Thread: Bolt strength for stupid people

Hmm - interesting reading above - thanks all.

On a related note then: So if I was putting new front recovery points on a 110, do I want 8.8 or 10.9 bolts?

Planned setup is always pull with an equaliser strap attached to a point on each chassis rail, each point is secured with 2 x M12 bolts all the way through the chassis rail, one of these bolts in the factory steering guard so it has an inner tube for added strength.

I had assumed 8.8 would be good enough...

Originally Posted by Gerokent

Is there a direct correlation between tensile strength and sheer strenght?

Yes as I posted. Shear strength of bolts is ~60% of tensile strength

Originally Posted by FeatherWeightDriver

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So if I was putting new front recovery points on a 110, do I want 8.8 or 10.9 bolts?
...

Firstly, the important thing in this case is to increase the bolt diameter is you can. I found on mine I could (just) drill out the chassis to 12 mm - which gives a big increase in bolt strength. Note there is a crush tube in the hole.

Given you may only have one bolt holding the load then a class 10.9 may be a better choice than 8.8.

Thanks again,
Totally agree with the fatigue view, most people will over look this.
I contacted the supplier, but at this point no reply.
It was through 4wdsupacentre, who I know are cheap, but it thought a rated hook they can't really get wrong. I've been corrected.
The retaining clip also snapped while tightening bolts to, so no flex what so ever in that. Should've spent an extra few dollars and not have the issues.
I am waiting for the response that they take into account the friction between the 2 items so the bolts don't need to meet the full rating.
Will let you know the outcome if I finally get a response.

Originally Posted by blindin

Thanks again,
Totally agree with the fatigue view, most people will over look this.
I contacted the supplier, but at this point no reply.
It was through 4wdsupacentre, who I know are cheap, but it thought a rated hook they can't really get wrong. I've been corrected.
The retaining clip also snapped while tightening bolts to, so no flex what so ever in that. Should've spent an extra few dollars and not have the issues.
I am waiting for the response that they take into account the friction between the 2 items so the bolts don't need to meet the full rating.
Will let you know the outcome if I finally get a response.

So the bolts are substandard, the clip has broken before you have even used it. On what grounds do you think the hook itself is any good?

If the hook is in fact "rated" it will have a SWL (safe working load) marked on it, and certification should be fully traceable.

If the hook doesn't have a SWL on it I would bin it, and put it down to experience.

Just my thoughts.

Tony

They're rated. 10,000lbs.
Although I don't really agree with that it must be rated to be used, I am very careful about what is used.
The reason I don't quite agree is for this exact reason - the hook itself is rated, but that's it, where and how it is bolted to the vehicle isn't.
How ever some people would refuse to use a 12mm plate attached to the vehicle with 4x 12mm h/t bolts, if not more, to a suitable place on the chassis and say its dangerous as it's not rated.
I do have a very good idea on how it all works, rated, and strengths of things.
As I mentioned before, I wouldn't use anything less than 8.8 bolts, although I was unsure exactly how much load they could take. The reason I asked the question is because the bolts were lower and I wanted to know exactly how much load they could take, as you would expect that they would supply a suitable bolt to at least match the rating.
I have mounted 2 hooks, and they will be used with a bridle, so unless I'm buried to the axel in mud, I shouldn't be able to max the ratings, and even then, the strap would ( or should ) fail first, even with the substandard bolts.
My preference is always being attached at the rear anyway, as I consider it to be a lot safer, and hopefully, they won't need to be used, they are there just in case.
Also, they are mounted to a xrox bar, I have just had the mounts redesigned as the way they were mounted was not adequate in my opinion. The old mounts only secured to the 2 end bolts on the chassis legs, I never trusted them, and personally would never have used anything bolted to just those. The new mounts extend backwards to the cross member and secured there with another 4x 12mm bolts.
The brackets have also been modified to allow them to clamp to the chassis rails, rather than just simply fitting over.
This is on a disco 2.
Very happy now that I trust the mounts, a bit gutted I have lost some approach angle compared to not having the bull bar.

I've done some work on recovery hooks before. All is not as it seems.

For a start the ratings are minimum failure. There is no safety factor which makes them nothing like the rating you get on lifting or rigging equipment.
I ran a FEA analysis on a 4,500kg rated hook and at the 4,500kg rating it's past yield and starting to permanently stretch. If you overload them they will open up and spit off the strap/rope.

These cheap hooks are not suitable for any hard recovery.

Now for the bolts.
You should never, ever be approaching the strength of the bolts. A proper bolted join carries the load in friction and the bolts are only loaded in tension to provide the clamp load. Using 2x 12mm grade 10.9 bolts torqued to 180Nm (providing 7.4t clamp load each) you can still only provide enough friction to meet the rating in a straight pull.
Any angled pull can still shift the hook and load the bolts in shear.

But to support a 12mm grade 10.9 bolt torqued to 180Nm you need crush tubes 25mm in diameter.

Put simply, 10mm bolts are rubbish and can never be safe.

Originally Posted by Dougal

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Put simply, 10mm bolts are rubbish and can never be safe.

Yes but try telling that to all the people who us "jate" rings for recovery...

Originally Posted by isuzurover

Yes but try telling that to all the people who us "jate" rings for recovery...

They're in double shear. Gives you twice the strength and it's not a bolted connection.