Hydraulics Glossary: P

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Hydraulic Engineering Glossary of Terms and Terminology

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A glossary of the key hydraulic engineering terms and related terminology you may encounter when working with hydraulic systems. Introducing the letter 'P'…

PACKING:
A sealing device consisting of bulk deformable material or one or more mating deformable elements, reshaped by manually adjustable compression to obtain and maintain effectiveness.
PARTICLE:
A piece of contamination or debris found in a fluid.
PARTICLE COUNT:
The number of particles, grouped by size, in a fluid sample of specified size.
PASSAGE:
A hole through which fluid passes in a fluid power system.
PETROLEUM-BASED OIL:
A hydraulic fluid developed from petroleum. Petroleum-based oils are the most common hydraulic fluids.
A term used to describe the quantity of cyclic electrical power sources in a high voltage system. Most commonly 1 phase or 3 phase.
A time difference between the input and output signal of a control unit or system, usually measured in degrees.
A hydraulic oil (fluid) that is made from an ester base. A synthetic fluid, manufactured to specific characteristics. Will not normally support combustion if heated to a specific temperature.
A passage in a fluid power system that is used to carry a fluid at a pressure lower than the normal operating pressure to aid the controlled shifting of spool valves.
A special check valve that may be opened against a check load by applying pilot pressure from a secondary source to open the check to free reverse flow.
The pressure in the pilot circuit.
A valve applied to operate another valve or control. The controlling stage of a two-stage valve.
A type of hydraulic fluid conductor that joins other hydraulic components usually in a straight line and on a permanent basis. When you are connecting up a hydraulic system, use the tubing or pipe that is capable of handling heavy pressures and loads required by the hydraulic system. Pipes should have a minimum number of bends and fittings, should be securely fastened and clean. Iron pipes are not recommended because they have particles that will flake off and contaminate a hydraulic system.
A device used for converting hydraulic power to mechanical energy. A rod inside a hydraulic cylinder that is moved by hydraulic pressure. In hydraulic pumps, the piston is responsible for pushing down and pulling up the ram.

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A rotating cylindrical mechanism that houses the pistons in a hydraulic piston pump.
A hydraulic pump that uses pistons driven by a rotating swash plate or cam to move fluid through a hydraulic system.
A metal ring that is used to seal high pressure fluid inside a passage to prevent (limit) leakage across the passage. Usually found in cylinders.
The swivelling end portion of a piston that maintains contact with the swash plate in a piston pump.
A material, usually metal applied to another metal by electroplating to reduce corrosion.
A hydraulic cylinder without a piston or with a piston without seals is called a plunger cylinder. A plunger cylinder can only be used as a pushing cylinder; the maximum force is piston rod area multiplied by pressure. This means that a plunger cylinder in general has a relatively thick piston rod.
The part of a type of valve that blocks flow when it closes against a seat.
A chemical compound whose molecules exhibit electrically positive characteristic at one extremity and negative characteristics at the other.
Machined, self aligning valve that incorporates an elastomer to provide a positive seal upon disconnection, no low pressure leakage, and generally provides higher flow that a ball valve.
The internal or external terminus of a passage. The point where the fitting is attached.
A port that provides a passage to or from an actuator.
A port that provides a passage for the purging of gas from a hydraulic system or hydraulic component.
A port that provides a passage to the atmosphere.
A characteristic of a pump or motor when a constant volume is delivered for each revolution or stroke.

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A hydraulic pump that continues to discharge fluid as long as it is powered.
A unit of pressure that measures the amount of load pressure per inch. Pounds per square inch is abbreviated “psi”.
Lowest temperature at which an oil or distillate fuel is observed to flow.
Work per unit of time measured in horsepower (hp) or watts (W). 
A hydraulic power pack is a combination of hydraulic pump, pump drive, reservoir, controls and conditioning components to supply (integral) hydraulic power to a hydraulic system.
The term used to describe a fluid power source.
A power take-off or power takeoff (PTO) transfers the mechanical power from a power source, such as a running engine, and transfers it to another piece of equipment or attached implement.
This allows for the power to move throughout the equipment to transmit energy even if it doesn’t have a motor or engine itself.  Most often, a PTO it is a splined draft shaft installed on a tractor or truck allowing attached implements to be powered directly by the engine.
Semi-permanently mounted power take-offs can also be found on industrial and marine engines. These applications typically use a drive shaft and bolted joint to transmit power to a secondary implement or accessory.
The pressure of compressed gas in an accumulator prior to the admission of the liquid.
A valve that is arranged so its inlet port is connected to a reservoir or tank in such a way that fluid will flow from the inlet of the valve into a cylinder or ram when opened. When closed, the valve must close off the ram or cylinder from the reservoir or tank to permit application of high pressure from another source on the cylinder side of the valve. Most commonly used to fill large rams on presses to take up non-operating stroke.
The force per unit area, usually expressed in pounds per square inch (psi), bars, or atmospheres.
The pressure encountered on the downstream or return side of a component.
The pressure that creates loss of fluid through the component envelope, resulting from failure.
A hydro-mechanical device fitted to a pump or other flow producing / controlling device that reduces flow when pressure rises and increases flow as pressure decreases, to preset limits.

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Pressure control valves are used to keep system pressures safely below a desired upper limit or to to maintain a set pressure in part of a circuit through relieving, reducing, sequencing or unloading fluid pressure; sub plate and in-line mounted options are available.
The most frequently used hydraulic pressure control valves include: relief, reducing, sequence, counterbalance and unloading pressure control valves.
The pressure at which a pressure operated valve begins to pass fluid.
The difference (drop) in pressure between any two points in a system or component.
A digital device that opens or closes a switch when the internal pressure differential changes. Most often used to detect clogging of filter elements.
A visual indicator of pressure that is set for `zero’ psi at atmospheric pressure and includes a dial that indicates the pressure level above atmospheric pressure. See also under VACUUM GAUGE and COMPOUND GAUGE.
A passage used to take fluid from the source of flow to various operating elements of a fluid power system. Rated for pressure at the maximum expected pressure of the system.
The maximum pressure at which a hydraulic component should be operated on a continuous basis, usually the relief valve setting at maximum flow rate.
The pressure at which a hydraulic system is operated.
The measure of pressure increase over the nominal setting of a device when additional fluid flow is passed over the device after it initially opens.
The maximum pressure encountered in the operation of a hydraulic component.
A side plate in a vane pump or motor cartridge on the pressure port side.
The qualified operating pressure which the hydraulic manufacturer recommends for a component or a system.
A pressure control valve whose primary purpose is to limit outlet pressure.
A non-adjustable control valve that regulates pressure in a hydraulic system. A pressure relief valve is a safety device and is required on all hydraulic systems. Once adjusted, the pressure relief valve opens whenever the pressure goes beyond the value set and allows oil to flow back to the reservoir.

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The pressure existing in a wave moving at sonic velocity. The peak value of a sudden increase in pressure in a hydraulic system producing a shock wave.
The pressure in a fluid at rest.
A digital device that opens or closes a switch when the internal pressure changes state.
An analogue device that produces a change in voltage or current when the internal pressure changes state. Normally a fast response device for use in servo control systems. See also under PRESSURE TRANSMITTER.
An analogue device that produces a change in voltage or current when the internal pressure changes state. Normally a slow acting device for use in display systems where update time is not crucial.
A container that holds fluid under pressure.
The pressure at which the working device normally operates.
The component of a hydraulic system that powers the main pump.
A valve which controls and varies pressure, flow, direction, acceleration and deceleration from a remote position. They are adjusted electrically and are actuated by proportional solenoids rather than by a force or torque motor. The output flow is proportional to the input signal. A PLC provides moderately accurate control of hydraulic fluid.
The condition, in a filter, where part of the flow passes through the filter element in proportion to the pressured drop.
A non-destructive level of pressure at which a component or fluid passage will not give during testing of the internal pressure. Normally twice working pressure.
A valve which controls and varies pressure, flow, direction, acceleration and deceleration from a remote position. Adjusted electrically and actuated by proportional solenoids rather to the input signal and provides moderately accurate control of hydraulic fluid.
A mechanical device used to move liquids and gases; The building block of any hydraulic system. The four most common designs are the vane pump, gear pump, gerotor pump and piston pump. All are well suited to common hydraulic uses with the piston design best suited for higher pressures. The variable displacement type is particularly well suited in circuits using hydraulic motors where variable speeds and the ability to reverse is needed.
A mechanical device that creates a flow of fluid when its shaft is rotated in the proper direction and when its inlet is connected to a chamber filled with fluid (a reservoir or tank). The outlet port may be connected to a passage leading to a fluid power system or exhausted into another chamber that is at a higher pressure. The higher pressure chamber must be equipped with a pressure limiting device. The output flow rate is fixed by the pump displacement per revolution.
Creates a pressure that is lower than atmospheric pressure at its inlet when the shaft is rotated. The outlet port is normally connected to a higher pressure chamber or atmosphere.
The output flow rate is fixed by the pump displacement per revolution but can be varied by the operator in a manual or servo controlled system, depending on the design.

If you want to learn more about hydraulic pumps, including what they’re for, how they work and why there are so many different types, our free “Introduction to Hydraulic Pumps” is a great place to start!

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