Bolts and washers ?

[Frank Catt]

A friend of mine in the UK, a cobra replica manufacturer, has just reported than one of his customer built cars has failed the SVA (Single vehicle approval) inspection and therefore has failed to get a registration. The reason for the refusal was that the rear cover on the differential housing was assembled with both a plain and a spring washer under the bolt head ! The inspectors attitude was that the bolt could loosen because the plain washer would allow the fastener to rotate. What is your view ? Do you think the tang on the spring washer is the locking mechanism, or do you think the tension imparted by the spring locks the bolt ? Frank

[John W]

IMO it is both tension and the locking Frank.

My dad used to work for a company making parts for aircraft and always swore by putting a plain washer under the head of the bolt but a spring washer under the nut.
The plain washer was to allow the bolt head to be turned if necessary to tighten it to the correct torque.
The idea was something along the lines of you are then measuring the true torque rather than including a greater friction without the washer.

He also told me that is why the spring washers have sharp ends. For places where this was not required they used spring washers that looked like corrugated flat washers.

My own opinion is that the nut would have had to be pretty lose to allow the flat washer to rotate anyway, and you could argue it might have a greater friction as it has more surface area, but I guess that might be hard to prove.

I'm surprised the car failed on this though. Sounds very picky on the detail. Next they'll be inspecting wiring crimp connections to make sure they can't come apart !

Cheers,
John.

[Bill Bayard]

Frank,

That's ridiculous! A flat washer, beneath a spring (lock)

washer? The added compression of the lock washer will

keep the bolt AND the two washers from rotating. The flat

washer only gives the lock washer a FLAT surface to grab

onto, it doesn't reduce the lock washer's effectiveness.

I don't think the tang has much effect on whether the

bolt will come loose or not. Once fully compressed, compression

is at work , not the tang biting into the flat washer or

the bolt head.

If it weren't for your friend's misfortune,

I'd think this was a joke!

Has this SVA engineer ever worked on a car before??? In the

engineer's opinion, what would have been the "PROPER" way

to secure the cover?? Spring washers with no flats?

In his defense(not really!), I have seen plenty of "factory

bolts" on engines, etc., without flat washers, where I

thought there should be one.

Perhaps there are supernatural forces involved here, that

we mere mortals are unaware of? [img]/ubbthreads/images/graemlins/tongue.gif[/img]


Bill

[Murray Cowley]

I'm not an engineer but I always thought it was the tension imparted by the spring washer that stopped any loosening. However I think the jobsworth at the SVA station is over enthusiastic. Did he impart any information on how it was supposed to be fixed? Which station was it, keep away from there!!
Murray

[Trevor Booth]

Tension in the spring washer. Personally I dont use them preferring to use a belleville washer instead

[91740Gt]

I hope now that Frank has posed this question some of you overseas members will begin to understand the problems we have in GB and Europe regarding the SVA test......
as for the question itself. The tester is, in theory, correct but in practice with this particular set up we will never know for certain.

Graham@ GTA.

[John W]

I've just been speaking with a couple of the guys here about this (aerospace industry).
They always use a flat washer below a spring washer.
Reason given is to protect the major part from any damage that could be caused by the spring washer digging in (remember that corrosion in space is a bad thing).
The spring washer is to provide tension only. If the nut were to be held in place they would use a serated washer (they don't use any of these at all though).

It was pointed out that they either use thread lock (not so often now) or scotch weld as well as the spring washers in all instances.
The scotch weld is like a blob on the thread and nut and acts as a witness to show (a) it has been done and (b) whether any movement has occured during testing.
I discovered not all threadlock is coloured too as the stuff they use here is clear, which is partly why they don't use it so much now.

Its interesting (to me anyway) that the space industry seem to do things differently to aircraft industry. Maybe this is because the aircraft industry can check things once in a while, but who knows...

Anyway, I wonder if your guy had used threadlock Frank whether that would have been acceptible ?

Cheers,
John.

[Murray Cowley]

Trevor,
what is a belleville washer?, I have never heard of them, is it native to your part of the world?
Cheers
Murray

[John W]

I didn't know either Murray so did a google:
This link tells you what they are & what they look like
http://www.rctek.com/fixings/bellville_washers.html

http://www.ecmweb.com/mag/electric_b...ers_correctly/

This link states "Many UL standards require using spring washers (Belleville) or lockwashers and a flat washer"
so it does seem common practice to do so.
http://www.ecmweb.com/mag/electric_belleville_blues/

Cheers,
John.

[Jim Pearson]

Murray,

It's a shallow cone shape which exerts a force when deformed to flat under the tightened nut. Can be used as a back to back stack for greater force. Readily available in the UK.

Jim

[Paul Thompson 'Hooligan']

Interesting site John - gives a good example of use on differentials too - having just replaced the six fitted to standard ATB diff from Quaife on Roys 40. I have also seen them used as a damper on Cosworth Throttle body assembly.

[Frank Catt]

I wonder if the SVA inspector, seeing what he believed to be a single plain washer, would accept that it was in fact a flattened Bellville washer he was looking at ! I also wonder if would even know what a Bellville washer was at all.

[Trevor Booth]

That set the cat amongst the pigeons!! (excuse the pun).
My experience -- commercial grade spring washers can and will break with high frequency vibrations. Mostly I have had this occur on specialist high speed machinery and have had it occur on some parts of racing cars and in particular the engine,trans area.
Simple brackets for alternators etc suffer HF vibrations. Another cause of failure is a stress reversal in the spring washer, say you have a bracket bolted to an engine block and the bracket , due to its loading, flexes a minute amount, the spring washer is alternately being "squeezed and relaxed" , it will ultimately fail. A correctly sized and torqued bolt will not come undone, the "holding tension" being provided by the proofing (stretching) of the bolt.
This is not always achievable on vehicles save for places like head bolts and like solid components. Most chassis related bolts need some form of security be it spring washers, belleville washers, conelock nuts, deformed nuts, nyloc nuts, loctite or lock wired. The choice is yours. My policy is to use hardened washers in all applications and particularly under belleville washers. If you dont use a hardened washer then the belleville (or spring washer) will "dig" into the soft washer and lose some of its installed tension.
The choice of bolt material also plays a part in the "staying tight" role. In lots of places I use bright machined MS bolts. I have seen many a car with million dollar bolts made from unobtanium come off a race track as a veritable "bucket of bolts"
Cheers

[Chris P.]

I agree with the responses so far... On the topic of fastening, I'd like to add the following links for anyone interested - they are very informative and helpful.

BoltScience - lots of good information

Short fastener descriptions at Machine Design

An engineering consultant's fastener overview

Gasket joints and fastening - goto pages 10-20

Although I am by no means an "expert", I am a degreed and employed engineer and have race car preparation experience. Gasket joints are prone to leakage due to relaxing of the gasket so a spring type tensioning device would intuatively help over the life of the joint, although I'm not sure if it's necessary. The ONLY type of spring washer that I can see someone arguing about NOT using a flat washer under is the split lock washer. The usual argument is that it is supposed to "bite" into the base material, as mentioned above. That is so if you want to chew your part up. Of course it will grab better, but it is wasted energy because a properly designed and fastened joint will not require it. Personally, the only people I have heard the "no flat washer" argument from are non-professionals. From professionals, I have ONLY heard that you always use a flat washer under the split washer, if you use one at all. Having said that, I will not personally ever use a split washer - they break. Carroll Smith, I believe, is a big supporter of NOT USING split washers. He is not the industry standard, but he is very knowledgeable with experience.

Hope the info helps.

P.S. - read the BoltScience "torque" info. I put torque in quotes because you will see that it's not about torque - it's about pretensioning - a very big difference. That's a very informative site.

[Jim Pearson]

Chris,

Interesting websites. Did you notice the Barrett Engineering Consultants' comments in Part Two on the LACK of locking capabilities of both non-serrated Bellevilles and of normal helical 'lock' washers?

http://www.tcnj.edu/~rgraham/barrett/manual1-B.html

Jim

[Murray Cowley]

Thanks John and Jim, now I know!! Very interesting.
Frank thats possible, do you know where the SVA centre was?
Chris, a lot more reading and learning I think for me.
I only wish I could work on mine and actually play with nuts and bolts!!
Cheers, Murray

[Mark Worthington]

[ QUOTE ]
What is your view ? Do you think the tang on the spring washer is the locking mechanism, or do you think the tension imparted by the spring locks the bolt ?

[/ QUOTE ]

Trick question, Frank...the answer is NEITHER. Trevor and Chris both correctly pointed out that a properly torqued fastener requires no locking mechanism. Flat washers serve to distribute the applied clamping load. Spring washers serve to make people feel better about themselves.

[Frank Catt]

I agree Mark, but - these inspectors insist on seeing a locking system on most items fixed with fasteners, and quite often there lies the rub !

[Brian Magee]

Frank,

Are SVA testers a law unto themselves or should they not be asked to show the person involved the engineering text book that upholds this theory? Do you have the right to appeal?

Brian

[Kalun D]

Two interesting (at least to me) topics in one thread, fasteners and inspectors.

Aircraft do not use split lock washers. Many auto manufactures generally do not use them either especially on critical assemblies.. From a structural standpoint they are a poor design having one cut across their section. I think the locking action is from both the spring and the sharp end of the cut. Any time you take apart a Honda or Nissan bolt that has them (not many) the sharp end digs into the piece and/or the bolt clamp surface and damages it. Another clue is the direction of the spring coil, if the cut wasn't used to lock you would see some split lock washers sprung in the other direction