JetMech has been pondering again, this time, about the fatigue life of bolted joints and the effect of bolt pretension. Pretension of bolted joints is important especially where the bolt takes tensile loads, and the joint members are compressed, such as engine mounts.
Photo © Dana Low
Photo © Agustin Anaya
The idea is to place a preload on the bolt which exceeds any foreseen load attempting to separate the joint members. Doing so reduces the magnitude of the cyclic loading on the bolt, which in turn, greatly extends the fatigue life of the bolt. Where I am getting confused however, is whether bolt preload - prior to joint separation - eliminates the bolt from "feeling" any of the external load, or whether it reduces the magnitude of the external load felt.
The "spring" model used to explain the behaviour of bolted joints seems to indicate - to me at least - that preload prevents the bolt from feeling any of the external load prior to joint separation. In other words, any external load - with a magnitude less than the preload - applied exactly equals the reduction in joint clamping force, the net effect on the bolt being zero.
However, if we take a more detailed look at joint behaviour using joint diagrams, we see that the bolt indeed "feels" part of the external load, albeit with a greatly reduce magnitude which itself depends on the relative stiffness of the bolt and joint members.
So what actually is occurring? Prior to separation of the joint members, does the bolt feel none or some of the external load?