A vane-type air motor spins fastest with no load and slows as the load rises, with its peak power at about half the free speed, so a grinder wheel must be rated for the motor’s unloaded speed, not its working speed. A 100 mm wheel on a motor with a free speed of 6,000 RPM has a rim speed of 31.4 m/s, faster than the 23 m/s rim speed of a 300 mm wheel at 1,500 RPM, for which the corpus already warns that a burst fragment is lethal.
Part of Pneumatic Systems
A rotary air tool is a vane motor with a chuck or a wheel on its shaft. The motor is simple; what takes care is matching the tool to the speed the motor reaches when nothing is loading it.
The Vane Motor Is a Rotor in an Off-Centre Bore
Following the carrier:
- A steel cylinder has an eccentric bore.
- A rotor sits offset inside it, with slots that hold sliding vanes of spring steel or fibre.
- Air enters at the widest gap, pushes the vanes round and leaves at the narrowest.
- The rotor turns at 3,000-10,000 RPM depending on load. Attach a chuck for a drill or a wheel for a grinder.
Vanes press out against the bore by centrifugal force and by air pressure behind them. They wear, so keep spares and oil the air with a few drops of light oil, as in Lubrication for oils generally.
Torque Falls As Speed Rises, and Power Peaks in the Middle
If the motor’s torque falls in a straight line from its stall value to zero at its free speed (an assumption that holds roughly for vane motors), then power = torque x angular speed = T_stall x (1 - ω / ω_free) x ω, which is greatest at ω = ω_free / 2 and equals T_stall x ω_free / 4.
Worked example (illustrative values, not a specification). Free speed 6,000 RPM = 6,000 x 2π / 60 = 628 rad/s; stall torque 1.0 N·m.
- Peak power = 1.0 x 628 / 4 = 157 W, at 3,000 RPM
- Air needed at best = 157 W / 263,700 J per m³ of free air from 6 bar gauge = 0.000595 m³/s = 36 L/min, with the energy figure from Pneumatic Tools and Devices. A real motor uses more because the air is not fully expanded.
For drilling, use the slow, high-torque half of the curve; for grinding, stay near the speed the wheel and the tool are rated for.
The Free Speed Decides the Wheel
A grinding wheel’s rim speed is π x diameter x revolutions per second. A 100 mm wheel at the motor’s free speed of 6,000 RPM (100 revolutions per second):
- Rim speed = π x 0.1 x 100 = 31.4 m/s
The corpus’s grinding-wheel article gives 23 m/s as the rim speed of a 300 mm wheel at 1,500 RPM and calls a fragment from a bursting wheel a lethal projectile (Grinding Wheels). It also gives 100-200 RPM for a sandstone wheel of 300 mm, which a direct-drive air motor cannot run at, so a sandstone wheel needs a heavy speed reduction. Never mount a wheel that is not marked for the motor’s free speed. A hand-made or unmarked wheel cannot be assumed safe. If the motor’s free speed is unknown, measure it under no load with a tachometer or a counted mark, with the wheel removed and a guard fitted. A throttle or governor on the air supply limits it.
Common Mistakes
- Mounting the wheel at the working speed’s rating. The motor races when the work is lifted off.
- No water or oil in the air. Wet air freezes in the exhaust as the air cools and a dry motor wears quickly.
- A heavy load for a small motor. The motor stalls; a stalled vane motor still passes air.
- Treating “safe in explosive atmospheres” as blanket permission. An air motor has no electrical spark, but the tool and the work can still spark, so do not use it in a flammable atmosphere.
Safety
A wheel faster than its rating can burst
The free speed of an air motor is higher than its working speed, so a wheel rated only for the working speed can burst when the work is lifted off. Fit a guard, ring-test every wheel by tapping, stand out of the plane of the wheel when starting it, and check the motor’s free speed before mounting a wheel. Shut off and bleed the hose before changing a wheel or bit. Do not point the exhaust at anyone; air from a motor can blow grit and oil into eyes.
What’s Next
- Learn next: Spray Equipment — an air tool with no moving parts
- Combine with: Drill Press — the machine-tool way to drill