Because it acts through a crank mechanism rather than through pressure on a sliding vane.

In a piston motor, air pressure acts on a piston face and the resulting force is transmitted to the crankshaft through a connecting rod, giving the mechanical advantage of the crank throw. That leverage is available from the moment the motor starts turning and does not depend on speed.

A vane motor develops torque from pressure acting on the exposed area of the vanes. At very low speed the vanes are held out mainly by air pressure rather than centrifugal force, sealing is less effective, and internal leakage reduces the torque actually delivered.

So a piston motor starts heavy loads more reliably and runs smoothly at low speed under load, while a vane motor is happiest in the middle of its speed range.

The practical rule: heavy start-up torque and slow running point to piston; light weight, moderate speed and low cost point to vane.

Two ways of laying out the cylinders, with different shapes and characteristics.

In a radial motor the cylinders are arranged like spokes around the crankshaft, pistons pointing inward at the crank. This gives a compact, roughly circular motor with a large diameter and short length, and it delivers very high torque - the classic layout for heavy low-speed duty.

In an axial or wobble-plate motor the cylinders lie parallel to the output shaft, and the pistons act on an inclined plate that converts their reciprocation into rotation. That gives a longer, slimmer motor, generally somewhat higher speed and lower torque than a radial of the same size.

The choice is often driven by the space available and the mounting arrangement as much as by performance. Both share the piston motor's characteristics of good starting torque and smooth low-speed running relative to vane motors.

It starts from any crank position, which is the practical advantage over some other designs.

With four or more cylinders phased around the crank, at least one piston is always positioned where admitting air produces useful torque. There is no dead point where the motor cannot start.

That matters on drives that stop under load and must restart - a mixer left standing in setting material, a hoist holding a load, a conveyor loaded at standstill. The motor is asked to break away from rest against full load, and it does so predictably.

Starting torque is normally quoted separately from running torque in the manufacturer's data, and it is the figure to size against for these duties.

Supply pressure matters directly: the whole torque curve scales with pressure, so a motor fed at less than its rated pressure - through an undersized hose, a restrictive fitting or a distant supply - will not deliver its rated starting torque. Size the supply line for the peak flow, not the average.

Yes - gearmotor versions combine the motor with a reduction gearbox for very high torque at very low speed.

Even a piston motor runs faster than many driven machines need, so a planetary or worm reduction stage is frequently fitted, producing an integrated pneumatic gearmotor.

The result multiplies the already-high torque and reduces the speed to something directly usable, which suits winches, capstans, turntables, valve actuators and slow mixing duties. It also means a smaller motor can do the work, reducing air consumption for the same output torque.

A worm reduction adds a further property that is sometimes wanted and sometimes not: it may be self-locking, holding the load when air is removed. As with any worm drive, that should not be relied on as a brake - it depends on friction, which varies.

Where holding is a safety requirement, specify a proper pneumatic brake in addition.

Generally better at low speed and high torque, which is the duty it is designed for - but air motors of any kind consume heavily.

A piston motor expands air more completely through the piston stroke and has less internal leakage past sliding vanes, so at its design point it converts a somewhat larger fraction of the air's energy into shaft work.

That advantage is real but modest. The dominant fact remains that pneumatic drives are an expensive way to produce shaft power compared with electric motors, and the compressor supplying them is usually the largest electrical load associated with the installation.

So the sizing exercise should establish the air demand at the actual operating point and confirm the compressed air system can supply it continuously without dragging system pressure down - a large air motor can easily exceed what a plant air system was designed for.

Where the motor runs continuously rather than intermittently, it is worth checking whether an electric drive with appropriate protection would serve.

Lubrication, clean dry air, and periodic attention to seals and bearings - with the air quality mattering most.

Most piston air motors require lubricated air, delivered by an in-line lubricator at the specified rate. Running them dry wears the cylinder bores, pistons and bearings, and the symptom is gradual loss of power.

Water in the supply is damaging in two ways: it washes lubricant off the working surfaces, and it corrodes them. Because air motors exhaust to atmosphere and expand air significantly, they also produce cooling that can freeze moisture in the exhaust, so a wet supply causes icing as well as wear.

Beyond that, the routine is inspection of seals, bearings and the exhaust muffler - a blocked muffler raises back pressure and reduces power noticeably, and it is a common and easily missed cause of a motor that seems to have weakened.

Check the supply filtration and drying before investigating the motor itself.

Heavy, slow, high-torque fixed installations - particularly where the drive must start under load or stall regularly.

Large mixers and agitators in chemical, paint, adhesives and food processing are a principal use, where the drive may have to break away from thickened or settled material.

Hoists, winches and capstans at the heavy end use them for starting torque and stall tolerance. Mining and drilling equipment uses them for the same reasons plus the absence of any ignition source.

Other applications include conveyor and rotary table drives, valve and damper actuation, tensioning duties, and marine deck machinery where an electric drive would be difficult to protect.

What they are not used for is anything hand-held or weight-sensitive - a piston motor is heavy - and anything requiring high speed, where a vane or turbine motor is more appropriate.