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.
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.
This blog post is originally written for the Formula Student India website and has been cross posted from here.
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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Does shank length matter for a single shear mount, such as the bolt goes into a threaded hole in the side of the chassis?
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