Axial piston pumps have a circular piston group which rotates against an angled swash plate. As the rotary group turns the pistons are pushed forwards and backwards. A grooved timing plate at the at the top of the pistons controls the way the fluid is drawn through the suction side of the pump and out through the pressure side.
A fixed displacement piston pump has fixed swash plate but a range of different controllers can be used to control the position of and adjustable swash plate to make a variable displacement version. Changing the angle of the swash plate will change the axial displacement of the pistons and therefore the flow from the pump. The angle of swash can be controlled manually although more commonly this a constant pressure, flow or power control.
The bearing and piston feet have hydrostatic lubrication that requires a constant leakage into the case. Draining this leaking fluid and maintaining a low, stable pressure in the pump casing is important for ensuring a long pump life.
Unlike gear pumps, piston pumps do not generate much contamination as they operate. But also piston pumps do not last as long if the fluid is dirty.
Piston pumps can also come with through shafts to run other piston or gear pumps in tandem.
Keeping the piston pump and case full of oil at start up is vital to avoid pump damage and extend pump life. For this reason, pumps must be primed during installation or bled if the reservoir is above the pump suction as recommended.
External gear pumps will generate light suction on the inlet side of the pump. NEVER start the pumps dry; they will not draw fluid when dry and will quickly fail. Prime the pump with fluid before you start them for the first time and ensure the do not drain down if they have a negative head.
Piston pumps create a fluid borne noise that will be carried around the hydraulic system by the fluid. To keep noise transmission to a minimum, isolate the pump on rubber feet and keep the distance between the pump and the first valve to a minimum. Hydraulic attenuators can also be used to at pump outlet.