<VV> Fuel Leak

[roboman91324 at aol.com]

Hugo,

 
Thanks for the interesting story and your theory.  I believe we may agree on more than you think.
 
First, a bit of info.  Chrysler products used to have left hand threads on the driver's side wheel lugs.  That's the American driver's side, not the British.  This ended, I believe, with the 1970 models.  There may have been a reason for it "way back when" but Mopar vehicles joined the rest of the car manufacturers without adverse effects in 1971.  Precession may not be the correct term for this but it is good enough for now.
 
Where we agree...  Like you, I believe there must be a technical reason for "wheel loss syndrome" and there must be a technical solution to the problem.  I would say that a simple phone call between Engineers of the British coach manufacturers and an American one might resolve the issue.  Professional courtesy would likely prevail and information would be shared.  It is probably an issue of thread pitch and/or thread profile and/or materials used.  In any case, with billions of perfectly happy lugs out there, somebody has probably solved this problem and that knowledge exists.  Of course, I may be wrong.  Despite the appearance of simplicity, the interface of so many dynamic and static parts in a wheel system makes the issue very complicated.
 
We also agree that these type of fastening systems are dependent on friction which is, among other things, a function of the contact force between surfaces.  As you torque a bolt or nut, the force between the thread surfaces increases.  The simplified formula is F = (mu) x N where "F" is the sliding force required, "mu" is the coefficient of friction and "N" is the normal force between the surfaces.  Further to this topic, we agree that the stretch/compression of the threads' contact surfaces is what increases the "N" factor in the equation.  As "N" increases, so does "F" if "mu" remains steady.
 
As an aside, I worked on a project whose purpose was to more precisely fasten components together,  Our customer was designing equipment to bolt heads to motors.  The way it was done previously was strictly with torque.  The new system would turn bolts/nuts to a preset torque to save time and then further tighten the fastener to the required stretch point.  The automotive industry has adopted this method universally.
 
That all has to do with the "N" part of the equation but you evolve into the "mu" part where we may not agree.  You state, "The problem with that is that a large part of the tightening torque is absorbed in overcoming the friction of the thread, and the stud doesn't stretch sufficiently to clamp everything together."  The friction derived from tightening the bolt creates the friction which prevents the threads from backing off.  It is a good thing, not a "problem."  As stated above, "the stretch/compression of the threads' contact surfaces is what increases the "N" factor in the equation.  As "N" increases, so does "F" if "mu" remains steady."  I believe what you are doing is reducing "mu" and then compensating by increasing the stretch of the system.  The increased stretch increases the "N" which compensates for your reduced "mu."  "F" is the critical factor.
 
Bringing this together ... The lug/lug nuts on your coaches are over designed for strength.  They designed them this way to guard against stripped threads when some guy goes crazy with an impact wrench.  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.  
 
As other examples:
 
1.  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.
 
2)  The application that started this thread has to do with fuel leaks.  Randy suggested that grease be used on the threads when tightening this interface.  He cautioned that you should only tighten finger tight to avoid stripping the threads.  Threads in softer metals like copper, aluminum/aluminium, malleable steel, etc. are easier to strip than the lugs on your coaches.
 
As an aside, there are many types of "thread-locker" products available.  Their uses and attributes vary wildly and care must be taken to select the right one for the application.  In any case, they are not lubricants.  In addition, the use of one of these products that is too viscous might give you poor fastening results.
 
Doc
 
In a message dated 9/27/2018 2:20:31 AM Pacific Standard Time, Hugo at aruncoaches.co.uk writes:

 
 An interesting hypothesis, but I'm going to disagree with most of what you 
say - apart from the fact that if it is proving so difficult to get a seal 
when there's only a couple of pounds pressure, there is clearly something 
fundamentally wrong. It just should not be that critical.
A bit of background - I am a coach operator based in the UK. we have a thing 
called 'wheel loss syndrome', whereby the left-hand rear wheels tend to fall 
off buses. At least that's what the politicians call it - I call it simply 
bad engineering practice.
Some years ago we switched to the European system of having right-hand 
threads on all wheel fastenings. British vehicles used to have left-hand 
thread lug nuts on the left side of the vehicle (as did quality cars also). 
This is to counter the effects of 'precession', which will tend to make them 
unscrew otherwise. A bicycle pedal will have left-hand threads on the left 
pedal for the same reason, which seems counter-intuitive at first, till you 
understand the principle of precession.
Anyway, back to bus wheels falling off - the coach and bus industry as a 
whole is paranoid about this 'syndrome'. But I am not. The main reason that 
wheels keep falling off is because people adopt the procedure you have 
outlined - they fit everything dry. Let me try to explain - a bolt or 
set-screw is like a spring. When you tighten the nut, it will stretch, and 
clamp everything together like a strong spring. On old British Triumph 
motorcycles, they didn't give a torque setting for the big-end bolts (conrod 
bolts) - you just used to tighten them till they stretched 1/8" or something 
like that.
On the back wheel of a bus, with twin wheels, you have a total of six mating 
faces. Any dirt, rust or paint on these faces when they are assembled will 
inevitably wear off in use, leaving the whole assembly loose. If the studs 
are not stretched sufficiently, the whole lot will fall to bits. So the 
first thing I do is clean all mating faces. Then I grease them lightly. I 
never put two metal surfaces together dry. Then I use copper grease 
(anti-seize grease) on the studs and lug nuts. That is to minimise the 
friction on the threads, and ensure that the stud is properly stretched and 
applying sufficient clamping force. Many if not most operators will fit the 
lug nuts dry. The problem with that is that a large part of the tightening 
torque is absorbed in overcoming the friction of the thread, and the stud 
doesn't stretch sufficiently to clamp everything together. The nuts are 
tightened till they are stiff rather than tight. These are the people who 
have to check their wheel nuts with a torque wrench every day and fit little 
plastic indicators so they can see when they are coming loose. These are the 
people who end up in traffic court when their wheels have fallen off, and 
everybody scratches their heads because "all the proper procedures have been 
followed". I never check mine from one year to the next, I never use a 
torque wrench, and they never come loose. The only remaining mystery is why 
American trucks & buses don't seem to suffer from this problem.
I realise this diatribe is of limited value when discussing fuel lines. 
Power-steering lines will hold a couple of thousand PSI without leaking, so 
as I said earlier, it should not be necessary to adopt all sorts of exotic 
practices to stop gasoline leaking out at a couple of PSI. I reckon you are 
missing something somewhere.