Originally Posted by hellosailor
Maine, all this sticking the shaft in the coupling, lathing everything, not just "bolting it up"...is this akin to "blueprinting" an engine instead of just building it? Is this really necessary every time a shaft is popped off?
Or is it more for the folks who simply prefer ironed shirts to permanent press?
If you remove an old straight coupling and try to re-use with a poor fit, as in not a light press fit, you will:
A) Eventually ruin the shaft because the set screw and ss key are not intended to take ALL the load. Often times the key will also fit sloppy due to the layer of rust broken free and this will result in hard loading the key from forward to reverse. A loose coupling can actually shear a shaft.
B) Potentially lose the shaft
I have seen both of the above scenarios happen when old couplings are re-used. When Buck or Walters or others ship a straight or split coupling they are normally undersized for the standard shaft sizes so they generally need to be fitted or you likely won't be able to get them on. If you don't want to face it that is not as critical if you don't mind vibrations. In lots of shaft jobs I have had one coupling, that when installed onto the shaft, did not need to be faced. Your shafting shop should always be willing to check it BEFORE facing. This is about a $10.00 job if facing is nor required. I have seen new shaft / couplings off by as much as 0.012" and that is more than enough to cause some pretty bad vibrations.
Considering the time and expense involved in doing this job and the potential safety issues the fitting and facing is a small portion.
The Sabre 34 I just finished was a ONE HANDED job due to piss poor access. Took about double the time it normally takes while working like an amputee. Things you take for granted as a 2 minutes job, like tightening hose clamps, but only blind & one handed when they just want to spin around the shaft log hose add up for the owners to the point that they become costly jobs. I don't think the owner would have been happy to do it over again if we had issues because I cut a corner and chose not to check the face or tried to re-use an old coupling.
I'm lucky in that I have a very reputable prop shop who will not ship out a shaft and coupling without doing it right. There are many shops, especially on-line who think nothing of taking your money and selling you junk.
I had one customer who insisted on buying his own shaft from an on-line source. No problem, right..? It was $137.00 cheaper than my shop excluding shipping so that's justifiable, if we're comparing apples to apples.. When I saw it there were no signs of it being faced, it was way off, and the coupling literally dropped over the shaft. The set screws were not spotted into the shaft as is required under ABYC P-06 and the keys on both ends were terribly sloppy and it came without prop nuts. Also the key at the flange end was brass and it should be SS.
All that wrong in just one on-line prop shaft adventure.
I had to decline to install it noting the potential safety issues. I sent the customer to my shop and they were horrified at the sloppy workmanship so much so that they installed a new coupling, spotted the shaft, fitted and faced it and supplied new keys all for $80.00 including the new coupling. they're good guys. Customer was still ahead but then had to buy some prop nuts and the shaft grade was still unknown. At the price he paid I am nearly certain it was not AQ22 as they said it was. You can barely buy the raw shafting from WBM for what he paid for the whole lot.
Having seen the failures I have I do take shafting work pretty seriously.. As always with your boat you can always do what you want. All I can do is share what I've learned...