I would have worded it in reverse to make the point more clear. The
purpose of the torque is to get the clamping force correct. It is
correct that that should also stretch the bolt the correct amount in
order to meet the primary goal, the clamping force. How much the bolt
stretches, or whether it goes into yield or even fails, is determined
by the size of the bolt. So first you pick the force, then you use
that to pick the bolt size. And that is why you see eight sizes of
bolt on the same lawnmower - it's not really to make you buy eight
different wrenches, that's just a side benefit.
You also alluded to the difficulty of getting the accuracy. From what
I've seen, nobody with a torque wrench does any better than 50% plus
or minus his desired amount of torque, so which end you put the wrong
torque on is probably moot.
re: So first you pick the force, then you use that to pick the bolt
Unless you have no choice as to the bolt size. Earlier I mentioned
that we are forced to single-source the bolt. Taking it to the next
level, we are required to purchase a *specific* bolt from that single
Once we have the parts, the torque used to install them is up to us.
If I could purchase bolts from any source I wanted, things might be
different from a torque value perspective, but the original question
would still stand - nut v. bolt.
re: nobody with a torque wrench does any better than 50% plus or minus
his desired amount of torque
Please elaborate. Let's say I'm using a click wrench set to 200 in-
pounds. Are you saying my actual torque is anywhere from 100 to 300?
I'll assume that's not what you're saying since my assembly will
either be flopping around (relatively speaking) at 100 or the bolt
would have snapped well below 300.
Yes, but how do you decide how much torque? Ideally, you know the
clamping force you want between the parts. Then you apply just enough
torque to get the clamping force. If you are forced to accept one
particular bolt, you run the risk that to get sufficient clamping
force you have to overtorque the bolt, risking failure, or undertorque
it, risking loosening in use. Do the calculation. If case 1, forget
it. If case 2, better use loctite, a very good lockwasher setup,
etc. Torque is there for a reason.
That seems strange to me too. But apparently that's what the studies
show. Also the torque can fail to have any correlation to the
By the way, torque is usually specified dry or wet. It's been decades
since I put a fastener on without lube, so I always look for wet. I
don't even like to put in a wood screw dry! <g>
And if I've got any of this wrong please correct me. Engineering
school was a .......long..........time ago and I may be
re: Yes, but how do you decide how much torque?
Trial and error over many years of experimenting. To prevent movement,
we go with the max before the bolt will fail, based on past
experience. In the other application, where we went to allow for
movement, that number might vary from installation to installation
based on how "loose" we want it - it's a case by case thing.
re: studies show...(the +/- 50% error)
Nothing personal, but I can't accept that without some substantiating
citations. If I were truly getting anywhere from 100 to 300 when my
click wrench says 200, then there is no way we could have zeroed in on
certain specific numbers that prove to work best time after time after
time. Why would we see bolts fail at a fairly consistant number that
is well below the upper range of a 50% error? I'll bet I could
consistantly snap the bolts at less than 10% above what we use as our
max number. Finally, the torque values we use work for our application
- and the bolt failure points are consistant - across multiple torque
wrenches from various manufacturers.
(I thought you were having some sort of "problem".)
Generally, I think, a condition of "over torque" might be expected to
produce a satisfactory result in many if not most "normal" instances.
You're using bolts of inferior grade for your application?
Whatever you do, don't trot out to the bolts acting all crazy and
confirm your assumption.
I'm no engineer, but 10% seems to me as if you may be using the wrong
grade bolt... depending, of course, on your application, which, in
this case, could scarcely be more vague..
Similar errors of tolerance might be expected to produce similar
I have only the slightest of ideas of what you are expecting of a
bolt, but my gut feeling is it's too much, and, to answer the original
question, again, from which end you torque it isn't going to make any
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