Examples of Bolt Failures

M24 Engine Mounting Bolt Failure

M24 Bolt Failure Bending fatigue failure of a M24 strength grade 8.8 engine mounting bolt. The bolt was 220mm long and was used to secure one the mounts of an engine to a supporting structure. The bolt was tightened using a torque prevailing nut and additionally used a split pin in an attempt to ensure the bolt did not come loose. This failure was due to a number of factors. Specifically:
1. The size of the bolt demanded a high tightening torque (660 Nm - 487 Lbft). This torque was not specified since, with the tools then available, this level of torque could not be achieved.

2. The bolt clamped through an aluminum housing. Because of the reduced stiffness the clamped parts the bolt sustained a larger proportion of the applied force.

3. The bearing stress under the nut head and nut face was well beyond the maximum bearing pressure of the aluminum. This resulted in creep occurring causing excessive preload loss.

M24 Bolt Failure Directly because of these factors the bolt lost its preload and came loose. The use of the split pin and the torque prevailing nut failed to prevent the nut loosening. This resulted in the bolt sustaining a fluctuating bending moment that it was not designed to take. These forces led directly to the fatigue failure of the bolt.

A full analysis of the joint was completed. Based upon the results of this analysis the problem was resolved by reducing the diameter of the bolt, increasing the strength grade and using flanged fasteners. This increased the stretch of the bolt resulting in less preload being lost from any embedding. The flanged fastener reduced the bearing stress under the bolt head and nut face to within the maximum level for the aluminum.

View another example of a bolt failure

You may also be interested in an article on this failure:


Failure of an M24 Engine Mounting Bolt

Published in the Fastener Technology International magazine in August 2006, this article looks into the causes of the failure of M24 engine mounting bolts. An M24 property class 8.8 bolt was used to secure one of four engine mounts to the chassis of a bus. Following the introduction of the bus into service and some operational experience, reports started to be received that bolts were occasionally found loose and, on a number of occasions, the bolts were failing. To prevent what was perceived to be a loosening problem, a split pin was introduced that passed through the bolt thread immediately below the nylon insert nut to prevent the possibility of the nut backing off. This fix proved to be only partially successful and instances were still being reported that the nuts continued to back off, leading to the split pin being completely sheared off in some instances. Fatigue failures continued to be experienced. This failure illustrates two problems that inadequate preload can manifest itself as. Fatigue failure is a common by product of inadequate preload; joint movement, because the friction grip was inadequate, results in stresses being induced into the bolt that it was never designed to sustain. This same movement, when the fatigue strength of the bolt is able to sustain the induced stresses, will result in the tendency for the fastener to self loosen.

The importance of achieving and maintaining an adequate preload is often the crucial factor in ensuring that the structural integrity of the joint is assured.