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I have a late model direct drive engine which is currently torn down for overhaul. During the removal of the transmission I noticed the key that goes between the output coupling and the transmission tailshaft was quite worn. Examining closer there is a slight amount of wear in the keyway of the tailshaft and a small piece of metal is missing from the tailshaft on the loaded side of the keyway. Is there any tried method of repairing this or should I just replace the key and move on? I would like to avoid the cost of replacing the tailshaft but would hate for it to become a point of failure in the future when maneuvering in a tight situation or such. Thanks!
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Evaluating the fit of an output coupling can be a rather tough call.
I would first acquire a new key and dry fit it into the slot in the shaft and output coupling. The key is (by design) a bit softer than the tail shaft and coupling, so you may find very little play left after replacing the key.
In my experience, the fit of the output coupling is slightly more important than the fit of the key. I would dry fit the coupling over the shaft first, without the key in the keyway. It's difficult to describe a proper "press fit", but if the coupling goes over the shaft all the way to its final resting place with only hand pressure, it's too loose and it will place too much of a burden on the key for the key to survive. It is common to only be able to get an output coupling on by hand far enough to get the key comfortably seated, and then it requires a mallet or pressing tool of some sort to get the coupling in far enough for the nut to be started to pull the coupling the rest of the way to its seat.
A couple suggestions might help to avoid the costly replacement of a shaft and/or coupling in cases where the combined condition of the keyway and output coupling are simply too close to call:
1) If the coupling is too loose for comfort, you can take a center punch and put a few center punch "dimples" around the shaft where the output coupling seats. Don't get too greedy on this one. It usually only takes 4 to 6 "dimples" spaced evenly around the shaft to snug up the coupling quite a bit. If you do end up with too much "dimple", you can take a small file and easily work them over a bit.
2) If the keyway is still a rather loose fit after replacing the key, you can work a bit of JB Weld in the slots (shaft and coupling slot) to snug up the key during installation. Don't worry about getting the coupling off in the future, it won't take that much more force to pull it off.
3) Be sure to tighten the end nut very securely. This usually means the use of a breaker bar. Like the key, if the coupling ever develops play, the nut itself will not be able to prevent continued increasing of the play until the key completely fails, taking the shaft and/or output coupling with it.
NOTE: Don't expect a proper machinist friend to bless these suggestions, but the principle at work is to prevent any play at all during normal service life. If you end up with an output coupling assembly that is "less than perfect", you may want to watch your shifting technique a bit more closely in the future. Most importantly, try to avoid shifting into either mode with the throttle above idle.
Regards,
Don
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