Driveshafts

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Awesomeness

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Yes I have read the published info about the shaft problem but I don't completely believe it. There had to be a serious problem with the manufacturing of the shafts for them to fail. Any insight into this appreciated.
You read the engineering reports and don't believe them?
 

m-35tom

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Right. There are to many other vehicles with worse angles and more power that have driveshaft slip joints last for over 100k miles. Someone screwed up and there was a cover up or something. Drive shafts do not just last 3000 miles and need to be replaced.
 

m-35tom

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From my experience, the slip joints wear out from highway driving. With the 2-1 hub reduction at 58mph those driveshafts are really humming away. Once you get play in the driveline you run the risk of throwing a driveshaft. This happens way more often than one would think. The manual has a hinging test it states .0015 inch of play is bad. The local unit that I know a few guys at say they cut that amount in half.
The driveshaft speed has nothing to do with hub reduction. It only related to the final ratio of transmission and engine rpm. So 2600 rpm in 7th is 3333 driveshaft rpm. not really so bad, just a little above most normal things. I routinely run my M-35 at driveshaft speed of 3623. These driveshafts should not have failed unless there was a defect in design or material.
 

coachgeo

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The driveshaft speed has nothing to do with hub reduction. It only related to the final ratio of transmission and engine rpm. So 2600 rpm in 7th is 3333 driveshaft rpm. not really so bad, just a little above most normal things. I routinely run my M-35 at driveshaft speed of 3623. These driveshafts should not have failed unless there was a defect in design or material.
think the problem comes from what it appears to be a solution taken by SStevens** to make best of front driveshaft angles. Since it is a shorter shaft... and would always be turning due to AWD..... my guess is SS decided to tilt the engine/tranny opposite of conventional*... Engine and the tranny point arse end up at a few degrees. This put the shorty front drive shaft at a fairly good angle... but left the rear face of transmission at a funky plane compared to face of the rear axle... which put the rear driveshaft ujoints at a funky angle.

Steeper slopes will increase existing funkyness thru a shaft ... thus if you start more funky than normal.. you end up... with even more than typical reverberation/vibes thru the system. This would appear to explain the results of the different test published. They didn't test "the reason" for the driveshaft wonkiness... only explained that it existed... and tried various different solutions for doing something about it.... from making things stronger (the D solution) and later testing alternative driveshafts. In real life use... my guess is this also explains the faster wearing rear driveshafts.

This would tend to explain why they swapped to the High Pinion rear axle in the A1P2? instead of one of the proposed different driveshafts ..... The high pinion solution; engineering wise, would seem to avoid the typical reverberation/vibration increase that happens the more a drive shaft is sloped. With High Pinion having a near horizontal driveshaft; this pretty much would seem to solve the problem . Get the feeling they may have changed the engine tranny slope also but got nothing but ancillary mentions of this.

Now in support of SS.... if they had put the engine tranny slope conventionally down... this would have put a pretty steep slope on the front shorty driveshaft instead. Also some time back someone mentioned that the original order for these trucks ..... and thusly the design based on that order...... was for 35ish mph trucks. Not sure how true that is.... but would explain the design that seems like a flaw....... originally it would not be a flaw for a 35mph truck. When order got changed to 55mph; AFTER truck was designed and moving into production.... that is when things reverberated.


*Most commonly trucks have the engine tranny on a slight downslope from front of truck to back... not upslope like our trucks. Discovered this upslope surprise when doing my high pinion axle swap. Did a request for folk to measure their angle and others reported their truck was same .... engine/tranny point up, not down.


**but this is only speculation.... only facts are... (as I understand them... but Im basically engineering dumb and only going by discussions with chassis fabricator)
> vibration etc. builds greater thru a sloped shaft compared to a horizontal one.
> ancillary review seems shows our engine tranny do slope up and conventional is down.
 
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Awesomeness

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tmy guess is SS decided to tilt the engine/tranny opposite of conventional*... Engine and the tranny point arse end up at a few degrees. This put the shorty front drive shaft at a fairly good angle... but left the rear face of transmission at a funky plane compared to face of the rear axle... which put the rear driveshaft ujoints at a funky angle.
The engineering reports specifically say the engine tilt was a mandatory requirement so the engine could keep proper oil pressure while negotiating the required 60% grade. Even though it seems like having it tipped up helps the driveshaft angle, it actually hurts it because it's moving the transfer case yoke higher (even though it's pointed a little better).

I don't think these trucks were ever intended to go only 35MPH, and it seems unrealistic that that speed would have been a design target. You might be confusing it with the 35MPH that the Army limited the trucks to after discovering the driveshaft issues, but before they had the new driveshaft solution.
 
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coachgeo

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The engineering reports specifically say the engine tilt was a mandatory requirement so the engine could keep proper oil pressure while negotiating the required 60% grade. Even though it seems like having it tipped up helps the driveshaft angle, it actually hurts it because it's moving the transfer case yoke higher (even though it's pointed a little better).

I don't think these trucks were ever intended to go only 35MPH, and it seems unrealistic that that speed would have been a design target. You might be confusing it with the 35MPH that the Army limited the trucks to after discovering the driveshaft issues, but before they had the new driveshaft solution.
the mention of the reason for the engine angle is suspect.... IMHO being that researcher main concentration was doing good study about proposals to improve an existing situation and not so much on why the situation exist in the first place... its not clear how much clarity he/she sought to know the actual reason, beyond it fits well with that opening part of the study material. Reason say that is additional search shows the CAT engine parameters allow for the 3116 engine to have a very wide range of engine install tilts that would seemingly completely negate this notion of oiling issues. If recall right the specs were for marine 3116 engines and ship/boats get pushed into as many odd positions as our trucks would off road.

but...... then again researcher may have well took the time to dig that out as fact.... so will go with it... ( with a tiny bit of suspect )

but in reality... does not matter why it's tilted like it is....... the end result is same.... funky angle of the plane between rear transmission face and rear axle face.... and that :

. the funky plane faces exist, thus more vibration created than a typical truck design..
. vibration/reverberation (not sure correct term here) is increased the steeper a drive shaft goes
. more vibration/reverberation you have the more it wears on components like driveshaft slip joints and other components..... and if it starts off worse than most... it is going to end worse down the line.
. changing the axle to high pinion (offering horizontal drive shaft) would seem to negate the issue.

your 35mph point is very plausible.
 
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Awesomeness

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Also described in one of the reports is that the original front driveshaft design was double-cardan, which would have handled the angle much better. It said they experienced issues with the strength of it, and had to switch to the regular style. Two steps forward, one step back, I guess.
 

coachgeo

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Also described in one of the reports is that the original front driveshaft design was double-cardan, which would have handled the angle much better. It said they experienced issues with the strength of it, and had to switch to the regular style. Two steps forward, one step back, I guess.
" they experienced issues with the strength of it, " Makes since... from others whom looked into doing cardan or other type high angle shaft designs for their FMTV have said.. .. Makes this statement in the study a round about way of saying....... no existing double-cardan shafts in production are strong enough and that is exactly what folk looking to upgrade their shafts found... no one manufactures the specialty high angle drive shafts for trucks size of FMTV's.
 

tobyS

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Having just built the driveshaft for my deuce, I was told anything more than 2 degree difference of the TC and the differential is too much. Does the rear differential face down to match the engine/tranny/TC angle?

I'm glad the M35A3 is parallel to the frame...I used a 2.8 degree block to bring it to about 1.4 degree (under the magic 2*).
 

coachgeo

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Having just built the driveshaft for my deuce, I was told anything more than 2 degree difference of the TC and the differential is too much. Does the rear differential face down to match the engine/tranny/TC angle?

I'm glad the M35A3 is parallel to the frame...I used a 2.8 degree block to bring it to about 1.4 degree (under the magic 2*).
engine tranny is at a 2% up angle.... assume the stock axle is set to match but didn't measure prior to removal.. only matched it when installing high pinion. Issue appeared to us that normal drop of the rear axle suspension moves the two planes of the faces (@tranny yoke and @rear axle yoke) more out of synch with one another compared to if the engine tranny was in typical downslope fashion. Least that was our hypothosis
 
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Two place I know of for drive shaft repair or rebuild in the Maryland area: In Lorton, VA, just outside DC off I-95 is Driveline Services, aka Drive Line Specialists (https://www.drivelinespecialist.com). I've had multiple custom driveshafts done there. It's an old shop that does lots of service. Yes, it's a ways away from you, but they can ship back to you.
Also, something I investigated but never used, is FleetPride. Find a local site, you drop off the drive shafts, they take them to their central repair/refurb area and when done send them back. They can also get lots of parts for LMTV and other trucks.
As an aside: I have a M1081 with about 11000 miles. Had my drive shafts rebuilt about three months ago by a local shop in Colorado Springs, CO, even though I had the D on my tags. Both front and rear were badly worn in the splines. New shafts were much smoother, and the 1000 miles last month between Colorado Springs, CO and Reno, NV went so smoothly!
 

Floridianson

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I do not see any dash plates with the D on my 97 M1088? Did they hide it? I did checked today and my front shaft did not have a weight and no play in the slip joint. Shaft to front tandem had a weight and no play in slip joint. Rear shaft no weight but some play in slip joint and hard to really to measure with it installed but not that bad I thought. Was wishing it was as tight as the others but o well. Thanks for the good info in the thread. Don't know if you remember but when I got the truck front drive shaft was out of phase and thanks to another thread got that straight. Guess I got lucky as there is no more growling or vibrations when I come off full throttle to half throttle. Even at full speed ahead no vibrations lucky again. Before I was thinking it was the front chunk as I know I had just set up all the hubs correct or dang close to it.
 
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gringeltaube

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...... Don't know if you remember but when I got the truck front drive shaft was out of phase and thanks to another thread got that straight.....
What thread was that and/or what exactly was done to fix it? Was it just one spline off, or did it have to be cut and re-weld?
 

DREDnot

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I do not see any dash plates with the D on my 97 M1088? Did they hide it?
This thread made me check mine...

20190503_134048.jpg

I could see a faint "D" stamped in.

The driveline angles on this thing are dictated by funky military requirements and not customer NVH complaints.
Other reading I've done mentioned that the research into the vibration would best be fixed with RZEPPA style joints but they also lacked strength and were nixed in favor of the bigger single cardan fix("D" stamped)

The main point is that the driveshaft life was traded for off-road prowess (and simplicity)
 

Floridianson

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No believe someone flat towed the truck and removed the front drive shaft. Some how guess they also pulled the slip joint apart and don't know why they would. Then when someone that did not care or did not know what they were doing when it came time to put the drive shaft back in the machine. They just put it out of phase on assembly by maybe six splines so she was way off. I guessed on the rotation to make the correction of the yoke and took it for a full speed drive and no vibration.
 

DREDnot

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Can these slip yokes be assembled out of phase? My HMMWV has an aligning spline that soldier proofs the shaft assembly. All the u-joints on the HMMWV front shaft are out of phase with each other on purpose.
 

Awesomeness

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I do not see any dash plates with the D on my 97 M1088? Did they hide it?
You shouldn't have to worry too much, they [supposedly] ALL got the upgrade, even if you have trouble seeing the "D" stamp. Then again it is the Army, so I'm sure there are a few trucks out there, but your chances of having one would be small still.

I'm not sure about this part, but I think that one way to tell is that the original driveshafts had yokes that the u-joints bolted to with u-bolts. So if yours are types with screw down caps, it should be good. I'm not 100% sure, but I think I remember that being one of the recommended fixes, to remove the u-bolt style u-joints.

I know it's a lot of money, but I still recommend you have your driveshafts checked and rebalanced. While it seems that only a small percentage of people have big issues, when they do it destroys the engine, or more. When my engine got wrecked, I couldn't feel an abnormal vibration from in the cab, even though I'd had the truck a year or so and put 1000-2000 miles on it. So it's not entirely obvious, and I'm usually quick to notice new sounds, smells, etc. that the truck is making.
Other reading I've done mentioned that the research into the vibration would best be fixed with RZEPPA style joints but they also lacked strength and were nixed in favor of the bigger single cardan fix("D" stamped)
The Rzeppa style joints are stronger. The engineering report said they failed the water intrusion test, but that the engineers suspected that could be overcome, and that the Rzeppa joints were the recommended solution.

I have yet to see a document detailing why, but I assume the Army just used the bigger shafts and u-joints because it was a cheaper, faster solution (little or no re-engineering involved, all off-the-shelf parts). You have to figure that they were desperate for a fast solution, because they had put a 35MPH limit on their entire new fleet of LMTVs. That had to be very frustrating, and embarrassing... good thing the USSR was gone and wasn't watching anymore.

Can these slip yokes be assembled out of phase? My HMMWV has an aligning spline that soldier proofs the shaft assembly. All the u-joints on the HMMWV front shaft are out of phase with each other on purpose.
Yep.
 

coachgeo

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I do not see any dash plates with the D on my 97 M1088? ..
your's is a 6x6..... those never had a driveshaft issue probably due the different rear axle. As far as I know the 6x6 never got the "D"... Could well be they off the shelf had the heavier duty components that became the D standard anyway (driveshaft and Tranny bell housing) just cause they were built to be 5 ton trucks from the get go and the others only 2.5ton??

either way if the 6x6 driveshaft was not initially changed along with the "D" of the 4x4/AWD LMTV's..... over time they probably got changed to upgraded driveshafts just cause motorpool didn't want cary both type driveshafts. (pretty sure long rear driveshaft is same length for Mtv and LMtv. )
 
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