*Editor's Note: This article was updated in February 2024.
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When a hydraulic pump operates, it performs two functions. First, its mechanical action creates a vacuum at the pump inlet which allows atmospheric pressure to force liquid from the reservoir into the inlet line to the pump. Second, its mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system.
A pump produces liquid movement or flow: it does not generate pressure. It produces the flow necessary for the development of pressure which is a function of resistance to fluid flow in the system. For example, the pressure of the fluid at the pump outlet is zero for a pump not connected to a system (load). Further, for a pump delivering into a system, the pressure will rise only to the level necessary to overcome the resistance of the load.
All pumps may be classified as either positive-displacement or non-positive-displacement. Most pumps used in hydraulic systems are positive-displacement.
A non-positive-displacement pump produces a continuous flow. However, because it does not provide a positive internal seal against slippage, its output varies considerably as pressure varies. Centrifugal and propeller pumps are examples of non-positive-displacement pumps.
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If the output port of a non-positive-displacement pump were blocked off, the pressure would rise, and output would decrease to zero. Although the pumping element would continue moving, flow would stop because of slippage inside the pump.
In a positive-displacement pump, slippage is negligible compared to the pump's volumetric output flow. If the output port were plugged, pressure would increase instantaneously to the point that the pump's pumping element or its case would fail (probably explode, if the drive shaft did not break first), or the pump's prime mover would stall.
A positive-displacement pump is one that displaces (delivers) the same amount of liquid for each rotating cycle of the pumping element. Constant delivery during each cycle is possible because of the close-tolerance fit between the pumping element and the pump case. That is, the amount of liquid that slips past the pumping element in a positive-displacement pump is minimal and negligible compared to the theoretical maximum possible delivery. The delivery per cycle remains almost constant, regardless of changes in pressure against which the pump is working. Note that if fluid slippage is substantial, the pump is not operating properly and should be repaired or replaced.
Needle valves are common in low-pressure hydraulic systems and are often automated, connected to a hydraulic motor or air actuator that automatically opens and closes the valve. Manual and automated valves of all types provide precise control of the flow rate and are used for regulating the flow of fluid in the system. However, in some cases a needle valve can cause excessive heat to build up and waste energy if not properly designed into a system. For instance, if the system is operating at 10 gallons per minute, pressure will flow to the actuator if the relief valve is set at less than the 3,000 PSI. If the pump is producing 10 gallons per minute and it becomes necessary to slow the cylinder down, the needle valve must be adjusted down to restrict the flow. As the flow to the cylinder is restricted, the relief valve will open allowing the flow to go over it at 3,000 psi wasting energy in the form of heat. At 3,000 PSI, with as little as 2 gallons per minute over the rated flow, it can generate as much as 9,000 BTU. Excessive heat will severely damage hydraulic components and waste energy.
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