Examples of Bolt Failures

M12 Bolt Fretting Failure

Bolt with worn shank Failure of several M12 bolts occurred as a result of a design problem on a commercial vehicle. The bolts secured a crossmember that sustained loading from the vehicle suspension to the main longitudinal channel sections. The threads of the bolts were coated with an anaerobic adhesive that prevented any possibiliity of the nut rotating loose.

The structural integrity of the design depended upon the suspension loads not exceeding the friction grip provided by the bolts. Partly due to embedding losses and partly due to the initial bolt preload being inadequate, friction grip could not be maintained and relative movement of the crossmember relative to the main frame occurred, resulting in fretting of the bolt shanks. Resulting damage also occurred to the nuts and the underhead of the bolts.

Worn Nut

The problem was resolved by ensuring that the clamped surfaces were flat, specifying an appropriate tightening sequence and increasing the strength grade of the bolts from 8.8 to 10.9 so as to increase the clamp force available. An appropriate increase in the tightening torque was applied to achieve the preload increase.

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