Piston units operate at higher efficiencies than gear and vane units and are used for high-pressure applications with hydraulic oil or fire resistant fluids. Several types of piston pump are available that use different design approaches and these include those having axial and radial piston arrangements.

The majority of piston pumps/motors are of the axial variety, in which several cylinders are grouped in a block around a main axis with their axes parallel as shown in Figure 1 which has variable displacement capability. The pressure force from the pistons is transferred to the angled swash plate lubricated slippers that are mounted onto the pistons with a ball coupling. Rotation of the cylinder block causes the pistons to oscillate in their cylinders by the action of the swash plate, which provides the conversion between the piston pressure force and shaft torque.

The piston cylinders are alternately connected to the high and low-pressure connections by a plate valve between the cylinder block and the port connection housing. Varying the swash plate angle allows the displacement to be changed over the full range from zero to maximum. The swash plate angular position can be arranged to vary either side of the zero displacement position so that flow reversal is obtained. This is referred to as over centre control.

Figures 2 shows a fixed displacement bent axis type of axial piston unit whereby the ball ended pistons are located in the output shaft. During rotation of the shaft there will be a rotating sliding action in the ball joint, and possibly, between the piston and the cylinder. Each cylinder is connected successively to the high and low-pressure ports by a similar valve to that used in the swash plate units.

In variable displacement units a mechanism is used to vary the tilt angle of the cylinder block from zero to maximum which, if required, can provide over centre operation to give reverse flow.

There are two types of radial piston pump; those in which the cylinder block rotates about a stationary pintle valve, and those with a stationary cylinder block in which the pistons are operated by a rotating eccentric or cam.

Many standard piston units of recent design operate at pressures of up to 450 bar. A wide range of types are available up to powers of 100 kW, although a number of manufacturers provide units having powers up to 300 kW with some available at powers of 1000 kW. Peak overall efficiencies in excess of 90% are usually obtained. The price of piston units varies from manufacturer to manufac­turer but may be as much as ten times the price of a gear pump of similar power.

Some pumps cannot draw the inlet fluid directly from the reservoir and may require boosting from a separate pump, often of the external gear type, that can accept low inlet pressures. However, for open loop circuits, variants are available that do not require separate boosting of the inlet which can be connected directly to the reservoir. These aspects also apply to motors that operate as pumps in hydrostatic systems when regeneration occurs (e.g. winch and vehicle drive ap­plications).

Variable displacement pumps provide a range of control methods, which in­clude pressure compensation, load sensing and torque, or power, limiting devices. Pump displacement controls incorporating electro-hydraulic valves are also avail­able.

In addition to their use to control outlet flow, variable displacement pumps provide considerable increase in overall system efficiency with the additional benefits of reduced heat generation and operating cost. This reduction in operat­ing cost can show an overall reduction in the total lifetime cost of the machine.