Running and starting torque from pressure and displacement, with speed and power.
Inlet minus outlet pressure, not supply pressure alone.
From the motor's data. Running figures are often near 90%.
Empty uses 75%. Typical range 70–80%.
Empty uses 95%.
A hydraulic motor does not hold a load. Internal leakage lets it creep, so a winch or lift needs a separate brake or holding device that applies when pressure is lost. Check starting torque against the load, using the maker's curve.
Example Result
Your result will appear here
The calculated result will appear here after you press Calculate.
A hydraulic motor's torque comes from the pressure drop across it and the oil it swallows per revolution. Enter both, with the efficiency, to get the running torque. The calculator also shows a lower starting torque, because a motor gives less torque from rest. Add flow to get speed and power.
Enter the pressure drop across the motor: inlet minus outlet.
Enter the displacement and the mechanical efficiency from the motor's data.
Leave the starting figure empty for 75%, or enter the maker's value.
Add flow to get speed and power.
By O.E. — Partner, chairman of the board and CEO of a metal products manufacturer for 21 years; the company exports to 45 countries.
Drafted with AI assistance; calculations checked against primary sources and tested with the site's own code. About FlowUtil
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Theoretical torque = Δp × D ÷ 2π
Δp is the pressure drop across the motor, and D is its displacement per revolution.
Real torque is lower. Multiply by the mechanical efficiency. Use the difference between inlet and outlet pressure, not the supply pressure alone: 10 bar of back pressure on a 200 bar motor costs 5% of the torque.
A motor at rest has little oil film and more leakage per turn, so it gives less than its running torque. Published guidance puts running torque for gear, vane and piston motors near 90% of theoretical, and starting torque at about 70–80%. Piston motors usually start best, and gear motors lose more as they wear.
| Efficiency used | Torque from 200 bar × 50 cm³ |
|---|---|
| Theoretical, 100% | 159 N·m |
| Running, 93% | 148 N·m |
| Starting, 80% | 127 N·m |
| Starting, 75% | 119 N·m |
| Starting, 70% | 111 N·m |
The load has its own number too. A stationary load needs more torque to break away than to keep turning. Size the motor so starting torque beats the load's breakaway torque, using the maker's torque curve. The general torque calculator helps turn a force and radius into the load torque.
To find a motor for a 300 N·m start at 200 bar and 75%, work backwards: D = 2π × T ÷ (Δp × η) = 126 cm³/rev.
Speed = flow × volumetric efficiency ÷ displacement. With 40 L/min and 95%:
In US units, 10 gpm through 3.05 in³/rev at 95% gives 720 rpm, and 112.9 lb·ft × 720 ÷ 5,252 = 15.5 hp.
Torque depends on pressure, and speed depends on flow. A larger motor gives more torque and less speed from the same supply. Check the supply with the hydraulic pump flow calculator, and the input power with the hydraulic pump power calculator.
A hydraulic motor does not hold a load. Internal leakage lets it creep, so a winch or lift needs a brake that applies when pressure is lost. A counterbalance valve does not fix this, because oil still leaks past the motor's own parts. Follow the winch maker's design and the lifting rules that apply to you.
Pressure ratings matter here too. Set the relief valve below the motor's limit, and check the case drain limit in its data sheet. The hydraulic pressure calculator covers the cylinders on the same circuit.
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