Vanoord, I remembered to check the strength of A4 (316) and A2 (304) bolts and they're the same. A2 are cheaper, A4 more corrosion resistant. If you intend to remove the nuts (as we did in Fron Boeth) it would be worth using normal steel nuts as these are far less likely to bind on the stainless bolt.

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I see cavers using 16mm rawlbolts and I see a new breed of explorer using industrial anchors and I'm not sure the latter are sensible from a physics point of view.

I can't be arsed to go into the physics of it in detail, but in essence, my thinking is based around bolt surface area and pressure exerted on the rock in order to achieve an acceptable level of friction.

A lot of mines I go in have borderline dodgy rock and I think it's probably a good idea to use bolts with a greater surface area.

I'm all for bodging and shortcuts, but bolts aren't an area to do that in.

IMO.

Not sure what you mean by industrial anchors, there's a huge choice. Bigger bolts will exert more expansive forces if done up to recomended torques so possibly more likely to crack the rock, where possible I prefer resin anchors and in some soft rocks are the only thing that'll hold.

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When I say industrial anchors, I'm talking about the sorts of things you'd bolt a compressor down with. ie:- previous photos.

Total friction would be proportional to force x area.

If you consider the relative areas of a 10mm bolt and a 16mm bolt (without considering the specific areas of friction-which varies considerably according to design) they are pretty different, so the forces required to give a specific level of.......

Anyway, formula 1 cars don't use bicycle tyres for a similar reason. :lol:

Look up coefficent of friction and hoop stress. The fastenings shown are also positively located in addition to friction, they expand the ring into the surface of the rock. Size isn't everything 😉 I bet you a jam butty my polo could beat an F1 car on wet grass because it has narrow tyres (and the landrover thrash it :lol: ).

Nothing is as clear cut as the two rules I've suggested would indicate, hense the width of F1 tyres. The manufaturers would suggest you do pull out tests to find the optimum diameter and torque in anything other than concrete (but how many of us have time). Just use more small anchors puts the least stress on the rock. Rawl do some "safety" rated anchors.

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My point was that with borderline rock, you want to spread out the stresses as much as possible. Clearly with things with a tiny contact point, the forces are going to be much larger. Think elephant and high heels!

My elephant in high heels will tow your landrover with it's mud tyres.

My elephant in high heels will tow your landrover with it's mud tyres.

I'm quite happy to stump up a pint for the winner of that challenge :lol:

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My elephant in high heels will tow your landrover with it's mud tyres.

Sadly I'll have to decline, it's got road tyres fitted and I feel your elephant may be off performing cabaret somewhere.

I agree about the surface area in theory, try (or better get someone else, the elephant perhaps) to pogo stick across a bog. Only problem is to get metal to bite in enough to grip takes a certain force per area so the bigger the area more force is needed (torque on the nut), bigger risk of bursting the rock. I've seen a large rawl bolt pull out where not done up tight enough due to iffy concrete. We got round the problem using studding with a nut (machined round, close fitting in the hole) in the bottom of the hole, rubber pipe slipped over it and compressed with a steel pipe (close fitting on studding and in hole) under another nut wound up tight. Err, I'd best add that my life wasn't hanging on this!!

Simon would agree with just add another bolt if you're not sure, it worked in Fron Boeth. Better load distribution and backup if one fails.

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I suppose that unless you have related torque of the nut to the peak tension force within a material, it's pretty academic. Taking a torque wrench with you would be total overkill. I'd like to see more resin in anchors really. 🙂