M24 Engine Mounting Bolt Failure

tom2005

M24 Engine Mounting 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.

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.