Friday, May 2, 2014

Shank Lengths of Bolts

Bolts and nuts are the most crucial part of any assembly. They hold the different components of the vehicle together by providing the structural strength required. Selection and the usage of bolts, nuts and other fasteners are a very critical aspect of engineering design and should be considered with high importance at the design phase of the vehicle.

A very common mistake on Formula Student and Baja cars that is seen is the incorrect selection of bolts in terms of their sizes and shank lengths. Before proceeding here is the clarification of the shank length and threaded length -
Bolts are designed to carry axial loads. It may carry bending loads as well however it is important to understand which areas of the bolt should see the bending load. To get a better idea of why this is - consider a rod that needs to be broken. A force is applied in bending because it is the weakest then (Why? -  Load PathsDesign Error Blog).
If the rod still does not snap the next best thing to do is create a notch like in the picture below. It should be easier to break this after that. The notch can be really tiny, but it has an associated stress concentration factor which will cause it to see higher stress in that region and break faster. The notch is also a perfect crack initiation point for the rod to break.

The threads on the bolts are nothing but a series of notches when the bolt comes under bending and the bolt is much likely to break when loaded in bending at the threaded region. 
The threaded length of the bolt is the section which should be used to fasten the nut onto the bolt and for that reason only, all the other bending load must be taken up by the shank region. Which means that a bolt that connects a rod-end to the chassis mounts must look like this -

The clamps and the spacers and the rod end see the shanked length of the bolt and the threaded length of the bolt only starts under the final washer after which the nut is tightened and torqued up. At least 2 threads should be visible on the other end of the nut to ensure that the bolt is properly fastened.
This not only applies to the suspension bolts but any critical bolt that might be used in the vehicle. This mistake is also often seen in brake balance bars there bolts or even studs are used and the brake forces travel right through the threaded region of the bolt / stud. That will break. Custom engine mounts often ignore this as well and we see these bolts bend and break with time.

This blog post is originally written for the Formula Student India website and has been cross posted from here.

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