OT - Torque the Nut or the Bolt?

[DerbyDad03]

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!

[mike]

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.

[DerbyDad03]

re: It does not matter. =A0The braking effect of the underside of the 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?

[mike]

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.

[dpb]

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.

[mike]

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.

[dpb]

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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[dpb]

DerbyDad03 wrote: ...

Then you get to set the torque specs... :)

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[dpb]

DerbyDad03 wrote: ...

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

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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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[Rick Brandt]

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.

[SteveB]

There is in a perfect world.

[dpb]

mike wrote: ...

As long as there is no a significant side load, yes...if there is, then it _is_ different.

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[mike]

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?

[Gordon Shumway]

Anyone that has an engineering background knows you are correct. However, I doubt that you will ever get that through to Mike.

[dpb]

Punch, usually...you never have anything difficult to line up? If not, you've never worked on much...

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[mike]

Rediculous. That's a great way to destroy the threads on a bolt.

[Gordon Shumway]

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.

[dpb]

mike wrote: ...

_Drift_ punch in the hole, not on the bolt, silly...

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[mike]

If you use tapered rods in neighboring holes to line things up... then there is no side load on your bolt! It drops right in.

[dpb]

That "tapered rod" is a drift punch.

Sometimes yes, sometimes not so much.