Gear Coupling Selection Guide — How to Choose the Right Size & Type

Gear coupling selection is a six-step check. Calculate rated torque as 9,550 × kW ÷ rpm, multiply it by the service factor (1.00–1.25 for centrifugal pumps and fans, 2.00–2.50 for rolling mills and crushers), choose the coupling type, then pick the smallest FG size rated at or above that design torque whose maximum bore and speed suit your drive.

This guide walks through each step with formulas, a service factor table, and worked examples. Free selection assistance is available from SMI engineers at +91 63510 70577.

What Service Factor Should I Use?

Use a service factor of 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 service factor (SF) is a multiplier applied to the motor's rated torque to determine the minimum coupling torque rating required. It accounts for shock loading, starting conditions, and duty cycle. Applying too low a service factor is the most common sizing error.

For VFD-driven applications: apply SF for the driven machine load type only (not for DOL starting torque, as VFD eliminates starting current spikes). For multiple starts per hour (>4/hour), add 0.25 to the base SF. The coupling service factor chart has more worked examples, including low-speed drives.

The Formula: Design Torque = 9,550 × kW ÷ rpm × SF

• Rated torque of the motor: T (Nm) = 9,550 × P (kW) ÷ n (rpm)
• Design torque the coupling must carry: Td (Nm) = T × SF
• Coupling size: First FG size with rated torque ≥ Td

Worked Example: 250 kW Motor at 1,480 rpm

Rated torque T = 9,550 × 250 ÷ 1,480 = 1,613 Nm. The same motor needs a different coupling depending on what it drives, because the service factor changes the design torque:

Then check the shaft. FG-102 takes finished bores up to 60 mm and FG-103 up to 75 mm (maximum bore D1). If a shaft is larger, move up to the first size whose D1 covers it: an 80 mm shaft, for example, needs FG-104 (D1 90 mm, rated 9,557 Nm) whatever the torque calculation says.

Run your own motor through the gear coupling torque calculator, then read the rated torque, bore and speed for each size in the FG-100 to FG-119 size chart.

Six-Step Gear Coupling Selection Process

Follow these steps in order. Each step filters the selection — a coupling that passes all six is correctly sized.

Determine Rated Torque of the Drive

Use motor rated torque — not peak torque and not starting torque. The service factor applied in Step 2 accounts for transient peaks.

Apply the Service Factor

Service factor accounts for shock loads, starting torque, cyclic loading, and number of starts per hour. See the service factor table above for your application type.

Select Coupling Type

Select Coupling Size by Torque

Always select the next size up if your design torque falls between two catalogue sizes. Never de-rate a coupling below its minimum service factor.

Verify Bore Diameter

Minimum bore is determined by keyway root stress — do not specify a bore smaller than the catalogue minimum for the coupling size.

Verify Speed Rating

Larger coupling sizes have lower maximum speed ratings due to centrifugal stress limits. At high speeds, consider a smaller-torque coupling of the next size down with a higher SF if space permits.

Worked Example — Selecting a Coupling for a Centrifugal Pump Drive

22 kW

1,450 rpm

Centrifugal pump (uniform load)

45 mm

Gear Coupling Selection — Frequently Asked Questions

What service factor should I use for a gear coupling?

Use 1.00–1.25 for uniform loads with low starting torque (centrifugal pumps, fans, centrifuges), 1.25–1.50 for uniform loads with moderate starting torque (conveyors, screw pumps, generators), 1.50–2.00 for moderate shock (mixers, multi-cylinder compressors, reciprocating pumps) and 2.00–2.50 for heavy shock with high starting torque (steel rolling mills, crushers, ball mills, cranes). For more than four starts per hour, add 0.25 to the base service factor.

How do I calculate the torque requirement for a gear coupling?

Rated torque (Nm) = 9,550 × power (kW) ÷ speed (rpm). Multiply it by the service factor for your application to get the design torque, then match that against the coupling rated torque. For example, a 250 kW motor at 1,480 rpm gives 1,613 Nm; at a service factor of 1.50 the design torque is 2,420 Nm, which FG-102 (rated 2,804 Nm) covers. To convert Nm to Kg.M, divide by 9.81.

How do I select the right gear coupling size?

Calculate the design torque (9,550 × kW ÷ rpm × service factor) and choose the first FG size whose rated torque is at least that. Then confirm that your shaft is within the maximum bore (D1) for that size and that your running speed is within its maximum rpm. If either check fails, move up a size.

What service factor should I use for a steel rolling mill?

Use a service factor of 2.00 to 2.50 for steel rolling mills, crushers, and ball mills — these are classified as heavy shock loads with high starting torque. Under-sizing the service factor is the most common cause of gear coupling failure in rolling mill applications.

What is the difference between full gear coupling and half gear coupling?

A full gear coupling has two gear hubs, so both ends are flexible and it takes angular and parallel misalignment. A half gear coupling has one gear hub and one rigid flange hub: it takes angular misalignment at the gear end only and cannot take parallel offset on its own, so it suits floating-shaft and flanged arrangements where the second flexible element is elsewhere in the drive.

What information do I need to size a gear coupling?

Provide motor power (kW or HP), speed (RPM), driver and driven shaft diameters, the application/duty type and any known misalignment — SMI then calculates design torque and recommends a size free of charge.

How do I measure shaft size for the coupling bore?

Measure the shaft diameter with a 0.01 mm micrometer, in two planes 90° apart, and note the keyway width and depth; SMI machines the coupling bore to an H7 fit (or your specified tolerance) on those dimensions. The SMI guide to measuring a shaft covers each reading.

Can SMI help me select the right gear coupling?

Yes. SMI provides free gear coupling selection assistance. Contact our engineering team at +91 63510 70577 or smigvs@gmail.com with your motor power (kW), speed (rpm), shaft diameter, and application type. We will recommend the correct coupling size, type, and bore within 24 hours.

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