An as-yet inconclusive discussion has developed on the MB Club forum about whether or not copper grease should be used on wheel nuts. Any opinions?
Secondly, having checked the manuals for both my MBs I find that the recommended torque for wheel nuts is 110Nm on both of them, and the owner of a different model has reported the same figure; it's the same for my Golf. Is this pretty much standard or does it vary widely between marques and models? . . . and a few minutes later, courtesy of Google I find a useful tabulation of torques at:
tinyurl.com/eylmq
(The associated PDF file seems to me unavailable, so this link displays the HTML version, imperfectlyj
And the Haynes manual shows different ranges (not single numbers) for front and rear on my Capri 2.8i.
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Wheel bolts / nuts / studs are safety critical. The manufacturers specified method of wheel nut tightening should be followed. Usually, that spec is for clean, dry threads, although I don't know if that is true for every type of car - check your manual.
The purpose of tightening a fastener is to introduce preload into the shank of the fastener, and corresponding compression into the joint members. This combined system then protects the shank of the bolt from enduring the full applied load. Typically, a properly preloaded bolt only sees 20% of any applied load. Properly preloaded is the important part - not excessive, nor insufficient preload - the correct preload.
Technically, by reducing the friction between the nut and thread you put more preload in the shank of the fastener per unit torque than for a dry set up. This reduces the margin between the operating preload and the yield preload for the fastener. i.e., it makes overload of the fastener more likely.
Also, by reducing the friction, one reduces the margin of the system to reversal. This is to do with the relationship between what is known as the friction angle*, and the wedge angle of the thread (imagine unroling the thread onto a flat sheet - you get a wedge). If the friction angle is greater than the thread angle, the fastener won't undo under static loading. Vibration and relative motion of the two joined parts is another complicating matter! Lubricating the threads reduces the friction angle.
Obviously, there is also the friction between the head of the nut and its contacting surface, which, acting at a larger radius is significant. In fact, if you do the sums for bolt tightening, the effect of this interface can dominate the solution (i.e., how much preload you get per unit torque).
So, lubricating the threads gives you more chance of yielding the fastener, and more chance of the system becoming reversible.
Stick to the manufacturer's recommendations is my only possible advice.
Number_Cruncher
* The friction anngle is related to the coefficient of friction by the following - mu=tan(theta), where mu is the coefficient, and theta the friction angle.
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NC,
didnt we have this discussion somehwere before?
www.honestjohn.co.uk/forum/post/index.htm?t=27764&...e
StarGazer
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You're right Stargazer! The topic did seem familiar as I typed my response in. I'm pleased I didn't contradict myself!
For anyone interested in how these calculations are carried out, these references may be useful;
1) VDI2230 - Systematic Calculations of High Duty Bolted Joints
(1988)
2) ESDU 86014 - Applying, Measuring, and Maintaining Pretension in Steel Bolts (July 1986)
3) ESDU 85021 - Analysis of Pretensioned Bolted Joints Subject to Tensile (Separating) Forces (August 1985, ammended August 1995)
4) BS7608 - Faigue Design and Assessment of Steel Structures
Number_Cruncher
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I have always understood that greasing or oiling a thread with conventional lubricants reduces the friction between the nut and thread thus applying the specified amount of torque will overtighten the fastening as NC says.
However I have also always understood that copper grease simply ensures smooth passage of both threads over one another without reducing the basic friction between the two items. This can ensure that a nut is not over tightened because with a dry thread there is the tendancy for it to stick requiring excessive torque to pass the sticking point which (if very near the final point of rotation of the thread prior to its ideal position) can cause the nut to rotate further on the thread than is ideal therefore causing over tightening.
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I think that applying anything to the threads, and thinking that the friction is unchanged is 'hopeful'.
During a project on a highly loaded rail vehicle bolted joint that I did some work on, I carried out some trials using strain gauged bolts. I wanted to see if there were any good methods of locking the fastener without introducing extra interfaces betweeen the bolt and joint, and without the hassle of lockwiring. In particular, I was checking a liquid thread locking compound.
Much to my horror, I found that the introduction of even a small amount of this fluid (which the sales rep had previously suggested would not affect the thread friction during tightening) resulted in up to a 40% drop in fastener preload when compared with a dry installation! I didn't specifiy it!
The manufacturer's advice is the only advice to follow in the case of wheel bolts.
Number_Cruncher
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Much to my horror, I found that the introduction of even a small amount of this fluid (which the sales rep had previously suggested would not affect the thread friction during tightening) resulted in up to a 40% drop in fastener preload when compared with a dry installation! I didn't specifiy it!
Surely though this 40% can be compensated for by amending the recomended torque setting?
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Yes, you are right, that is a possible work around in some circumstances.
However, in the particular application I was considering, bolt failures were common, the shanks were snapping in fatigue.
If I chose the locking compound, I would have to specifiy a higher torque to obtain the required preload. This would result in more torsional strain (hence shear stress) being locked into the shank of the bolt - the part that was failing.
While fatigue is driven by fluctuating loads, the amount of crack growth you get per cycle of applied loading, typically, increases with larger locked in, or residual stress. Hence my reluctance to specify the locking compound.
Number_Cruncher
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My experience in motorsport has taught me that as a general rule of thumb,most bolts,fasteners etc. should be lubricated with light oil.Obviously where the application involves high temperatures and/or some exotic materials ie.titanium then silver grease is used.Grease can act as a torque multiplier,so I use light oil (3-in-1 etc.)on my road wheel bolts and torqued to the figure in the car manual.
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so I use light oil (3-in-1 etc.)on my road wheel bolts and torqued to the figure in the car manual.
I tend to agree with NC that you are in danger of over tightening by following this procedure.
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According to my tin in the garage, it is an anti-sieze compound with added solid lubricant, not specifically a grease. It is designed, primarily, to stop corrosion between two different metals, in this case the ally wheel and the steel bolt/hub components.
I have seen an ally wheel siezed to the hub so tight that even belting the wheel hard with a copper mallet would not shift it. This was after the struggle to remove the well-corroded bolts!
I would have thought that the size and material chosen for both the bolts and the nuts/hub would be such that a large safety factor was applied by the manufacturer.
I have always used anti-sieze compound on both the hub/wheel interface and the bolt shanks and thread, so far without any problem. I must say that I have not noticed any significant reduction in the force needed to hand tighten the bolts before final tightening.
I accept that building cars/bikes for competition is totally different.
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This technical stuff is all very well but most wheel nuts/bolts are tightened to undoable if you are lucky,or overtightened using an impact wrench by garages who do not want to be sued for a wheel coming off.As mjm says the coppergrease is anti-sieze and,as someone who has often struggled with tight nuts,I would recommend its use,also where wheel meets hub.My worst one was on an Astra,a sledgehammer would not shift it and it was cut off with an angle grinder,steel wheel too.
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Going back to one of the original questions about the torque value - yes, most cars are very near to that torque setting. Possibly due to the similar sizes of bolts. There may even be attempts to get a standardised value to avoid confusion. A good yank with the vehcles standard wheel wrench is in that ballpark.
As to lubrication. I've never seen a new, dry wheel bolt. They all seem to have some form of coating that seems to be oily and corosion inhibiting. So yes, I dab a bit of copperease on the cones and threads. I've stopped using it on the hub/alloy wheel joint as it sometimes seems to dry 'lumpy' and now use a tiny smear of normal grease to prevent then corroding together.
Martin
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The industry standard torque recommendations are for non lubricated nuts.
Check www.norbar.com/Calculators/Torque-Tension-Calculat...x
Normally, the manufacturer will tell you the torque to use for your wheel nuts or bolts, and tell not to use grease, (or otherwise).
If they tell you not to use grease, they tell you the correct torque for non lubricated grease.
And vice versa.
Spray with brake cleaner; Leave brake cleaner to dry. Torque up. Do it right. Do it once.
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