Ever since a recipe change on some large mixing tanks, the drive shaft between the motor and its gearbox has been shearing. So have some of the shafts between the gearbox and the impeller. The recipe change calls for a longer time for the solution to sit and react before being mixed, resulting in a much more viscous fluid.
The system engineer has tried hardened shafts, but an old-timer says that will never work. He looked at the end of a broken shaft, said the break was torsional, and declared this an electrical problem. What if the old-timer's right?
If the old-timer correctly read the shear pattern, his conclusion is probably correct. Hardening a shaft makes it more scratch-resistant; the issue here is torsional strength.
Someone needs to evaluate the shaft diameter against the elasticity modulus of the steel. While most electrical engineers can vaguely recall this from Physics 101, a toolmaker or mechanical engineer is probably the best person to determine if the shaft is the wrong size or material for the rotational strength needed to handle torque required to move an impeller through that fluid.
But the twist here, so to speak, is the motor must start under load. The initial starting torque is greater than the running torque. That would make this an electrical problem, because the fix will be in the controls.
Install and adjust any these three devices, to eliminate that high starting torque:
1. VFD (input).
2. Soft-start (input).
3. Magnetic drive (output).