How to Use the Coupling Torque Calculator
Before selecting a coupling to connect a motor to a gearbox, or a gearbox to a driven shaft, you first need to know the actual torque it must transmit. This calculator converts transmitted power and rotational speed into nominal torque, then applies a service factor to give you the design torque you'll need to size against a coupling manufacturer's catalog rating.
It uses the standard power-torque relationship for rotating machinery: T(lb-ft) = 5252 ร P(hp) รท N(rpm). The constant 5252 comes from converting horsepower into an angular-speed basis (33,000 ft-lb/min per hp divided by 2ฯ), and it's the universal formula used across virtually all rotating equipment torque calculations.
Sizing a coupling to nominal torque alone risks failure under the momentary overload of motor starting or sudden load-shock events, so always check the catalog's rated torque against the design torque (with the service factor applied). Different coupling types โ gear, disc, or elastomeric โ also tolerate different amounts of vibration and shaft misalignment, so check alignment along with torque. For equipment with heavy vibration or rapidly changing loads, consult the coupling manufacturer to confirm the correct service factor.
Frequently Asked Questions
It builds in margin for momentary overload at motor startup, load fluctuations, and shock loading. Couplings should be selected against the design torque (nominal torque times service factor), not the nominal torque alone.
General industrial fans and pumps typically use 1.25-1.5, while equipment with heavy load fluctuation like conveyors or compressors often uses 1.5-2.0 or higher. Always follow the equipment manufacturer's specification for the exact value.