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CATEGORIES (articles) > Transmission > Components > Axial piston pump explained

Axial piston pump explained

An axial piston pump is one where the pistons are arranged in a circular housing that is driven by an integral shaft that is, more or less, aligned with the pistons. One end of the cylinders, housing the pistons, fits closely to a body part that is stationary.

The device can operate as a pump or a hydraulic motor.

The body has two semi-circular ports that allow inlet of the operating fluid and exhaust of the fluid (be it air, oil or whatever).

The opposite end of the cylinder and piston assembly has the heads of the pistons that bear against a swash plate, a circular collar that can be set parallel to the top face of the body, or at an angle to it. When set at an angle the swash plate forces the pistons to reciprocate (move up and down) as their housing is rotated. The swash plate angle varies the piston's stroke from zero (plate parallel to the body) to maximum (plate at maximum angle.

As each piston is drawn up its cylinder, to follow the up-slope of the swash plate, it draws fluid through the inlet slot in the body. As the piston's housing continues to turn the pistons reach full stroke, for the swash plate setting, and then are forced into the housing, forcing the fluid out through the body slot and to the delivery hose from the pump.

If the swash plate is set parallel to the body's face there is no movement of the pistons in their cylinders. Thus there is no output. Movement of the swash plate controls pump output from nothing to maximum volume. If the swash plate is arranged to adjust its angle according to output demand it can make the pump idle at no output when there is no flow demand in the system (swash plate parallel) and at maximum delivery when the swash plate is at the greatest angle.

In reality most systems use pressure as a control for this type of pump. The operating pressure reaches, say, 200 bar (2 MPa or 3000 psi) and the swash plate is driven towards zero angle (piston stroke nearly zero) and with the inherent leaks in the system allows the pump to stabilise at the delivery volume that maintains the set pressure. As demand increases the swash plate is moved to a greater angle, piston stroke increases and the volume of fluid increases, if the demand slackens the pressure will rise and the pumped volume diminishes as the pressure rises. At maximum system pressure the output is almost zero again.

If the fluid demand increases, beyond the capacity of the pump's delivery, the system pressure will drop near to zero. The swash plate angle will remain at the maximum allowed and the pistons will operate at full stroke. This continues until system flow-demand eases and the pump's capacity is greater than demand. As the pressure rises the swash-plate angle modulates to try to not exceed the maximum pressure while meeting the flow demand.

Designers have a number of problems to overcome in designing axial piston pumps. One is managing to be able to manufacture a pump with the fine tolerances necessary for efficient operation. The mating faces between the rotary piston-cylinder assembly and the stationary pump body have to be almost a perfect seal while the rotary part turns at, maybe, 3000 rpm. The pistons are usually less than half an inch (13 mm) in diameter with similar stroke lengths. Keeping the wall to piston seal tight means that very small clearances are involved and that materials have to be closely matched for similar coefficient of expansion.

The pistons have to be drawn outwards in their cylinder by some means. On small pumps this can be done by means of a spring inside the cylinder that forces the piston up the cylinder. Inlet fluid pressure can also be arranged so that the fluid pushes the pistons up the cylinder. Often a vane pump is located on the same drive shaft to provide this pressure and it also allows the pump assembly to draw fluid against some suction head from the reservoir, which is not an attribute of the unaided axial piston pump.

Another method of drawing pistons up the cylinder is to attach the cylinder heads to the surface of the swash plate. In that way the piston stroke is totally mechanical. However, the designer's problem of lubricating the swash plate face (a sliding contact) is made even more difficult.

Internal lubrication of the pump is achieved by use of the operating fluid—normally called hydraulic fluid. Most hydraulic systems have a maximum operating temperature, limited by the fluid, of about 120 °C (250 °F) so that using that fluid as a lubricant brings its own problems. In this type of pump the leakage from the face between the cylinder housing and the body block is used to cool and lubricate the exterior of the rotating parts. The leakage is then carried off to the reservoir or to the inlet side of the pump again. Hydraulic fluid that has been used is always cooled and passed through micrometre-sized filters before recirculating through the pump.

Despite the problems indicated above this type of pump can contain most of the necessary circuit controls integrally (the swash-plate angle control) to regulate flow and pressure, be very reliable and allow the rest of the hydraulic system to be very simple and inexpensive.

Axial reciprocating motors are also used to power many machines. They operate on the same principal as described above except that the circulating fluid is provided under considerable pressure and the piston housing is caused to rotate and provide shaft power to another machine.

A common use of an axial reciprocating motor is to power small earthmoving plant such as skid loader machines. Another use is to drive the screws of torpedoes.

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