nrpetersen
Curmudgeon
Think of a spline as a 1:1 gearset with one gear running inside of the other. The internal and external teeth of a spline are involute, just like regular external gears (i. e. curved profile). But splines are designed assume that all teeth will equally assume the load (actually contribute to the torque). The problem comes if the load varies every revolution as when the "gears" are not aligned precisely and rotating on the same centerline. This causes fretting when the lube eventuallyis rubbed out and things weld themselves metal to metal. This causes the characteristic red (ferric) or grey (ferrous) debris in failed clutch spline housings.On some of our Cat hoes we could cook the pump drive splines every few thousand hours. Alignment and material defect turned out to be the real causes but the big guys in the office wanted to blame the operators. When I was a Cat tech-rep, I saw the same thing on a wide variety of machines including drive axles and every once in a blue moon, the drive coupling on their compact machines. It was always alignment and material defect.
Practically, my R1100RT has serrations, clutch ID and tranny OD, defined by the lack of radius on the outer diameter. Just a distinction that doesn't matter unless someone thinks they know better. Good lube is important and not to add a whole pile of controversy because so many are so smart, I use wheel bearing grease. Works for me, all the way back to my old bikes.
BMW, like Caterpillar and all the others don't make machines to inherently fail but, I've seen stuff come off the factory floor "close enough" and I have even seen dealership technicians/mechanics at PDI say, "really, we are going to give this to a customer?"
The best lube is some sort of antiseize. Moly greases do well here but it also has to be very sticky and have a lot of extreme pressure material in it.
Note that if the shaft supports are stiff (and they are here!), misalignment will cause the clutch disk to be drug around the flywheel face and pressure plate, placing a massive radial load on the spline that rotates with the spline axes. This is what causes the fretting wear - whenever the engine rotates with the clutch out.