<VV> Fuel Leak
roboman91324 at aol.com
roboman91324 at aol.com
Thu Sep 27 21:43:26 EDT 2018
Following your format:
1) Though related in some applications, the term coriolis effect would be more appropriate than precession in this context. While gyroscopic effects do occur with wheels and their component parts, the forces are minor. Yes, wheels are significant gyroscopes especially at higher speeds and larger wheels. However, the effects are minuscule on lug nuts which are the components of interest here.
Yes, the method of centering wheels has evolved. This is due to the constant drive to reduce manufacturing costs and the ability to maintain accuracy of parts in process. Error build-up isn't the problem it once was. However, I don't think the conical vs spigot system should have a bearing on things. I am willing to admit that I don't have deep knowledge of this as it pertains here.
2) Again, I believe we agree more than you think. Yes, the materials in stress operate much like a spring. As you compress or stretch them, they store energy which results in opposing forces. I think we are saying the same thing in different words. You are also correct that there is a friction component between the nut and the wheel. I ignored that because the discussion started with greasing threads. I do not know which gives more friction but both the threads and "shoulder" contact contribute significantly. Neither should be ignored.
3) I said that you are using excessive force on the studs because you are stretching them further than their design anticipated. As I said, the studs and lugs on this type of application are designed to withstand forces beyond the designers intentions. It occurs in both locations. If you have effectively eliminated the friction from the threads, you must compensate with additional shoulder friction. What seems to be a flaw in your logic is that the corrosion, heat cycling, etc. only occurs on threaded contact. Of course, much depends on materials and application. If you have steel nuts/washers or spigots on aluminum alloy wheels, there is more of an issue.
4) Obviously, if the manufacturer has designed his process around oiling threads, that is the process that should be followed. I am unaware of this as a recommendation on any of the cars I have worked on. I guess that must be just another difference between the Brits and the US. If this process is common practice over there, why haven't they adopted your solution to the "wheel loss syndrome" you described?
In a message dated 9/27/2018 3:55:32 PM Pacific Standard Time, Hugo at aruncoaches.co.uk writes:
A couple of brief responses;
1) You say; “Precession may not be the correct term for this but it is good enough for now.”
Precession is indeed the correct term – here’s a link to good old Wikipedia to explain how it works. https://en.wikipedia.org/wiki/Precession_(mechanical)
Going back to wheels & lug nuts, the effects of precession will tend to loosen the wheel nuts on the left side of the vehicle. The effect is obviously minimal, since practically all modern cars have right-hand threads all round, and they manage ok. BUT ... car wheels are located by the lug nuts. Up till the 1980’s, commercial vehicles had the same set-up; conical lug nuts, relatively coarse threads, and left-hand threads on the left side. And wheels NEVER used to fall off. But since then, we have gone over to ‘spigot-mounted’ wheels, which are located on a central register, using fine threads (= easy to strip) and right-hand threads all round. I own one vehicle, a 1989 Leyland, which has spigot-mounted wheels with left-hand threads on the left side. That is the only time I have seen that hybrid arrangement. Clearly precession is indeed a problem with spigot-mounted wheels that rotate anti-clockwise and have right-hand thread nuts. Or else they wouldn’t keep falling off.
As I said earlier, I am not aware of this being a problem with US commercial trucks. I have seen trucks pull into a truck stop to have tires fitted, watched the fitter do the lug nuts up DRY with a 3/4” air gun, drop the jack & drive straight out. I asked the guy what torque he was setting them to, and he muttered something about 120 psi & just shrugged. That would almost be enough to get you arrested in the UK, but it doesn’t seem to be a problem in the US. The mounting system is identical, threads are similar if not identical, and European wheel studs are further out from the centre than American ones, which theoretically puts the US ones at a disadvantage. So that remains a bit of a mystery.
2) You say; “We also agree that these type of fastening systems are dependent on friction... “....”The friction derived from tightening the bolt creates the friction which prevents the threads from backing off.”
I’m not sure that we do agree on this. A stud or bolt or set-screw is effectively a spring, a very strong spring, that stretches when the nut is tightened and holds everything together literally by spring force. The more friction that exists between the threads, the more torque is going to be absorbed by this friction, and the less the ‘spring’ is going to be stretched. Obviously the less the ‘spring’ is stretched, the less the clamping force between whatever it is you’re trying to hold together. It is not, or at least it should not be, the stiffness of the thread that stops the nut unscrewing; it is the friction between that flat surface of the nut and whatever it is pressed up against (ideally a spring washer of course).
3) You say; “Forgive me but the only reason your system doesn't result in disaster is because the threads are capable of withstanding the increased stress to which you subject them. Other fastening applications would not survive. By the way, if that crazy guy with an impact wrench works on your greased lugs, you may have an accident waiting to happen.”
What makes you think I subject my wheel studs to excessive force? I do them up by feel. In any case, although I would never use an impact wrench to fully tighten a wheel nut (or any other nut for that matter), it wouldn’t hurt the threads – as I said earlier, the vast bulk of the friction occurs between the nut and the wheel, not between the threads. And it is that friction which stops them unscrewing. The problem is, if you do them up dry, you may not have enough clamping force, so that as soon as there is any wear between the mating surfaces (due to heat cycles, dirt/rust/paint wearing off) you have lost whatever clamping force you had, and consequently there is not enough friction between the nut and the wheel to stop it all unravelling. If the stud is properly stretched, it can withstand a lot more wear between the mating faces before it becomes slack.
4) you say; “If applied to attaching heads to our Corvair motors, your method would likely result in multiple studs ripping out of the crank cases. The additional stretch of the nut/stud interface would transmit to the stud/crank case with disastrous results. Never lubricate these threads.”
Most British engine manufacturers advise oiling the threads before fitting the head nuts. I fully accept your point about the other end being in aluminium, so I wouldn’t argue this point. I do have a related experience however; I have a Bristol 2 litre, with a cast iron block and aluminium head. Because of the bizarre valve gear (it’s based on a pre-war BMW) it is impossible to get a torque wrench anywhere near the head nuts. The threads are ‘cycle thread’, which may not exist in the US – it is a VERY fine thread. Somebody did suggest an appropriate notional torque would be 25 lb/ft, which gives you an idea how fine it is.
Anyway, I had the head off mine once, duly refitted it, and went to re-torque the head after a hundred miles or so. I was surprised to find one of the nuts barely finger tight. It turned out that the thread had stripped, solely from the expansion of the aluminium head when it got hot. And of course, it couldn’t be the nut that stripped – it was the stud, so the whole damn lot had to come off again!
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