2nd Anniversary Update.
Well, despite all the care the old sticking problem resurfaced after another year. A 1cm strip of corrosion on the square shaft seems to have been a focus for collecting gunk and even though it was almost imperceptible, it could have been enough to stick into the matching aluminium face of the driven clutch plate. Also, it seemed that the outer pulleys were showing play at the limit of their tolerance (5mm total side to side at the rim, an informal limit discussed in other threads) and so I took the plunge to replace nearly everything - square shaft, inner plates, outer pulley and springs (see pic). I retained only the outer transmission mount bearings (relatively new) and the shims. Expensive, but cheaper than a new mower.
Net result, works well again. Two factors that might have helped this time around:
- The new driven plates have extended bosses that meet at the centre of the transmission. Apart from giving increased contact area on the square shaft (which reduces friction loads) the driven plates now sit inside the yoke bearing with a slight interference. On my original transmission the centre of the yoke bearing was filled with an aluminum spacer which I had to remove - a single cut with a hacksaw was sufficient. This is a significant improvement because the yoke bearing can now help to pull the driven plate back off the clutch pulley, helping to overcome the stick problem.
- I found that on assembly of the new components and tightening the clutch pulley nuts that the clutch pulleys were not trued to the centre shaft. This could be seen by sitting the transmission up on a vice and watching the outer edges of the clutch pulleys as the square shaft was rotated. The out of true shows as the rim of the clutch plates moving back and forth a couple of milimetres as the square shaft is rotated. Given that there is only a couple of milimetres in the clutch contact travel, this could make a significant difference to the clutch engagement process. I corrected the wobble of one clutch pulley by giving it a calibrated (light!) tap with a rubber mallet. Unfortunately I was in a rush to finish and didn't do both (kicking myself now) because there is a slight pulsing engagement at near neutral which means that the other plate is not riding true either. If I pull it out again I'll pay a lot more attention to truing up the clutch pulleys to the square shaft.
Hopefully this fix will be enduring so that I won't have to provide yet another update.
Cheers, Matt
