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Well I learnt the trade in a proper 'old fashioned' style garage and they always used to copper slip the mating surfaces of the wheel/hub and the roadwheel bolts/studs. It is a practice that I still use to this day on the array of vehicles that I look after, whether it be techinically right or wrong. I have never had a problem with a wheel coming loose, studs/bolts breaking or wheels cracking - equally so I have had anyone not be able to subsequently remove a wheel that I have fitted themselves due to stuck nuts/bolts or a wheel stuck to the hub. I shall carry on copper slipping them until I see evidence to persuade me otherwise.
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I agree, Simon, and I have never used a torque wrench on wheel nuts/bolts. I tighten to a nip and then a bit more, never had a problem in 36 years of DIY motoring.
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Keith
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Good for you, probably nothing wrong with that, although not best practice.
You have to realise that we deal with all sorts of customers and most of the general public think that wheel bolts have to be very tight and will try to tighten like their life depended on it. So lubing everything with copper grease can result in some much overtorqued bolts.
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Good for you probably nothing wrong with that although not best practice. You have to realise that we deal with all sorts of customers and most of the general public think that wheel bolts have to be very tight and will try to tighten like their life depended on it. So lubing everything with copper grease can result in some much overtorqued bolts.
I agree WT, I lubricate the studs and wheel centre (spigot) to ease wheel removal on my own vehicles, on which I remove the wheels at least every 12 months, to do my own checks and I am very careful not to overtighten them.
I know that dealing with other peoples' vehicles on a commercial basis will involve written practices which should be followed and as a technician you no doubt will.
regards
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Copper grease can often be used between wheel and hub (not on the threads or bolt/nut contact areas) to prevent the wheel becoming stuck on the hub. This is recommended for my BMW rear wheels.
Yes, copper grease should be put on the hub spigot, this is where they normally stick. Ideally you shouldn't put any grease on the threads or on the contact face with the hub - as number cruncher says this is basically a friction coupling to your wheel. Torque should go through this and not through the wheel bolts. The bolts are to provide compression, not to carry braking/acceleration forces.
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The wheel bolts provide a clamping force wheel to hub face.
Provided the torque applied across the bolts (accelleration, braking etc.) does not exceed the sum of the clamping force (capped bolts clamping wheel to hub), everything's dandy.
Friction of hub face cannot come in to play provided this clamping force isn't overcome (and if it is overcome you've lost a wheel bolt, nasty!).
NB: Clamping force is not cleanly related to how much torque is applied to fastening the wheel bolt, for all the reasons listed above about clean vs. greased threads it's impossible to *accurately* tell clamping force that will be exerted from the torquing force applied to the fastner.
The threading gives you a mechanical advantage, so for energy applied through 100Nm twist onto the bolt, there will be even more clamping force bolt cap to wheel to hub.
EDIT: friction of the hub face *does* come in to play, but i can't think how to measure or model it. It does contribute however, so WT is right.
Edited by CraigP on 21/01/2010 at 15:31
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I'll step through your post point by point;
The wheel bolts provide a clamping force wheel to hub face.
Yes.
Provided the torque applied across the bolts (accelleration braking etc.) does not exceed the sum of the clamping force (capped bolts clamping wheel to hub) everything's dandy.
No. You're trying to equate a torque with a force - it's like trying to equate apples with oranges.
The correct statement would be that providing the torque across the interface is less than the friction torque which is generated by the clamping force provided by the bolts then all is well.
Friction of hub face cannot come in to play provided this clamping force isn't overcome (and if it is overcome you've lost a wheel bolt nasty!).
No. Friction at the hub face is what should transmit all traction and braking torques from the hub to the wheel, or from the wheel to the hub.
The bolts provide the clamping force. The N in F = mu * N
NB: Clamping force is not cleanly related to how much torque is applied to fastening the wheel bolt for all the reasons listed above about clean vs. greased threads it's impossible to *accurately* tell clamping force that will be exerted from the torquing force applied to the fastner.
Yes, any change in friction level from the ideal means that you get more or less tension in the bolt per unit torque applied.
As a first step, you can use Torque = 0.2*(preload)*(bolt dia) to estimate how the bolt works - as a first step in deciding whether you need M6 bolts or M16 bolts.
The threading gives you a mechanical advantage so for energy applied through 100Nm twist onto the bolt there will be even more clamping force bolt cap to wheel to hub.
No.
Firstly, energy can't come from nowhere.
Secondly, you're trying to equate torque with force again.
EDIT: friction of the hub face *does* come in to play but i can't think how to measure or model it. It does contribute however so WT is right.
You can model it just like you would model the torque capacity of a clutch with one friction face rather than two - that's fundamentally what it is.
When you get your head around how this joint works, and do the sums, you'll realise that a greased contact at this interface *can* result in slippage under extreme loading, which will load the bolts in shear and bending which they are not designed for.
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There is a further aspect to this discussion which has so far not been mentioned. On a well designed steel wheel, the bolt seats are pressed in such a way as to provide a slightly raised contact area at the hub or disk interface.
The resulting clearance around the bolt seat means that the wheel steel itself is subject to some elastic deformation when the bolts are tightened. I'll leave the arithmetic to our honourable and learned friend, but I would suggest that this deformation has at least as great an effect in maintaining the joint preload as does the elongation of the (short, stiff) bolt.
Wheel bolts/nuts are almost universally provided with tapered seats which are not always strictly required for the maintenance of concentricity - many wheel hub interfaces have a spigot. I suspect that the inclusion of a taper adds materially to the friction under the bolt head and inhibits loosening. (NC please comment). This will serve to make the preload difference greater between the lubricated and unlubricated conditions for a given tightening torque.
I use copper loaded grease but don't overtighten them. There's a huge margin of safety in the design of this joint (the hub is less strong and an Al wheel much less strong - see what breaks after a serious RTA) and I've never ever had a lubricated bolt come loose. I do check them though.
Of course, "my" wheel preload is lost completely with Al wheels - but I don't use them. If I had to, I'd be more cautious with grease, but at the same time aware of the bi metallic corrosion implications. I'll stick to steel wheels.
659.
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No doubt a well lubricated, greased, thread can overtighten because the lubrication enables the specified torque to create additonal tension by overcoming friction.
However the addition of copper particles to the grease ensures that the amount of torque required to achieve the required tension is not affected greatly at the same time ensuring that mating surfaces move smoothly against each other, and that the thread does not corrode to the extent that it cannot be undone.
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at the same time
You can't have both.
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>> at the same time You can't have both.
You can if they are two different things.
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But the two things you're talking about ARE the same.
If something moves more smoothly, the friction is lower. If the friction is lower, the torque versus preload relationship has changed. You cannot have one without the other.
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There can be a specified level of friction which is not achived due to machining inaccuracies, surface imperfections, dust, grit, corrosion etc. Copperslip counters this enabling smooth torquing (as opposed to smooth talking ;-) ) without increasing the tension relative to torque significantly.
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>>without increasing the tension relative to torque significantly.
No - you're kidding yourself.
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There can be a specified level of friction which is not achived due to machining inaccuracies surface imperfections dust grit corrosion etc. Copperslip counters this enabling smooth torquing (as opposed to smooth talking ;-) ) without increasing the tension relative to torque significantly.
Even Mr Scott cant do that. "Ye cannae change the laws of physics!"
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>>without increasing the tension relative to torque significantly.
No - you're kidding yourself.
Even Mr Scott cant do that. "Ye cannae change the laws of physics!"
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I think you are both ignoring the word "significantly".
I had a chart on this though cant find it currently, the effect of various lubricants on a dry thread, IIRC a moly grease, hypoid 90, a multigrade, WD40, copperslip characteristics were nearer a totally dry thread than any of the lubes.
I think it is to do with copper having a high coefficient of friction against other metals.
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Copperslip counters this enabling smooth torquing (as opposed to smooth talking ;-) ) without increasing the tension relative to torque significantly.
For a given torque, the tension in a clean and dry bolt even depends on whether the components are plated or not.
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