Bolts and studs with slender shanks
For design engineers working on bespoke engines, the job of designing special-purpose fasteners is a familiar task. There is often a need for a very long and thin fastener, for example to pull together the stages in a multi-stage pump, or possibly bolts that run right through the engine from top to bottom (anyone who has designed upgrades for the four-cylinder Rover K-series engine will be familiar with the through-bolt concept).
In terms of their tightening, slender bolts and studs behave in the same as any other bolt. For example, with identical dimensions, surface finish and coefficients of friction, a stout bolt and a slender bolt should develop the same pre-load for a given torque.
In service, however, the slender bolt is much more likely to be affected by resonance. This is a condition where the bolt is excited and vibrates at one of its natural frequencies by a stimulus coming from the engine. There are all kinds of vibrations in an engine, and these have various harmonics acting at multiples of the fundamental speed of the component or system generating them. Slender fasteners are more likely to be affected because their natural frequencies are lower than those of stouter fasteners. When a resonant condition is reached, the maximum amplitudes in a slender fastener are greater than for a less slender one.
The life of a fastener may be reduced significantly owing to the higher stresses encountered. The resonant condition will usually be one of bending, and the fastener will often show signs of bending fatigue at the points at which it is restrained, which is often in those parts of the threaded portions of the bolt which are most highly stressed in service.
Fortunately there are a number of easy remedies to this problem. These revolve around increasing the natural frequency of the fastener, which can easily be done, or by limiting the amplitude of vibration and hence lowering the stresses involved.
There are a number of ways to change the natural frequency of the shank. It can be increased markedly by even a slight increase in the shank diameter. Changing the length of the shank is also effective; for waisted (reduced diameter) shanks this can be done by increasing the length of the threaded portions on the fastener (decreasing the length of the waisted section) or adding an increased diameter section close to the ends of the fastener.
Another way to change the natural frequency is to add a location diameter part of the way down the shank. This effectively creates two short shanks on the same fastener, increasing the natural frequency significantly. In practice, however, it is difficult to guarantee location. In order to get the parts to go together, it is far more likely that a clearance condition between the location and the hole will be created. In this case, what will happen is that the natural frequency can actually be reduced slightly, owing to the effective addition of a mass to the slender beam. What will happen is that the amplitude of vibration and hence the stress will be reduced.
Written by Wayne Ward