pilot valve Small-scale valve for controlling main valve via adjustment valve, electromagnetic valve and safety valve.
Piston A cylinder part which operates upon exertion of fluid pressure onto the inner surface of the cylinder and transmits mechanical force/motion.
Piston pump A pump converting input shaft's rotary motion to movements of axially-arranged pistons by sucking in and discharging fluid.
Piston rod A co-axial part installed to a piston for force/motion transmission from pistons.
Plunger Mechanical part with a length longer than its diameter for acting and reacting to fluid pressure during piston movements inside the cylinder.
Poppet valve A type of valve in which the body moves vertically from the sheet surface of the valve base to open/close the passage.
Positive control A type of control in which the pump's oil flow increases porportionally according to the signal pressure from the control valve.
Power take-off Output port for power from devices other than the engine shaft. Also used for retrieving hydraulic power from the reserve control valve.
Pressure compensation A function for maintaining the pressure at a default level in devices/circuits.
Pressure loss/ drop Loss of fluid pressure due to resistance occurred against fluid flow inside the oil passage.
Pressure reducing valve Pressure control valve with regardless of the pressue at the entry port.
Terms Explanations
Rated flow Fluid flow determined by tests and, for maximising the device's/pipe's lifespan, applied as a standard for design and use.
Rated pressure Pressure level determined by tests and, for maximising the device's/pipe's lifespan, applied as a standard for design and use.
Receipt pressure Pressure level at which, when fluid flow from the valve drops to a default value, the pressure at the entry port decreases and the valve starts closing.
Reduction gear/ Reduction unit Mechanical device for reducing gears' rotation speed to increase output torque.
Relief valve Pressure regulating valve for keeping the pressue inside the circuit at the default level by partial or full fluid discharge.
Response/ reaction time Time required for the output pressue to reach a predetermined level after an input signal is sent to the valve/circuit.
Restrictor/ throttle A device for reducing cross-sectional area of the passage, thus creating resistance against the fluid flow inside.
Rod side The side of the cylinder on which the piston rods protrude outwards.
Terms Explanations
Seizure A phenomenon involving occassional suspension of smooth movement at the spool/ piston sliding surface.
Self-priming Property/ Ability of sucking oil by hydraulic pump.
Sequence valve Pressure regulating valve which allows fluid flow from the entry port to the exit when the input pressure or exterior pilot pressure reaches the preset level.
Shuttle valve With two entry ports and one common exit port; A valve in which either one of the entry ports is connected to the exit according to pressure actions.
Spool A tandem constituent sliding against the internal surface of the hollow shaft for opening/ closing oil passages.
Spring-centred Another name for "spring return" when the 3-position valve's initial position is the same as the normal (neutral) position.
Stroke Displacement (moving distance) of the valve rod or the operating shaft.
Swing motor Machine using oil pressure for rotary motion in construction machinery.
Swing unit Machine consisting a swing motor and a reduction unit combined in a parallel way.
Terms Explanations
Unload valve Safety valve for preventing damage to hydraulic equipment and pipes when fluid pressure reaches a certain level.
Terms Explanations
Valve Collective term for devices controlling/regulating the flow direction, pressure and velocity in a fluid system.
Vent Passage opening to the surrounding pressure.
Volumetric efficiency Ratio between the theoretical flow and the actual oil flow in a hydraulic motor/pump.
Terms Explanations
Wrap Overlapping of the spool's land and port/ Extent of the overlapping.



Click for catalogYou can view our catalog


Inquiry about productsFor catalog requests and quotations.


+81-3-5212-1153Telephone reception hours:9:00 ~ 17:30