A stone crusher had a V-belt that squealed every time it started. The maintenance crew tightened the belt three times. Each time it was quiet for a week, then squealed again. They went from a 5V belt to a 8V belt (heavier duty). The squeal got louder. They were about to buy a variable-speed pulley drive.
The crusher was a 15 kW motor driving a flywheel through two SPB belts. The crusher started empty (no stone in the chamber). The flywheel had to accelerate from 0 to 600 RPM in about 3 seconds. That’s a lot of inertia. The belts slipped during acceleration. The squeal was the slip — not the running tension. Tightening the belts only made the slip louder (the friction coefficient drops when the belt is over-tensioned, because the belt crowning loses contact). Up-sizing to 8V belts didn’t help because the problem was the starting torque, not the running load.
The fix was a soft-start. We installed a VFD on the motor and ramped the crusher from 0 to 600 RPM over 10 seconds. The starting current dropped from 6x to 2x. The belts never slipped. The squeal disappeared. The VFD also let the operator start the crusher with a small amount of stone in the chamber (the flywheel carried the load). The crusher ran more smoothly. The belts lasted 4 years instead of 6 months.
The customer had been tightening the belts because that was the obvious thing to do. But belt tension doesn’t fix a starting-torque problem. A belt that slips at start needs either a softer start (VFD), a bigger flywheel (more inertia storage), or a clutch that engages after the motor is up to speed. Tightening the belt just wears the bearings faster. The belts were fine. The start was brutal.
