I didn't know there were so many variations of shaft tapers. http://littlemachineshop.com/Reference/Tapers.php
Lap Fitting A Prop To A Shaft
- Thread starter Maine Sail
- Start date
like i said "strange breed of cat." lot's of fandango here... for 30+ years all my friends were machinist, tool and die makers, even a navy master gear maker. one fellow came straight from wm.r. moore, didn't know a chuck from a chick. i helped him learn the trade. the right way. told him to get skin like an aligator, you'll need it. i'm proud to say he is the owner of a multi million dollar manufacturing company, with over 20 cnc machines running round the clock. Go Jimmy Murphy! if i ask him to make me a 2.5" propshaft 92" long, he would do it right. he would know what the small end of the taper had to be, withen .001", t.i.r. .001. every aspect comes from a little handbook. there would be no toolmarks on the taper or from the steady rest. there would be no 4 jaw chuck marks on the coupling end. of course if he had the coupling he would fit it to the end and face it true. in short it would be a nice piece of work. maby that's why he's makeing surgical implants for humans. if you want to lapp this shaft to the prop when you get it, fine... no shaft would leave my shop like the one in the first picture. we call that a pittsburg finish. probably said oops! if i file and polish it will be too small... just for curiosity's sake put a mike on the small end and see what you get. jimbob
post script, hope i offended no one except my fellow aligators.
post script, hope i offended no one except my fellow aligators.
Fortunately it did not leave the shop like that. It has had three different props on it and has been removed from the vessel four times since installation. Each removal requires cleaning the outside of it of growtha nd ocean gunk with a ScotchBrite pad so I can slide the PSS rotor off it.... It looked a lot better when I first installed it with the original prop.
one more quick question.. with the prop and hub all fastend down did you check the balance. lots of shops have a set of balanceing wheels handy for just such an assembly. particularly if the do fan shafts or air handlers. jimbob
Balance
Thereisabelle makes a good point about balance. I wouldn't just do a static balance though. The shaft and prop should be dynamically balanced to avoid any mass moment imbalance. Car tires used to get static balanced, now they all get spun up. Someone mentioned at low RPM's why worry, how many RPM's does a car tire make and a lot of effort goes into balancing and alignment. I'd suggest no less care be put into a propeller installation, I'd actually think a lot more given the precision of the parts and the expense.
So rather than a bunch of math I looked up a car tire and got a table that shows for a 16 inch tire at 65 MPH it is spinning about 12 revolutions per second. 12 x 60 seconds/min. = 720 RPM's at 65 MPH. So if a prop is spinning at say 1200 or more I would think balance and alignment would be pretty important. I have worked with balancing rotors that spin up to 120,000 RPM and the effort to balance those perfectly is enormous, along with the cost.
Thereisabelle makes a good point about balance. I wouldn't just do a static balance though. The shaft and prop should be dynamically balanced to avoid any mass moment imbalance. Car tires used to get static balanced, now they all get spun up. Someone mentioned at low RPM's why worry, how many RPM's does a car tire make and a lot of effort goes into balancing and alignment. I'd suggest no less care be put into a propeller installation, I'd actually think a lot more given the precision of the parts and the expense.
So rather than a bunch of math I looked up a car tire and got a table that shows for a 16 inch tire at 65 MPH it is spinning about 12 revolutions per second. 12 x 60 seconds/min. = 720 RPM's at 65 MPH. So if a prop is spinning at say 1200 or more I would think balance and alignment would be pretty important. I have worked with balancing rotors that spin up to 120,000 RPM and the effort to balance those perfectly is enormous, along with the cost.
IMHO - Is it Better - Yes. Is it Necessary - No.
Over the last 50 years or so I have built up a large selection of prop pullers to suit all types of prop. People come from far and wide and ask to borrow them when they can't get their props off. First they try hitting them, then simultaneously with two hammers then, as a last resort they use heat, then they come round and borrow my bag of pullers.
Nobody will convince me that it is necessary to lap to get a good enough fit to prevent rotation.
Again, anyone ever heard of a prop key shearing, even after entanglement with a chain. Typically keys are 1/4" square, 2" long and made of steel. No the gearbox or the coupling lets go first. The key is there as a last resort. Makers wouldn't go to the trouble and expense if it was not necessary.
Occasionally one hears of a prop which fell off. This could not have happened if the prop was properly lock nutted or had a cotter pin so it would be impossible to attribute this to a badly fitting taper.
Now lapping only puts circular rings onto both parts because this is the only direction that lapping motion is possible. Proper lapping needs movement in all directions.
As was said previously one can never get a perfect fit so there will always be a molecular wide gap. ANY gap means no contact between the surfaces - and no contact means no transmission of torque from that area. So we are back to square one. The torque is transmitted only where there is metal to metal contact and there is not much of that however 'good' the fit.
It all relies on the deformation of both parts resulting from the compression caused by the nut to get any appreciable contact area.
Over the last 50 years or so I have built up a large selection of prop pullers to suit all types of prop. People come from far and wide and ask to borrow them when they can't get their props off. First they try hitting them, then simultaneously with two hammers then, as a last resort they use heat, then they come round and borrow my bag of pullers.
Nobody will convince me that it is necessary to lap to get a good enough fit to prevent rotation.
Again, anyone ever heard of a prop key shearing, even after entanglement with a chain. Typically keys are 1/4" square, 2" long and made of steel. No the gearbox or the coupling lets go first. The key is there as a last resort. Makers wouldn't go to the trouble and expense if it was not necessary.
Occasionally one hears of a prop which fell off. This could not have happened if the prop was properly lock nutted or had a cotter pin so it would be impossible to attribute this to a badly fitting taper.
Now lapping only puts circular rings onto both parts because this is the only direction that lapping motion is possible. Proper lapping needs movement in all directions.
As was said previously one can never get a perfect fit so there will always be a molecular wide gap. ANY gap means no contact between the surfaces - and no contact means no transmission of torque from that area. So we are back to square one. The torque is transmitted only where there is metal to metal contact and there is not much of that however 'good' the fit.
It all relies on the deformation of both parts resulting from the compression caused by the nut to get any appreciable contact area.
OK, not to start a big hoo-haa here but have you considered that the prop nuts won’t come loose and a prop won’t fall off unless the prop itself is loose enough to cause that?? It is very tough to get something to snag the aft end of the prop, at a smaller diameter than the hub..
On the lapping, consider that ya have mismatched tapers.. There will be line contact in a ring around the tapers.. As you grind them together, the line will “grow” to a band as the prop moves along the shaft axis very slightly.. that is how ya get two kinds of motion, one circular and one along the shaft axis.. that same motion doesn’t happen on a straight shaft. This stuff is long established shaft fit practice; the guides are in the handbook.... If ya want a machine to work as designed and last as long as it can, those machinery practices were developed over years of trial and error.. When you hold it in your hands and watch the machinists blue (how a lapped fit is checked) go from a line to a band, it is clear that the lapping is doing what it is supposed to.. Again, it is always up to the owner to make a decision as to what work is necessary and what is not.. Like Ross says, I have peed on a few fences but I can read standards and make a choice to, or not to wet the fence..
I used to work closely with a great old machinist whose eye would twitch when I (his engineer) would suggest shortening steps or doing something that would compromise a practice when we were rebuilding things.. I could tell then that I needed to not wet that fence !
On the lapping, consider that ya have mismatched tapers.. There will be line contact in a ring around the tapers.. As you grind them together, the line will “grow” to a band as the prop moves along the shaft axis very slightly.. that is how ya get two kinds of motion, one circular and one along the shaft axis.. that same motion doesn’t happen on a straight shaft. This stuff is long established shaft fit practice; the guides are in the handbook.... If ya want a machine to work as designed and last as long as it can, those machinery practices were developed over years of trial and error.. When you hold it in your hands and watch the machinists blue (how a lapped fit is checked) go from a line to a band, it is clear that the lapping is doing what it is supposed to.. Again, it is always up to the owner to make a decision as to what work is necessary and what is not.. Like Ross says, I have peed on a few fences but I can read standards and make a choice to, or not to wet the fence..
I used to work closely with a great old machinist whose eye would twitch when I (his engineer) would suggest shortening steps or doing something that would compromise a practice when we were rebuilding things.. I could tell then that I needed to not wet that fence !
They don't! And Mainesail is as fussy about that fit as he is about the prop to shaft fit.
Your statement highlights the differences between essentially the old British "Whitworth System" and the "American Standard - ANSI". The "American Standard" largely depends on the perfect (forced together) mating surfaces of a ~10 degree taper to transfer ALL the torque load by friction between the mating surfaces; the Whitworth also depending (necessarily) on 'keys', etc. Machining tolerances, although improved by modern tooling and methods, still are 'tolerances' which imply surface 'mismatch' with the probability of 'point-to-point-contact; whereas, lapping lessens the point-to-point contact and better insures total surfaces contact .... and more predictable or reliable 'friction/intereference fit'.
Crevice corrosion, nowadays better understood, is usually solved by the correct choice of less vulnerable alloys, better 'surface finish', etc. etc. 300 series Stainless Steels were formulated primarily for chemical resistance in chemical processing equipment; the newer 'duplex' stainless steels, etc. are better for 'load carrying' as they have far better 'fatigue resistance thus better corrosion resistance. Usually 300 series stainless forms 'microcracks' from fatigue, usually starting at the very FIRST loading, and additively accumulating from there on. Forensically I find that although most 300 series SS have an ultimate tensile capacity of ~90000 psi, any loading over ~30,000 psi (typical 'fatigue endurance limit') will rapidly produce fatigue; and, the microcracks from fatigue being the 'entry point' for crevice corrosion. A two-part simultaneous failure mode: fatigue AND crevice corrosion
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rotational shear
I believe they call that torque or just plain old shear. It should be taken primarily by the key. The lap fit is just to keep everything solid.
The shear force on the key is the toque supplied x radus of the shaft / area of the key. The area being what would be left of the key if you sheared it off. Which is why you should not skimp on the key.
The torque on the coupling is torque supplied X radus of the connecting bolt holes from the center of the coupler / sum of the bolt areas along the shear line. A much larger area and radus to shear through so this part will probably hang in there as the prop key goes south. Course if you have a very high strength steel in the key and low strength in the coupling bolts you could get the coupling to go first.
I believe they call that torque or just plain old shear. It should be taken primarily by the key. The lap fit is just to keep everything solid.
The shear force on the key is the toque supplied x radus of the shaft / area of the key. The area being what would be left of the key if you sheared it off. Which is why you should not skimp on the key.
The torque on the coupling is torque supplied X radus of the connecting bolt holes from the center of the coupler / sum of the bolt areas along the shear line. A much larger area and radus to shear through so this part will probably hang in there as the prop key goes south. Course if you have a very high strength steel in the key and low strength in the coupling bolts you could get the coupling to go first.
ONLY applicable on non-tapered shafts. On properly designed/engineered tapered shafts the key is only a 'back-up or safety'.
Inclined planes
Like an inclined plane (think of a wedge used to split a piece of oak, chime in Ross, you must have used one) the taper on the shaft will be applying a tremendous force on the ID of the propeller trying to make the propeller get larger in diameter. This is known as a hoop stress, the same stress you would get in a pressurized vessel. This stress applied at the shaft/prop interface will result in enormous friction that by itself will drive the propeller just fine. There is also a phenomena called metallic bonding where metals share their electrons so there is likely a joint strength factor also attributed to metallic bonding of the two metals. Kind of like wringing two gage blocks together, they actually bond to each other so you have to mechanically separate them.
I worked with an old designer of propeller (airplane) hubs and one of the designs had two dowel pins in the output shaft going in two holes in the hub. He explained how the propeller was completely driven by the friction between the two mating surfaces and the dowel pins were there just as a security measure, but didn't carry any of the load. I believe the key in a sailboat prop has the same purpose.
Like an inclined plane (think of a wedge used to split a piece of oak, chime in Ross, you must have used one) the taper on the shaft will be applying a tremendous force on the ID of the propeller trying to make the propeller get larger in diameter. This is known as a hoop stress, the same stress you would get in a pressurized vessel. This stress applied at the shaft/prop interface will result in enormous friction that by itself will drive the propeller just fine. There is also a phenomena called metallic bonding where metals share their electrons so there is likely a joint strength factor also attributed to metallic bonding of the two metals. Kind of like wringing two gage blocks together, they actually bond to each other so you have to mechanically separate them.
I worked with an old designer of propeller (airplane) hubs and one of the designs had two dowel pins in the output shaft going in two holes in the hub. He explained how the propeller was completely driven by the friction between the two mating surfaces and the dowel pins were there just as a security measure, but didn't carry any of the load. I believe the key in a sailboat prop has the same purpose.
Re: Inclined planes
Large, high horse power drives typically use splines. The pto's on farm tractors are always a spline with a notch for the retainer. it is not unusual to apply a hundred hp to the machine behind the tractor.
Large, high horse power drives typically use splines. The pto's on farm tractors are always a spline with a notch for the retainer. it is not unusual to apply a hundred hp to the machine behind the tractor.
So if it required to have a tapered lapped fit on the prop end of the shaft, than why not on the coupling end as well?
No place for the nuts! Not all propellers are fitted to a tapered shaft some outboard motor props slid easily onto a straight shaft and were driven by a shear pin that could be easily replaced. High power outboards and stern drives are usually spline driven. With a straight shaft you must have a shoulder to support the prop against the clamping force of the nut. On outboards with shear pins the nut tightened against a shoulder on the shaft and served only to keep the prop on the shaft but the drive was strictly by the shear pin.
Most larger and high horsepower shafts do have a taper and nut at the coupling end.
Because builders and customers don't want to pay for it. Sabre Yachts / Back Cove specifies double tapers on all their shafts and they are all made at my friends shop here in Maine and all are double taper, sail & power. I have seen double tapers on lots of higher end sailboats.
Ross this is not the case, the nuts are well hidden inside the coupling and there are a number of ways they can be attached without the typical "prop" type nuts.
This is a double taper on a 4" type coupling which was likely for a Sabre..
