The torque value in a gearbox catalogue is given for a defined operating condition. If the machine works harder than that, running longer each day, taking shock loads or starting and stopping often, gears and bearings fatigue sooner. The service factor (SF) is the coefficient that covers this difference: required gearbox torque ≈ machine torque × SF.

Main factors that raise the SF:

  • longer daily running time (continuous operation instead of a single shift),
  • load type: uniform (belt conveyor), moderate shock (mixer), heavy shock (crusher),
  • number of starts and stops per hour and sudden reversals,
  • high ambient temperature.

The numerical SF is read from the gearbox manufacturer's own table; this guide does not give a general figure. The HEM gearbox pages also state that the final selection is made according to speed, service factor and running time. Stating the operating condition clearly in the quotation request ensures the right frame is chosen.

Note: the SF on an electric motor nameplate (1.15 for HEM motors of 75 kW and above) is the motor's continuous overload margin; it is not the same concept as the gearbox service factor.