Shaft Failure

Did the new gears run for while before the shaft snapped or was it the first time running?

What do the gears look like? Tooth evenly spaced? Constant profile? off centre shaf?

Check the face of the broken shaft. We have faced this problem in other areas - The failure after 13 months of running (co-incidental or unlucky 13 in operation?) is almost certainly due to fatigue failure

Any abnormal vibrations ? Bearings replaced while shaft change ? If all factors are OK, check the shaft material (spectro, and micro structure for any inclusions) and the hardness of the shaft.

In certain cases, like in ours, it was a combination of notch effect (stress raiser) + faulty machining process (leaving high residual stress)

You may have even problem with the heat treatment process - must be hardened and tempered shaft - check the tempering temperature. Certain materials have a banned tempering temperature range. I am not conversant with En-8, you can google and find out.

the shaft failure is most likely due to fatigue near a stress concentration point. You should examine the face of failed shaft. you should see a clean smooth outer ring and a rough patch in the center. you should check for alignment, vibrations, bearing problems , improper heat treatment as the reasons of failure.

Photo's welcome

There could be many reasons for braking. It could be due to torrsional failure due to jamming, if there was brake fitted in the drive then it could be due to failure of the brake and motor was still in running condition. Material was defective, failure of bearings in gear box. To understand problem in detail it is always better to take a photo of complete drive and attach to the this thread. Also picture of the sheared shaft.