It is the proportion of time the compressor may run, and exceeding it is the commonest way these machines are destroyed.

A piston compressor generates heat in the cylinder and relies on air flowing over cooling fins and on the pauses between running periods to shed it. It also has rings, valves and bearings in contact and impact throughout every revolution.

Manufacturers therefore state a maximum duty cycle - for example that the machine may run no more than a stated fraction of any period. Run it beyond that and cylinder and valve temperatures climb, the oil degrades, valves carbon up and fail, and wear accelerates sharply.

The practical failure is a machine bought on its delivered air figure for an application whose demand is nearly continuous. It appears adequate, runs almost all the time, and fails early - and the cause is invisible unless someone checks the duty cycle against the demand.

If demand is close to continuous, the answer is a screw compressor.

Large enough that the compressor starts a limited number of times per hour and runs for a reasonable period each time.

The receiver stores air so the compressor can stop. Its size, together with the pressure band between cut-in and cut-out, determines how much air is available between starts.

Too small and the machine short-cycles - starting and stopping frequently, with each start drawing high current and each stop leaving the machine hot. Motor starting is hard on both the motor and the supply, and manufacturers specify a maximum number of starts per hour for exactly this reason.

Too large is rarely a problem beyond cost and space, and generous receiver capacity is one of the cheapest improvements available - it reduces starts, allows a lower average system pressure, and buffers demand peaks.

Widen the pressure band as well as enlarging the vessel where short cycling is a problem; both increase the stored volume between starts.

Above the pressure a single stage handles efficiently - and the second stage is about temperature as much as pressure.

Compressing air in one step to a high pressure produces a large temperature rise, and hot air is harder to compress, so efficiency falls and the machine suffers. Beyond a moderate pressure, a single stage becomes both inefficient and thermally punishing.

A two-stage machine compresses partway in a large cylinder, passes the air through an intercooler to remove the heat, then compresses it the rest of the way in a smaller high-pressure cylinder. Each stage does less work, the intercooling reduces the work required in the second stage, and discharge temperature stays acceptable.

The result is markedly better efficiency and much longer valve and ring life at higher pressures, plus the ability to reach pressures a single stage cannot manage at all.

For ordinary workshop pressures a single stage is adequate and cheaper; above that, two stages are the norm.

Oil changes, valve attention, filter and belt checks, and draining the receiver - with the drain being the one most often neglected.

Compressing air condenses water out of it, and that water collects in the bottom of the receiver. Left there it corrodes the vessel from the inside, and a receiver is a pressure vessel whose failure is dangerous. Draining should be routine, and an automatic drain removes the reliance on somebody remembering.

The oil is a lubricant in a hot, contaminated environment and degrades, so it is changed on schedule with the correct compressor oil rather than a general lubricant.

Valves are the characteristic wearing part. They open and close every revolution and eventually leak or break; symptoms are reduced output, longer running to reach pressure, and higher discharge temperature.

Intake filters, belt tension and cooling fin cleanliness complete the routine. A compressor packed with dust cannot shed its heat, which brings the duty cycle problem back.

At small sizes, on intermittent duty, at high pressure, and where simplicity and serviceability matter more than efficiency.

Workshops, garages, tyre bays, small fabrication shops, farms, sites and mobile applications are the natural home. Demand there is intermittent by nature, so the duty cycle is not a constraint, and the low purchase price dominates the decision.

High-pressure applications are the other case: reciprocating machines reach pressures far beyond screw compressors, which is why breathing air, cylinder filling and specialist high-pressure duties are reciprocating.

They are also easy to repair - valves, rings and bearings can be replaced with ordinary workshop skills and modest tooling, which matters where specialist service is not available.

What they are not is a continuous industrial air source. Above small powers with steady demand, a screw compressor costs less to run, lasts longer and is quieter, and the difference compounds quickly.