Does it make any difference in joint tightness if the tightening torque is applied to the bolt or the nut?
This edition of the Fastener Training Minute with Carmen Vertullo was originally published July 28, 2017 as “How to Torque Bolted Joints” during episode 118 of Fully Threaded Radio.
I had a question recently which has actually come up many times in the past and its one that’s often put to suppliers of hardware nuts and bolts and the question is this: does it make any difference in joint tightness if the tightening torque is applied to the bolt or the nut in a connection which has a bolt and a nut? Well intuitively it would seem like it does not matter, is that true or not that is the question.
For the answer for that to this question we first have to think about what happens to the energy or torque that goes into the bolted joint in terms of how it applies to creating tension in the joint. Well as we know or we may know, most of that energy goes into overcoming friction. That friction occurs in two places, in the threads and under the bearing surface of the turned component. So if that turned component is the bolt then it would be under the head of the bolt, and if that turned component is the nut it would be under the nut. So the question is; is there a difference in friction under the head of the bolt or under the bearing surface of the nut?
And the answer is: for the standard connection there is no difference. In other words the bearing surface of the nut is about exactly the same as what would be the washer surface or the bearing surface of the bolt. This has actually been proven by experimentation, I’ve done the experiment myself to prove it and the Industrial Fasteners Institute has done the experiment (the I.F.I.). So if you ever need an authoritatively citation for this, the Industrial Fasteners Institute has a White paper called “Tightening the Bolt or the Nut makes no difference”, and you can get that from their website or you can contact me at firstname.lastname@example.org and I will get it for you.
Now obviously this is a generality, or a generalization. Could there be a case where maybe this does make a difference if we tighten the bolt or the nut and should we care?
If we were to change the nut, let’s say from a standard hex nut to a flanged nut, that could make a difference. If we did not have the same exact surface against which the bolt head is being tightened down and the nut is been tightened down. In other words, a hardened washer. If they weren’t the same, that could make a difference. At the end of the day though, one of the things we do in assembly technology as engineers is to eliminate variables. So if we can, let’s do it the same way every time, and by basic practice I recommend whenever we can, we tighten down the nut. We hold the head of the bolt and we tighten down the nut. That way we do it the same way every time, and if we ever have a problem, thats one of the variables we don’t need to consider.
All of this can be done easily by the user as well in terms of experimentation just with a torque wrench and a Skidmore-Wilhelm bolt tension calibrator. I recommend all fastener suppliers have that equipment.
Well, tightening the bolt or the nut makes no difference as it turns out.