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MG Midget and Sprite Technical - Hub to disc bolts

Nearly finished my 63 Mk 1 midget, and there is one thing I need to tidy up that has been in the back of my mind for a while. On the front wheel disc brakes, the wire wheel hub is bolted to the disc by 4 bolts, the ones with the funny head. They didn't have spring washers on them when I got the car, and on all the diagrams I have looked at there are no spring washers or anything. Is that correct, or should they have spring washes or locktite? The book says they "must be torqued up" but there is no way I can get a socket on them cause the hub is in the way???
Mike Quilter

I guess you need to be seriously anal to have hunted down the tech reason for the lack of washers (like me?)

The bolts have a slightly concave (or convex?) head face so that they sit onto the hub in the same way as a rubber suction cup does. As you torque it down it squashes the concave-ness down and mushes into a flat shape so that the torque is what deforms the bolt to the 'correct' shape, just like a rubber suction cup does. This helps the bolts to absorb vibration without undoing themselves. No washer required or wanted. Clean out the threads with a new bolt of correct thread and whizz 'em in.

IIRC, you need a decent quality thin-walled socket for them. I've used NAS grade aerospace bolts since my hubs are alloy ones but I'd put money on the original bolts being the best for the job, if used correctly.


rob multi-sheds thomas

Don't use spring washers... use locktite if you want additional peace of mind.

Normally tabbed washers or split-pins are used to secure torqued bolts... I can't recall seeing spring washers under these circumstances.

A
Anthony Cutler

Technically it is the result of TORQUE that secures (correctly) torqued bolts because torquing to between 60-80% of the yield point places the bolt squarely within its 'elastic deformation' range. The resultant elastic deformation (stretch) causes the male threads to lock against (and thus grip) the female threads.

All the split pins (or safety wire) in the world will not stop a bolt undergoing torque relaxation (eg because of vibration) - they'll just stop it falling off.


Tab washers are the work of the Devil! - The very fact that they are designed to bend means they will crush as the fastener is tightened with the result that you can NEVER know the actual torque being applied!

Spring washers are just as bad for the same reason and should (IMHO) NEVER be used on 'safety critical' fasteners.
Deborah Evans

I can't completely agree with that.
When I was still working on rolls royce olympus gas turbines, there where bolts that where torqued AND had a spring washer. Mind you, a bolt, nut, washer and spring washer costs about 65 GBP per set.
Alex G Matla

Hi Deborah

I was referring to tab washers like on the brake callipers. They are there solely to lock the bolt (also used on some b/e bolts, too; they are deformed post torque being applied); what's bad about them?

A
Anthony Cutler

Thanks for that everybody, I will go with locktite and no spring washes. What a great thing this BBS is!
Mike Quilter

Anthony,

To clarify any misunderstanding regarding torquing of bolts or studs:


Any given metal or metal alloy has what is known as a 'Young's Modulus' which is a measure of that metal's stiffness. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds.

Where:

Stress is defined as Force per unit Area.

Strain is defined as Percentage Elongation.

'Hooke's Law states that the extension of a spring is in direct proportion with the load added to it as long as this load does not exceed the elastic limit.



The upshot if this is that if you plot a graph of Stress against Strain, eg for a fastener, then the graph will have 4 distinct phases:

1. The first phase is the 'Hookeian Phase'. Here a given amount of Stress will result in a proportional amount of Strain. Thus if x Stress produces y Strain, then 2x Stress will produce 2y Strain. Furthermore, once the Stress is removed the material will return to its original length. Thus the metal in this part of the graph is said to be in its 'Elastic Phase' because it is undergoing Elastic Deformation.

2. There comes a point on the graph where Stress no longer produces a proportional amount of Strain. This is known as the Elastic Limit or 'Yield Point' of the sample. This is, generally not a sudden point but a small area of the graph.

3. After the Yield point, as previously stated, Strain is no longer proportional to Stress but, more importantly, the sample WILL NOT return to its original length. In effect the sample has become permanently distorted. Thus the metal in this part of the graph is said to be in its 'Plastic Phase' because it is undergoing Plastic Deformation.

4. At some point the Strain applied will overcome the strength of the sample and it will break. This is known as the ’Failure Limit’ and defines the ‘Ultimate Tensile Strength’ of the sample (usually expressed in psi).


Normally when we torque a bolt we are Stressing that bolt to between 60-80% (dependent upon the material) of its Yield Point. What we are actually doing is setting up a given amount of Strain (elongation) in the bolt. This causes Elastic Deformation in the bolt causing the male threads to lock with the female threads – it is this ‘locking action’ that provides the necessary clamping load to hold together whatever it is we are bolting up.

Now, it is the Strain (elongation) that results in the desired clamping load. Thus the most accurate way of assessing this is to measure the amount of stretch in the fastener. If the fastener is not ‘blind’ such as a con rod bolt for example, then this is relatively easy to do with a DTI, Micrometer, or Vernier Calipers. Indeed, high-grade fastener manufacturers such as Cosworth or ARP will generally give stretch figures for such fasteners.

The trouble comes when the fastener is blind, such as a main bearing bolt. Here the only recourse is to use a torque wrench and torque the bolt to a given figure. Torque of course, is Force x Distance tangential to the axis of rotation.

However, this throws up a whole raft of pitfalls:

1. How accurate is your Torque Wrench? Buy a good one and get it calibrated regularly. Torque Wrenches should ALWAYS be returned to their ‘zero setting’ after use and NEVER be used to de-torque a fastener (use a breaker bar for this!). Failure to observe these rules will mean that your torque wrench will rapidly go out of calibration.

2. The Stress/Strain relationship in the Elastic Phase of the graph (and remember we are now applying Stress in order to get the required Strain) is based upon Tensile Stress, however as we turn the bolt we set up friction which results in a Torsional Stress in addition to the Tensile Stress (actually this Torsional Stress can be resolved as a ‘Shear Stress’). This Torsional Stress disappears as soon as you stop turning the bolt. Your torque wrench cannot determine the difference here and will add both together when giving a final torque figure. This is why you should torque in stages and why, when the final torque is reached, you should ‘double click’.

3. Allied with Serial 2 the friction developed (and thus the Torsional Stress) will depend upon whether the fastener is lubricated and what with. Hence, for eg, A Series main studs should be torqued to 95 ft.lbs with SAE 40 grade oil or just 75 ft.lbs with ARP moly based lube (note: studs should always be put in no more than hand tight so that it is the nut that is torqued – this is to ensure no residual stresses are placed in the stud such that the correct clamping load is applied). Thus, when you look up your torque figures you need to know whether this is for a dry fastener or a lubricated fastener.

As stated earlier, the torqued fastener lies squarely within its Elastic Deformation phase and is perfectly re-usable so long as it is not damaged. The reason things like rod bolts are not re-used is because they undergo many cycles of stress reversal and are susceptible to fatigue failure (but that’s a different essay for another day!).



Now, given that when we torque a fastener what we are trying to achieve is a set level of strain in that fastener (stretch), then if we use a lock tab we can not determine from the torque wrench whether the correct torque (and hence, elongation) has been applied. This is because tab washers, by their very nature, are designed to deform ie they must be 'bendable' and are, hence, in their area of Plastic Deformation. Thus as we torque the fastener the tab washer deforms and we have no real idea of the elongation in the fastener itself (which is what we are trying to achieve.

Tab washers are all very well in non safety critical areas but for things like caliper bolts then lock wiring plus 'loctite' would be my method of choice. The lock-wire will not stop any torque relaxation, eg from vibration, but it WILL stop the fastener from falling off.
Deborah Evans

Alex,

I spent the first 20 years of my working life around aircraft and high performance jet engines. Granted these were earlier types like the Avon, Spey, Viper, Adour, yet not one of these used spring washers in critical areas - ALL used lock-wire.

It may be with the advent of the Olympus and later types such as the RB211, Trent, etc, that the metallurgy was that much better such that spring washers could be produced that lay squarely within the Elastic Deformation area of the graph. I suspect this is the case given the prices you quote!

Having said that I have yet to see ANY spring washer in the Automotive Industry that do not permanently distort when the fastener to which they are attached is correctly loaded. That being the case then said spring washers are undergoing Plastic Deformation and are of no more use in safety critical areas than are tab washers for the same reason as stated above.

Furthermore, I know of no one in the professional Race Car-building Industry who uses either tab washers or spring washers on safety critical fasteners.
Deborah Evans

Mh, the bolts 'n nuts I was reffering to where on the HP compressor, maybe not critical enough. Then again, the stuff looked expensive, and when the bolt fretted in the nut we had to break them.(By overtightening) The breakage looked funny, with a little twist in the middle.

And thank you for your ellaborate explanation on torqueing.

Alex"I'll shut up now"Matla
Alex G Matla

Deborah has it exactly (if not in nutshell!)
I think the most imortant point for the layman such as myself to get our heads around is Deborah's point about lubrication of threads; its CRITICAL for acurate torquing of fasteners, also to understand that when the thread or nut is lubricated (also under the head not just the thread) the torque figure given in workshop manuals must be reduced, because all of the them are dry figures... please correct me if I'm wrong. But importantly this does not reduce the actual torque/ stretch being applied to the fastener. I generally reduce workshop manual figures by 15-20% when using the original spec studs/bolts + lubrication.
Ian
Ian Webb '73 GAN5

Deborah,

Very good write up but I would disagree with the paragraph.

<<Now, given that when we torque a fastener what we are trying to achieve is a set level of strain in that fastener (stretch), then if we use a lock tab we can not determine from the torque wrench whether the correct torque (and hence, elongation) has been applied. This is because tab washers, by their very nature, are designed to deform ie they must be 'bendable' and are, hence, in their area of Plastic Deformation. Thus as we torque the fastener the tab washer deforms and we have no real idea of the elongation in the fastener itself (which is what we are trying to achieve.>>


As we are talking about using tightening torque to determine the strain and therefore clamping load then I don't see that plastic deformation of the tab washer effects the overall torque result. There will come a point in the tightening where the load on the tab washer , under the bolt head, will exceed its elastic limit and it will deform plastically until a point where it can support the load. If the washer was unable to achieve this then it would continue to deform and fail and the desired torque not reached. The desired torque is reached though, which is an indication that the clamping load has been reached, as far as torque can be used as an indication of that, regardless of any plastic deformation of the washer. I am sure you're aware that plastic deformation usually results in an improvement in material properties.
David Billington

Hi Deborah

Familiar like many with Young/Stress/Strain.

Like many (all?!) here I would never use spring or shakeproof washer in an attempt to lock a torqued bolt. I can't recall seeing these on any car I've worked on... including engine and box.

Nylocs, split pin and tabs seem fine to me. I don't see the problem with a suitable tab washer (notionally flat before torqued up, and with additional para from DB). Engines and boxes (well, older ones) frequently use(d) tabs, e.g. on nuts to secure bearings on shafts and big-end bolts. I'm happy to use tabs also e.g. on the brake calliper bolts, fly-wheels and say rear wheel bearing. I like the visual feedback they bring (it's not easy to be sure the Locktite on the calliper bolts is doing its stuff).

Of course, it's best to use a new tab if you're able, and they cost very little in addition (I keep spares e.g. for the callipers).

BTW - can't remember ever working on a wired nut or bolt... very 40s and earlier.

A

Anthony Cutler

I've used wire-locking on certain car parts, especially where heat is a problem. These exhaust nuts are brass and run on mild steel studs. They have a nasty habit of snapping if you torque them up very high and have a tight interference fit. I run a tap bown the bolts a few times to releave the thread and then wire them. This makes them easier to remove without damaging the head.
Getting hard to do with only one working hand!

The other good idea is what we call nappy pins. Makes splitpins a bit obsolete. Easy to remove and to reuse but best with castelated nuts.
http://www.pegasusautoracing.com/bigpicture.asp?RecID=775
If you put them through the very end of the thread of a bolt it will prevent a nut from coming off the end.


rob multi-sheds thomas

This is a very interesting and educational discussion, but could you clarify what you are talking about when you mention tab washers? To me, this means the washer with a tab that is bent up against a flat on a nut or bolt head to secure it. Is that correct? There has been no mention of flat washers. Is the problem with tab washers the same for flat washers, as far as inaccurate torque, etc? I thought the flat washer would spread the force over a larger, more even area than just the small hexagonal area of the nut/bolt head. Are you using flat washers with no tab or spring washers, or are all washers bad for mission critical areas?

Jack "not an engineer" Orkin
Jack Orkin

Hi Jack

from memory, on the ASeries the head nuts (and rocker shaft nuts) have plain washers underneath. I always replace such washers when I encounter as standard fitting.

For tab washers, I mean either single, double or multiple washers that have an extension tab that bends up to lock the nut/bolt once torqued. Again, I always refit these or use new.

I'm the sort of person that does not like to see any washer, ball-bearing, nut, bolt, ... going spare when I've rebuilt an engine, box, suspension,... the only items I like to see on the bench after a job are ones I've replaced!

A
Anthony Cutler

normal flat washers won't crush like tab washers as the latter have to be made of softer material. With a few notable exceptions I've always thought the torque settings on the Spridget are to prevent the fastenings being over-tightened, as the figures are all rather low, lower than a b good heave on a half inch ratchet handle. I now use the smaller 3/8" ratchet and sockets and rarely use the torque wrench. Thinking more about it, I cannot remember when I last found a fastening coming undone, whether with or without tab, spring or lock washer. So while I find the thread interesting I don't think it's that important and I certainly won't worry over it :-)
David (davidDOTsmithAT stonesDOTcom)

I once had the long bolts that secure the Armstrong damper work loose which created an interesting pre-MOT handling problem.

I throw away spring washers whenever I find them (found a couple the other evening on something that hasn't been apart for a decade). I never find lock tabs anymore on my car.

I like to lock wire fasteners and when I can't I like to loctite - anything going into a blind hole including the sump plug. If a bolt has a nylock nut on the other end I leave it as it is. Some stuff I never loctite like ARP flywheel bolts and thermostat housing nuts.

The Nappy clip Rob refers is too is an aircraft cowling pin and I get mine from Aircraftspruce.com whenever I'm ordering AN bolts and threaded fittings. Trident racing supplies also do them. Here's a picture from a Spridget book that shows just the place to use them.

The name for the disc to hub bolt is the 'place bolt' and it's covered in Carroll Smith's Nuts, bolts and Fasteners book.


Daniel Thirteen-Twelve

This thread was discussed between 25/08/2009 and 28/08/2009

MG Midget and Sprite Technical index

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