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Drive Shaft U-Joint
- Thread starter lmo1131
- Start date
U-joint 101
The U-joint is lubricated by gear oil picked up by the coupling bell where it naturally pools near the final drive - and “Spun-up” the shaft by centrifugal force until it covers, coats and permeates the bearings of the U-joint completely. Excess oil is then thrown off into the boot or runs off the shaft against the transmission seal and eventually runs back down into the reservoir at the ‘bell’ end of the shaft housing where it begins this circulation pattern again.
Drive line angularity is critical to ensuring the service life of the U-joint. The specific limit of movement is 30 degrees total with 15 degrees above and below 0 (0 degrees is where the shaft is lined up with the output shaft in the transmission). If that number is exceeded, the U-joint binds and self destructs. Note that the mounting position of the U-joint flange to the output flange of the transmission [on a /6 for instance] is forward of the pivot point for the swing-arm. The pinion gear that the coupling bell captures has a radius that allows the coupling and drive shaft to move in a separate arc from the swing arm itself.
That separate arc is not a problem until the swing arm bearings seize from lack of lubrication or the pins holding the swing arm in the frame are too tight OR the shaft is not centered and hits the shaft housing. If the swing arm binds on the bearings, it will cause the U-joints to wear rapidly. Another failure mode is from a T-bone accident. In a straight on crash, the final drive / swing arm move forward instantaneously and the mass of these heavy components comes to bear on the needle bearings in the U-joint. This in turn can cause dents from the needle bearings in the races, damage to the individual needle bearings, rapid wear and failure. In some cases, the U-joint becomes loose from a hit like this and in other cases it becomes tight. One other failure is from lack of lubrication or use of improper lubricants…
If I were to hazard a guess as to why the later style drive shafts with a U-joint at both ends fail it is because of three things: Drive line angularity exceeded, insufficient lubrication and something the 70’s airhead drivers don’t need to worry about~ timing the U-joints.
The U-joint is lubricated by gear oil picked up by the coupling bell where it naturally pools near the final drive - and “Spun-up” the shaft by centrifugal force until it covers, coats and permeates the bearings of the U-joint completely. Excess oil is then thrown off into the boot or runs off the shaft against the transmission seal and eventually runs back down into the reservoir at the ‘bell’ end of the shaft housing where it begins this circulation pattern again.
Drive line angularity is critical to ensuring the service life of the U-joint. The specific limit of movement is 30 degrees total with 15 degrees above and below 0 (0 degrees is where the shaft is lined up with the output shaft in the transmission). If that number is exceeded, the U-joint binds and self destructs. Note that the mounting position of the U-joint flange to the output flange of the transmission [on a /6 for instance] is forward of the pivot point for the swing-arm. The pinion gear that the coupling bell captures has a radius that allows the coupling and drive shaft to move in a separate arc from the swing arm itself.
That separate arc is not a problem until the swing arm bearings seize from lack of lubrication or the pins holding the swing arm in the frame are too tight OR the shaft is not centered and hits the shaft housing. If the swing arm binds on the bearings, it will cause the U-joints to wear rapidly. Another failure mode is from a T-bone accident. In a straight on crash, the final drive / swing arm move forward instantaneously and the mass of these heavy components comes to bear on the needle bearings in the U-joint. This in turn can cause dents from the needle bearings in the races, damage to the individual needle bearings, rapid wear and failure. In some cases, the U-joint becomes loose from a hit like this and in other cases it becomes tight. One other failure is from lack of lubrication or use of improper lubricants…
If I were to hazard a guess as to why the later style drive shafts with a U-joint at both ends fail it is because of three things: Drive line angularity exceeded, insufficient lubrication and something the 70’s airhead drivers don’t need to worry about~ timing the U-joints.
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K
keelerb
Guest
How could they possibly have ever maintained the Thousand Year Reich, if they weren't clever enough to make drive shaft U-joints REBUILD-ABLE?!?!?!?
Well, the Thousand Year Reich only lasted 12 years, so your driveshaft did considerably better....
lmo1131
New member
Well, that went just about as expected ... got bit by an unexpected glitch in the process. I dry-fit the bolts, but didn't do a dry-run with my new Northwood Airhead Torque Wrench Adapter...
After applying Blue Loctite and running all the new short 12pt bolts into the transmission flange, I blue-torqued them by hand using the 12pt wrench in the tool roll. When I applied the Adapter (to see how close I was (Kurt)), it wouldn't fit over the bolt heads. I rotated the drive shaft to three different positions thinking that that might open up some clearance... no deal, no go.
I "doubled" the wrench and got a little more rotation on the bolts; at that point I called it good.
I have now determined that the mechanical detail of the u-joint yoke on the drive shaft is of two configurations ... the one most of us are familiar with with the yoke "recessed". The NATW Adapter would probably work fine with this one...
problem is ... my '73 /5 has this configuration u-joint. The yoke surface is flat and there was insufficient clearance for the Adapter to fit over the bolt. Lucky me. This would seem like a later model production mod; maybe for a 900cc engine with more torque?? The BMW part numbers are identical. Anyone have a clue?
After applying Blue Loctite and running all the new short 12pt bolts into the transmission flange, I blue-torqued them by hand using the 12pt wrench in the tool roll. When I applied the Adapter (to see how close I was (Kurt)), it wouldn't fit over the bolt heads. I rotated the drive shaft to three different positions thinking that that might open up some clearance... no deal, no go.
I "doubled" the wrench and got a little more rotation on the bolts; at that point I called it good.I have now determined that the mechanical detail of the u-joint yoke on the drive shaft is of two configurations ... the one most of us are familiar with with the yoke "recessed". The NATW Adapter would probably work fine with this one...
problem is ... my '73 /5 has this configuration u-joint. The yoke surface is flat and there was insufficient clearance for the Adapter to fit over the bolt. Lucky me. This would seem like a later model production mod; maybe for a 900cc engine with more torque?? The BMW part numbers are identical. Anyone have a clue?
Lew -
I can see how the torque adapater would fit on the top picture but not on the bottom figure. Notice how in the top picture that the forward flange is dished out all the way down to the screw hole...this will allow the rounded portion of the adapter to fit.
In the bottom picture, this area is not dished out but seems to be flat all the way down to the base. I suspect this is why the adpater didn't fit. Not much you can do about it, unless you make the adpater very thin all the way around the circumference, which might severely weaken it. No way you're going to be able to modify the driveshaft flange.
I can see how the torque adapater would fit on the top picture but not on the bottom figure. Notice how in the top picture that the forward flange is dished out all the way down to the screw hole...this will allow the rounded portion of the adapter to fit.
In the bottom picture, this area is not dished out but seems to be flat all the way down to the base. I suspect this is why the adpater didn't fit. Not much you can do about it, unless you make the adpater very thin all the way around the circumference, which might severely weaken it. No way you're going to be able to modify the driveshaft flange.
lmo1131
New member
I'd like to bring this to Northwood Airhead's attention. The $20 I spent isn't going to kill me, but I think they should be aware of the situation. The 12pt wrench has a thinner wall section than the adapter; forged vs. machined. BIG difference is strength.
Does anyone have a clue as to what year/models this "flat yoke" configuration is? I'm going to give Rusty Gill a call tomorrow ..... Rusty knows everything.
Does anyone have a clue as to what year/models this "flat yoke" configuration is? I'm going to give Rusty Gill a call tomorrow ..... Rusty knows everything.
