Lap Fitting A Prop To A Shaft
- Thread starter Maine Sail
- Start date
Good Details/Nice explanation!
Maine, I wish you were in the slip next to mine. I'm always seeking advise and sometimes get the wrong kind. Your pictures and explanation were excellent!
Good work and thanks for taking the time and effort to get into such detail.
The sport of sailing needs all the help it can get with the economy like it is.
Maine, I wish you were in the slip next to mine. I'm always seeking advise and sometimes get the wrong kind. Your pictures and explanation were excellent!
Good work and thanks for taking the time and effort to get into such detail.
The sport of sailing needs all the help it can get with the economy like it is.
Mainsail,
Thank you for your detail procedure. Despite all the different takes on it, I’ll definitely use it when replacing my broken prop over the winter. I can't see how it will hurt and it doesn't add any noticeable cost or time to do the process. Discussions and disagreements are good for this forum because we all learn from it. So keep posting your detail processes, it has helped me in the past and will continue to in the future.
Thanks
Salty
Thank you for your detail procedure. Despite all the different takes on it, I’ll definitely use it when replacing my broken prop over the winter. I can't see how it will hurt and it doesn't add any noticeable cost or time to do the process. Discussions and disagreements are good for this forum because we all learn from it. So keep posting your detail processes, it has helped me in the past and will continue to in the future.
Thanks
Salty
Mainesail, I agree with Salty. keep up the good work. The post about balance was interesting though. Any ideas on how one would go about balancing the whole coupling, shaft and prop together, while installed in the boat? Cheers, Ian
Interesting discussion and thanks to Maine Sail for the effort and expertise.
I'm puzzled though. I can see the benefit of lapping when the machine parts stay matched when torqued. But when a prop is lapped to a tapered shaft with hand pressure, the place on the shaft that is nicely matched to that particular prop is quite different from the fully torqued final resting place of the prop on the shaft. The prop could easily move forward on the shaft 10 to 20 thousandths.
Early
I'm puzzled though. I can see the benefit of lapping when the machine parts stay matched when torqued. But when a prop is lapped to a tapered shaft with hand pressure, the place on the shaft that is nicely matched to that particular prop is quite different from the fully torqued final resting place of the prop on the shaft. The prop could easily move forward on the shaft 10 to 20 thousandths.
Early
The idea is to remove any imperfections between the two surfaces so they mate together better and actually get on the shaft to the proper depth. Often times shafts come from one shop and props from another and they can fit together poorly, despite industry standards. If you get a prop and shaft from reputable shop they will have already lap fitted it for you using this same technique.
The idea is to get the two surfaces mating well with no high/low spots. This is the same principle used on US Navy & USCG shafting. They try for a very high percentage fit which I am not privy to know but suffice it to say that by hand lap fitting you won't hit what they do nor do we have the same tapers they do. Those tapers can take days to fit right.
First, a disclaimer: I’m not a machinist, and can’t afford the book I reference below. I read the stuff I mention from the preview on Google books. My sole professional experience is kludging fixes to large machinery in the field in order to get it home to the real professionals.
After a bit of research I think that if you hand-lap for a good fit each time you mount a prop, (-perhaps even the first time if you’re big, strong and aggressive) it could create a ridge at the forward limit of the lap. When you torque the prop nut, the leading edge will sit high on the ridge, defeating the purpose of the lap fit.
The Shaft Alignment Handbook, 3rd, Piotrowski, p.172 describes a procedure that first identifies the high spots using Prussian Blue paste, laps the high spots only if contact is less than 80%, and cautions that the machine shop will have to re-turn the shaft if you develop a ridge at the edge of the lap.
Early
After a bit of research I think that if you hand-lap for a good fit each time you mount a prop, (-perhaps even the first time if you’re big, strong and aggressive) it could create a ridge at the forward limit of the lap. When you torque the prop nut, the leading edge will sit high on the ridge, defeating the purpose of the lap fit.
The Shaft Alignment Handbook, 3rd, Piotrowski, p.172 describes a procedure that first identifies the high spots using Prussian Blue paste, laps the high spots only if contact is less than 80%, and cautions that the machine shop will have to re-turn the shaft if you develop a ridge at the edge of the lap.
Early
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Props are soft and shafts hard. Manganese Bronze vs. AQ17-22 or Nitronic 50. Even in the old days when you could still get Tobin Bronze shafting, which you' can't anymore, I never once witnessed someone developing a ridge unless the prop was loose. As the title implied you are lap fitting the "prop" to the shaft taper and it is the prop that gives up the most. As I mentioned this is standard industry practice and my shafting/prop shop does it to every combo that goes out the door using the same procedure.
While nowhere near as accurate as the military shafting they fit this is still a well accepted procedure.
Maine, I agree with you method of lapping the prop to the shaft, but one thing you are removing more metal from the shaft than the prop. When you lap the lap is always softer than the part being laped. the grit sticks in the soft and cuts the hard. Machinery's Handbook has a good section on laping, (28 edition pg 1100). I'm not nick picking, but I had to look it up to make sure that I remembered right, because you almost always have your ducks in a row and I value your posts and advice.
Fred Villiard
Fred Villiard
I have read that and I agree with it but when you are doing this lightly by hand the prop is what tends to smooth out and the shaft just seems to change finish a bit. Never, ever put a groove in a shaft. In fact I called my prop shop today and asked him about it and he's never seen it happen either, a groove in the shaft from lapping a prop and he does a lot more of them than I do...
I have physically watched three of the best prop shops in the NE do this though some use special taper bores but none that I have seen do this on a lathe always by hand and with light pressure. I agree if using a lathe you would see that but by hand it works great and smooths out the prop taper well and cleans up the shaft.
Old thread but I just saw it and the graph and two video links above it are quite telling....
http://www.boltscience.com/pages/twonuts.htm
Ken
http://www.boltscience.com/pages/twonuts.htm
Ken