Axial Piston Pump

What is modelled

Pistons in a rotating cylinder block slide against a tilted plate. Each piston moves in and out once per turn, drawing oil from the inlet half of the turn and pushing it to the outlet in the other half. A larger tilt gives a longer stroke.

Equations

• Stroke s = 2 R tan α, with pitch radius R = 64 mm.
• Displacement Vd = N π r2 s, bore radius r = 11.8 mm.
• Flow Q = Vd n ηv, η_v = 0.92.
• Torque T = p Vd / (2 π ηm), η_m = 0.89.
• Ripple from the sum of the positive half-sines of N phase-shifted pistons.

Checks run in the page

• The mean of the kinematic flow equals displacement times speed.
• Flow ripple peak and trough against closed forms for odd and even N.
• Piston travel equals 2 R tan α.
• Flow scales with speed and efficiencies close the power balance.

Honest simplifications

Constant efficiencies, no leakage dependence on pressure, no compressibility or valve-plate timing effects, and a symmetric kinematic flow ripple.

• Set the angle to zero: the pistons stop stroking and the pump delivers nothing even though the shaft still turns.
• Compare 7 and 8 pistons: an odd count gives a much smaller flow ripple.
• Raise the pressure and watch the shaft torque rise while flow stays the same.