How an axial piston pump works: the swashplate, the valve plate and the case drain

In simple terms

Inside the pump is a round block called the barrel, with nine holes bored along its length. Each hole has a piston in it. The barrel is turned by the drive shaft.

The feet of the pistons rest against a plate called the swashplate. If the swashplate is flat, square to the shaft, the pistons go round without moving in or out, and the pump delivers nothing, even though it is spinning.

Tilt the swashplate and everything changes. As the barrel turns, each piston’s foot slides round the tilted plate, so the piston is pulled out of its hole for half a turn (drawing oil in) and pushed back in for the other half (pushing oil out). The more you tilt, the longer each piston’s stroke, and the more oil per turn.

At the back of the barrel is the valve plate, which you can see end-on in the animation. It has two curved slots, called kidneys. Each piston’s hole passes over the suction kidney while it is filling, and over the delivery kidney while it is emptying. That is how oil is let in and pushed out without any valves opening and closing.

Some oil always leaks past the pistons and the valve plate. That is deliberate: it lubricates them. The leaked oil collects in the pump case and returns to the tank through a separate pipe, the case drain. As the pump wears, more oil leaks, and the case drain flow goes up. Measuring it is the quickest way to check a piston pump’s health.