Hydraulic Terms

We have compiled a comprehensive list of commonly used hydraulic terms which may prove helpful in purchasing, troubleshooting and repairing your hydraulic unit.

• Accumulator - A container in which fluid is stored under pressure as a source of fluid power.
  
• Actuator, pneumatic/hydraulic - A device in which power is transferred from one pressurized medium to another without intensification.
  
• Additive - A chemical added to a fluid to impart new properties or to enhance those which already exist.
  
• Air bleeder - A device for the removal of air.
  
• Air breather - A device permitting air movement between atmosphere and the component in which it is installed.
  
• Cavitation - A localized gaseous condition within a liquid stream which occurs where the pressure is reduced to the liquid’s vapor pressure. It is caused by an insufficient supply of oil.
  
• Circuit, open - A circuit in which return fluid is directed to the reservoir.
  
• Contaminant - Any material or substance which is unwanted or adversely affects the fluid power system or components, or both.
  
• Cushion - A device which provides controlled resistance to motion.
  
• Cylinder cap - A cylinder end closure which completely covers the bore area.
  
• Cylinder capacity, extending - Volume required for one full extension of the cylinder.
  
• Cylinder capacity, retracting - Volume required by one full retraction of the cylinder. (Generally, less than extending)
  
• Cylinder force, theoretical - The pressure multiplied by the effective piston area, ignoring friction. For double acting cylinders the value must be given for both direction of stroke. (PSI x A² = F)
  
• Cylinder, bore - The internal diameter of the cylinder body.
  
• Fluid power - Energy transmitted and controlled through use of a pressurized fluid.
  
• Heat exchanger - A device which transfers heat through a conducting wall from one fluid to another. (Typically to cool a system.)
  
• Hose, wire braided - Hose consisting of a flexible material reinforced with woven wire braid.
  
• Hydraulic motor - A device which converts hydraulic fluid power into rotary motion.
  
• Hydraulic motor, fixed displacement - A hydraulic motor in which the displacement per unit output motion cannot be varied.
  
• Hydrostatic transmission - Combination of one or more hydraulic pumps and motors forming a unit.
  
• Line, return - A pipe (conductor) to return the working fluid to the reservoir.
  
• Intensifier - A device which converts low pressure fluid power into higher pressure fluid power.
  
• Manifold - A conductor which provides multiple connection ports.
  
• Port - A terminus of a passage in a component to which conductors can be connected.
  
• Power unit - A combination of pump, pump drive, reservoir, controls and conditioning components to supply hydraulic power to a system.
  
• Pressure - Force per unit area, usually expressed in pounds per square inch (bar). PSI
  
• Pressure, atmospheric - Pressure exerted by the atmosphere at any specific location. (Sea level pressure is approximately 14.7 PSI absolute.
  1bar = 14.5 PSI).
  
• Pump - A device which converts mechanical torque and motion into hydraulic fluid power.
  
• Pump, fixed displacement - A hydraulic pump in which the volume displaced per cycle cannot be varied.
  
• Pump, variable displacement - A hydraulic pump in which the volume displaced per cycle can be varied.
  
• Quick disconnect coupling - A component which can quickly join or separate a fluid line without the use of tools or special devices.
  
• Reservoir (tank) - A container for storage of liquid in a fluid power system.
  
• Seal, dynamic - A sealing device used between parts that have relative motion.
  
• Seal, static (gasket) - A sealing device used between parts that have no relative motion.
  
• Switch, pressure - An electric switch operated by fluid pressure.
  
• Transducer, flow - A device which converts fluid flow to an electrical signal.
  
• Trunnion - A mounting device consisting of a pair of opposite projecting cylindrical pivots. The cylindrical pivot pins are at right angle or normal to the piston rod centerline to permit the cylinder to swing in a plane.
  
• Tube - A conductor whose size is its outside diameter. Tube is available in varied wall thickness and material.
  
• Vacuum - Pressure less than ambient atmospheric pressure.
  
• Valve - A device which controls fluid flow direction, pressure or flow rate.
  
• Valve, directional control - A valve whose primary function is to direct or prevent flow through selected passages.
  
• Valve, flow control (flow metering) - A valve whose primary function is to control flow rate.
  
• Valve, flow dividing - A valve which divides the flow from a single source into two or more branches.

Definitions from 1994/1995 Fluid Power Handbook & Directory by the editors of Hydraulics & Pneumatics