What is the formula for torque from power and speed?

The formula is T = P × 9549 / n, where T is torque in N·m, P is power in kW, and n is rotational speed in RPM. The constant 9549 comes from 60000/(2π).

Why do we need a service factor for coupling torque?

Service factors account for dynamic loads beyond the steady-state nominal torque. Shock loads, cyclic variations, and starting torques can significantly exceed nominal values. The service factor ensures the coupling can handle peak loads without damage.

What is T = F × r and when do I use it?

T = F × r calculates torque from a tangential force F (in Newtons) applied at radius r (in meters) from the shaft center. Use this when you know the force acting on a lever arm, gear tooth, or coupling bolt circle.

How do I convert between N·m and other torque units?

Common conversions: 1 N·m = 0.7376 lb·ft = 8.851 lb·in = 141.6 oz·in = 10.20 kgf·cm. For kW to HP: 1 kW = 1.341 HP.

What service factor should I use for my application?

Uniform loads (fans, pumps): 1.0–1.5. Moderate shock (compressors, conveyors): 1.5–2.0. Heavy shock (crushers, mills, presses): 2.0–3.0+. Always verify with the coupling manufacturer's catalog for your specific application.

Free Engineering Tool #084

Coupling Torque Calculator

Calculate shaft torque from power and speed using T = P×9549/n. Includes service factors for coupling selection and direct force application T = F×r.

T = P×9549/n T = F×r Service Factors

Power (kW)

Speed (RPM)

Service Factor (K_A)

Quick presets

Results

Design Torque (with Service Factor)

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Nominal Torque

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Torque in lb·ft

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Torque in kgf·cm

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Angular Velocity

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Torque from Power & Speed

The constant 9549 = 60000 / (2π), converting kW and RPM to N·m.

Design Torque with Service Factor

Torque from Force & Radius

Where F is tangential force in N and r is the lever arm in meters.

Service Factors Table

K_A	Load Type	Driven Machine Examples
1.0	Uniform	Fans, centrifugal pumps, generators
1.25	Light shocks	Belt conveyors, blowers, light mixers
1.5	Moderate shocks	Reciprocating compressors, screw conveyors, positive displacement pumps
1.75	Heavy shocks	Ball mills, rubber mills, heavy mixers
2.0	Very heavy shocks	Crushers, sugar mills, steel rolling mills
2.5	Severe reversing	Reversing rolling mills, vibrating screens
3.0	Extreme	Presses, shears, hammer mills

Tip: When the driving machine (motor) also has pulsating output (e.g., single-cylinder engine), multiply K_A by an additional motor factor (typically 1.25–2.0).

Example — Pump Drive

Given: P = 15 kW, n = 1500 RPM, K_A = 1.5

T_n = 9549 × 15 / 1500 = 95.5 N·m

T_design = 1.5 × 95.5 = 143.2 N·m

Coupling Torque Calculator | T = P×9549/n | Free Online Tool

Published by Nikolai Shelkovenko on February 15, 2026 February 15, 2026