Motor to gearbox couplers

[FFMan]

My E46 conversion has just knocked out the second version of the coupler.

For context, i removed the 2l diesel lump from the bmw and fabricated an adapter plate for an Outlander motor so the shafts had a 1cm gap between the tips. Line up felt good, with 1mm or so in 1 plane, spot on the other.

The first coupler was solid, splines from motor and clutch centre welded solidly together. That lasted maybe 2000 miles but eventually knocked the splines off the motor and gave up, In fact splines both end had taken some wear.

Second iteration (new motor) had a rubber doughnut/rotoflex coupler between the shafts. This has just knocked all the splines off the clutch centre that was on the gearbox shaft. Other splines all look ok.

I am wondering what the issue is and what next and wondered what others had done ?

I wonder is some of the issue related to the the fact the gearbox input shaft would have been supported at its tip by a spigot bearing in the flywheel when mated to the engine, and now has no such support. Maybe at speed, imbalance cause wear ?

Any tips and advice much appreciated.

[barracuda816]

This is an interesting issue.
Is there much back lash in the splines? and how much engagement is there on the splines?

Even if the run out was too much I would expect there to be bearing damage way before the splines being stripped.

Is it only the motor splines that are showing damage? It Could be worth ensuring there is as much engagement on the splines as possible and shrinking the coupling so it is more of a press on fit.

[FFMan]

so since my first post i've discussed with a few people, inc a gearbox specialist, a custom motorbike builder and a fellow racer.

The consensus seems to be that at speed the harmonics across the shafts exacerbate the natural spline movement. And by making the coupling flexible this in fact makes that worse, plus if the coupling has much weight, at 8000 rpm the forces could be quite intense.

So the current thinking is maybe, get the alignment spot on, make up a new solid coupler, possibly using two clutch centres back to back on the gearbox side to give more depth and load spread, and find someway of clamping to the splines. The BMW prop has such a clamping mechanism where it joins the back section, hard to replicate, the easy option is on one end, weld it on. Preventing the natural play in the splines at both ends is the ideal, but not sure how as yet, as on assembly access is limited due to the transmission tunnel (motor is in the car), and access because of the bell housing.

[jrbe]

My best guess from the info is that there is a misalignment issue. Any chance of mounting a dial indicator on one of the shafts and rotating it around to measure to the other shaft?
And if you mean the splines were off by 1mm that's a lot.

You might also be able to put 2 dial indicators on the transmission to motor without alignment pins or bolts so you can measure up/down and side to side movement from the spline play. Idea being you measure both and find the middle before bolting it down. If you know you got it really well aligned you can drill in 2 alignment pins.

This would be much easier and more accurate to do with the shaft / motor axis pointed straight up.