Hydraulics Glossary: Key Terms and Definitions
Hydraulics Glossary
A
Actuator
A device that converts hydraulic energy into mechanical energy (e.g., engine or cylinder).
Linear Actuator
An actuator that transforms hydraulic energy into rectilinear movement (a cylinder).
Rotary Actuator
A device that transforms hydraulic power into rotational movement (hydraulic motor).
Accumulator
A vessel in which a fluid can be stored under pressure, as a source of hydraulic power.
Aeration
The presence of air in the hydraulic fluid. Excessive aeration causes foaming in the oil, leading to irregular operation of the components.
B
By-Pass (Derivation)
A secondary passage for the flow of a liquid.
C
Cavitation
Excess steam in a liquid stream that occurs when the pressure is lower than the vapor tension of the liquid.
Center Open (Valve)
All openings of the valve are connected to the tank in the central or neutral position.
Center Closed (Valve)
All openings of the valve are isolated from each other in the central or neutral position.
Charge
Energy referred to the unit weight.
Circuit Differential
A piping schematic for a differential cylinder in which the discharge from the annular section of the cylinder is combined with the flow from the pump.
Closed Center Circuit
A circuit in which the pump’s discharge flow is blocked in the central position, and the pressure at the pump outlet is maintained at a maximum set by the safety valve.
Component
A single hydraulic unit.
Compensator Control
Control for variable displacement pumps and motors that adjusts according to pressure variations in the system and preset pressure settings.
Contamination
Any substance external to the hydraulic fluid that exerts a detrimental effect on the system’s performance.
Control
There are various types of control in hydraulic systems:
- Hydraulic Control: Control performed by hydraulically induced forces.
- Manual Control: Control actuated by the operator, regardless of the actuation method.
- Mechanical Control: Control driven by gears, screws, cams, and other mechanical elements.
Cylinder
Elements that transform hydraulic power into linear motion and forces. The force is proportional to the cross-sectional area and the hydraulic pressure acting on it.
- Double Acting Cylinder: A cylinder in which fluid pressure can be applied in both directions.
- Single Acting Cylinder: A cylinder that produces hydraulic force or movement in one direction only.
- Differential Cylinder: Any cylinder in which the forward and return areas are not equal.
D
Damper
A device sometimes placed at the ends of a hydraulic cylinder to reduce oil flow at the outlet, thereby reducing the speed of the cylinder rod at the end of the stroke.
Deposit
A container for storing liquid.
Differential Cylinder
A cylinder with different areas on either side of the piston, resulting in different force and speed capabilities.
Discharge Valves
Valves that send fluid to the tank when a predetermined pressure is maintained at their pilot line.
Displacement
The amount of liquid that can pass through a pump, motor, or cylinder in a single revolution or stroke.
Double Acting Cylinder
A cylinder that can be pressurized from either side, allowing for movement in both directions.
Drainage
A passage in a hydraulic component or preceding it that independently returns leakage flow to the tank.
E
Electrohydraulic Servo Valve
A directional valve that receives an electrical control signal and regulates the flow accordingly.
Energy
There are two main types of energy in hydraulic systems:
- Kinetic Energy: Energy possessed by a solid or fluid due to its mass and velocity.
- Potential Energy: Energy stored in a physical system due to its position or configuration, such as the pressure in a hydraulic system.
F
Filter
A device whose primary function is to retain contaminants insoluble in the fluid.
Air Filter
A device that allows clean air from outside to pass into the reservoir to maintain atmospheric pressure.
Filtration Level
The smallest particle size that the filter can retain.
Flow
- Volume of fluid discharged by a pump in a given time, usually expressed in liters per minute (l/min).
- The volume of fluid passing through a conduit per unit of time.
Flow Control Valve
A valve that controls the flow rate of hydraulic fluid.
Fluid
- A liquid or gas.
- A liquid specially prepared for use as a means of transmitting power in a hydraulic system.
Force
Any cause that tends to produce or modify motion. In hydraulics, the total force is expressed by the product of pressure and area.
G
Gauge Scale
A pressure scale that does not take atmospheric pressure into account, with zero coinciding with atmospheric pressure.
H
Heat Exchanger
A device that transfers heat from one fluid to another through a dividing wall.
Hydraulic Control
Control achieved by using hydraulic forces.
Hydraulics
The science that deals with the pressure and flow of liquids.
Hydrodynamics
The science that deals with fluids in motion, particularly their kinetic energy.
Hydrostatic
The science that deals with the energy of liquids at rest.
L
Laminar Regime
A flow regime in which fluid particles move along parallel paths.
Linear Actuator
An actuator that produces linear motion.
M
Manual Control
Control actuated directly by a human operator.
Mechanical Control
Control achieved through mechanical means, such as gears, levers, and linkages.
Micron (Micrometer)
One millionth of a meter, or one thousandth of a millimeter.
Motor
A device that transforms hydraulic power into rotational mechanical motion.
O
Open Center Circuit
A hydraulic circuit where the pump flows to the tank when no actuators are active.
Opening Pressure
The pressure at which a pressure-actuated valve allows fluid to flow.
P
Pilot Pressure
Pressure used to actuate or control other hydraulic components.
Piston
A cylindrical component that fits inside a cylinder and transmits or receives motion through a connected rod.
Positive Displacement
A characteristic of a pump or motor where the output is isolated from the inlet, preventing fluid from flowing back within the element.
Power
- Work per unit of time, measured in horsepower or watts.
- The rate at which energy is transferred or transformed.
Pressure
- Force per unit area, usually expressed in bar, kilopascals (kPa), or pounds per square inch (psi).
- In hydraulics, it refers to the force exerted by the fluid on the system components.
Absolute Pressure
Pressure measured relative to a perfect vacuum (zero pressure). It is the sum of gauge pressure and atmospheric pressure.
Atmospheric Pressure (Barometric Pressure)
The pressure exerted by the Earth’s atmosphere. At sea level, it is approximately 1.01 bar (14.7 psi or 101.34 kPa).
Pressure Drop
The difference in pressure between two points in a system or component.
Pressure Gauge
An instrument used to measure pressure, typically calibrated in units like bar, psi, or kPa.
Pressure Line
The hydraulic line carrying pressurized fluid from the pump outlet to the actuator.
Pressure Point
A sudden, localized increase in pressure within a hydraulic circuit.
Pressure Reducing Valve
A valve that limits the maximum pressure at its outlet, regardless of the input pressure.
Pressurize
To apply pressure above atmospheric pressure to the inlet of a pump.
Pump
A device that converts mechanical energy into hydraulic energy by displacing fluid.
R
Race
- The working length of a cylinder.
- Sometimes denotes the change of motion of a rope or variable displacement engine.
Refrigerator (Cooler)
A heat exchanger used to remove heat from a hydraulic fluid.
Relay
An electromagnetic device that allows one electrical circuit to control another without a direct connection between them.
Restriction
A reduction in the cross-section of a line or passage that causes a pressure drop.
Return Line
The hydraulic line carrying fluid from the actuator back to the reservoir.
Rotary Actuator
An actuator that produces rotary motion.
S
Safety Valve
A pressure-actuated valve that diverts flow from the pump to the tank, limiting system pressure to a predetermined maximum value.
Sequence
- The order of a series of operations or movements.
- To direct flow for a subsequent operation or movement.
Sequence Valve
A pressure-actuated valve that directs flow to a secondary actuator when a specific pressure is reached.
Servo
A mechanism controlled by a control device, operating as if directly actuated by the control device.
Shank
A cylindrical part of constant diameter used to transmit thrust.
Single Acting Cylinder
A cylinder that acts under pressure in only one direction.
Solenoid
An electromagnetic device that converts electrical energy into linear mechanical motion.
Speed
- The rate at which fluid moves in a hydraulic line.
- The rotation of a motor, measured in revolutions per minute (RPM).
Static Load
The height of a column of liquid above a given point, expressed in units of length.
Suction Line
The hydraulic line connecting the reservoir to the pump inlet.
Swashplate
A rotating, oscillating plate in axial piston pumps that pushes the pistons within their bores during their movement.
Switch
An electrically powered interrupter activated by fluid pressure.
T
Tank (Reservoir)
A container holding the hydraulic fluid.
Torque Converter
A hydraulic rotary coupling capable of multiplying motor torque.
Turbine
A rotary device driven by the impact of a moving fluid against its blades or paddles.
Turbulent Regime
A flow regime in which fluid particles move in irregular paths, creating eddies and vortices.
U
Unsubscribe Area
This term seems to be incorrect. It likely refers to the Annular Area, which is the area on one side of a cylinder rod (the area of the piston minus the area of the rod).
V
Vacuum
Pressure lower than atmospheric pressure. It is usually expressed in millimeters of mercury (mmHg), inches of mercury (inHg), or Pascals (Pa).
Valve
A device that controls the direction, pressure, or flow of a fluid.
- Check Valve (Non-Return Valve): A valve that allows fluid flow in only one direction.
- Directional Control Valve: A valve that directs or blocks fluid flow in predetermined directions.
- Flow Control Valve: A valve that regulates the flow rate of hydraulic fluid.
- Pilot Valve: A valve used to control the operation of another valve.
- Pressure Control Valve: A valve that manages the pressure in a hydraulic system. Examples include pressure relief valves, pressure reducing valves, and sequence valves.
Viscosity Index
A measurement of the variation of a fluid’s viscosity with changes in temperature.