How do I calculate torque for a gear coupling selection?

Take the driver's rated power and speed and convert them to nominal torque — for kilowatts and revolutions per minute the working constant is 9550, so nominal torque in newton metres is 9550 × kW ÷ rev/min. Multiply that by the service factor for the driven machine to get selection torque, and compare that against the coupling's published continuous rating. Peak events are then checked separately against the coupling's peak rating.

Where do peak torques actually come from?

From transients that never appear on a nameplate: direct-on-line motor starting, a short circuit at the terminals, a two-phase reclosure, compressor surge, and a driven machine jamming. They are brief, poorly documented and frequently several times larger than anything the service factor contemplates, which is why they are checked against a separate rating rather than folded into the continuous calculation.

Does a gear coupling need derating at high temperature?

Yes, and it is easy to miss because the derating applies to the coupling rather than to the drive. Elevated ambient or radiated heat reduces the material's allowable stress and shortens the grease's service life at the same time, so both the torque rating and the lubrication interval move. A coupling adjacent to a furnace, kiln or hot process line is in a different service from an identical coupling in a ventilated motor room.

Selection & Sizing

Torque calculation: continuous, selection and peak

Rated power and speed give you nominal torque. Multiply that by a service factor and you have the selection torque a coupling must carry continuously. Peak events — motor starts, short circuits, jams — are a separate check against a separate rating, and collapsing the two is how undersized couplings get specified.

Author: Priyansh Thummar, Editor
Dates: Published 6 August 2026 · Last updated 6 August 2026
Reading time: 5 minutes

1. Nominal torque

Nominal torque is the least interesting number in the calculation and the only one nobody argues about. It falls straight out of the relationship between power, torque and angular velocity: power is torque multiplied by angular velocity, so torque is power divided by angular velocity. Convert the units for kilowatts and revolutions per minute and the constant works out at 9550.

What that number describes is the average torque the driver delivers at its rated condition. It is not what the coupling experiences. A motor nameplate is a steady-state statement about a machine that spends part of its life starting, part of it loaded beyond nameplate, and occasionally a few milliseconds in a fault condition that dwarfs everything else on the datasheet.

1.1 The formula

T_nom = 9550 · P / n

Nominal torque in newton metres, where P is driver rated power in kilowatts and n is speed in revolutions per minute. The constant is 60 000 divided by 2π, carrying the conversion from kilowatts and rev/min into newton metres. Selection torque follows as T_sel = T_nom · K_s, with K_s the application service factor.

2. Selection torque

The service factor is the bridge between nameplate and reality for continuous running. It accounts for the character of the driven machine: a centrifugal pump presents a smooth, predictable load, while a rock crusher presents an unpredictable one with impacts that never appear anywhere in a power rating.

The factor is not a safety margin, and treating it as one leads to double-counting. It is a translation from a smooth nameplate figure to the roughness of a particular duty. Adding a further margin because the number feels small produces an oversized coupling — and an oversized coupling is not automatically a safer one. A heavier hub raises the overhung moment on the shaft, adds inertia the driver must accelerate, and can shift a torsional natural frequency into the operating range.

3. Peak torque is a separate check

Peak torque is where the interesting failures live, because peaks are transient, poorly documented and frequently several times larger than anything the service factor contemplates. They are checked against the coupling’s peak or momentary rating — a separate published figure — and the check is a comparison of two numbers, not an adjustment to one.

A coupling can pass the continuous check comfortably and fail the peak check outright. The correct response is a larger coupling or a softer starting arrangement, not a larger service factor.

3.1 Where peaks come from

Table 1 — Transient torque sources and their order of magnitude
Event	Typical multiple of rated	Duration	What it governs
Direct-on-line induction start	2–3 ×	Seconds	Usually the sizing peak
Star-delta or soft start	1–1.5 ×	Seconds	Reduces the sizing peak markedly
Synchronous motor start	Pulsating, several ×	Seconds	Needs torsional analysis
Motor terminal short circuit	6–10 ×	Milliseconds	Absolute momentary maximum
Driven machine jam or stall	Driver stall torque	Until protection trips	Crushers, mills, conveyors
Reciprocating machine excitation	Cyclic, superimposed	Continuous	A fatigue case, not a single peak

Orders of magnitude for orientation only. Actual transient figures come from the driver manufacturer and, on a critical train, from a torsional analysis of the whole machine.

4. A worked example

A 250 kW induction motor at 1480 rev/min drives a reciprocating compressor through a gear coupling, started direct on line.

Table 2 — Worked example: 250 kW compressor drive
Step	Calculation	Result	Compared against
Nominal torque	9550 × 250 ÷ 1480	1 613 N·m	—
Selection torque	1 613 × 2.0 service factor	3 226 N·m	Coupling continuous rating
Start peak	1 613 × 2.5 breakaway	4 033 N·m	Coupling peak rating
Verdict	Continuous 3 500 N·m, peak 7 000 N·m	Passes both	—

The service factor of 2.0 is illustrative of a reciprocating-compressor application class; the governing figure comes from the standard or catalogue named on the order. Note that the two checks compare against two different published ratings.

5. Temperature derating

Temperature is the last adjustment. Gear couplings run in grease, and both grease behaviour and seal material degrade as sleeve temperature climbs. Above roughly 90 °C at the sleeve a derating factor is applied to the continuous rating. The value comes from the manufacturer rather than from any general rule, because it depends on the lubricant and the seal compound actually fitted.

6. Three recurring mistakes

Applying the service factor twice — once from the catalogue and again as engineering judgement. This produces an oversized coupling and every problem that comes with one.

Checking peak torque against the continuous rating — comparing the wrong pair of numbers. The result usually survives commissioning and fails on the third or fourth start.

Starting from absorbed power rather than driver rated power — the coupling has to survive what the driver can deliver, not what the driven machine normally asks for. On a train with a generously sized motor those two figures differ substantially.

Frequently asked

How do I calculate torque for a gear coupling selection?: Take the driver's rated power and speed and convert them to nominal torque — for kilowatts and revolutions per minute the working constant is 9550, so nominal torque in newton metres is 9550 × kW ÷ rev/min. Multiply that by the service factor for the driven machine to get selection torque, and compare that against the coupling's published continuous rating. Peak events are then checked separately against the coupling's peak rating.
Is the service factor a safety margin?: No, and treating it as one leads to double-counting. It translates a smooth nameplate figure into the roughness of a particular duty — the shock and reversal content of the driven machine. Adding a further margin on top because the number feels small produces an oversized coupling, and oversizing is not automatically safer: a heavier hub raises overhung moment on the shaft, adds inertia the driver must accelerate, and can shift a torsional natural frequency into the operating range.
Where do peak torques actually come from?: From transients that never appear on a nameplate: direct-on-line motor starting, a short circuit at the terminals, a two-phase reclosure, compressor surge, and a driven machine jamming. They are brief, poorly documented and frequently several times larger than anything the service factor contemplates, which is why they are checked against a separate rating rather than folded into the continuous calculation.
Does a gear coupling need derating at high temperature?: Yes, and it is easy to miss because the derating applies to the coupling rather than to the drive. Elevated ambient or radiated heat reduces the material's allowable stress and shortens the grease's service life at the same time, so both the torque rating and the lubrication interval move. A coupling adjacent to a furnace, kiln or hot process line is in a different service from an identical coupling in a ventilated motor room.

References

AGMA 9002-B04, Bores and Keyways for Flexible Couplings (Inch Series), for the shaft interface that follows from a size selection.
IEC 60034-1, Rotating electrical machines — Rating and performance, for induction motor rated and breakaway torque definitions.
Editorial note: service factor tables and coupling continuous and peak ratings are properties of the governing standard and the manufacturer’s catalogue. The factors used above are illustrative of an application class, not reproductions, and are to be replaced with the governing figures at technical review.

Selection & Sizing
Service factor guide
What the factor represents, and why two similar machines can carry different ones.
Selection & Sizing
Misalignment capacity
The three ratings that constrain a selection independently of torque.
Standards
AGMA 9002: bores and keyways
Whether the shaft can physically take the coupling the torque calculation asks for.