How air motors work and where they suit
Air motors convert compressed-air energy into rotation. The common types are vane motors, radial and axial piston motors, and gear motors; turbines are used for very high speeds. They are chosen where electric motors are awkward: explosive or dusty atmospheres, wet wash-down areas, applications that stall frequently, hand-held tools, mixers, hoists, winches and conveyors in hazardous areas.
- They can stall under load without damage and restart when the load reduces.
- Speed and torque are easily adjusted with a flow control or a pressure regulator.
- They are light for their output and generate no sparks from windings.
- They are less efficient than electric motors in energy terms, so air consumption and compressor cost must be considered.
Understanding the torque–speed curve
An air motor at constant inlet pressure has a characteristic curve. Torque is highest near stall and falls roughly in a straight line to zero at free speed. Output power is zero at both ends and peaks at around half the free speed. Air consumption rises with speed and is highest at free speed. These relationships are why selection starts at the working point.
| Term | Meaning | How to use it |
|---|---|---|
| Free speed | Speed with no load | Not an operating point; the motor uses most air here and does no useful work |
| Stall torque | Torque at zero speed under full pressure | Not the same as reliable starting torque, which is usually lower and varies with vane or piston position |
| Starting torque | Minimum torque the motor can produce from rest | Must exceed breakaway torque of the load, with margin |
| Maximum power | Peak output, typically near half free speed | Good design point for continuous duty |
| Air consumption | Free air used at a given speed and pressure | Determines compressor capacity and running cost |
Makers usually publish curves at a reference pressure, commonly about 6.3 bar (91.4 psig). If your supply at the motor is lower, both torque and power fall. Use the maker’s correction data or curves at your pressure.
A working-point selection method
- Define the load. Running torque and speed at the driven shaft, breakaway torque after a stop, any inertia to accelerate and how torque varies through the cycle.
- Include the transmission. Gearboxes, belts and couplings change torque and speed and introduce losses. Convert the load to the motor shaft.
- Find the available pressure. Measure or estimate pressure at the motor inlet under flow, after FRL, hose, valves and couplings.
- Select from curves. Choose a motor whose curve at that pressure passes through your working point, ideally between about a third and two thirds of free speed for good power and controllability.
- Check starting. Confirm minimum starting torque exceeds breakaway with margin.
- Check air consumption. Read consumption at the working point and add it to the compressor demand, considering duty cycle and simultaneous use.
- Check the installation. Shaft loads, mounting, exhaust silencing, lubrication, braking and environment.
Worked example (illustrative)
A mixer needs 20 N·m at 300 rpm at the shaft, with a breakaway torque after a dwell of 30 N·m. Mechanical power at the working point is torque × speed: 20 × (300 × 2π ÷ 60) ≈ 630 W. A motor with a free speed around 600–700 rpm and a maximum power somewhat above 0.63 kW at the available inlet pressure would place the working point near the middle of its curve. Its minimum starting torque must exceed 30 N·m with margin. If the pressure at the motor is lower than the curve’s reference pressure, re-read the curve or choose a larger motor or a gearbox. These figures illustrate the method only; real selection uses the specific motor’s published data.
Speed control and reversing
- Inlet throttling reduces both speed and torque; simple but makes the motor more sensitive to load changes.
- Exhaust throttling gives a stiffer speed under varying load for some motor types and is common for reversible motors with care for back-pressure.
- Pressure regulation lowers the whole curve, reducing stall torque; useful where torque must be limited.
- Reversing needs a reversible motor and a suitable directional valve; check that both directions meet the duty.
- Braking and holding are not inherent; an air motor will back-drive or coast unless a brake or self-locking gear is fitted, which matters for hoists and winches.
Air supply and preparation
Motor performance depends on the air arriving at its inlet. A properly sized supply line, FRL and valve are as important as the motor.
| Supply item | Common mistake | Good practice |
|---|---|---|
| Hose and fittings | Small bore quick couplings causing large pressure drop | Size for flow at the working point; minimise length and couplings |
| Filter regulator | Regulator too small, pressure collapses under flow | Select by flow curve, not port size |
| Lubrication | Lubricated motor run dry, or lubricant-free motor flooded | Follow the maker; lubricated vane motors need correct oil feed |
| Moisture | Wet air causing corrosion and ice at the exhaust | Dried air and drains on low points |
| Exhaust | Unsilenced or restricted exhaust | Correct silencer or piped exhaust with low back-pressure |
Air expands and cools strongly through the motor; in humid conditions, such as the monsoon months in Telangana and Andhra Pradesh, moisture can freeze at the exhaust of a hard-working motor if the air is not dried.
Compressor capacity and energy
Air motors are often the largest single air users on a machine. A motor running near free speed or left running unloaded wastes compressor energy continuously. Size the compressor for the sum of motor consumptions at their working points multiplied by realistic duty and simultaneity factors, and add leakage. Shut off motors at the valve when not in use. Running the network at a higher pressure to compensate for undersized hoses costs about 7 percent more compressor power per extra bar; fixing the hose is cheaper.
Installation and environment checks
A motor that matches the load on paper can still fail early if the installation is wrong. Check these points before ordering:
- Shaft loads: radial and axial loads from pulleys, sprockets or direct-mounted impellers must be within the motor’s limits, or a separate bearing support is needed.
- Mounting position: some motors and their lubrication arrangements are sensitive to orientation.
- Hazardous areas: air motors are widely used where flammable gases or dust are present, but the complete installation, including any sensors or solenoid valves, must suit the area classification.
- Wash-down and corrosion: stainless or coated motors are used in food plants; confirm exhaust routing so that wash water cannot enter.
- Noise: exhaust noise can be high; silencers or piped exhaust may be needed to meet workplace limits.
- Heat and dust: on outdoor sites in Telangana and Andhra Pradesh, summer heat and dust affect seals, lubricants and filters; protect the air inlet and service filters more often.
Common problems and causes
- Motor will not start under load: starting torque below breakaway, low inlet pressure, worn vanes.
- Speed drops badly under load: working point too close to stall, supply restriction, undersized regulator.
- Excessive air use: motor oversized and throttled, running near free speed, leaks at couplings.
- Icing or water at exhaust: wet supply air.
- Short vane life: lack of lubrication, dirty or wet air.
How Reddy Compressors helps
We focus on the air supply behind air motors and air tools. As an independent multi-brand specialist based at Tarnaka, Secunderabad, with more than 20 service engineers and technicians covering Telangana and Andhra Pradesh, and supply across India, we can:
- Size compressors for air-motor duties, including duty cycle and simultaneous use, and supply new or used/refurbished machines from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes.
- Measure pressure and flow at the motor inlet to find supply restrictions.
- Recommend dryers, filters, receivers and distribution changes.
- Provide rental compressors, including diesel portables, for site work with air motors, winches and hoists.
- Service and maintain the compressors and treatment that feed them.
Motor selection itself rests on the motor maker’s published curves; we help you check that the air supply matches them.
What to send us
- Load description with running torque, speed and breakaway torque.
- Motor make, model and curve if already chosen.
- Number of motors, duty cycle and how many run together.
- Existing compressor nameplate and network pressure.
- Hose, FRL and valve sizes from header to motor.
- Environment: hazardous area, wash-down, dust or outdoor site.
Frequently asked questions
How do I size a pneumatic air motor?
Start with the torque and speed needed at the driven shaft, convert through any gearbox to the motor shaft, and find the pressure available at the motor inlet under flow. Choose a motor whose curve at that pressure passes through your working point, ideally near the middle of its speed range. Then check starting torque, air consumption and installation separately.
Why does my air motor lose power under load?
Usually the pressure at the motor inlet falls when it draws full flow, because of undersized hoses, quick couplings, regulators or valves. Worn vanes and lack of lubrication also reduce torque. Measure pressure at the motor inlet while it is loaded. If supply is fine, the motor may be working too close to stall for that duty.
How much air does an air motor use?
Consumption depends on motor size, inlet pressure and speed, and is highest near free speed. Makers publish consumption curves, normally at a reference pressure around 6.3 bar (91.4 psig). Read the value at your working point, then multiply by duty cycle and the number of motors running together to estimate the compressor capacity needed.
Vane or piston air motor: which should I choose?
Vane motors are compact, light and suit medium to high speeds. Piston motors give higher starting torque and smoother low-speed running, which suits heavy starting loads, winches and hoists. Gear motors are robust for continuous duty. Choose from the load’s speed and starting needs, then confirm against the specific motor’s published curves at your inlet pressure.
Can an air motor be stalled without damage?
Yes, a correctly installed air motor can stall under overload without damage, which is one of its main advantages. It restarts when the load reduces. However, stall torque is not the same as reliable starting torque, and holding a load still needs a brake or self-locking gearbox, because the motor will not hold position by itself.
What compressor do I need for air motors?
Add up the air consumption of each motor at its working point, apply duty and simultaneity factors, and add an allowance for leakage and other users. Supply that flow at a pressure high enough to give the required inlet pressure after line losses. We can size and supply a suitable electric or diesel compressor, new, used or on rental.
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
Translate the driven machine’s speed and torque through the gearbox to the motor shaft, then check one candidate motor’s published running and starting torque and its air use against the supply.
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 17 October 2026. How our engineering content is reviewed.