BIZOMAT MAKEOS · Hydraulics
Hydraulic Motor Torque, Speed and Power
What torque, speed and power will this hydraulic motor give for this flow and pressure?
Output torque, speed and power of a hydraulic motor from displacement, pressure difference, flow and volumetric and mechanical efficiencies.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Displacement | cm³/rev | Geometric displacement per revolution from the motor data sheet. |
| Pressure difference across motor | bar | Inlet pressure minus outlet (case or return) pressure. |
| Flow to motor | L/min | Flow supplied to the motor. |
| Volumetric efficiency | % | Volumetric efficiency at this speed and pressure, from the maker. |
| Mechanical efficiency | % | Mechanical (hydromechanical) efficiency from the maker. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Displacement
- What it is
- The volume of oil the motor takes in to turn its shaft through one revolution, from the motor data sheet.
- Units and format
- Enter a number in cm³/rev: cm³/rev. The form accepts 0.000001 cm³/rev to 100000 cm³/rev. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 cm³/rev as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Torque rises in direct proportion; speed falls in inverse proportion.
- Get it right
- Use cubic centimetres per revolution; some data sheets list cubic inches.
What this means: Pressure difference across motor
- What it is
- The pressure at the motor inlet minus the pressure at its outlet or return port, which is what actually produces torque.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 5000 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 150 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Torque and hydraulic power rise in direct proportion.
- Get it right
- Subtract the return line back pressure; using inlet pressure alone overstates torque.
What this means: Flow to motor
- What it is
- The volume of oil delivered to the motor each minute by the pump or valve.
- Units and format
- Enter a number in L/min: litres per minute. The form accepts 0.000001 L/min to 100000 L/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 L/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Speed and output power rise in direct proportion; torque does not change.
- Get it right
- Use the flow actually reaching the motor after any relief or flow control.
What this means: Volumetric efficiency
- What it is
- The share of the oil supplied that actually turns the shaft, the rest leaking internally, from the motor maker at this speed and pressure.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 95% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Speed rises in direct proportion.
- Get it right
- Type the percentage itself; it falls at high pressure and low speed.
What this means: Mechanical efficiency
- What it is
- The share of the theoretical torque the motor delivers after internal friction, also called hydromechanical efficiency, from the maker’s data.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 92% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Torque rises in direct proportion.
- Get it right
- Starting torque is lower than running torque; use the maker’s starting efficiency for breakaway loads.
Method and formulas
- Torque T = Vd × Δp × η_m ÷ 20π (cm³, bar → N·m).
- Speed n = Q × 1000 × η_v ÷ Vd (rpm).
- Hydraulic power = Q × Δp ÷ 600 (kW); output = 2π n T ÷ 60 000.
- Example: 100 cm³/rev at 150 bar and 60 L/min gives 219.63 N·m at 570 rpm.
Limitations
- Efficiencies vary with speed, pressure and oil viscosity; starting torque is lower than running torque.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Select a hydraulic motor for a winch, wheel or conveyor
- Check pump flow for a motor speed
- Estimate output power
Who it is for
- Hydraulic and process engineers
- Maintenance engineers
- Design engineers
What you get
- Output torque
- Output speed
- Output and hydraulic power
- Overall efficiency
Interpreting the result
- Check starting torque against breakaway load using the maker’s starting efficiency.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 352–368. Topic reference; no source tables or recommended settings are reproduced.
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