First establish how many rpm the machine actually needs, then divide the rated speed on the motor nameplate by that figure. Using the rated speed under load (e.g. 1,450 rpm) rather than the synchronous speed (1,500) brings the result closer to reality.

Example

  • Motor: 2.2 kW, 4-pole, rated speed 1,450 rpm (100L1-4 cast-iron motor)
  • Target output speed: 72 rpm
  • i = 1,450 / 72 ≈ 20.1 → standard ratio 1/20
  • Actual output speed: 1,450 / 20 = 72.5 rpm

If the calculated ratio falls between two standard values, check whether the machine tolerates running slightly faster or slightly slower. The HEM worm gearbox pages list ratios of 1/7.5, 1/10, 1/15, 1/20, 1/25, 1/30, 1/40, 1/50, 1/60, 1/80 and 1/100, with approximate output speeds for a 1,400–1,500 rpm motor. The ratio only sets the speed; the torque the chosen frame carries at that ratio is checked separately.