It converts the air's velocity into rotation rather than its pressure into displacement.

Vane and piston motors are positive displacement machines: they admit a quantity of compressed air into a chamber, let it expand against a moving surface, and exhaust it. Force comes from pressure acting on an area, and torque is substantial.

A turbine accelerates air through a nozzle into a high-speed jet and directs it at blades on a rotor. The rotor is driven by the momentum change of the airflow. Nothing is trapped and nothing expands in a closed chamber.

The results are opposite in almost every respect: very high speed and very low torque instead of moderate speed and high torque; negligible starting torque instead of maximum torque at stall; and almost no wearing contact instead of sliding vanes or piston rings.

They are therefore not interchangeable products - they answer different questions, and choosing between them is really choosing between speed and torque.

Extremely high - into the tens and hundreds of thousands of revolutions per minute depending on size, with the smallest units the fastest.

Because the rotor is small, light and does not have to seal against anything, there is little to limit its speed beyond bearing capability and rotor stress. Smaller rotors run faster, since the peripheral speed for a given rotational speed is lower.

At those speeds the bearings become the limiting component and are frequently the reason for the motor's service life and its cost. Air bearings are used in the most demanding spindle applications, removing mechanical contact altogether.

Speed is also strongly dependent on load: a turbine slows markedly as load is applied, and free speed with no load is much higher than working speed. Quoted speeds should therefore be read carefully - free speed and speed at rated power are very different numbers.

Overspeeding an unloaded turbine can be damaging, which is why some designs incorporate governors.

Because the mechanism converts momentum rather than pressure over an area, and a small rotor has very little of it.

The force on the blades comes from changing the direction and speed of a stream of air. The mass flow is small and the rotor radius is small, so the torque produced is correspondingly small - even though the power can be respectable, because power is torque times speed and the speed is enormous.

That is the key to using them correctly: a turbine motor may have useful power, but it delivers it as speed rather than as force.

So it works well where the tool itself provides the mechanical advantage - a small burr or dental bur removing material by surface speed - and badly where a load must be turned against resistance.

It also means the motor must be brought up to speed unloaded and the load applied gently. Stalling a turbine is not damaging in the way stalling an electric motor is, but it does no useful work at all.

Precision high-speed tooling and medical work, where surface speed matters and torque does not.

Dental handpieces are the most familiar application - a very small, very fast, very light turbine in the head of the instrument, driven by compressed air from the chair.

Surgical and veterinary handpieces work similarly. In industry, high-speed die grinders and pencil grinders for deburring and finishing, engraving spindles, PCB drilling spindles, and small machining and finishing spindles all use them.

They also appear in test and instrument work - small turbines driving optical choppers, centrifuges and rotating test rigs, where the absence of electrical noise and heat is valuable.

What they are not used for is any drive requiring torque: hoists, mixers, conveyors and general machinery are vane or piston territory. Confusing the two produces a motor that spins freely and stops the moment it is asked to do work.

Clean and dry above all, delivered at the specified pressure and flow - and cleanliness matters more than in other air motors.

A turbine's blades and bearings are small, precise and turning extremely fast. Particulate in the air stream erodes blades and damages bearings quickly at those speeds, and the effect is cumulative and irreversible - a turbine that has lost performance through blade erosion cannot be restored by servicing.

Moisture is equally problematic. The large pressure drop through the nozzle cools the air significantly, so a supply near its dewpoint will condense and can freeze at the exhaust, and water reaching the bearings destroys them.

Many turbine motors, particularly medical and cleanroom ones, must also run on oil-free air, since the exhaust vents at the working point.

Specify the filtration and drying to the manufacturer's requirement and treat it as part of the tool, not as general plant air practice.

By supply pressure and flow, with governors used where speed must be held.

The simplest control is the supply: reducing pressure or restricting flow lowers the speed, and this is how most hand tools are operated - the trigger is a variable throttle.

Because turbine speed varies so strongly with load, however, throttling alone gives poor speed regulation. A tool that is running fast in air slows substantially when it engages the work.

Where speed must be held within limits - for surface speed control on a grinder, or for safety on a tool with a rated maximum - a governor is fitted, which restricts flow as speed rises. Governors are a safety feature as much as a control one, since an unloaded turbine can otherwise overspeed.

On precision spindles, closed-loop control with a speed sensor is used where the application genuinely requires a stable speed.

Never remove or disable a governor to gain speed - the rotor's speed limit is a structural one.

The bearings, almost always - and the air quality that protects them.

There is very little else to wear. The rotor does not touch the housing, there are no vanes sliding in slots and no rings on a cylinder wall. What is working continuously at extreme speed is the bearing set.

Bearing life at those speeds is inherently shorter than in ordinary machinery, and it is dramatically shortened by contamination, moisture and incorrect lubrication. In medical and precision applications, bearing replacement at defined intervals is a normal part of the maintenance regime rather than a repair.

Blade erosion from particulate is the second life-limiting factor and is not repairable.

So the practical measures are: filtration and drying to specification, the correct lubrication regime for the specific motor, avoiding overspeed, and replacing bearings on schedule rather than on failure - since a bearing failure at very high speed can damage the rotor and housing as well.