Re: Shear strength of 3/4" square steel?


"Dave Baker" <DaveBaker@xxxxxxxxxxx> wrote in message
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"Tim Leech" <duttondock@xxxxxxxxxxxxxxxxxx> wrote in message
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I've got to do the final tightening of some cylinder heads tomorrow.
I've been using a torque multiplier but for some of the bolts there's
no good way to secure the reaction arm & it's been getting damaged.
(it's the type designed for waggon wheel nuts which uses a second
socket on another nut)
My big torque wrench has lost the 3/4" square shaft which connects it
to the sockets, they're a special order item so I can't just buy one
off the shelf tomorrow.
If I square up a piece of mild steel bar, will it take the 500 lb-ft
torque, or just twist off? I could find a bit of stainless or EN8 if
that would do better.
I would just try it & see if the job was here, but it's 30 miles away,
it must be finished tomorrow, & I'd struggle to find something here to
test it on. OK, I could weld a big nut to the side of a boat on the
dry-dock and use that, but we've just finished painting it and the
owner is coming to collect tomorrow!

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.

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.
--
Dave Baker
www.pumaracing.co.uk
Usenet - a collection of people who only open their mouth to change feet.


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