Calculate hydraulic motor output speed, torque, and power based on displacement, flow, and pressure drop.
A hydraulic motor converts pressurized fluid flow into rotational torque, driving conveyors, mixers, augers, and traction wheels. This calculator solves output torque, shaft RPM, and required input flow from displacement, working pressure, and target speed, including the mechanical and volumetric efficiency that separate ideal performance from field results. It is the companion sizing tool to the pump-flow calculator.
Theoretical torque T = (P × D) / (2π) where P is differential pressure across the motor and D is displacement per revolution. In US units, T (lb·in) = P (psi) × D (in³/rev) / (2π); in SI, T (N·m) = P (bar) × D (cm³/rev) × 0.01592. Output shaft speed N = (Q × η_v) / D with Q in lpm or GPM and N in RPM. Mechanical efficiency (0.85–0.95 for gear motors, 0.92–0.97 for piston) reduces actual torque, while volumetric efficiency drops shaft speed by 3–10% at rated pressure. Required input flow at a target RPM is Q = (N × D) / η_v. Always check the manufacturer's drainage requirements for case-drain motors — case flow above 5% of rated flow can indicate seal wear.
A material-handling engineer designing a roller conveyor drive at 50 RPM and 600 lb·in torque selects a gear motor with 7.5 in³/rev displacement, then verifies that 2,500 psi supply delivers the torque with 10% reserve.
A process technician sizing an agitator drive at 120 RPM needs 250 N·m starting torque; the calculator picks a 100 cm³/rev motor at 175 bar with 92% mechanical efficiency to clear the worst-case startup load.
A vehicle integrator checking the top speed of a hydrostatic wheel drive enters pump flow, motor displacement, and final-drive ratio, then confirms the computed shaft RPM produces the target ground speed at full pedal.
Gear motors are low cost and tolerant of contamination but have lower efficiency (~85%). Vane motors offer smoother torque and 88-92% efficiency. Piston motors deliver 92-97% efficiency and handle the highest pressure but cost more.
Volumetric efficiency drops with rising pressure as internal slip increases — typical gear motor goes from 95% at 1,000 psi to 88% at 3,000 psi. Always size at maximum operating pressure for accurate speed predictions.
Bent-axis and inline piston motors require case-drain lines back to tank to relieve internal leakage. Gear and vane motors typically do not. Check the manufacturer datasheet — running a piston motor without drain can blow shaft seals.