rotor shaft fracture 2

[chumet1]

vibrating screen motor rotor fractured at bearing journal shoulder (rotor side). set-up of the system installation is shown in one of the pics. motor is mounted on a rigid frame that is mounted to the ground while driving a vibrator through v-belts. my initial assessment is that during vibration, the v-belts exert a radial force on the shaft and due to the motor's rigid mounting it will eventually fail due to the forces. engineers, i am kindly asking for different views on possible causes and solutions to prevent this failure from recurring

 

[Tmoose]

"-made sure the machinist employed larger radius on shaft dimensional transitions "

Can it be determined what the radius was on the shaft(s) that broke?
It sounds like you are having replacement shafts made locally. What material are they using? What was the OEM motor shaft made of? A material like 4140 is often an option right from the motor manufacturer, so I wonder if that is the material that should have been supplied with the motor, and if so, used for replacement shafts.

Correct geometry is at least as important as the correct material. Bad geometry can kill the best material in a week at 900 rpm (~ 9 million load reversals from belt drive). The radius should be a continuous curve with ends tangent to the journal diameter and the shaft shoulder. NOT like this -

http://www.clubcobra.com/photopost/data/500/25X_RADIUS.jpg

It is difficult or even impossible to create a good radius by turning in a lathe. It (like the bearings seat) really should be ground. And Dressing the grinding wheel corner to generate a true radius is not a trivial task, or done freehand. If a hard working radius is poorly formed, and has steps, corners, torn material (inherent with turning processes) or tooling scratches (inherent with turning processes) it needs to be re-worked. It is possible Using a small grinding stone (with diameter equal to or slightly larger than OEM radius) to create an acceptable radius, whose scratches will be in a benign direction (parallel to bending stress), but the preserving the shoulder that is part of the bearing seat is tough. Even machine grinding must be carefully controlled, as it is easy to generate heat "checks' ( CRACKS!!) in the ground surface, eager to propagate to failure.

There are processes like shot peening (not the same as sand or grit blasting) that can be used to process a finished part that really boost the endurance limit/fatigue strength of a part