Thread: Lets get down to the nuts and bolts

Originally Posted by Mick_Marsh

My towbar is bolted on with grade 8 bolts.
I have been advised to change them to high tensile by a knowledgeable fellow.
I always thought high tensile has the potential to be too brittle for some applications.
Should I swap them for high tensile? Why/why not?

All advice considered and appreciated.

Grade 8 bolts are high tensile heat treated bolts. They have six radial lines on the bolt head and a minimum tensile strength of 150,000 psi.

concur



without getting into the nitty gritty of how the load was applied to cause failure....

lower tensile bolts stretch more before failing so youd expect to see something like this
or this
the bolt will turn into an elongated shape as it stretches, I call it hourglassing if you get bolt thats thinner in the middle its stretched, throw him away.

higher tensile bolts hold on longer but dont give you a lot of warning prior to failure

I asked the rep at ABC Bolts the same question re putting higher than grade 8s on my series tow fitting. He said he would sell me grade 12 (??from memory) but that it was illegal to fit them to tow bars because they did exactly that - fail without warning under high loads.
Dave

Originally Posted by praxis

I asked the rep at ABC Bolts the same question re putting higher than grade 8s on my series tow fitting. He said he would sell me grade 12 (??from memory) but that it was illegal to fit them to tow bars because they did exactly that - fail without warning under high loads.
Dave

He neglected to mention that the lower grade bolts would have failed long before a grade 8 or better would. Use Grade 8. Anything more is overkill. Most chassis fittings on heavy duty trucks ( Kenworth, Western Star, Mack) are Grade 5 or Grade 8 with Nyloc nuts, three threads through.

It is most likely that the bolts holding the towbar are sized so as to have a very large safety margin, and increasing the strength of the bolts is unnecessary, plus, as noted, the higher the strength the less warning of failure, and the more susceptible the bolt will be to fatigue failure in the right circumstances.

John

as a side note....

lower grade bolts are better at resisting vibration type loadings than higher ones, IMHO this is why you dont want anything over a grade 8 bolt for a normal vehicles towing gear, below grade 8 and you just dont get the strength, above grade 8 and the shock loading is too much for the bolts. sure they'll last for a while but the constant impact loading causes them to fracture.


look at it this way. hit a bit of spongecake lightly with a meat tenderizer and the cake springs back, hit it a bit harder and the cake takes some damage. smack it really hard and you get a tenderizer shaped hole in the middle

now grab a bock of ice and try again.

what happens?

Interesting stuff.
They're metric bolts which means they're really grade 5.

Too soft for a towbar?

if they're metric and they're unmarked then they are grade 3-6 (if its a bunnings special its 3, if its from somewhere like bolt masters, blackwoods or similar then its grade 6.

metric bolts have the grade stamped on them as a number....

Id pull the bolts and inspect them, IMHO yes, they are too soft and should be replaced with grade 8 bolts.