Shear strength of 3/4" square steel?

T = 0.208fS^3 for a square shaft, T = twisting moment, where a safe f for MS is given as 9000 psi.so:

0.208x9000x0.75^3 = 789.75, but im not sure what the units are, or if this helps....

values are for generic 1944 MS, alloy steel isnt covered, but the sheer strenght is listed as

15000 psi safe working stress.

hth a bit

Dave

The 3/4" Britool Torque Wrench here has a hardened drive piece, you might get away with MS but you'll have to keep an eye on it in case it starts to go when you are finely balanced on top of the pots! :-))

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

Well I suppose logically they ought to be lb-in, as the pressure is in psi & you've done the shaft size in ins, but I can't believe the figure would be as low as that!

Well it would if I knew the units

Thanks Tim

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I wouldn't be on top of the pots, they're at shoulder level while I'm on the floor plates. 500 lb-ft is pretty much a 2-man job at that level, even with a big torque wrench, & it could be uncomfortable if the link did fail :-(

I'm not sure whether hardness has much bearing on the matter, or does it? Not my subject, which is why I've asked the question!

Cheers Tim

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Tim,

I have never seen a torque multiplier, so I don't quite understand what you are up to, but 500 lb.ft sounds like a scaffold pole with a heave on the end to me - so if you are in a cramped space that must be the explanation.

Any chunk of steel as part of the gubbins of a torque mutiplier would have a far higher yield stress than mild steel. EN8 would be better, but the genuine article is going to be hardened and heat treated to give it big strength and toughness compared to EN8 in its annealed (machineable state).

Any 3/4 inch extension shaft in a crap socket set will take at least 3 times the abuse of mild steel, and a good one will take far more, maybe

7 to 10 times the torque, maybe more. It's be an alloy steel hardened and tempered for the job.

Maybe if I had a torque multiplier I wouldn't need the land rover track rod arm and spring balance that I often use to check torques (cheap torque wrenches have cost me a diesel head gasket before!).

Steve

as a rough and ready measure 500 lb-ft on a 3/4 inch piece of square stock would equal 16000 lb at a 3/8 inch radius, and that is asuming no safety margin, which would equate to a material able to withstand 43,000 lb per square inch, as a rule of thumb plain MS works out as around 16 to 20 tonns per square inch. where as the load you are want to exert is in the reigeon of 20 tons on the 3/4 square drive, so I suspect it would shear. Guage plate on the other hand has a capacity of somewhere abouts 40 tonns per inch if memory serves so should give a safety margine of 2 to 1. Hope this helps

Here's my take on it if my calcs are correct. The amount of torque a bar can take is hopefully proportional to the cube of the diameter. If I've got that bit wrong we're stuffed.

The most I've seen listed for torque and impact wrenches is as follows.

3/8" 100 ft lbs 1/2" 250 ft lbs 3/4" 800 ft lbs 1" 2000 ft lbs

That pretty much ties in with the cube of the size. However these are going to be made of high tensile steel with a tensile strength of (I'm guessing) around 60 or 70 tons per square inch. At least twice that of mild steel.

On that basis you'll be lucky to get 400 ft lbs out of a 3/4" mild steel square bar. If the professional wrenches have enough safety margin on their design specs then maybe you'll just reach 500 but it'll be touch and go. With something a bit higher tensile I think you'd be in business. EN8 might just do the job.

Dave.

Sounds like we are up the same street, but you'll have to use the yield stress (or proof stress) rather than the ultimate tensile stress when comparing steels in this use. I have no books at hand, but i think mild yields at about 12-15 tsi in tension and finest piano wire at 100 to

120 tsi (which is heat treated high carbon steel).

The really good stuff in a tool set should surely be up in the 60-90tsi range, and I reckon annealed EN8 maybe half this.

The reason the tensile strength for mild steel is not much use is that it stretches like putty before reaching it. A square shank will become round very easily with a lot of ductlility.

Steve

Allowable twisting moment (T) = Ss x Zp.

Ss is the allowable torsional shearing stress for which they quote

8500 psi for small short shafts and although they don't state the material I am sure that they are referring to mild steel (or "best laundry iron" if you are a Frazer Nash man)

Zp is the polar section modulus: for a square section = 0.208 x a^3 where a is the length of each side.

That makes the result 8500 x 0.208 x 0.75^3 = 746 lb.in i.e. 62 lb.ft

Unless the 8500 psi figure has a safety facor of at least 8 your bit of metal will go spong.

Bob

There's a chart here.

On that basis if EN24 (817M40) is somewhere close to the sort of steel tools are made of then either mild or EN8 are not going to do the job. Best bet is going to be to buy a bloody big high tensile cap head screw and machine it into a 3/4" square bar.

Thats a handy chart Dave, but a bit short of different species - we ought to have a go at gathering this sort of stuff together on a website.

If you look at the 080M40 (0.4% carbon) it shows it in normalised and heat treated forms. Not much difference in the tensile strength (550 to

660), but the yield stress goes up from 280 normalised to 530 heat treated. Mild steel yield stress is 215, 0.4% carbon is 280 - not much improvement ! Shame the chart doesn't have the same figures for 0.8% plain carbon steel (I would guess 350 to 370 - though I am lost with these metric units).

Once these higher carbon steels are heat treated then they have a much higher yield stress - you can see circa 1000 MPa or more. Thats what a decent chunk of steel in a socket set should give. But by then it will be pretty hard to machine. However if you have a chunk of 0.8% carbon steel stock waiting for action, the chances are its annealed - so it will not be more than double the (yield) strength of mild steel. And once it has yielded its suffering permanent deformation - just like when a bolt yields - we all know the horrible feeling and what happens next !

Time for bed. Hope this helps, Tim (or have I just sown seeds of confusion) ? Steve

Been having another think about it. Easiest way to test what a stick of 3/4" mild would cope with is try a bit of 1/2" and apply the calculations. 3/4" will take 3.375 times as much torque.

That means 1/2" square mild would have to go nearly 150 ft lbs if you want to reach 500 with 3/4". That puts it all into better perspective and it ain't gonna happen I'm afraid. 3/4" tooling is a bit out of my normal line of work but I know what 1/2" will take and 150 ft lbs is a hell of a heave even for proper equipment. Maybe the best equipment will reach 250 ft lbs at that size but that'll be hardened and tempered impact quality gear. My own decent chrome moly stuff will do 150 easily enough and maybe a good bit more but you start bending socket bars when you head into that territory.

Gut feel is that 1/2" mild would go maybe 60 or 70 ft lbs and therefore 3/4" about 200.

I think I've got some 1/2" square mild somewhere. If I clamp a big nut onto the mill and put a socket on that I can test the 1/2" bar between the socket and the torque wrench. Be interesting to see what happens.

Thought of an easier way. Clamp the 1/2" bar in the 4 jaw chuck, lock the lathe backgear up and use the torque wrench on the bar until it gives. 5 minute job if I can find some bar.

Done that. More fun than sleeping. Found a 2 foot length of something old rusty and 1/2" square. No idea what it is and being colourblind doesn't help but the paint on one end looked light green to me (EN1A). I can't imagine it's anything else though because all my bits of scrap nicked from my mates scrap bin are mild steel.

Well all our calcs and guesstimates can go straight out of the window because full scale on my torque wrench (110 ft lbs) didn't faze it in the slightest. I even wound it off scale to the dead stop, maybe 120 ft lbs, which is not good for torque wrenches, but the bar wouldn't budge. In fact it didn't feel like it was anywhere near giving. There was no twist taking place that I could feel and the torque wrench just gave a solid click each time. I have no way of putting any more load on it because my breaker bars have a 1/2" stub on them already and also I'd rather not strip the teeth off the lathe gears. Only the torque wrench has a 1/2" hole and a removeable bit of bar. Next step, purely for interest's sake, might be to clamp a length of it to the bed of the mill and try again with a bigger torque wrench if I can borrow one.

On that basis I revert to my first post. 3/4" mild could well just do the job and anything a bit stronger ought to manage fairly comfortably. Looks like the chrome moly professional gear has a hell of a safety margin built in if mild steel can take that much abuse.

So the book says 3/4" mild is only good for 62 ft lbs eh? Hmmmmm.

Tim,

With your big TOS lathe etc you are bound to have some old and big

3/4" square lathe tools lying arround which will be something a bit tougher than mild steel.Out with the angle grinder and chop a bit off the end of one.

AWEM

Me neither, which is part of the reason Im not sure, but lb/sq inch x inch^3 does imply lb ins

789lb in doesnt sound like a lot, however the book is about safe, and longterm machine desing, not a do the job for 5 mins at the edge of its envelope...

Dave

A torque multiplier is basically a reduction gearbox, in this case

5:1, which allows the use of ordinary-sized wrenches etc for a job like this. Unfortunately mine isn't perfectly suited for this job & it was suffering damage, so I've had to revert to the simple bl**dy big spanner method.

I've got spare 1/2" extensions which could be butchered for a special job, but unfortunately no spare 3/4".

Torque multipliers are generally more expensive than even good torque wrenches, which is why mine is one that I found at a reasonable price rather than the perfect type for this job. Next time I'll put some planning into adapting the reaction arm before starting the job.

Cheers Tim

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I'll certainly be looking in that box

I was in a tool place yesterday where there was a box of BIG new brazed tip lathe tools on the floor, various shapes, £1 a piece. More like 2" shank, though!

Cheers Tim

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I'm going to see what material I can find in a few minutes, I'm tempted to make something up from MS as well as something better, just so I can see whether the MS works!

Thanks to all for suggestions, I'll report back later.

Cheers Tim

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