Selection & Sizing
Service factor guide
A service factor is not a safety margin and not an allowance for uncertainty. It is a classification of the character of the load — how much shock, reversal and cyclic variation the coupling will see — on both the driven and the driving side. That is why two machines of identical rated power can carry factors that differ by a factor of two.
- Author
- Priyansh Thummar, Editor
- Dates
- Published · Last updated
- Reading time
- 4 minutes
1. What it is not
A safety factor covers uncertainty — you apply it because something about the load or the material is not precisely known. A service factor covers a load you do know about: the shock, reversal and cyclic content of a particular class of driven machine.
Two couplings sized for the same average torque, one on a centrifugal fan and one on a rock crusher, need different capacities because the crusher genuinely applies different loading to the tooth flanks — not because the crusher is less well understood.
It is also not an allowance for future capacity, not a hedge against a power figure that might be wrong, and not a substitute for the peak torque check. Each of those is a separate question with its own answer, and folding them all into one multiplier means none of them has been answered.
2. The three things it prices
- Torsional shock — how abruptly torque is applied, and whether the driven machine can jam.
- Reversal — whether torque changes sign, which unloads and reloads the same flanks and drives fretting at the fit.
- Duty cycle — starts per hour, whether the machine starts under load, continuous running versus short cycles.
A machine can be gentle on one of these and severe on another, which is why load classification groups machines by behaviour rather than by industry.
3. The driven machine sets most of it
Centrifugal machinery — fans, blowers, centrifugal pumps and compressors — applies torque smoothly and sits at the low end. Positive-displacement machinery applies torque in pulses at shaft frequency or a multiple of it, and sits higher. Machinery that processes solid material — crushers, mills, screens, shredders — can meet an uncrushable object and reach stall torque within a fraction of a revolution, and sits highest of all.
| Load class | Character of the torque | Representative machines | Relative demand |
|---|---|---|---|
| Uniform | Essentially steady; varies only with process conditions | Centrifugal fans, blowers, centrifugal pumps, generators | Lowest |
| Moderate shock | Regular variation, no reversal, occasional transient | Conveyors, mixers, machine tools, kilns | Moderate |
| Heavy shock | Pulsating at shaft frequency or a multiple; peak well above mean | Reciprocating compressors and pumps, positive-displacement blowers | High |
| Severe / jamming | Can reach stall torque within a fraction of a revolution | Crushers, mills, shredders, vibrating screens | Highest |
3.1 Two pumps, two factors
A centrifugal pump and a triplex reciprocating pump of identical rated power impose entirely different loading. The centrifugal machine's torque is essentially steady, varying only with system conditions. The triplex pump's torque varies at three times shaft speed with every plunger stroke, so the coupling sees a peak-to-mean ratio well above unity on every revolution — for the whole of its life, not occasionally.
The same coupling that runs indefinitely on the first can wear progressively on the second. Nothing about the average power distinguishes them; only the load class does.
4. The driver contributes too
This is the half most often left out. An electric motor started across the line, an engine with a firing pulse every cycle, a steam turbine and a variable-frequency drive all impose different torsional characteristics on the same driven machine.
Internal combustion drives are the notable case: the torsional excitation from cylinder firing is continuous, not transient, so a coupling on an engine drive carries a higher factor than the same coupling on the same driven machine behind an electric motor. Some rating systems handle this with a separate driver multiplier and others fold it into the load class — either way, the driver has to be named before a factor can be chosen.
5. Do not stack margins
Where the driven machine's own rating already contains an allowance — a gearbox with its own service factor, a motor selected one frame larger for future duty — applying a coupling service factor on top of it multiplies two independent allowances together.
The result is a coupling two or three sizes above what the physics calls for, carrying extra inertia and overhung moment at the shaft end, with a torsional natural frequency the train designer never analysed. Oversizing is a change to the machine, not an absence of one.
6. Four ways it goes wrong
- Applying it to the peak check as well. The factor converts nominal into selection torque, compared against the continuous rating. Transients are their own calculation against a separate peak rating — see torque calculation.
- Choosing a large factor instead of asking. Does the load reverse, does it start loaded, are there pulses at shaft frequency, how many starts per hour, can it jam. Five questions place the drive in a class.
- Selecting for the industry, not the machine. Two drives in the same plant routinely belong in different load classes.
- Carrying a factor over from last time. Same site, same size, different duty — the previous selection is a starting point, not an answer.
Frequently asked
- Is a service factor the same as a safety factor?
- No. A safety factor covers uncertainty in the load or the material — you apply it because you do not know something precisely. A service factor covers a load you do know about: the shock, reversal and cyclic content of a particular class of driven machine. Two couplings sized for the same average torque, one on a centrifugal fan and one on a rock crusher, need different capacities because the crusher genuinely applies different loading, not because the crusher is less well understood.
- What service factor should I use if the duty is unknown?
- Characterise the duty rather than inflate the number. The useful questions are narrow: does the load reverse, does it start under load, are there torque pulses at shaft frequency or a multiple of it, how many starts per hour, and can the driven machine jam. Answering those places the drive in a load class. Choosing a large factor because nobody asked those questions produces a coupling that is oversized for the running condition and still unprotected against the specific event that will break it.
- Can I use a lower service factor if the coupling is oversized anyway?
- That is the same calculation done twice, and it is worth being explicit about which one you are relying on. If the size was set by bore rather than torque — which is common on general-industrial drives — the spare torque capacity is real and it does cover a heavier duty than the calculation assumed. What it does not cover is the peak case, which is checked against a separate rating and is not improved by the service factor at all.
- Does the service factor apply to the peak torque check as well?
- No, and applying it twice is a common error. The service factor converts nominal torque into selection torque, which is compared against the continuous rating. Transient events — direct-on-line starting, short circuit, surge, a jam — are calculated as their own torque values and compared against the coupling's separate peak rating. A drive can pass the factored continuous check comfortably and still fail on peak.
References
- AGMA 922-A96 (reaffirmed 2025), Load Classification and Service Factors for Flexible Couplings — the information sheet that classifies the character of the load and from which industrial service factor tables derive. It is explicit that where its values do not apply, the manufacturer's recommendation governs.
- ISO 14691:2008, Petroleum, petrochemical and natural gas industries — Flexible couplings for mechanical power transmission — General-purpose applications, for the driver-side torsional considerations in §4.
- Editorial note: this article explains what a service factor classifies and deliberately does not reproduce a numeric table. Published factors are the property of the standard or the manufacturer's catalogue they appear in, they differ between rating systems, and a number copied out of context is the specific error this article exists to prevent. Use the load class established here to read the correct row of the table that governs your order.
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