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Axial Piston Pumps
The axial piston pump is used in hydraulics to build up the pressure in the hydraulic system and transfer power to the connected drive elements using hydraulic oil. The characteristic feature of axial piston pumps is a construction technique in which the pistons act more or less parallel to the driving axle and not perpendicular to it. Axial piston pumps are used in hydrostatic gears, for instance, to drive slow-moving work machines.
Structure of axial piston pumps
Internally, axial piston pumps consist of a cylindrical drum which is either aligned with the drive shaft or installed at a slight incline to it. Finger pistons, which are positioned around the drive shaft and jointly rotated by it, engage in the cylinder bores of this drum. It is designed in such a way that a piston stroke occurs in the jointly rotating cylindrical drum when the shaft is rotated.
There are two different models of axial piston pumps: bent axis pumps and swash plate pumps.
In the bent axis pump, the entire cylinder housing with the cylindrical drum is slightly inclined to the drive shaft, whereas in the swash plate pump, the cylinder and piston are actually completely parallel to the drive shaft. In the swash plate pump, the piston stroke is produced by a sliding disc, which acts as a counter bearing for the pistons and has either a fixed or a variable angle of incidence with respect to the piston axis. As the shaft rotates, this disc pushes the pistons back and forth. The dead centres of the piston movement remain fixed in relation to the pump casing during this process. An advantage of the swash plate design is that several pumps can be driven by the same shaft via a single through-drive mechanism.
The piston stroke and thus the delivery rate can be changed by altering the angle of incidence of the plate in the case of the swash plate design. However, in some models of bent axis pumps, it is also possible to change the piston stroke by bending the cylinder housing to a greater or lesser extent relative to the drive shaft. The prerequisite for this is a suitable drive for the cylindrical drum. This can be achieved using bevel gears or a cardanic drive. The drum can also be carried along by the pistons.
Properties of axial piston pumps
Axial piston pumps have a small size, are self-priming and generate a continuous pressure. The direction of flow of axial piston pumps is reversible. Due to the circular pistons arranged around the drive shaft, the axial piston pump works very smoothly. Here the pressure build-up is always on the same side of the cylinder housing. The axial piston pump can also be used as an axial piston motor in an almost identical design.