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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 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 harvester hydraulic pump is not operating properly and should be repaired or replaced.The positive-displacement principle is well illustrated in the reciprocating-type pump, the most elementary positive-displacement gear pump, Figure 1. As the piston extends, the partial vacuum created in the pump chamber draws liquid from the reservoir through the inlet check valve into the chamber. The partial vacuum helps seat firmly the outlet check valve. The volume of liquid drawn into the chamber is known because of the geometry of the pump case, in this example, a cylinder.
Figure 10. Pressure-flow curve of fixed-displacement oil hydraulic pump.
Bent-axis pumps — This pump consists of a drive shaft which rotates the pistons, a cylinder block, and a stationary valving surface facing the cylinder block bores which ports the inlet and outlet flow. The drive shaft axis is angular in relation to the cylinder block axis. Rotation of the drive shaft causes rotation of the pistons and the cylinder block.
Figure 11. Pressure flow curve of variable-displacement hydraulic oil transfer pump with ideal flow and pressure compensation.
In radial-piston pumps, the pistons are arranged radially in a cylinder block; they move perpendicularly to the shaft centerline. Two basic types are available: one uses cylindrically shaped pistons, the other ball pistons. They may also be classified according to the porting arrangement: check valve or pintle valve. They are available in fixed and variable displacement, and variable reversible (over-center) displacement.
A pump's mechanical efficiency is also less than perfect, because some of the input energy is wasted in friction. Overall efficiency of a hydraulic oil pressure pump is the product of its volumetric and mechanical efficiencies.
Pumps are generally rated by their maximum operating pressure capability and their output, in gpm or lpm, at a given drive speed, in rpm.