Fasteners: Back to basics - part 5
In previous articles we have looked at how to calculate fastener and joint stiffness, and seen how these values of stiffness can affect the proportion of the working load experienced by the fastener by calculating the ‘load coefficient’.
See here
The forthcoming article in Race Engine Technology on the subject presents some simple examples of how variations in fastener stiffness can affect the levels of stress in service, and more importantly the mean stress and stress amplitude. The combination of mean stress and stress amplitude are important factors in determining fatigue life.
By decreasing the stiffness of the bolt in relation to the joint stiffness, we decrease the proportion of service load taken by the bolt. It follows from examining the equation for load coefficient that, should we also be able to increase joint stiffness, that we can achieve the same effect. A simple way to do this might be to make a longer joint by increasing the distance between the nut and bolt (note that this won’t have the desired effect if there is a strange load path, or if the joint is essentially two hollow cylinders of the same diameters as the nut face. This would clearly add mass to the assembly and might not be possible within the constraints that we have. Another method might be to increase the modulus of the joint materials. Again this might neither be possible or desirable. The change to a stiffer casting material may bring negligible benefits if the same kind of material is used and in many cases we will have to work with production castings where we have no control over the material. A change from cast aluminium to cast steel would be disastrous in terms of added mass and so that option is a non-starter.
A very simple way to achieve an increase in joint stiffness is to increase the volume of clamped material, and this can be done with a washer having a diameter greater than the bolt-head outside diameter or the nut-face outer diameter if a nut is used. Washers are often used with threaded fasteners, but a lot of people ignore their technical significance in decreasing stress. Many people think that a washer is simply to prevent damage to a casting or low-strength component by the action of tightening. The head of the bolt or nut can indeed gouge a casting owing to the relative movement, or become slightly embedded due to pre-load. A washer can indeed prevent this damage, but more importantly, if it is stiff enough and is larger than the head of the bolt, it can increase the joint stiffness by effectively clamping a greater volume of material. If we look at the diagram in the previous article on the subject of joint stiffness (Part 4) we can see a joint with the two cones of clamped material, where a thin washer has been used. Thin washers deform easily and we don’t see the benefit of increased joint stiffness. However, if we use a thick washer made from a stiff material, we increase the clamped volume of material and consequently the stiffness of the joint. In terms of the diagrams in the previously referenced article, we increase the diameter of the clamped cones. This can have a significant effect for little increase in mass.
Ignore washers at your peril, and beware of the pitfalls of using thin washers.
Written by Wayne Ward.