I think it's a number of things. One, the diesel engines are rubber mounted and move a bit under torque, misaligning the shaft. Two, the rag joint design really can't seem to handle all the torque in my opinion. It's not so much the driveshaft itself that is the problem; the 5/8" CRS shaft is good for about 35-40 hp according to faint memories of some calculations I did awhile back. What happens is, the spherical ball bushings in the couplers on both ends wear, causing the ends of the shaft to wobble around. I think there is a certain engine RPM that happens to have a multiple of the natural frequency of the driveshaft assembly, as both my 782D and the 1512 I used to have had a HORRIBLE vibration at about 1/3 throttle before I replaced the driveshafts in both of them. When the engine runs at that speed, the shaft vibrates even more than at other speeds. The Kubota also shakes a bit at idle, which doesn't help, either. If this isn't fixed, the rag joints don't last much longer, and all of the vibration eventually wears out/crack the spirol pins holding stuff to the driveshaft. The wobbling driveshaft also wiggles the coupler on the trans input shaft, destroying the spirol pin, and eventually, the coupler itself and the trans input shaft if left unrepaired. This usually deforms all of the spirol pin holes in the driveshaft and causes weird wear on the ends, requiring it to be replaced, as well. I had to replace everything except the front coupler on both of my diesels, to the tune of more than $200 per tractor. The biggest problem is the rear coupler on the trans input shaft; if it wobbles, it needs immediate replacement. Buy the QL one, it's the same thing as the 82 series one at 1/3 the price. I've been making driveshafts out of 4140 Chromoly steel lately. If there is any vibration at WOT, something needs attention. My 782D is smooth as glass at full throttle...you cannot tell it is running when sitting on it, save for the noise.
I can't say for sure that this is what's happening, but it's my opinion as to what's going on from what I've observed owning and repairing two of these tractors.
This is also the most annoying repair I've ever done to a Cub Cadet in the 7 years I've been into these...there is very little room to work to remove and reinstall the driveshaft.
|