Thread: Bolt strength for stupid people

Firstly, no question is a stupid one! You are right to ask! Simply put, don't use those 4.8 grade bolts!!! (It does prompt me to question the structural integrity of the recovery points that the bolts were going to be holding....). Ok, 4.8 is not much stronger than STD mild steel. You then have 8.8. These are used on most things in manufacturing, tractors, loaders, Land Rovers etc. You then have 10.9. These are stronger again, and what I'd be using for your recovery points.. These are found on Agricultural Wheel centers, drawbars etc.. Lastly you have 12.9. These are (I believe) the strongest available bolts over the counter. Specialist applications.. Prop flanges, PTO shafts, heavy duty axles.. They are usually in 'Cap Head' form, which means they use an Allen Key type tool instead of a spanner. Hope that gives you a rough idea. Bolts on the left are Metric. Bolts on the right are imperial.

If you have 2x 10mm ISO Class 4.8 bolts holding the recovery point on then those 2 bolts can hold:

~3.8 tonnes in shear
~6 tonnes in tensile load

For ISO classes the first number is the strength of the bolt /100 in Newtons per mm2 cross sectional area

So your Class 4.8 bolt is 400 N/mm2 (or about 40 kg/mm2 - though really it is slightly higher as you divide by 9.81 (gravity) to convert between N and kg).

a 10 mm diameter bolt shank is 78.5 mm2 in cross sectional area. Shear loads are about 60% of tensile loads, hence the reduced number.

In short - buy some good quality ISO Class 8.8 bolts.

Thanks for the bolt info.

Question to Blindin - what brand recovery hooks? I believe the ones sold in Super Cheap etc are described as being for towing and not for recovery.

anything below a 8.8 bolt is what i call butter bolts because there about as strong as butter i allways try to use 10.9 where possible,
where a lot of people get lost is they know the old imperial system and think a 8.8 is the same as a grade 8.
as for what bolts i would be using 10.9's

Thanks for the help, I knew about the ratings, and what they meant, just didn't know how to work it out in real terms.
I've contacted the supplier and told them that they are supplying under rated bolts, last thing you want to do is have your recovery hook flying towards someone else car, or even worse, some one. We've all seen recoveries done badly.
I have always worked on the opinion that it should be a minimum of 2x 8.8 10mm - not that I know what shear rating that would give though.
It is a worrying thought that people think they are doing the right thing by getting a rated point, but the supplier giving a much lower rating bolt.
Like I mentioned, I knew the bolts were pretty much the lowest rated, but wasn't sure if it would meet the rating of the hook.
Thanks again
Simon

I'm interested to hear the reply of the supplier.

I think they should send you the appropriate strength bolts and thank you for letting them know.

Happy Days

I would add as a caution - using unnecessarily high tensile bolts is not necessarily an advantage, since, as a general rule, the higher strength means harder and more susceptible to fatigue failure. This will not usually be a problem, for example on a recovery fitting, where load cycles are likely to be very few, but may well be a problem on a tow bar, which may be in continuous use.

John

Originally Posted by JDNSW

I would add as a caution - using unnecessarily high tensile bolts is not necessarily an advantage, since, as a general rule, the higher strength means harder and more susceptible to fatigue failure. This will not usually be a problem, for example on a recovery fitting, where load cycles are likely to be very few, but may well be a problem on a tow bar, which may be in continuous use.

John

X2. Agreed, that's why I usually favour 10.9's for this job over the 12.9's. I'd also be interested to hear what the suppliers reply was...