Guides
Hydraulic Pumps are any of a class of positive displacement machines used in fluid power applications to provide hydraulic flow to fluid-powered devices such as cylinders, rams, motors, etc. A car’s power-steering pump is one example where an engine-driven rotary-vane pump is common. The engine’s gear-type oil pump is another everyday example. Hydraulic pumps can be motor-driven, too, or manually operated. Variable displacement pumps are especially useful because they can provide infinite adjustment over their speed range with a constant input rpm.
Starting with variable displacement pumps, the principal types of hydraulic pumps in use include:
Pumps produce flow. Pressure is resistance to flow. Whereas centrifugal pumps can run against blocked discharges without building up excess pressure, positive-displacement pumps cannot. Hydraulic pumps, like any positive-displacement pump, thus require overpressure protection generally in the form of a pressure-relief valve. Over-pressure relief is often built into the pump itself.
Hydraulic systems are used where compact power is needed and where electrical, mechanical, or pneumatic systems would become too large, too dangerous, or otherwise not up to the task. For construction equipment, hydraulic power provides the means to move heavy booms and buckets. In manufacturing, hydraulic power is used for presses and other high-force applications. At the heart of the hydraulic system is the pump itself and the selection of a correct hydraulic pump hinges on just what the hydraulic system will be expected to do.
Axial piston pumps use axially mounted pistons that reciprocate within internal cylinders to create alternating suction and discharge flow. They can be designed as variable-rate devices making them useful for controlling the speeds of hydraulic motors and cylinders. In this design, a swashplate is used to vary the depth to which each piston extends into its cylinder as the pump rotates, affecting the volume of discharge. A pressure compensator piston is used in some designs to maintain a constant discharge pressure under varying loads.
Radial piston pumps arrange a series of pistons radially around a rotor hub. The rotor, mounted eccentrically in the pump housing, forces the pistons in and out of cylinders as it rotates, which cause hydraulic fluid to be sucked into the cylinder cavity and then be discharged from it. Inlets and outlets for the pump are located in a valve in a central hub. An alternative design places inlets and outlets around the perimeter of the pump housing. Radial piston pumps can be purchased as fixed- or variable-displacement models. In the variable-displacement version, the eccentricity of the rotor in the pump housing is altered to decrease or increase the stroke of the pistons.
Rotary vane pumps use a series of rigid vanes, mounted in an eccentric rotor, which sweep along the inside wall of a housing cavity to create smaller volumes, which forces the fluid out through the discharge port. In some designs, the volume of the fluid leaving the pump can be adjusted by changing the rotational axis of the rotor with respect to the pump housing. Zero flow occurs when the rotor and housing axes coincide.
External Gear pumps rely on the counter-rotating motion of meshed external spur gears to impart motion to a fluid. They are generally fixed-displacement designs, very simple and robust. They are commonly found as close-coupled designs where the motor and pump share a common shaft and mounting. Oil travels around the periphery of the pump housing between the teeth of the gears. On the outlet side, the meshing action of the teeth decreases the volume to discharge the oil. The small amount of oil that is trapped between the re-meshing gears discharges through the bearings and back to the pump’s suction side. External gear pumps are very popular in fixed-displacement hydraulic applications as they are capable of providing very high pressures.
The internal gear pump uses the meshing action of an internal and external gear combined with a crescent-shaped sector element to create fluid flow. The axis of the external gear is offset from that of the internal gear, and as the two gears rotate, their coming out of and into mesh create suction and discharge zones. The sector serves as a barrier between suction and discharge. Another internal gear pump, the gerotor, uses meshing trochoidal gears to achieve the same suction and discharge zones without needing a sector element.
This article presented a brief summary of some of the common types of hydraulic pumps. For more information on additional topics, consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products.
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