Nut screws washer and bolts

No, not the apocryphal headline following a madman's assault in a laundry and subsequent flight, but a request for enlightenment.

Simple question, but I suspect the answer isn't. Why are shakeproof washers used under nuts, but are rarely seen under bolt (or screw) heads?

More puzzles. 'Keps' nuts or nuts with external star washers clearly 'lock' the nut to the substrate, but, once loosening starts, achieve virtually nothing. Spring washers also 'lock' the nut to the substrate, but also load the thread, increasing friction, should loosening occur. This seems a better idea, so why use K-nuts? And why not add some anti-loosening device to the bolt head too?

I'm asking this because we tend to take familiar things for granted, and tend to accept things as 'normal practice', without thinking. I've just been working on some machinery which used many K-nuts and 'cone nuts' (an all-metal nut, similar in principle to a nylock nut), but no spring washers. None of the mating screw heads had any washers. Also, on other machinery, some plain nuts on the end of bolts were used with no washers, others had plain washers, and some had spring washers. There seemed to be no pattern to this. Nylock, Philidas, stiffnuts, etc. I understand - I think.

Where can I find the answers?

My theory is that washers were invented as a cosmetic statement to spawn a whole new fashion industry 'cos someone was too mean to buy a longer spanner. Then along came the nyloc, but washer buffs (like i-phone owners) stay set in their ways of paying for jobs.

pgk

Lock washers are not placed under the head of a bolt in a nut and bolt assembly because the shank of the bolt provides additional friction.
Spilt / spring lock washers do add tension as well as friction but put an off-set load on the nut and thread which can be a fatigue issue in high load applications. Star washers are virtually useless in high load applications. They flatten and are just high friction. Both types damage protective coatings /surfaces. There are "Wave" type spring washers that are better with small fasteners. and don't damage the surface.

Critical nut and bolt designs lock the nut to the bolt, either directly or both to the substrate. Really critical ones like aircraft controls have thread locking and a secondary means of preventing the joint coming apart.

Google aircraft thread locking.

Robert.

I don't even ask the question, unless there is a good engineering answer why not, if I want a nut and bolt that won't shake loose, I just go for nyloc every time.

Have a look at Nord Lock Washers Here I have used these on my IC engines. Mike.

We’ve trialled those at work in lieu of a Nylock nut, but without huge success. I suspect that may be due to our application though as it is a little........ perculiar. I find Nylocks to be quite reliable though.

'Lock washers are not placed under the head of a bolt in a nut and bolt assembly because the shank of the bolt provides additional friction.'

I find that hard to believe if the bolt is in a clearance-diameter hole. Apart from that, I'm heartened to see that Robert Atkinson 2 shares the same ideas that I do.

Should serrated flange nuts be regarded as similar to nuts + star washers? They're popular these days.

I still suspect that common assembly practice owes rather a lot to dogma and questionable beliefs...

On a nut and bolt assembly the lock washer is usually placed under the nut, the bolt is held in place, and the nut turned to tight and the washer bites in, but if for some reason the nut can't be turned, the washer is placed under the bolt head, the nut is held in place while the bolt is turned to tight. You could probably start something like the argument about lock nuts, and which goes on top and bottom, thick or thin-----Don't go there please.

Ian S C

Having worked on life time critical designs in the past and seeing all the calculations required to just prove a fixing method, I think the best is to do it the way you are comfortable with. There are numerous documents out there that prove by calculation and test how things should be fitted. Working in a model environment we don't have any safety critical fixings, except perhaps for those things attached to pressure vessels. I have attached a link which some may find interesting.

**LINK**

Another +1 for Nord-locks. We had a chappie in some years ago at work to demo them. He had a little vibration rig, to which he affixed a correctly torqued-up M8 nut and bolt. It loosened within seconds. So did a pair of locknuts. And a bent-beam nut, and a Nyloc. The nut & bolt fitted with Nord-Locks just sat there and took it. Since then we've used them on all sorts of critical applications with great success, provided that you fit them correctly - IN PAIRS with the coarser serrations facing each other. (We had a complaint once from a client that screws kept coming loose - despite the instructions, the twits had used just one Nord-Lock per nut and thrown the "spare" ones away...)

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?

Wonder if a split common lock washer on both ends of the bolt would offer any advantage?

The discussion thus far completely ignores the situation with studs where there are of course no bolt heads. In the past, lock nuts onto washers were used with 'legs' one of which could be bent onto the component with the opposite leg bent back over the nut.

Or for wheel bearings the situation differs again with a wide plain washer, bent one way onto the nut that sets the preload and back the other way to lock it against the lock nut of the assembly. Then of course there are aircraft wired nuts, it is not all about spring washers and nylocs

Brian

I want to know who decided the roof rails on a VW Passat should be attached without any vibration proofing. Just nuts, bolts,and plain washers. We stopped on the A26 in France to try and find what was rattling round annoyingly in the roof load. Then somebody pulled on the roof rail. AARRGGHH. FIxed using a Leatherman and gaffer tape. Fortunately, a hire car on the company account, specfically ordered to take a roof load, so no bill to me for the proper repair..

I was told by my grandfather that the use of plain washers was to reduce friction between the nut and the substrate to allow more accurate torquing of the nut/bolt assembly. I don't know about star washers, i refuse to use them. I do occasionally use lock washers on larger nuts where there is some vibration as the washer will bite into the nut and substrate and reduce the potential for loosening. I also use 'Nyloc' or similar generic locknuts in smaller sizes as i find them more convenient, although they lose their effectiveness if removed and re-fitted more than twice.

My preference for critical items is castellated or slotted nuts with split pins, or wired nuts. All the big-end bolts and main bearing bolts on my old 1949 Ford Prefect had wired nuts. They never came apart and they did not need a lot of torque to keep them in place.

I was told by my grandfather that the use of plain washers was to reduce friction between the nut and the substrate to allow more accurate torquing of the nut/bolt assembly. I don't know about star washers, i refuse to use them. I do occasionally use lock washers on larger nuts where there is some vibration as the washer will bite into the nut and substrate and reduce the potential for loosening. I also use 'Nyloc' or similar generic locknuts in smaller sizes as i find them more convenient, although they lose their effectiveness if removed and re-fitted more than twice.

My preference for critical items is castellated or slotted nuts with split pins, or wired nuts. All the big-end bolts and main bearing bolts on my old 1949 Ford Prefect had wired nuts. They never came apart and they did not need a lot of torque to keep them in place.

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

Half nut on first.

'orses for courses

Surely much depends on the materials you're bolting together?

I was also told it was friction related, but that only applies to plain washers. They might also spread the load over a larger area than the face of the nut on softer materials.

Almost all the nuts I encountered on agricultural machinery seem to have used spring washers, and I was told they should be replaced with new whenever the were undone . . . but I can't remember that ever happening.

Mike.

Kiwi Bloke 1, sorry no. the full nut on first and torqued down, the half nut is then put on to just tight. In aero engine use, mostly cylinder base nuts on air cooled engines the full nut goes on and torqued (special ring spanner required to fit torque wrench} then the lock nut, these are a special nut pressed from thin steel(about ,020" thick) is put on and pulled up tight, usually with a ring spanner as you can't get an ordinary socket on the nut.

There was a long series in Model Engineer in the 1990s, and I think the same occurred many years before, The conclusion was as above, 1/2 nut on top.

Don't use a split pin on a stud, it won't lock it, it just becomes a bolt.

Ian S C

My views are based on the articles on nuts etc in books such as Machinery's Handbook, along with years of experience. In the first place, star washers (= shakeproof) are only fit for one application - the earth (ground) connection to the painted surface of a fridge or cooker. Spring washers are little better - see comments above about side loads etc. Also, spring washers can cut into the nut and component when removed, destroying flatness and providing nasty spikes for mechanic's fingers.

But I start from the position that a properly flat surface at right angles to the thread at both ends is key - get that right and tighten the nut properly (a full-page topic on its own) and you don't need a shakeproof or a spring washer. Fine threads can also improve reliability against backing off with vibration - just look at a modern big-end bolt.

Nyloc nuts can be a much better option, except in situations like inside a crankcase where the temperature can melt the nylon and it then goes through the (often cooler) oil pump. Loctite does the same job cheaper, but is also limited by temperature problems.

A flat washer can help avoid problems with surfaces that can pick-up under heavy loads, but a much better, neater solution all round is the Belleville Washer (= cone washer). This is the only way I know to provide a degree of extra load in the event of bolt stretch or material compression (caused by over tightening or over-revving, for instance), while not being affected by high temperatures and not damaging the assembly or the washer when re-used. And they can provide a good guide to the right torque if the washer thickness and diameter are specified carefully.

Damage to fingers is also a severe problem with locking wire and with split pins. RR engineers have a way with split pins which reduces this problem, but I rarely see it used.

I hope this helps some people and adds to the knowledge of some, too.

Cheers, Tim