Hydraulics Glossary: V

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A glossary of the key terms you may encounter when working with hydraulic systems. Introducing the letter 'V'…

VACCUM:
The state of negative pressure. Pressure less than atmospheric pressure. A hydraulic pump works by creating a vacuum in the closed hydraulic system.
VACCUM GUAGE:
A visual indicator of pressure that is set for `zero’ psi at atmospheric pressure and includes a dial which will continue to indicate the level of pressure below atmospheric pressure.
VALVE:
A device used in a fluid power system, which is used to control fluid flow rate, direction or pressure.
Has the inlet port blocked from the outlet and works in neutral.
A valve whose primary function is to direct or prevent flow through selected passages.
A 4 port valve (in, out and 2 work) used with double acting hydraulic cylinders, bi-directional hydraulic motors.
A valve whose primary function is to control flow rate
A valve which divides the flow from a single source into two or more branches.
A device which prevents a load from dropping when a valve is shifted, until ample pressure and flow is available to hold or move the load.
Has the inlet port connected to the outlet (tank) port in neutral.
A multiple spool valve in which the inlet oil is connected to all spools simultaneously. If more than one spool is actuated, the function requiring the lowest pressure will operate first.

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A predetermined position maintained by a holding device acting on the flow-directing elements of a directional control valve.
The valve position when signal or actuating force is not being applied.
A sleeve attachment which permits the oil flow from one valve (when in neutral) to be used by another valve downstream. Hence, a 3-spool valve could be connected to a 2-spool valve to create a 5-spool valve, The first valve takes priority and must have a separate outlet port to return oil from an activator back to the reservoir.
Provides the valve stop. It’s precise location allows maximum flow when mating quick-action halves are connected.
A multiple spool valve which has all spools connected to the open centre passage in neutral. However, when actuated, the upstream valve takes full priority. The return oil is directed to downstream spools as in a series type valve.
A 3 port valve (in, out ~r,d work) normally used with a single acting hydraulic cylinder or uni-directional motor.
A 2 port valve with inlet and outlet ports.
The number of positions in which a hydraulic valve can be positioned.
That area of the quick-action coupling that comes in contact with either the ball or poppet valve, allowing a positive sealing surface. Shape and surface varies with valve style.
A valve whose primary function is to direct flow in a predetermined sequence.
A multiple spool valve in which the return oil from the first spool is directed to the inlet of the second spool (and from the second to the third, etc.). This type of valve permits simultaneous operation of two or more functions with the same oil flow. However, the total pressure requirements of all functions are accumulative.

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A retractable spring-loaded mechanism that extends radially from the centre of a hydraulic pump rotor.
A hydraulic rotary actuator that directs energy in a circular motion through the use of a pivoting mechanism.
A hydraulic pump that uses a flat protrusion to trap and move liquid through a hydraulic system.

The measure of pressure at which a specific fluid will change to a gas.
A factor or condition which can be measured, altered or controlled, e.g: temperature, pressure, flow, liquid level, humidity, weight etc.
A hydraulic pump that can be adjusted to increase or decreases the amount of liquid that is moved in one pump cycle.
The speed of fluid flow through a hydraulic line. Expressed in feet per second (fps), inches per second (ips), or meters per second (mps). Also, the speed of a rotating component measure in resolutions per minute (rpm).
Pressure in a hydraulic system caused by kinetic energy.
To remove trapped air from a component or permit the opening of a pressure control valve by opening its pilot port (vent connection) to atmospheric pressure. An air breathing device on a fluid reservoir.
A valve that may be manually opened to allow air or fluid or a combination of both to be exhausted into a lower pressure chamber or to the atmosphere.
The size of a space or chamber in cubic units. The term is also loosely applied to the output of a pump in gallons per minute.
A measure of internal friction or the hydraulic fluid’s resistance to flow. As temperature increases, viscosity decreases.

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