How much compression with 12mm bolt?

Answer, lots.
Assuming what you are bolting together is not compressible, that the cross sectional area of a 12mm bolt is very approximately 75x10^-6 m^2, and that the yield strength of stainless steel is approx 520 MPa (depends on the grade), and that you have the strength to wind the nut on, the load applied would be in the region of 39kN (or around 40 tonnes force).

That load is then applied over the contact area provided by the washer, and if you consider the contact faces of the two components you are joining then the area of contact between those components is the relevant figure to work with.

Obviously in practice most things will distort as you load them - the further away from the bolt, the less the load. So in theory two bolts will apply twice the load, in practice you'll find that they will just provide a bit extra over a bigger area.

If anyone wants to correct my figures please do.
 
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There's no short answer to this question. Depending on which material the bolt is made from, you will get different values for the ultimate tensile stress (UTS) the bolt can sustain (different Stainless Steels have different properties). The maximum Force exerted by the bolt will be the UTS, multiplied by the cross sectional area of the bolt. Two identical bolts, side by side will both exert the same force, however they will exert at two points, instead of one.

If you are concerned about the shear load, rather than the elastic load, however, two bolts will double the sustainable shear load. (shear failure is the probability of bolt failure perpendicular to the bolt length)

The only way to double the tensile force is to double the cross sectional area of the bolts. The closest standard bolt which would give you double the force would be 18mm.

If you are trying to compress two things together, the more bolts you use, the greater the pressure between the two things your are trying to compress. This is a function of the individual loads in the bolts and the cross sectional area of the Washers used between the bolt heads and the things your are trying to compress. Two bolts will double the pressure. Bearing in mind that whatever you are trying to compress will have some measure of rigidity in its own right, there will be a lower, different pressure exerted between their mating surfaces and if this is what you wish to maximise then, two bolts will double this interstitial pressure.

Perhaps if you were to explain what you were trying to do, it might help you get a clearer explanation.
 
Topcat47 said:
Perhaps if you were to explain what you were trying to do, it might help you get a clearer explanation.
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Yes, I couldn't help thinking that as well. I had in mind that the op could be bolting a couple of bits of plywood together, or maybe two long strips of 3x25 steel, or a couple of huge cast iron blocks. In each case the results would be very different.
 
LittleSister said:
I want to know
(1) with how much force(?) I can compress two things together using a 12mm stainless bolt and nut, and
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Guess - maybe a couple of tonnes per sq.in. ?
(2) if I use two bolts adjacent to one another (a few inches apart), do I get twice that value?
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Yes.

My guess is based on a vehicle side-lift jack - they are good for lifting a tonne, with a good safety margin built-in.

The limiting factor is going to be the nut - if you were to use a stud connector (Ebay # 200541797183) instead of a normal nut, then the load would be spread across many more threads, and the limiting factor would then become the tensile strength of the bolt itself.
 
srp said:
If anyone wants to correct my figures please do.
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I'll do it myself.....:o
I'm talking botty waffle - it's probably not the tensile strength of the bolt that is the limiting factor but the shear strength at the root of the threads. If you can be bothered to work out the cross sectional area of the root of one turn of a 12mm metric thread, multiplied by the number of threads in the nut, and then use the shear stress figure rather than the tensile yield stress that would give you a more accurate figure. Just off the top of my head I think the shear stress is about 200Mpa and the total csa subject to shear in a 12mm bolt is around 38mm^2, so the load would be around 7.6kN or around 8 tonnes force. Roughly. I think.
 
Topcat47 said:
Perhaps if you were to explain what you were trying to do, it might help you get a clearer explanation.
Click to expand...

Ah! Sorry to be obscure, but I was deliberately trying to avoid explaining the situation so that I might get an answer to that particular question, as opposed to all the other potential issues surrounding it! Apologies if that's made it frustrating and impossible to answer.

I'm not trying to 'do' anything, but trying to understand the potential source(s) of some distortion which has appeared in the boat. Some bolts in the vicinity have been replaced with bigger ones (12mm, previously 8 or 10), so have come under suspicion. I wanted to know roughly what sort of load they are likely to be putting on the bolted together structures. If each bolt is capable of anything like 40 tons, that may be the root of the my problem.

The bolts haven't broken, so presumably it's the 'elastic load' mentioned above that I am interested in. There is negligible sideways force concerned, which if I understand it correctly is the shear strength also referred to above, so that's not an issue. It is a purely about how much compression is there likely to be, with bolts tightened by a strong man with a socket spanner.

I'll probably post a separate thread explaining the wider problem more fully once I've had a chance to have another look at the situation armed with your answers.

In the meantime a couple of supplementary questions, if I may:
(1) What would be the relative compression of 8mm, 10mm and 12mm bolts (actually pairs of bolts)?
(2) More general question, just out of curiosity: srp mentions the strength of the cross section of the bolt. If one could keep tightening a bolt until it failed, would it not be the thread rather than the shaft of the bolt that went first?

Thanks for your help.

P.s. Started this reply earlier, new posts arrived unseen before finished, too tired time to amend to suit just now but you get the drift.
 
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LittleSister said:
Some bolts in the vicinity have been replaced with bigger ones (12mm, previously 8 or 10), so have come under suspicion. I wanted to know roughly what sort of load they are likely to be putting on the bolted together structures.
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It's impossible to say what load is being applied, based just on the diameter of a bolt ... it all depends on how much they've been tightened. To find that out, try applying a torque wrench.
 
If it's any help, my ref book shows that a 12mm bolt with a minimum tensile strength of 520 MPa (as in 316 stainless) will have a clamping force of 30.67 kN and a standard dry torque of 73.61 N/m.

What this all means, I have no idea.
 
LittleSister said:
The bolts haven't broken, so presumably it's the 'elastic load' mentioned above that I am interested in. There is negligible sideways force concerned, which if I understand it correctly is the shear strength also referred to above, so that's not an issue. It is a purely about how much compression is there likely to be, with bolts tightened by a strong man with a socket spanner.


(2) More general question, just out of curiosity: srp mentions the strength of the cross section of the bolt. If one could keep tightening a bolt until it failed, would it not be the thread rather than the shaft of the bolt that went first?
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I corrected my own garbled thinking in my third post - you have to consider the shear strength of the bolt material at the root of the thread - as you correctly point out this is the bit that will fail, ie the threads will strip.

If it's any help, my ref book shows that a 12mm bolt with a minimum tensile strength of 520 MPa (as in 316 stainless) will have a clamping force of 30.67 kN and a standard dry torque of 73.61 N/m.

What this all means, I have no idea.
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This refers to the tensile strength of the bolt, and (very) roughly agrees with my first calculation - it differs in that it gives the clamping force achieved by applying a specific torque figure, in this case 73 Nm (about 54 ft lbs), whereas my figures show the yield strength - though the threads might strip before this point is reached.

As the others have said, without more information there is no answer. I very much doubt that replacing an 8mm bolt with a 12mm one would cause any problems as such, but it depends on the size/shape of the components, and what material they are made of. In any case, many materials are capable of withstanding extremely high compressive loads before failure, much higher than shear, tensile or torsional loads.
 
It is more or less standard with machinery bolting to tighten to at least 70% of yield strength, mainly to avoid fatigue fracture. With high strength bolting, which has no measurable yield point, it is quite common to tighten to yield.
 
Salty John said:
If it's any help, my ref book shows that a 12mm bolt with a minimum tensile strength of 520 MPa (as in 316 stainless) will have a clamping force of 30.67 kN and a standard dry torque of 73.61 N/m.

What this all means, I have no idea.
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It means I'm getting some very useful information!

Do you have the figures for 8mm and 10mm bolts of the same material, John?
 
From memory (my copy of Shigley is at work), torque=0.2 x force in bolt x diameter, if you know what they are torqued to this will give you an idea of the load. Bolted connections are actually quite complicated!
 
gordmac said:
From memory (my copy of Shigley is at work), torque=0.2 x force in bolt x diameter, if you know what they are torqued to this will give you an idea of the load. Bolted connections are actually quite complicated!
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Seems to tally with the figures Salty John quoted. That's really useful, as now I can work out what torque will give me the same compression as the previous smaller bolts (unless of course, I've completely misunderstood the whole thing!).
 
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