Ball Mill Auxiliary Gearbox Input shaft damage

[S.Chakraborty]

Dear Sirs,
I am finding an unusual occurrence in the Auxiliary gearbox of a Ball Mill.
The input shaft of the auxiliary gearbox once broke into two pieces on Nov'20 and then I replaced the input shaft with a new one which got bend last week. All alignments were duly reviewed while refitting the new input shaft into the gearbox and also the gearbox reinstallation in the main drive arrangement. The teeth of all the other helical gears inside the auxiliary gearbox are intact and there are no impression marks on them to show any signs of shock.
The jaw-clutch arrangement was working fine too.
The incident took place right after the operator switched from auxiliary to main drive from the PLC.
I am unable to determine the Root Cause of this failure. Please help me with this case.
Thank you.

 

[desertfox]

Hi

We need a lot more information about the gearbox and its duty,also pictures of the failed parts.
From the information given we can’t tell you anything.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[S.Chakraborty]

I tried attaching more information, but for some reason, the drawings and photos did not get enclosed in this thread. I am sorry. I am trying again.

 

[S.Chakraborty]

Main Drive Arrangement dwg.

 

[S.Chakraborty]

This picture is of the original input shaft which came along with the aux. gearbox when the gearbox was first installed. This broke into two pieces as shown in the photo.

 

[S.Chakraborty]

The picture shows the broken piece of the input shaft stuck in the shaft coupling (tyre coupling).

 

[S.Chakraborty]

picture of input shaft failure before dismantling the gearbox.

 

[S.Chakraborty]

After the original input shaft broke, I thought it was probably due to the fact that the gearbox has been utilised for more than 25 years now and so may have lost its ductility and broke off. Then I procured this new input shaft (now failed and bent as shown in the picture dated 30th march 21) from the gearbox manufacturer and replaced it. This one was installed on Dec'20 and failed on 30th Mar'21.

 

[desertfox]

Hi S.Chakraborty

Thanks for the information, off the top of my head and assuming everything was checked and aligned as you stated, then I am wondering if allowance was made for thermal expansion of the shaft? If the shaft end was touching the other shaft end inside the then failure would occur like you are seeing ie the shaft bending. The shaft wants expands during operation but if there is no room because the shaft is hard up against another shaft and hence it buckles.
That’s the only conclusion I can come too because there are no marks on the gear teeth or the keyway to suggest overload. Have you spoken too the gearbox people who supplied the shaft regarding this?

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[EdStainless]

And have you double checked all of the mounts? Is everything tight and stable?
Thermal expansion would be my first thought though.

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P.E. Metallurgy, consulting work welcomed