OT - Torque the Nut or the Bolt?

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This question came up while working on a project this weekend and I'm looking for some input.
Let's say I've got a nut and bolt holding something together and I want it torqued at XXX inch-pounds. I can hold the nut with a standard wrench and use the torque wrench on the bolt, or I can hold the bolt with a standard wrench and use the torque wrench on the nut.
One of the guys I was working with said "When given the chance, you always torque the nut." When I asked why the answer was "Because that's what I've always been told." You know how *those* conversations go!
So what's the deal? Does it matter? And if so, why? Could it be different in different cases? And if so, why?
Thanks!
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It does not matter. The braking effect of the underside of the nut is equivalent to the braking effect of the underside of the bolt head.
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nut is equivalent to the braking effect of the underside of the bolt head
Even if the material on the underside of the nut is not equivalent to the material on the underside of the bolt head?
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In such a case the difference would still likely be minimal. It's not like you're going to use an aluminum nut with steel bolt.
Torque specs vary on bolt size, bolt strength, application (permanent, non-permanent, etc.) and lubrication or lack thereof. They don't vary on which half of the fastener is experiencing sliding friction.
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mike wrote: ...

Correct, so _if_ one portion of the fastener (namely the bolt) _does_ have a friction load, the net torque applied will be low as compared to the spec.
For most relatively small bolts and applications, the effect is probably minimal, granted, but the general principle holds.
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No the torque spec will not be low. Both the nut and the bolt would have frictional resistance on the underside if turned. See the picture in this thread so you can see the symmetry of a bolted joint. The placement of the threads doesn't affect the frictional symmetry.
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mike wrote:

There can be friction along the _LENGTH_ of the bolt which must be turned which, if present, is _NOT_ symmetric as turning the nut doesn't require turning the bolt against that load.
--
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Anyone that has an engineering background knows you are correct. However, I doubt that you will ever get that through to Mike.
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DerbyDad03 wrote: ...

Then you get to set the torque specs... :)
--
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DerbyDad03 wrote: ...

Because he didn't read the original question??? :)

Choose the nut if you can. Again, if there's no side load on the bolt (that is, it turns freely in the hole) there will be virtually no difference. _IF_ (again, the proverbial "big if") there is a resistance on the bolt, whatever that torque that resistance contributes is will contribute to the reading of the torque wrench so the actual torque applied to the fastener will be low by that amount.
In normal situations, again, as noted, this will be small, but it's more consistent to stay w/ the nut end.
--
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SteveB wrote:

These are discussion groups. You make a new post and a discussion is started. If your original post asks a question then (if you're lucky) you will get some answers somewhere in the thread. However; having the discussion branch off in other directions is part of the package. There is no requirement that the conversation stick to your original premise.
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There is in a perfect world.
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mike wrote: ...

As long as there is no a significant side load, yes...if there is, then it _is_ different.
--
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If there is significant side load on your bolt from the holes in the plate stack, then how'd you get the bolt in the hole in the first place?
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mike wrote:

Punch, usually...you never have anything difficult to line up? If not, you've never worked on much...
--
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Rediculous. That's a great way to destroy the threads on a bolt.
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On Wed, 2 Jul 2008 08:16:44 -0700 (PDT), mike

Mike,
Get your head out of your butt. "Punch" is a noun not a verb. You use the punch to line up the holes before inserting the bolt.
Sit back and just read. You could learn a lot.
G.S.
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If the hole is lined up with taper rods, then the bolt drops in and there is no "side load".
From Bolt Science.com:
"Typically only 10% to 15% of the overall torque is actually used to tighten the bolt, the rest is used to overcome friction in the threads and on the contact face that is being rotated (nut face or bolt head). "
Notice that it doesn't say anything about the side of the bolt being squeezed by the hole. It seems that you're the one with your head up you butt.
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mike wrote: ...

Notice it says "typically"...
Notice I have continually used "if" for the cases where it is/can be a problem and have also continually pointed out it isn't significant where there isn't a problem.
In addition to the misalignment problem, there are also assemblies where the bolt may, in fact, be designed to carry a load (as in a hanger, for example).
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I'm sorry, but you can't have it both ways. If you can install the bolt, then there is no squeezing effect. If there is a squeezing effect during installation, then you cannot install the bolt.
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