Shear strength of 3/4" square steel?

8500 psi is 60Mpa I think (so about 100MPa tensile - 6 tons per square inch), so pretty cheesy stuff. I think that may have a safety factor of 2 in that, otherwise fatigue could fail it.
Not sure how you do the sums for square stuff, but for round I came up with figures as follows for the torque to cause it to start to take a permanent set. I read that the shear yield stress is about 58% of the tensile yield stress.
1/2 inch round, mild steel (215 Mpa Tensile Yield) = 40 lb.ft 3/4 inch round, mild steel = 125 lb.ft 3/4 inch round, carbon steel (guessing 350 Mpa tensile yield) = 215 lb.ft 3/4 inch round, carbon heat treated to 1000MPa tensile yield = 590 lb.ft
If you can use the ultimate tensile strength instead of the yield stress (for a once off emergency), then I get the following
1/2 inch round, mild steel (360 Mpa UTS) = 60 lb.ft 3/4 inch round, mild steel = 210 lb.ft 3/4 inch round, carbon steel (guessing 600 Mpa tensile yield) = 350 lb.ft 3/4 inch round, carbon heat treated to 1000MPa tensile yield = 590 lb.ft (uts and yield stress are pretty much the same for HT steel).
Looks like we are in the same ball park.
However Dave has been up half the night torturing a bit of half inch square steel and says it is happy with 120 lb.ft - so either its not mild, or our sums are wrong (and this is NOT my strong suit !). Though it is interesting to sort out and may come in useful later.
Steve
I was pleasantly surprised to find not only a short 3/4" sq length of (probably) EN1A mild and a similar length of gauge plate in my store, though no 3/4 sq lathe tools that I was willing to butcher. If I'm not too pressed for time when I get to the job, I'll do some tests with the EN1A at progressively increasing torques. If it fails, I'll have to do the same with the gauge plate.
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
Having just returned from the job,
I had forgotten that the last time I worked on these engines, I borrowed a 500 lb-ft torque wrench, as mine only goes up to 420. The engine has two sets of studs/bolts, one is 1 1/8" BSF with a recommended torque of 500 lb-ft, the other is 7/8" BSF @ 400 lb-ft.
Luckily the 500 lb-ft jobs are OK to do with the torque multiplier. I tried my torque wrench at various stages up to 400 lb-ft with the 3/4" mild steel square. Not even a hint of twisting, it walked it! I then pulled down the 18 bolts & studs to that torque, again no visible distortion & it felt 'solid'. I've checked the source, it was a 'budget pack' from GLR distributors, described as 'BMS'. it cut and ground like mild steel would. My wrench is marked up to 420 lb-ft, I wound it beyond the scale to what would represent about 450 & tried it on the 1 1/8" nuts, no apparent distortion there either.
Another question, re heat treatment. For the amount this big wrench gets used, after today's experiment I don't think I'll bother ordering the proper bit from Britool. Having cut & ground to fit a piece of 3/4" square gauge plate, is there a rough & ready way that I can treat it to ensure it stays the course? I'm used to simple hardening & tempering, but not to treating for maximum strength.
Incidentally, this is (more or less) the engine in question. Installed in 1964.

Thanks
Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
Big bugger then - I thought the straight six 3 litre in my Merc was on the large side.
John
Well I am delighted to hear the job went OK - thats the important thing. But I think we have to figure out why the calculations don't match the experience (think of all those people studying mech eng - precious few these days !).
BMS is bright mild steel (unless its Birmingham Mighty Steel), so it is stronger than the annealed state due to the a little cold work, but it is still mild steel. I am frankly surprised it did the job, after all we spent donkeys years developing the high strength steels and if we suddenly find that any old bit of wrought iron will do the job, then why did we bother ?
Its a mystery !
Steve
Tim, Missed this original post but I'm sure I have a box of 3/4" drive bits ex army. When we had the truck garage we used to buy these in bulk, they didn't last long when you are on bonus
Might not get chance to look before the show but I will drag the big socket set out and look first chance. Not a meant feat as it's in a wooden chest 3' x 2' x 2' think it goes up to 4" A/F. -- Regards,
John Stevenson Nottingham, England.
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John
Many thanks
Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
Could the length of exposed shaft have something to do with it? Perhaps the calcs are based on a shaft which has at least some space in which to twist like barley sugar. In this case the gap between wrench and socket is very small, probably zero at a couple of points. Mind you, there didn't seem to be any hint whatever of deformation.
Cheers Tim Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
Well forgetting all the wonderful figures the simple fact is that when the socket and torque wrench were close together it is getting nearer a shear load as opposed to twist and to shear 0.75" square mild steel needs 14 tonnes.Now big as Tim is reputed to be I don`t think he weighs that much even hanging on the end of a 5ft pole. Mark.
Indeed it does. The shaft is in pure shear when there is no gap between the load and counter load. Twist is proportional to the length of a torsion bar. If you had a 1 foot long piece of 3/4" mild steel square and put 500 ft lbs on it it would probably wind up like a rubber band. Again I might test that at 120 ft lbs with the 1/2" bit I did my empirical night work on :)
However even a long bit should still take a lot of load but not without distortion. The distortion weakens the bar somewhat as I understand it. (I probably understand only three fifths of sod all about all this torsional calculation business actually and wish I knew more).
It's very much like a valve spring. The more coils in a given length the weaker the spring is because each coil compresses a certain amount under a given load. Remove some coils and the spring becomes stiffer. Shorten a torsion bar and the same thing happens.
Anyone who regularly works with 500 lb-ft bolts is in a different league from me thats for sure. I wonder what the bolt diameters were, as clearly they didn't shear, though I suppose they would be nuts rather than bolts ?
I think 14 tonnes would be the figure for shearing it as in a guillotine. When you are trying to twist it the main shear load if furthest from the axis, the steel in the middle does get much abuse, so its not the same as shearing the whole lot at once.
The amount of twist in the shaft will depend on the length, but the shear load shouldn't be affected, however when it is very short you are limiting its abilty to go out of square. Maybe that is a factor, but we have a big gap between theory and experiment to explain.
Steve
See my post yesterday. 1 1/8" BSF, studs @ 500 lb-ft 7/8" studs and bolts @ 400 lb-ft
Cheers Tim
Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service
I wonder what they have to torque these head studs up to.
Doubt if they use mild steel in the tooling though :)
It would be 14 tonnes regardless of whether you were shearing it in a guillotine or punching a hole.Actually for shearing a 3/4" sq. bar it would be more as the shear would be from the centre neutral axis to a corner,twice.Assuming of course there was neglible gap between the socket and the wrench. Mark.
I'm not qualified to comment on the various shear strength calculations used by the wise folks on the group, but here is a suggestion which might be of help.
How about grinding down a 3/4" square bit onto an extension bar, thus making it 3/4" square both ends ? If you've only got good tools you could buy a crappy draper one speciallly. I'm thinking of a rough and ready job with a hand-held angle grinder, or bench grinder rather than anything sophisticated. The item is obviously made of a suitably strong steel to start with, and unlike a blade, there shouldn't be too much risk of spoiling any heat-treatment if you dunk in water from time to time.. Just a thought.
Cheers
Hywel

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