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Dealing with thrust loads in drive shafts under high torque

Russ, I’d be concerned about just plain friction first. Confirmed by the Machinery’s Handbook (rock solid source of information), the laws of friction basically say for dry or unlubricated surfaces, that until you get to abnormally high pressures, that friction in both the total amount and the coefficient are independent of the areas in contact. That means until it gets to some undefined high pressure, the surface area or psi or clamping force are not important for calculating forces due to friction as long as its not close to galling. Just multiply the total force pushing the surfaces together times the coefficient of friction.

There are plenty of different values for the coefficient for metal to metal, the handbook lists .8 for static and .4 for sliding. Thats 80% of the pressure forcing the surfaces together is needed to start the shaft sliding and 40% to keep it moving! They do say when pressures get abnormally high the friction does increase at a rapid rate until seizing takes place, but give no indication of what “abnormally high” is. Anyway, that would only make the thrust numbers much higher than what you get using the coefficients they list!

Using the lower 3,000 lb-ft number for driveshaft torque, the thrust from your driveshaft would be anywhere from around 14,700 lbs to get the shaft moving to around 7,400 lbs once its sliding. I don’t know about anyone else, but to me thats way too high and thats before figuring in any binding or other real world factors making matters worse!

While the laws of friction for lubricated metal sound much more complicated, we can still use the coefficients to get a rough comparison. The handbook lists .16 for static and .03 for sliding. Thats only 16% of the pressure forcing the surfaces together is needed to start the shaft sliding and 3% to keep it moving! So if the shafts were well lubricated the numbers drop to roughly 3,000 lbs and 550 lbs respectively. Granted thats over simplifying it and most real world conditions probably make matters worse not better. If nothing else it sure makes a case for using lube of some kind!
 
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are we overlooking the fact that 9,000 lb of torque at the shaft is gonna be o, like 46,000 after the diffs (5.13)...
at 46,000 ftlbs... wont the tires spin? 554,000 inlbs/4 tires 138,510 inlbs per tires/ 42"= 3,297... ok, i guess i answered my own question.... if all the weight is on one tire, and the tire is only exerting 3300 lbs force, then the truck wont move... right? /forums/images/graemlins/thinking.gif edit: if you are headed straigt up in the air and under no acceleration.. /forums/images/graemlins/smirk.gif

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Ryan, good question. If we are assuming all four tires have equal grip then the 9000 lb-ft of torque is split between the two driveshafts 4500 lb-ft on each. 5.13 gears = 23,000 lb-ft each rear. 11,500 lb-ft at each wheel. The tire’s radius is the lever that the axleshaft uses to push against the ground. Figuring the tires squished 3" into the ground gives 18" as the loaded radius. 18" =1.5' so 11,500 / 1.5 = 7666 lbs. Can a 42" swamper, aired down and wrapped around a rock generate a peak load of 7600 lbs before losing traction? I guess it depends on how much force is pushing the tire against the ground and how the tread interlocks with the rocks.

Don’t forget this is a extremely short duration event, not a constant load. On the “landing” the tires bite and try to accelerate the truck while decelerating the drivetrain, the loads will build up until some combination of the following events take place; the tires loose traction, the rig moves enough to lessen the load on the driveline or things flex and/or break. Trying move the rig means overcoming the inertia of 5,000 lbs of weight, so its not that the rig won’t move, but that it won’t move fast enough to relieve the stress on the drivetrain before something breaks.

But for all I know I could be missing something important in all this /forums/images/graemlins/doah.gif Where are all the engineers out there?
 
Wow, good points.

About all I can add is a point related to the tire traction. When figuring contact pressure to estimate peak load before slipping you would have to also figure in pressure from decelerating 5,000 lbs of mass falling off the waterfall. So I would guess (WAG) that between deformation of the low pressure tire, and high momentary pressure on the contact patch, the peak/shock load could go quite high.

But I think you have further convinced me that the slip is not slippery enough running dry. Most of the guys running them out here do run them dry, and seem to have few problems, or at least I have not been aware of any consistent issues. On the other hand, most are linked Toy based rigs too, which are rather different than a 5000 lb truggy with a V8, 42" TSLs, and leafs. I'm starting to think I was lucky to get this far without more carnage… /forums/images/graemlins/thinking.gif
 
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So I would guess (WAG) that between deformation of the low pressure tire, and high momentary pressure on the contact patch, the peak/shock load could go quite high. /forums/images/graemlins/thinking.gif

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This is the point that none of the math (although impressive/forums/images/graemlins/waytogo.gif) has yet captured. The dynamic loading of a real world situation multiplies this by a factor that this discussion is only guessing at... /forums/images/graemlins/thinking.gif .... and I think we're all underestimating it.

What choices do you have.../forums/images/graemlins/dunno.gif ... Perhaps a [] shaft bit you in this case. Perhaps that coupled w/ inertial loads, Torsional stresses, and "Bud Lite style" driving all stacked up to cause it.

...or perhaps it was cuz the guy who owned the tranny before had an FI Big Block that would roast 35's on the street like a Top Fueler /forums/images/graemlins/ignore.gif...

Anyway... you should go w/a conventional HAD style driveline in the rear next time. Leave the [] for the front. Then you can do >40 mph!... /forums/images/graemlins/laugh.gif

Marv
 
Hey, what you talking about! I've been clocked running over 45 (based on functional speedo in another truck that is reportedly a bit low) on the TM roads and had no significant vibrations! /forums/images/graemlins/dunno.gif At least nothing worse than I got with the old round shaft (before it spiraled) and unbalanced 42s…

But your right, the rear square shaft seems to have been my problem, mostly due to the weight/power in my truggy. And maybe due to Dave's heavy foot. Yeah, that's it, I'll blame it on him... /forums/images/graemlins/laugh.gif At least there will be no more tweety birds chirping away... /forums/images/graemlins/whistling.gif

But anyway, I think I may try to figure out a way to lube it and combined with links that may take care of it. Dry lube or heavy high-film grease with a seal of some sort should do just about as good, if not better, than a splined shaft. Seems the larger diameter of the contact surfaces should drop the loads quite a bit I would think, and since the load * coef is all that is important, the profile should not matter if I can get grease in it. I just had no idea the type of forces that were coming into play with the dry shaft.

Looks like anecdotal evidence and "thinking things through" let me down this time... But without an ME or similar, what do you do? /forums/images/graemlins/doah.gif <beating head on wall>
 
Russ,

http://www.finishing.com/59/81.html
http://www.google.com/search?hl=en&ie=UTF-8&q=sandstrom+dry+film

sandstrom is the stuff we use on the Helos. i cant find any product info thats why i included the first link. it has the Mil number that we get from sandstrom.

it works well. the main use for us on the helos is to make the syncronized elevator system be a .002 to .005 slip fit. the only problem we have had with it is if oil based lubricants contact it and it stays installed for a long period of time then you will never get it appart. schedualed maint for them to come appart is normaly every 200 flight hours. application instructions for us is to use a lube to assemble the spar tubes into the horn. you can see where this is a problem for us. on a drive shaft app movement would be happening more offten. i wouldnt worry about the oil problem due to the movement.

it is also a corrosion prevention deal as well. no dissimilar metal contact or extra play.
Grant
 
For a square shaft lube, how about powdered graphite? The same stuff you're supposed to use in door locks and deadbolts. Won't attract dirt, and you probably won't have to come up with a seal.
Or a little moly grease? Just enough for a thin film on the surfaces.
 
jim... lol, in a tired stupor, i left the lever arm as 42" /forums/images/graemlins/rotfl.gif
 
Damn, I was just about to post that until I saw you already mentioned graphite. I think that is perfect and you can get it in a aerosol can and apply it before every run. That one can will last a long time too, so not much expense.
 
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snippage......
This would explain why HD drivelines often use coarse splines (less overall surface area)

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If less surface area in the 'spline' is the solution then the square drive shaft would be the ultimate or nearly so. I am inclined to suggest the opposite, go with fine Involute splines. With Involutes you can have some clearence in the system and still have it self-center under load.
 
UPDATE....................

Square Drivelines have caused another failure.

77K10Chevy (Andrew) called me today and told me he has become victim #2 of unlubricated square drivelines.

He said he ripped and stripped some bolts on the back of the tranny or tcase. I don't have all the details, he will I am sure when his internet is up again.

He got lucky and should be able to drill and retap the holes in the trans and be OK. His failure was not extreme like Russ's. He has not has a chance to get any good trail time yet on the square shaft, just play around stuff so far.

Harley
 
drill and tap for grease zerks, grease b4 every trail run /forums/images/graemlins/waytogo.gif
 
what if you mill down the 2X2 from a .250" wall to a .200" wall and braze it up in two spots 2" long and mill it to a nice fit with the 2.5x2.5. this would make the contact area less or would it make it worse.

Andrew
 
If you do that I would figure out where those patches sit at ride height and install some grease zerks there so you can grease the 2" areas of the shaft.
 
i have read much of this post but with out knowing the exact details on bad dogs distruction but from what i undersand he was climing the waterfall and busted his drivetrain preaty bad and there is a suspition that the square drive shaft was the culprit. all these point are very interesting but theres one thing that hasen't been mentioned yet /forums/images/graemlins/thinking.gif(if it has i must of missed it or maby its not a issue but i'll through it into the equation anyway) no one mentioned putting a vent hole in the drive shaft to pre vent a hydrolock or air lock situation. what if baddogs suspention was at full extention then it was collapsed at a very fast rate if the vent hole could't relieve the preasure at the speed in wich the shaft collapsed then maby it could hydrolock before it actually bottomed out /forums/images/graemlins/dunno.gif
well what do you guys think would that be something to consider or am i just talking out my a$$ /forums/images/graemlins/thumb.gif
 
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no one mentioned putting a vent hole in the drive shaft to pre vent a hydrolock or air lock situation. what if baddogs suspention was at full extention then it was collapsed at a very fast rate if the vent hole could't relieve the preasure at the speed in wich the shaft collapsed then maby it could hydrolock before it actually bottomed out

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/forums/images/graemlins/thinking.gifThis thread got you thinkin', eh /forums/images/graemlins/screwy.gif.

I'm not trying to shoot your theory down, but if we were dealing with an "incompressible" liquid - then this would be a possiblity. But the only thing that could be trapped in BadDog's driveshaft is air.... and I think the forces we're talking about would have no problem compressing a little Nitrogen /forums/images/graemlins/wink.gif...

Marv
 
yea i guess air can compress alot but how much preasure can you build in a 2x2 tube with about 8" or so stroke not much i guess if my calculations are right ( with a 24" drive shaft with 10"of stroke 2x2x19" length=76 inch cubed for the inner shaft and the outer shaft would be 2.5x2.5x10"fully extended=62.5 so 76+62.5=138.5 total cubic inch so 138.5 /76=1.82 compression ratio )
i don't know if that's right but i guess that is minimal compression inside the shaft for just compressing air.
/forums/images/graemlins/doah.gif
even if he was mudbogging and filled it with water i don't know if that would be enough to lock it /forums/images/graemlins/dunno.gif
i guess i never thought that over very good /forums/images/graemlins/screwy.gif
 
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and I think the forces we're talking about would have no problem compressing a little Nitrogen

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I agree, but it could act to delay the collapse of the driveshaft since the compression is not likely to be instantaneous. We've probably all come across the splined slip yoke that is fully compressed and won't easily slide back out b/c of the grease sealing out the air. Low compression ratio or not I have reasonable confidence that this could be a factor in the total picture.

As an aside, I just don't see any redeeming features in a square driveshaft, but then I don't drive the way Russ and others do.
 
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