Okay. I see many, including Tmoose, want to know the real deal behind my scenario.
When I say "machinery", I mean underground mining machines. So yes, the environment is *that* severe. This mining machine has
a standard Berco undercarraige system. i.e. It has the tension wheel, tension cylinder, track set, and finally, "dolly
wheels/or road wheels". In either case, refer to my photograph attachment depicting the seven "road wheels" at the bottom of
the Dozer.
The problem is that the entire machine is undamped so we are talking about a 4 ton machine being tossed and turned like a toy
with the bottom road wheels (more specifically the front road wheel) taking the full impact. Before you know it, at best the
first wheel breaks off at 400 hours and at worst breaks off at as little as 100 hours of operation. The cost of one a piece
is pretty hefty. And that is the problem. Before I move onto my solutions, let me say that by me implementing a solution to
this would be a massive achievement in my junior engineering career. It really is a challenge, and its relevant to my
studies.
Nevertheless, onto my thinking of how to go about this... Any criticism you have, please dont be afraid to hammer home some
points, I will love any help.
I see many problems:
1) Bearing surface embedment. Being in the mines, it is inevatible that dust grit or flakes of paint (from new road wheel)
will be ever present between the mate. So that when you tighten the bolts, the joint will "relax" quickly after that? Apart
from the obvious solution to clean and make sure no grit is on bearing surface, I see no other way around this. And just by
the way, Im not too sure as what literature means by which mating surface. They only seem to mention bolt head- part
interface. What about the part 1, part 2 interface? And the nut-part interface? I will provide a sketch if anybody requires
to understand.
2) Undertorquing. Using simple spanners, how can the technician "know" what is the right tightness. If they undertorque, the
preload will be low and the bolt may loosen quicker. Yes, I know I know, even without the bolt loosening, the preload may
drop but ill get to that now.
3) Overtorquing resulting in tapped hole deformation/stripping. They may at times use a pipe extension and tighten so much
that the thread on the machine strips. By the way, the road wheel bolts are bolted into a tapped hole and therefore do not
have a nut on the other side. If the thread is damaged (deformed or stripped), it may loosen quicker, or it may deform and
thus loosen pre-load without ever loosening.
4) Overtorquing resulting in bolt necking. In a desperate attempt to keep it as tight as possible, the bolt may neck. To be
honest I have not seen or heard of this. I even wonder if this is a possibility because the bolts used are 10.9, and the
thread into which they bolt into (directly into the machine) is mild steel. So the thread will always be the weaker point and
the thread will strip due to overtightening before the bolt necks?
5) My worst fear is the invetable and what maybe gadkinsj was pointing out. The tapped hole into which the bolt goes into,
simply gets deformed over time no matter how the hole is torqued. Due to the severe impacts, the hole plastically deforms
each time. Its mild steel afterall. This worst nightmare of mine may be true. Upon inspection of undercarraige with road
wheels broken off, the holes looked simply terrible. The steel almost looked like chewing gum on the surface. If this is
inevatable, I can surrely to some degree mitigate this?
Ultimately the bolts break due to loss of preload which is a combination of bolt loosening and surrounding deformation.
One of my solutions, and correct me here, is to use a torque wrench and adjust the bolt to a value *JUST* high enough before
the bolt necks OR before the surrounding thread strips or deforms. Like I mentioned before its one of these and most prob the
thread because its mild steel. But what value to use? Well... solely judging on the bolt (M12 and 10.9) the literature says
that a guideline value of 129 N.m ("dry thread") is to be used considering failure to shear and tension combined with a 90%
of the actual yield value (safety factor... if you will). But my problem with this is that I cannot only consider the bolt yeild strength. What about the thread which should give way first??? Infact the bolt science website (very comprehensive by the way, I recommend this site to all of you... its simply biblical in its breakdown of bolt mechanics) for some reason never
mentions how to take thread strength into account. If I can know what the thread can stand, then I know the limiting factor we can then tighten it to that limiting factor. Further, if it is indeed inevitable that even this value loses pre-load after many hours (initial pre-load loss due to embedment), we can pick it back up to that value after, I dont know, say, 50 hours.
It is at this very point that I had a heated debate with one of my colleagues. It was the typical "young engineer" (me)
versus the "old veteran" (colleague) of the company.
I argued my torque wrench idea that it may have *some* help with this and he flat out rejected it. His argument is that there
are too many variables. By this he consistently pointed to the fact that "we cannot know the value" "too much dirt" "what
about deformation over time?" etc etc. Im sure he has a point to a degree. But I find it hard to accept prefering the use of
a spanner over a torque wrench. Atleast with the wrench , you can consistently hit *a* value and not guess. Yes I understand
that thread fricion has play here and grit etc but this can be put under certain control. The colleague mentions that "with
nordlock washers, using a clean thread/surface, using loctite and tightening the hell out of the bolt, the road wheel WILL
NOT COME LOOSE". I cannot beleive this as im sure thats been done and never worked therefore I am to conduct this procedure
and see if it holds out. Ultimately, how true are points 1 through 5? And how does my line of reasoning hold? as you see, its very interesting and stimulating topic
