Lock nuts / Jam nuts - MEW 311

Ah! That's got the hornets going again... To address the earlier point (which appears to have been removed) if you put the thin nut on first, it will strip when you dissemble the joint. If you consider what happens, this is clearly not the case.

I.e. install the (thin) lock nut first and tighten to, say, 25% torque. This gives roughly 25% preload. Add the main nut & torque fully. This unloads the thin nut (and actually reverses the load that it sees, providing the locking function) and gives full preload to the assembly.

If the main nut is removed, it goes back to a thin nut applying the 25% preload: it cannot retain the full preload because that was introduced by the (now removed) full nut.

We do seem to have moved into two spheres here: the "technically correct" thin nut on first, and the "historically or aesthetically correct" thin nut last!

One point that might have been missed, asI found from the ESDU Data Sheet that I referenced, is that this all applies to bolts that are predominantly loaded in shear. For those in tension, a lock nut is not required (have you seen many on a car engine?).

And lastly, as others have pointed out, lock nuts are not an acceptable locking mechanism for safety critical applications anyhow. So is the order really that important?

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That’s “interesting” … considering that it is generally bad practice to load a bolt in shear

MichaelG.

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P.S. __ The vey long nuts on my bridge example are clearly required [and presumably ‘engineered’] to apply the requisite load … and it is most likely that the thin lock-nuts are there to discourage tampering by person or persons unknown. 

Edited By Michael Gilligan on 07/01/2022 14:01:09

Why interesting, Michael? A modern steel building is held together by shear bolts, and the average aeroplane is peppered with them. I agree that applying shear through the threads is a no-no, but they bolt shank is fine.

If used in steel buildings the bolts are likely to be high strength friction grip where the shear is taken by the friction between the 2 clamped surfaces.

Yep, like I said t'is a ripe ol' chestnut.

DrDave says:

• ' A modern steel building is held together by shear bolts' and

AStroud says

• 'If used in steel buildings the bolts are likely to be high strength friction grip where the shear is taken by the friction between the 2 clamped surfaces.'

I think they're both right because it depends on the circumstances. Deciding what matters is surprisingly complicated when you don't already know the answer.

One of the things I like about engineering is it's full of hidden depths. Even simple nuts and bolts get the old grey cells going. It's a good thing most jobs aren't critical and I can just whack a fastener of about the right size on and do it up tightish!

Dave

As I said before this all depends on the length (stretchyness) of the bolt and the clearance in the thin nut. Most unlikely to give the required preload.

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"interesting" was deliberately within quotation marks [for the sake of a little sarcasm]

Yes ... proper 'fitted bolts' are O.K. in shear, provided that they are effectively 'dowel pins'

But any bolts used in clearance holes should be considered a clamping device, and the side-loads should be taken by friction between the clamped surfaces.

MichaelG.

In proper engineering, "standard" bolts and studs go through clearance holes and do not resist shear, unless something moves to take up the clearance. Resisting shear is meant to be done by friction (produced by the clamping force) at the interface. The thought of a portal frame structure squirming under varying load as the connections shift to and fro is quite scary. And yes I do know about flexure under load and wind conditions, but that flexure is accommodated by stress, and consequent strain, in the members, not in movement at the joints. The only bolts that are intended to resist shear are "fitted" bolts that are the full diameter and in reamed holes.

Oops. My “steel buildings” was a bad choice…. I did the analysis of a couple of lifting frames last year. Bolted together with clearance holes. By reference to Shigley’s book, we have assumed that the holes yield until all the bolts are active at ultimate load. But we digress.

Duncan, I see your point. Tightening the top nut gives a preload on the lock nut, which in turn transmits part of this to the bolt shank. This suggests that you should only use lock nuts where the preload is unimportant.

Hi, I'm with AStroud here as far as steel structures are concerned, friction grip bolts is what I was taught during my fabrication and welding technicians course.

As far as bolts that are just in tension not needing a locknut, the four bolts holding the spring steel laths between the motor plate and the tube on these vibrating conveyors, always had to have two nuts of the same thickness, the bottom nut was torqued up to full load and the top nut was then locked on, but both bottom nuts on each block had to be fully tightened before the locknuts were put on and this was the only way to be sure they would not come loose, in fact the motor plate and or the laths would fail. Only knew one failure of the bolts due to someone who decided to use a pair of Nyloc nuts, one on each bolt, which came slightly loose, which made two or three of the laths and the spacers to rattle enough to cause enough heat to spot weld them together in several places and the noise it produced was horrendous and I even had to cut the bolts off to replace all the laths and two new bolts, a single nut on the bolts would not stay tight.

Regards Nick.

Edited By Nicholas Farr on 07/01/2022 20:27:57