Why air motors are selected by duty, not by air use
Air motors turn compressed air into rotation. They are used where electric motors are awkward or unsafe: hazardous areas in refineries and paint shops, wet or washdown areas, mixers and agitators, winches and hoists, tensioning equipment, mining and tunnelling tools, and drives that must stall without damage.
Two motors can consume similar airflow yet deliver very different torque at the working speed. So the selection starts at the shaft, with the mechanical work required, and only then turns to the air supply. A motor chosen from a cfm figure or a kW rating alone often stalls on start-up, overspeeds when lightly loaded or starves the rest of the air system.
The worksheet keeps five quantities separate, because confusing them is the commonest cause of a wrong selection:
| Term | Meaning | Common mistake |
|---|---|---|
| Running torque | Torque needed while the machine runs at its target speed | Read at the wrong speed on the curve |
| Starting torque | Torque available from standstill, compared with the breakaway load | Ignored, so the motor cannot start a loaded mixer or winch |
| Peak torque | Short process demand above running torque | Treated as continuous |
| Stall torque | Torque at zero speed | Used as if it were a working rating |
| Free speed | Speed with no load | Used as the loaded operating speed |
The core mechanical relationship
Shaft power, torque and speed are linked by an exact relationship, provided consistent SI units are used:
- P = 2 × π × n × T ÷ 60, where P is power in watts, n is speed in rpm and T is torque in N·m.
- T = 60 × P ÷ (2 × π × n), to find torque from known power and speed.
- n = 60 × P ÷ (2 × π × T), to find speed from known power and torque.
If a gearbox sits between motor and load, translate the duty through the ratio and an explicit efficiency:
- Motor speed = load speed × gear ratio.
- Motor torque required = load torque ÷ (gear ratio × gearbox efficiency).
Gearbox efficiency should come from the gearbox maker. Where it is only an estimate, the worksheet shows it as a preliminary assumption rather than hiding a default.
Air motor types and where they fit
| Type | Character | Typical uses |
|---|---|---|
| Vane | Compact, light, high speed; torque rises as speed falls | Mixers, tools, small winches, conveyors |
| Radial or axial piston | High starting torque, good low-speed control | Hoists, winches, heavy starting loads |
| Gear | Robust, high power for size | Mining, heavy industrial drives |
| Turbine | Very high speed, often oil-free exhaust | Starters, grinders, specialist drives |
For a typical vane motor, power output peaks somewhere around the middle of its speed range and falls towards zero at both stall and free speed. The best working point is usually near peak power, but always confirm it on the manufacturer’s curve for your model and pressure.
How to use the worksheet
- Describe the driven machine and its duty: continuous, intermittent or cyclic, one-way or reversing.
- Enter the required speed and running torque at the driven shaft, plus starting or breakaway torque and any peak torque.
- Add gear ratio and gearbox efficiency if a reduction is used.
- Enter the pressure available at the motor inlet while air is flowing, not the compressor discharge pressure.
- Select a candidate motor and enter its torque at the target speed, starting torque, stall torque and air consumption from its published curve at that pressure.
- Read the running torque margin and starting torque margin separately.
- For several motors, total the air demand using each motor’s own consumption and how many run together.
The worksheet returns pass, hold or fail. Pass means the motor meets running torque, starting torque, speed and air supply on model-specific data. Hold means data is missing or provisional. Fail means at least one requirement is not met.
Worked example: geared mixer drive
A paint mixer must turn at 300 rpm with a running torque of 40 N·m. When full and settled, it needs 60 N·m to break away. A 5:1 gearbox with an assumed efficiency of 90% is used. The motor inlet pressure under flow is 6 bar(g) (about 87 psig).
- Shaft power at the mixer = 2 × π × 300 × 40 ÷ 60 ≈ 1,257 W, about 1.26 kW.
- Motor speed = 300 × 5 = 1,500 rpm.
- Motor running torque = 40 ÷ (5 × 0.9) ≈ 8.9 N·m.
- Motor starting torque required = 60 ÷ (5 × 0.9) ≈ 13.3 N·m.
| Check | Motor A | Motor B |
|---|---|---|
| Torque at 1,500 rpm, 6 bar(g) | 9.5 N·m (pass) | 11 N·m (pass) |
| Starting torque | 11 N·m, below 13.3 (fail) | 16 N·m, above 13.3 (pass) |
| Result | Fail on start duty | Pass, subject to air supply check |
Motor A meets the running duty but cannot start a settled batch. Running power alone would have chosen the wrong motor. The figures are illustrative; your candidates’ published curves decide the result.
Pressure at the motor, not at the compressor
Air motor torque and speed fall as inlet pressure falls. The pressure at the motor while it runs can be well below compressor discharge because of the dryer, filters, regulator, valves, pipework, hoses and quick couplings. A motor checked at 7 bar(g) on paper may only see 5.5 bar(g) in service.
- Build the supply path in the pressure budget builder.
- Size the branch and hose with the pipe sizing calculator; undersized hoses and couplings are a frequent cause of weak motors.
- Do not raise the compressor set-point to compensate. Each extra bar costs about 7% more compressor power across the whole plant.
Air consumption and air quality
Air consumption must come from the manufacturer’s curve or table for the chosen model at the selected pressure and operating point. Air motors are relatively inefficient converters of compressed-air energy, and the ratio of air to shaft power varies widely between designs, so estimating air use from kW is unreliable.
For several motors: total demand = sum of each motor’s consumption × its simultaneity factor. Enter this total in the compressed air demand calculator.
Follow the motor maker’s rules on air preparation. Many vane motors need lubricated air through an FRL; some are designed for lubrication-free running, and food or pharmaceutical areas may require oil-free exhaust. Water in the air causes corrosion and vane wear, so dry, filtered air protects the motor.
Duty cycle, reversing and speed control
- Continuous duty and frequent starts affect wear and lubrication; record start frequency and check it against the maker’s permitted regime.
- Reversible motors suit winches and positioning drives, but reversing under load creates torque transients; record how often and whether the load can drive back.
- A pressure regulator alone does not give precise speed control. Where speed matters, use flow control, a governor or feedback and state the method.
- Exhaust noise and icing at the exhaust can be significant on large motors; plan silencers and exhaust routing.
How Reddy Compressors helps
We do not manufacture air motors. Our part is the compressed-air supply that makes them work. Reddy Compressors, based at Tarnaka, Secunderabad since 2002, sells new and used compressors, rents from our own 50–2,000 cfm fleet, and services and supplies spares for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands as an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands.
- Checking that compressor capacity covers the motors’ combined demand at the required inlet pressure.
- Pressure-drop surveys from compressor to motor inlet, including hoses and couplings.
- Air treatment: aftercoolers, dryers and filtration suited to the motor maker’s requirements.
- Portable diesel or electric compressors on hire for winches, hoists and site drives.
Our service engineers cover Telangana and Andhra Pradesh, and we supply compressors, rentals and spares across India.
What to send us
- The driven machine, its speed, running torque and starting torque.
- Air motor make, model and data sheet, or the motor you are considering.
- Number of motors and how many run at the same time.
- Compressor make, model, nameplate photo and working pressure.
- Pipe and hose sizes and lengths to the motor.
Frequently asked questions
How do I size an air motor for my application?
Find the speed and torque the driven machine needs at its shaft, including starting or breakaway torque. Calculate power with P = 2πnT ÷ 60, allow for any gearbox, then pick a motor whose published torque–speed curve meets the running point and whose starting torque exceeds the breakaway load at the pressure available at its inlet. Finally, check that your compressor can supply its air consumption.
Can I select an air motor from kW alone?
No. Two motors with the same power rating can give quite different torque at your working speed, and one may not start the load at all. Power is only the product of torque and speed. You need both at the actual operating point, plus the starting torque requirement, and you must read them from the specific motor’s curve at your inlet pressure.
Why does my air motor lose power under load?
Most often, pressure at the motor inlet falls when it draws air, because of undersized hoses, quick couplings, clogged filters or a regulator set too low. Worn vanes, lack of lubrication where it is required and a restricted exhaust silencer are other common causes. Measure pressure at the motor inlet while it is running under load, not at the compressor.
How much compressed air does an air motor use?
It depends on the motor design, size, inlet pressure and operating point, so the only reliable figure is the manufacturer’s consumption curve or table for that model. Estimating from shaft power gives poor results because air motor efficiency varies widely. Once you have the figure, multiply by how many motors run together and add it to your plant demand calculation.
What is the difference between stall torque and starting torque?
Stall torque is the torque a motor produces when held at zero speed; it is not a safe continuous working condition. Starting torque is the torque available to begin turning a load from rest, which may be lower than stall torque on some designs. Compare starting torque with the load’s breakaway requirement and running torque with the load at working speed.
Do air motors need lubricated air?
Many vane and piston motors need lubricated air, usually through a filter–regulator–lubricator at the point of use. Some designs are built to run lubrication-free, which suits food, pharmaceutical and clean areas. Always follow the motor maker’s instructions, and supply clean, dry air in either case, because water and dirt are the main causes of internal wear and corrosion.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Air motor duty check
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Required speed at the driven shaft (rpm): 300
- Running torque at the driven shaft (N·m): 40
- Breakaway (starting) torque at the driven shaft (N·m): 60
- Gear ratio (motor speed ÷ load speed; 1 for direct drive): 5
- Gearbox efficiency (%): 90
- Pressure at the motor inlet while running (bar g): 6
- Pressure at which the motor curve was read (bar g): 6
- Candidate motor: torque at the required motor speed (N·m): 11
- Candidate motor: starting torque (N·m): 16
- Candidate motor: air consumption at that point (L/s free air; 0 if not yet known): 0
- Number of these motors running together: 1
- Spare compressor capacity at that pressure (m³/min free air; 0 if not yet checked): 0
Result
- Shaft power at the driven machine: 1,257 W (1.26 kW)
- Motor speed: 1,500 rpm
- Motor running torque required: 8.89 N·m
- Motor shaft power required (incl. gearbox loss): 1,396 W (1.4 kW)
- Motor starting torque required: 13.33 N·m
- Running torque margin: 23.8 % (pass)
- Candidate motor output at that speed (from its curve): 1.73 kW, against 1.4 kW required at the motor shaft
- Starting torque margin: 20 % (pass)
- Air demand: not calculated enter the consumption from the maker’s curve at this pressure and point
- Result: Hold torque checks done so far pass; still needed: air consumption
Method. Shaft power P (W) = 2 × π × n × T ÷ 60. Motor speed = load speed × gear ratio. Motor torque required = load torque ÷ (gear ratio × gearbox efficiency); the same for the breakaway (starting) torque. Margins = motor torque from the curve ÷ torque required − 1. Air demand = motor consumption × number running together. Result: Pass when running torque, starting torque and air supply are all met on data read at or below the pressure available at the motor inlet; Hold when data are missing or read at a higher pressure; Fail when any requirement is not met. For a direct drive enter ratio 1 and efficiency 100%. Peak and stall torque are not checked here: compare them with the maker’s curve separately.
What to take from it. Running power alone can choose the wrong motor. Check starting torque against the breakaway load, and read every figure at the pressure the motor actually sees while running.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.