ModelWorks Foden 
Builder's Diary

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Tightening Bolts - Do not over do it!

Introduction

Kit 1 account

Most of the bolts used in assembling the Foden are either M5 or M6 and it is all too easy to over tighten these to destruction.  I usually am very careful to only apply moderate force but there have been times in the past when I have felt a bolt go.  By this I mean that as the head turns the resistance no longer increases. I wanted to know how close I was taking them to failure so I did a little experiment. 

I took a brand new bolt and threaded on three washers followed by a nut.  I held the nut in the 3 jaw chuck of my dividing head and positioned a spanner on the bolt's head the other end of which was resting on my bathroom scales.  The set up is not ideal but the scales reads the force applied to the end of the spanner as stress is applied by winding the dividing head.  Knowing the length of the spanner I was able to calculate the torque.  I took readings every 9 degrees until the bolt failed or I knew it was about to fail as witness by a reduction in measured torque.

The results are plotted below and they deserve careful study.  The lower pair of traces were obtained from M5 high tensile bolts I purchased off the internet.  The next pair of traces are from a pair of M6 bolts purchased from the same source.  The upper trace is from a M6 bolt supplied in one of the kits. All bolts were class "8.8"

In all cases the torque rises smoothly as the bolt is wound round.  It can be seen that this is approximately linear until the nut has been turned through 90 degrees.  In can then be seen that for the next 1/4 to 1/2 turn the force remains constant followed by a sharp drop upon which catastrophic failure occurs.

These results look similar to the classic stress/strain curves in the text books and I believe we are seeing the bolt reaching its elastic limit after 90 degrees of turn.   We should not read anything in to this occurring at 90 degrees as the angle for a particular bolt will vary according to a number of factors particularly the distance between nut and bolt head.  The interesting feature of my experiments was that there is a definite transition point between a linear rise in torque to a plateau region where the torque remains constant.

On some of the tests I stopped before visible failure occurred.  What was particularly worrying was that the these bolts looked normal despite having been at the point of failure.   The picture on the right shows the MW bolt which I stopped testing after it started to offer reduced resistance to being turned.  It looks in good condition however a vernier measurements showed that the end near the head has been reduced in diameter by 6 thou. 

Interesting results but what do they mean?

I am not sure!  Looking at the recommended torque for class "8.8"  M5, M6 bolts they should be tightened to 4 & 7 ft lb respectively (5.8 & 10Nm).  As expected this is within the linear region but towards the top.  The difference between the black and plated bolts is down to the coefficient of friction - Black bolts are not stronger, they just need slight more torque to overcome friction.  What I found surprising was that I found references to sophisticated bolt tightening systems that seek to tighten a bolt to the nee at the top of the linear region.  What more it suggested that's how a skilled mechanic would do it buy feel.  I am confused, surely this is the point where the bolt begins to be permanently distorted and any additional load will take it towards failure.  As I don't use a torque wrench this is academic.

My M5 spanner is 4 1/2 inches long and the recommended torque it achieved with just 11 lbs force at its end.  Failure occurs with only 16 lbs applied.  With a 5" spanner M6 bolts were failing at 25 lbs applied force.

The message is simple.  Bolts of this size can be easily broken with modest force that could easily be applied by hand without additional leverage or exertion.   What is worrying is that a bolt could be taken near to breaking point such that it fails in service.

Go easy on M5 & M6 bolts otherwise you will break them!