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Nut screws washer and bolts

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Howard Lewis25/10/2018 22:25:00
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In my Industrial experience, most lock washers were pretty ineffective. Prevailing torque nuts, such as Nyloc, were pretty good, but only really reliable for single use. "Nylon" inserts in threads were pretty good, as were anaerobics.

Tightening to yield creates so much friction in the thread that the fastener does not slacken (!/2 UNF in W range, tightened to yield, produces a tensile load of 9 tons, and a permanent extension of 0.002 - 0.004". I've never heard of one of those cylinder head bolts coming loose!)

For the ultimate in retention, wire locking, a tab washer, or a split pin through a castellated nut, are very effective, if properly done. Rolls Royce tab washers were Stainless DDQ!

Howard

Hopper25/10/2018 22:44:59
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Posted by Sam Longley 1 on 25/10/2018 09:38:30:

Question:-

If one is using a pair of nuts as a locking nut & one of the nuts is a half nut which nut goes on first?

Does the half nut go on first with the full nut on last or is it the other way round?

I have a small model which will have that & this thread has me wondering. I did read about it years ago ( I think that it was in RCME mag) but cannot recall the correct answer.

Edited By Sam Longley 1 on 25/10/2018 09:46:52

This is a bit of a vexed question. In recent years there has been an ISO standard published, backed by science, that says the half nut should go on first and the full nut on top. This is because as you tighten the top nut, it takes all the strain, the half-nut beneath it becoming in effect a washer between the top nut and the casting below.

HOWEVER, the common practice up until very recent times was to put the full nut on first with the half-nut on top to "lock" it. I still have my tech. drawing books from when I studied marine engineering in the 1970s and it shows in several examples the half nut on top in the old manner. This book was a prep guide for the MOT marine engineer's examinations for chief engineers in charge of sea-going vessels so they took such details very seriously. (EG you could fail the exam if your drawing did not include a split pin where required.) Other old books I have concur with this old method.

I read somewhere that the reason the thick nut went on first and the thin nut second was simply because a standard spanner back in the day was wider than a standard nut so could not be fitted on to the thin nut once the thicker nut had been screwed on above it. Thus there was no way to hold one nut still while tightening the other. Makes sense but the source is apocryphal.

So I guess the situation is if you want best modern practice with the most chance of stopping the nuts coming undone, put the thin nut on first and the thick nut second, and use a thin, or ground down, spanner to hold the thin nut while nipping up the thick one.

But if you want your 19th or early 20th century model engine to look authentic, put the thick nut on first wiith the thin nut second.

Kiwi Bloke26/10/2018 08:55:12
461 forum posts
1 photos

Ian, it was your suggestion earlier that the lock-nut order debate shouldn't be re-ignited, but then you did...

As Hopper says, it is (now) generally accepted that the half nut goes on first. The reasoning is only a few clicks away, should you choose to search out the science, rather than regurgitate dogma. The locking nuts to which you refer, sometimes known as Palnuts, are obviously much more elastic than conventional half-nuts, and compress and deform when tightened, rather than causing significant elongation of the stud. The deformation under compression 'bites' into the stud thread, thereby directly increasing nut-to-stud friction. They therefore work very differently from the conventional nuts being asked about. Stud elongation, resulting from tightening the upper (conventional) nut, unloads the lower nut, transferring stress to the upper nut, hence the upper nut should be the stronger one. When a Palnut is used, the lower nut remains the fully-loaded one.

Apart from any other considerations, if, as you suggest, you were to use a conventional nut on top, torqued 'just tight', it would tend to loosen more easily than a fully-torqued, non-locked, single nut, thereby defeating its intended purpose.

This whole subject is far more complicated than it seems at first sight, and it shouldn't be assumed that industrial practice is optimal at all times. I started the thread in the hope that it would get people to think.

Thread-locking compounds are probably the answer... ; )

Edited By Kiwi Bloke 1 on 26/10/2018 08:56:49

Ian S C26/10/2018 10:00:53
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With the thin nut on the bottom, and the standard nut tightened down on top, the bottom nut becomes a washer once the load is taken by the top nut. The item in ME had the stress diagrams etc, and the modern science, rather than the 19th century science repeated in 20th century text books.

One reason that our metal work teacher gave for putting the thick nut on the bottom was that you could never find a thin spanner to tighten and hold the thin bottom nut, he trained as an engineer in the UK before WW2. AN military specifications say that the AN-315(thick nut)shall be placed on first, the AN-316 check nut is then placed on top, the AN-316 L and R nuts are also used for locking turnbuckles.

Ian S C

Edited By Ian S C on 26/10/2018 11:15:56

Michael Gilligan26/10/2018 10:05:10
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16358 forum posts
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Posted by Kiwi Bloke 1 on 26/10/2018 08:55:12:

... I started the thread in the hope that it would get people to think.

Thread-locking compounds are probably the answer... ; )

.

I did think ... and remembered that, in the vibration test-house we never used, or recommended the use of, any form of spring lock-washer.

[the exception that proves the rule being the Earthing point which someone referenced earlier]

MichaelG.

Tim Stevens26/10/2018 11:55:40
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1268 forum posts

Ian - the thin lock nut does not 'become a washer' as you suggest, it should be locked by tightening against the thicker outer nut, so that in the event of the system developing slackness (no load on the bolt/stud itself) the two nuts remain in place.

The 'correct' placing of the thinner nut was a concern from the beginning of last century at least. I quote from the ICS Reference Library Textbook on Machine Design (vol 5) 1907:

A common device is the locknut ... Two nuts are used, one of which is about two thirds the thickness of the ordinary nut. The load comes on the outer nut, which should therefore be the thicker one. In practice, however, the thin nut is usually placed on the outside, because the spanner is generally too thick to act on it when placed below the other.

So there, great-grandad was right all along ... And of course, spanners are now made of alloy steel and can be much thinner, removing any excuse for not doing it right.

Tim

Hopper26/10/2018 12:21:01
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This document outlines current science in a plain and interesting way. The two graphs at the bottom show results from a Junkers vibration tester and seem pretty conclusive. The do make the proviso that the thin-nut-first method while locking better, requires more time consuming and more skilled assembly. **LINK**

Yes, methinks Loctite etc is the truly modern way to go, so all this is a bit academic. I can't think of anywhere I've actually seen double nuts used except on vintage machinery, where period-correct assembly is the "wrong" way round with thin nut on last.

Edited By Hopper on 26/10/2018 12:33:37

David T26/10/2018 14:16:12
74 forum posts
14 photos
Posted by Nigel Bennett on 24/10/2018 14:31:30:

Possibly David T's application wasn't suited to the way they work - the hardness (or otherwise) of the mating materials can upset them. Did you try approaching Nord-Lock themselves for advice, David?

Hardness may well have something to do with it. Our application includes securing a crimp to maintain electrical continuity, so a relatively soft material is involved. I don't know if Nordlock were ever contacted to begin with as they were introduced before I moved into my current job.

Ian S C27/10/2018 11:53:08
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In one artical I read that the thin nut on the bottom should be softer than the full nut.

In one of my books the method of assembly is, tighten down the 1/2 nut and hold with a spanner, tighten down the full nut, then hold the full nut and back off the thin nut against the full nut

Isn't Loctite great.

Ian S C

Eddy Curr27/10/2018 22:46:46
39 forum posts

A Thank You and some comments.

That 1/2 height Jam Nuts function better underneath full height Hex Nuts than on top comes as news to me - I have never seen an OEM configuration where the Jam Nut wasn't on the outboard side of the thread. However, I have since seen some authoritative reports confirming that an inboard placement is superior. Thank you for drawing this to my attention.

On my side of the pond, there are names for thin wrenches/spanners intended for use on Jam Nuts. Stanley Proto offers the following:

- Check Nut Wrench: Single, open end wrench with head set at a 15° angle

- Service Wrench: Single open end with head set at a 30° angle

- Tappet Wrench: Double open end with heads set at a 15° angle

Regarding Pal Nuts mentioned earlier. The unknowing should be made aware that the formed from stamped sheet metal Pal Nut is physically different from the machined from solid bar stock Jam Nut.

Regarding Nylok Nuts. The nylon locking component is not suitable for use in elevated temperatures - one example stipulates a ceiling of 140°C (284°F) in ideal conditions. There are specials rated for higher temperatures, but the improvement is insufficient for many applications.

An alternate form of self-locking nut that is suitable for much higher temperature service is the Stover Nut. The outboard side of a Stover Nut is intentionally deformed after threading, this creates an interference fit between the nut's deformed threads and those of a bolt or stud. Stover Nuts are reusable.

Lastly, an reference source for fasteners.

"Nuts, Bolts, Fasteners and Plumbing Handbook" Smith, Carroll

Most recent edition: http://carrollsmith.com/books/nutsbolts.html

Bill Pudney28/10/2018 01:21:31
464 forum posts
16 photos

Where I worked in the 80s at a company making electronics for the defence industry, company standards insisted on using wavy washers, under every fastener, bear in mind these were all small fasteners under about M5. There had long been a feeling that wavy washers were a waste of time, but the standards stood. Then I saw an electronics box, full of PCBs and other stuff, all held together with an approved fastener scheme on a shake table. It only took a minute or so for the screws to start to release, and about 3 or 4 minutes for most of the screws to fall out.

We still had to use wavy washers though, but on final assembly we started using Loctite.

I will continue to put full size nuts on first, and locknuts last.

cheers

Bill

Alan Waddington 228/10/2018 08:15:17
507 forum posts
87 photos

30 years ago i had a short stint selling nuts and bolts, around that time Unbrako brought out a product called Durlok, which were nuts and bolts with a serrated flange built in.

They worked really well, i sold loads to local stone crushing plants that had battled for years with bolts coming loose. Also picked up a large order from a bus manufacturer who used them for bolting in seats, which had previously worked loose over time when fitted with all manner of traditional fasteners.

They were a brilliant and simple idea, however the cost was eye watering, which is probably why they never took off in the mainstream.

Howard Lewis30/10/2018 17:05:00
3605 forum posts
2 photos

I remember Durlock setcrews and bolts. They were extremely difficult to undo, The "ratchet" serrations under the head really worked. Once tightened, you never needed to worry about them slackening.

Howard

Bazyle30/10/2018 18:24:17
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The airborne electronics I made used wavy washers typically on M2.5 screws and they survived the random vibrator that was driven by its own 1 megawatt generator for a 1 hour test. I can't remember for certain but I think we used Loctite on the satellite flight hardware. An Arianne rocket vibrates harder than a Harrier during take off but only does it once and it is kind of hard to check if any of the screws came loose.

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