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Posted by Steve on August 11, 2006, 5:07 am
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BobKellock wrote:
> 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
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
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Posted by Tim Leech on August 11, 2006, 5:37 am
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wrote:
>
>BobKellock wrote:
>> 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
>
>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.
>
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
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Posted by Tim Leech on August 11, 2006, 4:12 pm
Please log in for more thread options 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.
http://www.kelvindiesels.co.uk/Model_TA6.htm
Thanks
Tim
Dutton Dry-Dock
Traditional & Modern canal craft repairs
Vintage diesel engine service
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Posted by John Blakeley on August 11, 2006, 5:00 pm
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> Incidentally, this is (more or less) the engine in question. Installed
> in 1964.
>
> http://www.kelvindiesels.co.uk/Model_TA6.htm
>
>
Big bugger then - I thought the straight six 3 litre in my Merc was on
the large side.
John
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Posted by Steve on August 11, 2006, 5:14 pm
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Tim Leech wrote:
> Having just returned from the job,
>
> 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.
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
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>
> Unless the 8500 psi figure has a safety facor of at least 8 your bit
> of metal will go spong.
>
> Bob