Coupling Service Factor Chart: Values by Driven Machine

Service factors from 1.00 to 2.50 by driven machine, how to apply them, and two worked sizing examples.

A coupling service factor is the multiplier you apply to the motor's rated torque to get the torque the coupling must be rated for. Use 1.00–1.25 for uniform loads such as centrifugal pumps and fans, 1.25–1.50 for conveyors, screw pumps and generators, 1.50–2.00 for moderate shock such as mixers, multi-cylinder compressors and reciprocating pumps, and 2.00–2.50 for heavy shock such as steel rolling mills, crushers, ball mills and cranes.

The design torque is Td = 9,550 × kW ÷ rpm × SF. Choose the smallest coupling rated at or above Td, then check its maximum bore and speed. These are the values Super Mech Industries uses to size its FG-100 to FG-119 gear couplings. Where a coupling maker's catalogue gives its own table, that table governs its couplings.

Coupling service factor chart

Load characteristic	Typical driven machines	Service factor
Uniform load, low starting torque	Centrifugal pumps, fans, centrifuges	1.00 – 1.25
Uniform load, moderate starting torque	Conveyors, screw pumps, generators	1.25 – 1.50
Moderate shock, medium starting torque	Mixers, multi-cylinder compressors, reciprocating pumps	1.50 – 2.00
Heavy shock, high starting torque	Steel rolling mills, crushers, ball mills, cranes	2.00 – 2.50

Two adjustments apply on top of the chart. If the drive starts more than four times an hour, add 0.25 to the service factor. If it runs on a VFD, use the service factor for the driven machine's load type only, because the VFD removes the direct-on-line starting spike. When you are unsure where a machine falls within its band, use the upper value.

How to apply the service factor: a 110 kW worked example

A 110 kW motor at 1,480 rpm has a rated torque of 9,550 × 110 ÷ 1,480 = 710 Nm. The coupling it needs depends on what it drives:

Driven machine	Service factor	Design torque	Smallest FG size
Centrifugal pump	1.25	887 Nm	FG-101 (1,127 Nm, bore to 50 mm)
Belt conveyor	1.50	1,065 Nm	FG-101 (1,127 Nm, bore to 50 mm)
Reciprocating pump	2.00	1,420 Nm	FG-102 (2,804 Nm, bore to 60 mm)
Crusher	2.50	1,775 Nm	FG-102 (2,804 Nm, bore to 60 mm)

All four run at 1,480 rpm, well inside the 6,300 rpm limit of FG-101 and 5,000 rpm of FG-102. Last, check the shaft: if it is larger than the size's maximum bore, move up to the first size whose bore covers it, whatever the torque says.

Why low-speed couplings come out so much larger

Torque rises as speed falls. The same service factor on a slow shaft gives a far bigger coupling. A ball mill driven at 50 rpm through a gearbox from a 250 kW motor carries 9,550 × 250 ÷ 50 = 47,750 Nm on the low-speed shaft. With a heavy-shock service factor of 2.50 that is 119,375 Nm, which needs an FG-111 (123,065 Nm, bore to 260 mm). The high-speed coupling between motor and gearbox, at 1,480 rpm, carries about 1,613 Nm and at the same factor needs only an FG-103 (5,047 Nm).

Common service factor mistakes

Starting from peak or starting torque. Use the motor's rated torque. The service factor already allows for the peaks; adding it to starting torque counts them twice and oversizes the coupling.
Using the motor nameplate service factor. A motor's 1.15 is about the motor's thermal margin, not the load the coupling sees.
Choosing by the motor instead of the driven machine. The shock comes from the machine being driven, so its row in the chart sets the factor.
Rounding down between sizes. If the design torque falls between two sizes, take the larger one.

To run the numbers for your own drive, use the gear coupling torque calculator. The full six-step method, including type, bore and speed checks, is in the gear coupling selection guide, and every size's ratings are in the gear coupling size chart. Send us the motor kW, rpm, driven machine and shaft sizes and SMI will confirm the size.

Frequently asked questions

What is a service factor in a coupling?

A coupling service factor is a multiplier applied to the motor's rated torque to find the torque the coupling must be rated for. It allows for shock loads, starting torque, cyclic loading and the number of starts per hour. Design torque = 9,550 × kW ÷ rpm × service factor.

What service factor should I use for a conveyor?

Use 1.25 to 1.50 for a belt conveyor, a uniform load with moderate starting torque. Take the upper end for long, heavily loaded conveyors and add 0.25 if the conveyor starts more than four times an hour.

What service factor is used for crushers and rolling mills?

Use 2.00 to 2.50. Crushers, steel rolling mills, ball mills and cranes are heavy-shock, high-starting-torque drives, and an undersized service factor on them is the most common reason a gear coupling fails early.

Is the motor service factor the same as the coupling service factor?

No. The service factor on a motor nameplate, such as 1.15, tells you how far the motor can be run above its rated power. The coupling service factor depends on the driven machine. Size the coupling from the motor rated torque times the coupling service factor, not from the nameplate service factor.

Does a VFD change the coupling service factor?

A VFD removes the direct-on-line starting torque spike, so apply the service factor for the driven machine's load type only. The shock from the driven machine itself, a crusher for example, is still there, so its service factor does not drop.