Pulley diameters determine speed, and belt tension determines the side force on the motor shaft; together they affect motor life.

Speed calculation

n2 = n1 × d1 / d2. Assumptions: 7.5 kW 4-pole motor, rated speed 1,460 rpm, motor pulley 160 mm, machine pulley 320 mm. Machine speed: 1,460 × 160 / 320 = 730 rpm. The calculation uses the rated speed on the nameplate, not 1,500; belt slip may lower the actual speed slightly further.

Effect of pulley diameter on belt force

The rated torque of the same motor is 49.1 Nm. The force transmitted at the pulley rim is roughly F = 2 × T / d:

  • With a 160 mm pulley: 2 × 49.1 / 0.160 ≈ 614 N
  • With a 100 mm pulley: 2 × 49.1 / 0.100 ≈ 982 N

To transmit this force without slipping, the belt is fitted with pre-tension; so the actual load on the shaft is greater than the transmitted force and rises quickly as the pulley gets smaller.

  • Fit the pulley as close to the motor as possible, against the shaft shoulder; an overhanging pulley lengthens the lever arm of the load.
  • Keep the belt at the tension recommended by its maker; over-tensioning stresses the bearing, while a loose belt slips and heats up.
  • Align the pulleys in the same plane.

If a very large speed reduction is needed, a gearbox may suit better than a belt; this is covered in gearbox selection and use questions.