How to use this calculator
- Enter displacement. Enter the motor displacement per revolution.
- Enter pressure drop and speed. Use the pressure drop across the motor and the target rpm.
- Set efficiencies. Enter mechanical efficiency for torque and volumetric efficiency for flow.
- Read torque and flow. Read output torque, required inlet flow, shaft power and hydraulic input power.
How it works
A hydraulic motor converts pressure and displacement into torque: T = Δp · Vd · η_m / (2π) With Δp in MPa and Vd in mm³/rev, the unit product is N·mm per revolution; the calculator divides by 1000 to report N·m.
Flow comes from the same displacement and the desired speed: Q = Vd · n / η_v Volumetric efficiency is in the denominator because leakage means the pump must supply more flow than the ideal geometric displacement would suggest.
Worked example
Verified against the live calculator
A 50 cm³/rev motor at 160 bar pressure drop, 300 rpm, 90% mechanical efficiency
and 92% volumetric efficiency produces
16 MPa × 50000 mm³/rev ÷ (2π × 1000) × 0.90 ≈ 115 N·m. It needs
50 × 300 ÷ 1000 ÷ 0.92 ≈ 16.3 L/min of inlet flow.
Frequently asked questions
How do you calculate hydraulic motor torque?
Hydraulic motor torque is T = Δp·Vd/(2π), with pressure drop Δp and displacement Vd per revolution. The calculator multiplies the theoretical torque by mechanical efficiency.
How do you calculate hydraulic motor flow?
Required flow is displacement × speed ÷ volumetric efficiency. Leakage means the supplied flow must be higher than the ideal geometric flow.
Should I use pump pressure or pressure drop?
Use the pressure drop across the motor. Valve losses, line losses and back pressure reduce the pressure available to make torque.
Why are there two efficiencies?
Mechanical efficiency reduces torque because of friction. Volumetric efficiency increases required flow because some oil leaks internally instead of moving the shaft.
Method & assumptions
- Pressure is the drop across the motor, after valve and line losses.
- Efficiency is treated as constant; real motors vary with speed, pressure and oil viscosity.
- Does not check case drain limits, minimum speed stability, starting torque, cavitation or continuous thermal rating.