Bolts sheering

I monitor a reciprocating compressor fairly large motor 890 rpm about 400 HP.

The input shaft cover to the crank arm box has 8 bolts holding it on.

They are torqued to 40 Foot pounds each. Every few months one will sheer the head. The same on each time. Has anyone a theory as to why?

 

Thanks

Original Post

I agree with John regarding needing more details....but it strikes me very odd that THE SAME BOLT LOCATION  <--- do I have that right??  is the one that shears the head? 

If that is the case, you should make sure the bolt is not bottoming out or find out what's different about that particular hole.

Any bolt not tensioned properly will experience alternating stresses likely resulting in what you are seeing. The exact fatigue mechanism will depend on the material differences between the bolt and the mating hole. 

Regards

Jim P

No I don't they just called me and asked me if i had an idea. He said torque was relatively light. Honestly I am thinking the other 7 bolts bottom out just before torque is reached. The 8th bolt takes the full 40 ftpounds of torque and is taking most of the load and eventually breaks the head.

Jack,

"Every few months one will sheer the head. This is a general description.

Bolt shear failure occurs when the clamped components slide, so the force (translation or torsional) is perpendicular to bolt axis. A tension failure occurs when the two clamped components are trying to separate with force inline with bolt axis. What type of failure do you have? Can you provide a photo or drawing/sketch of the assembly?

Walt

Rational thinking tells me bolt heads do not typically break unless something is moving or you over torque the bolt. If the cover was changed to a different thickness this can cause bending modes on the cover and over stress the one bolt. As others stated more details are needed

Dave

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