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Spline Failure: Why Does Radial Offset Matter?


2wheelterry

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I have disassembled my 2002 R1150RT with nearly 31,000 miles. As suspected, I found failure of the splined connection between the clutch disk and transmission input shaft. I had noticed that the low gear downshifts were very sticky over the last several thousand miles. I had planned to perform a spline lube during the coming winter.

 

Now that I have it apart and can see these parts, I can better understand the past discussions about potential causes of the spline failures. I have read many of these posts attempting to get up to speed. Some past posts suggest that a radial offset misalignment needs to be corrected. Per the following two posts, here and here, these fellows have had success making this alignment correction, (which was impressive work).

 

I do not understand why this makes an improvement. The disk is sandwiched between the pressure plate and housing cover, (unlike a traditional clutch) and is centered on the transmission input shaft. The pressure plate and housing cover are centered on the crankshaft. If these two centerlines do not line up, it seems the disk would ride offset between these with no additional loading on the splines of the input shaft and disk.

 

I want to understand this so that I can decide how to best address my own issues. I can see how an angular misalignment can cause loading of these splines. But, can someone explain how a radial offset misalignment creates extra loading on the splines of the input shaft or clutch disk?

 

I would attach a Max BMW blow-up of the clutch assembly, but have never figured out how.

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2wheelterry, in a nutshell the rotating instant center of the crankshaft & the rotating instant center of the transmission don't line up with each other while in operation so they try to find a common center while spinning, even though they don’t line up properly.. That trying to find center places a very high load on the connecting coupler (that would be the clutch disk & it’s center hub in your case) so spline wear results..

 

Lets take the misalignment to the extremes.. You have two shafts that run in the same direction mounted end to end only they are not on the same plane (lets say they don’t line up by 4”).. You must connect them together so the front shaft will spin the rear shaft.. With a 4” offset that is going to be difficult as the shafts are close enough end to end to not allow a 3rd shaft with 2 U-joints to be placed between them.. So all you have room to do is pry both of them 2” towards each other then connect with a collar (that would be the clutch disk center).. How long do think that would work before something wears or breaks?

 

Twisty

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Joe Frickin' Friday
The disk is sandwiched between the pressure plate and housing cover, (unlike a traditional clutch) and is centered on the transmission input shaft. The pressure plate and housing cover are centered on the crankshaft. If these two centerlines do not line up, it seems the disk would ride offset between these with no additional loading on the splines of the input shaft and disk.

 

Begin with the driveline at rest. Assuming an offset between flywheel-center and gearbox input shaft-center, if the clutch disk is centered with respect to input shaft, then it's not going to be centered WRT flywheel-center; as the flywheel spins, the center of the clutch disk will orbit about flywheel-center. Once per revolution, it will be centered on input shaft center; half a rev away will be worst misalignment and greatest stress.

 

Stress alone does not cause wear. But stress, coupled with the sliding action as it goes from centered to eccentric every time the flywheel (and input shaft) spins through a rev, does. The splines are not a zero-clearance thing, and that tiny bit of radial movement, coupled with the high stress as described above, will have those surfaces fretting against each other. With perfect alignment between shaft-center and flywheel-center, the spline teeth (for the most part) simply push on each other, with little sliding action (and hence little wear).

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