Fitting & Facing A Marine Shaft Coupling

[Maine Sail]

What the heck is that..???


I am currently doing a shaft, cutlass, coupling and alignment job and when I arrived at my buddies shop the other day the job was not yet done. Cool.:cussing: Actually, it was cool because he allowed me to make a short video of how facing a coupling is actually done.

Here's the video: Facing A Marine Shaft Coupling (LINK)

Faced:

So what is "fitting & facing".

Fitting: To fit a straight or split coupling to the shaft for an interference fit. Good shops will usually strive for lighter than a bearing press fit but more than what you can slide on easily by hand.

Before you can face the coupling you first need to fit the new coupling to the new shaft or old shaft. Rusted old couplings can rarely, if ever, be re-used due to loss in tolerance from the layer of rust broken free during removal.

Sailboat couplings from reputable manufacturers such as Buck Algonquin & Walters usually always ship a little under sized so that they can be "fitted" to the shaft and not be loose. Even though it was a split coupling it still requires fitting and facing. My shafting shop is VERY picky. This fit is most always a light tap fit meaning you need often need to tap the coupling on with a wood or lead mallet.

The proper fit of the coupling to the shaft is critical, especially with straight couplings, but even with split couplings. If the fit is sloppy it will work the set screw holes, and the key, if the draw bolts come loose, and in a worst case a shaft can pull out of the coupling. A more likely scenario is that both become ruined from slop movement and in extreme cases the key works the shaft so much that it fails. This failure is often right at the end of the keyway. A loose prop can do the same.

Fitting is often done by hand with the aid of a machine shop lathe to hold the coupling. The reaming tool for this is very precise and they are quite expensive. It is used for the rough fitting. The coupling is most often inserted into the head of the lathe and the reaming tool end centered so the reamer goes in perfectly straight. The machinist makes a couple of turns of the reamer with a wrench and then removes the coupling and test fits it. This rough fit can usually be done in two or three tries. This one took three fittings. Care is taken to not to go to big because you can never make the hole smaller. Multiple fittings is usually the reality. This one took about 12-15 minutes to fit to the shaft.

Because this was a split coupling it was shimmed square to maintain the "fit" all the way through. The shimming was measured as accurately as possible before hand reaming.

The reamer is then adjusted by hand and inserted & centered into the coupling. It is then turned by hand with a wrench. After a couple of turns it is removed and the fit is checked. If not good this is repeated.

A good final fitting requires a lead mallet to tap the coupling onto the shaft.

Once it is fitted the coupling is installed on the shaft, with key or without, does not really matter, and with the set screws in place and is then ready for facing.

Facing: This makes the flange face rotate perfectly square to the shaft with no wobble.

Facing is rather easy compared to fitting and the video can be seen above. If you listen carefully you can hear the woo, wooo, wo as it is ground. The noise is not smooth because the cutting head was taking more in some areas and less in others due to wobble in the flange face.

Shaft flanges are aligned to roughly .001" for every inch of coupling diameter. Thus, a 4" coupling should be aligned to somewhere between .003" & .004". If the face is out .001" - .003" because it was not faced this can get inaccurate and a become a bear to align.

Gear box flanges are faced at the factory to spin true. Provided you have not bent it through pounding on it, or using it to press off a coupling, it should be darn close to rotating true. Unless your coupling is like new, with no rust between it and the shaft, I don't advise pressing the coupling off the shaft with it. Some gear boxes, like the small Hurth's (HBW-50 etc.), have very flimsy output shafts and they can easily fracture or be bent. It doe not take much to achieve a .003" or greater bend in a gear box coupling.

Often times with old shafts the easiest way to remove the coupling is to cut the shaft. This 1987 shaft had bad cutlass wear as well as bad stuffing box wear, and the coupling was not coming off in the boat any other way due to very limited access. A new shaft & coupling is far less costly than ruining your gear box!!

 

[Richard Bryer]

Thanks for your effort, Maine Sail- very timely. I have my coupling in the basement ready to go to the shop for this very operation and a new shaft and probably a new CS prop (based largely on your recommendation.

 

[tommays]

Good write up as always

What do they think of honing ?

 

[jibes138]

Actually if you have a simple way to spin a hone or lap like a small bench lathe you could do these processes right in your garage and it wouldn't cost much at all and give a much better result. I remember Mainesail describing the lapping process to fit a propeller to the taper on the shaft for instance, so lapping is already a part of the fitting process. The problem with this is you don't have a taper to put the parts together and lap to fit them. In this case the parts have to be made independently so they can be assembled with the right fit.
If it was me I'd consider using a boring bar to size the bore rather than a reamer and put an indicator on the cross slide to precisely measure how much stock is being removed with each pass of the tool. You need a dial bore gage to accurately measure the size of the hole and a good micrometer to measure the shaft size in ten thousandths of an inch.
Obviously the process Mainesail decribes is good and works, Just trying to suggest some possible improvements to the process based on experience.

 

[Maine Sail]

MaineSail
I am surprised at you. This is a recipe for what we used to refer to as a "free running press fit." Take that term to your machinist and ask him what it means.

I've spent a good part of my life making extreme precision parts with matched fits, parts that are not interchangeable for fuel systems and actuators. You can not get a fit the way you are describing it by reaming it, in my opinion. Tommays is right on when discussing honing but I'll take it a step further and discuss lapping. I have made in production many fits of .0002 to .0004 diametral clearance and they had to free fall by their own weight when wet with hydraulic fluid. There should be no need to smash them together with a lead hammer. If you need a hammer for assembly you have an interference fit. So why?

First of all is the hole and shaft perfectly straight? A human hair is about .003 in diameter. So a diametral fit of .0002 only has .0001 clearance on a side. The parts have to be perfectly straight measured in millionths in order to have a clearance fit of this size. We typically called out straightness within .0001 total so a .0001 diameter circle defines the zone that the entire surface has to fit inside.

Then there is roundness. Both parts have to be perfectly round in our case within .000050 inches. That is 50 millionths. Then there is surface finish, the roughness of the peaks and valleys caused by the tool used to cut the surfaces. A reamer might give you a 32 microfinish but this is typically a RMS finish and you will have individual peaks and valleys much larger. We would typically hone the bore to make it straight then lap it to make it round. If you do all these things you will end up with about a 2 microfinish.

So follow this process and you will have parts that will slide together without a hammer, be less susceptible to corrosion because of the fine surface finish, and you will be quite proud of your work. A hammer does not belong in this process at all. High precision work is done with materials like jewelers rouge, not hammers. The final product will cost about $1500.00 for the fitting process, but you will be very happy with the results.

It's not quite that precise and I doubt need be. He did do some other type of finishing of the coupling to the shaft but I had to take a phone call so don't know what it was and therefore did not talk about it. I also missed the shot of the machine cutting the keyway and a couple of other steps. I know the rough fit was done with the reamer. The inside of the coupling, when done, had a similar finish to the shaft but did not after reaming. I don't however know whether he lap fit it or honed it and did not ask but I will the next time I see him. I came back in at the final fit where he was tapping it onto the shaft with the lead mallet hence the pic.. When it came back off there were no "drag marks" from high spots which I have seen before. It came off leaving a smooth surface to the shaft and coupling. I guess he is fitting it to "interference", because this is always what it is, but I was told by someone that interference is an incorrect term as well..?

I buy and have bought shafting from four shops mostly. NE Prop & Machine, H&H Prop, Bracket Machine and my buddies shop who does mostly military work and shafting for military vessels, they don't really do trade or retail stuff but do make shafts for many high end boat builders here in Maine. I am lucky they do my shafting work for me and the price is REALLY good (half what H&H was). All the straight couplings I get require a light tap fit but not a hard press fit like a cutlass bearing so I guess you'd call that interference.

I have seen him working the military stuff and he uses lap fitting processes with varying grits as the 30+" tapers on those shafts (real taper length is info I am not privileged to know) need to be at or near 100% contact over the majority of the length (again the actual specs I am not allowed to know). He is the only shop/machinist in the US except for one other that does this work for the military and has the military supplied equipment to do so, on these specialized prop shafts. These shafts take days to get the tapers just right before the government engineers show up for the final inspection and up to four days to get them as straight as the spec calls for over their 30' +/- length (I am also not allowed to know that).

He's a pretty darn good machinist but did shoot from the hip when I asked him about the fit tolerance. I have to take him at his word but perhaps it is tighter than that, or looser, he did not physically measure it.

All I know is that every reputable shop I use fits the coupling to the shaft with a light press or light tap fit or as your calling it an interference fit. Ron Peck at NE Prop is rather adamant about this and quite vocal about certain other shops doing sloppy fits and having owners lose shafts and damage rudders etc. etc.. Tap or light press fit is probably not the correct machinist term but it is what I have heard it referred to as. The couplings/shafts I buy press on with a lot less pressure than a cutlass bearing but they do not just slide on. I stopped using one shop because the fit was tight but caused by high or low spots which, from what I have been told, is not a sign of a good fit and more a sign of poor fitting..

Does not seem like rocket science but I do know that a straight coupling that just slides on can wreck a shaft or come lose from the coupling and I have seen the results. I try to use splits whenever I can as I find them a tad more secure. I'd really prefer a double taper, Sabre, Morris, Hinckley and other use double taper shafts, but there is often not enough room for the longer coupling in many older sailboats to do this after the fact..

P.S. Just got back from installing this shaft and when I went to align it the motor mounts just spun the bolt in the mount. I HATE those types of mounts for this very reason! Cha-ching, the owner now he needs new motor mounts before we can proceed... Gotta love boats. Everything went smooth up until the alignment process, which was REALLY, REALLY way out.....

 

[tommays]

It its not a hammer fit you will get Fretting on the shaft which is why i love taper lock bushings

Always a press fit and never any shaft dammage

 

[jibes138]

Now we're getting somewhere

Ok so the idea is to have no relative motion between the shaft and the coupling in service so as to avoid fretting? Then it sounds like the machinist is striving for a really light interference (or press) fit. A .001 press fit for a shaft this size will take an enormous amount of force, more than a light tap with a hammer. So it sounds like they are trying for an interference fit, not a clearance as originally suggested, and trying to make it extremely small on the order of about .0002 diametral interference. I'd still suggest either freezing the shaft with dry ice or liquid nitrogen or heating the coupling to about 250 degrees to get them to slide together and then shrink in place. This will make sure the bore or shaft is not scored during assembly. In fact this might be a lot easier than going to all the trouble to get the perfect fit. The stainless shaft probably has a coefficient of thermal expansion of about 10 millionths per inch per degree F. So a 1 inch diameter shaft will shrink 10 millionths in diameter for ever degree F you make it colder. Dry Ice is about 105 degrees below zero. So from room temp of 65 a temperature differential of 170 degrees. So 170 times .000010 times 1 = .0017 change in size. If you made it a .001 interference fit you could easily shrink it together. This would be completely secure, then face the coupling square. The next problem is how to install it in the boat once it is shrunk fit and faced. I guess you'd have to pull the motor and put it (the shaft) in then put the engine back in. This is called "GETTING THE SHAFT":dance:

 

[jibes138]

So with a nice interference fit coupling how do you install it on the shaft with the engine already installed? I assume you put the shaft through the cutlass bearing and stuffing box first. Doesn't seem a good idea to hit it with a hammer or risk impact damage to the cutlass bearing or shaft log from side loading. A shrink fit seems like the way to go, wrap the shaft in some dry ice for a while, heat the coupling in a pot of boiling water, then just slide the two together. Since I've never done this how did you install the shaft in the boat you are working on?
Saving a coupling. If a coupling is in good shape but the bore is a little corroded or scored it would be very easy to hone it out or bore it on a lathe and then electroless nickel plate it to bring it back to size. Then hone the nickel plate to the fit size you want. You might want to also consider using a galvanic primer on the shaft before installation. In a shrink fit most of the primer would stay on during assembly and reduce the corrosion of the interface of the two dissimalr metals. I've installed steel sleeves in aluminum bores this way and it works well. You paint on the primer first then freeze it, slide the parts together and when they warm up you have a nice protected joint.

 

[Witzend]

Jibers138, you bring up a good point about how your going the get the assembly installed without pulling the engine, if the fit is too tight.

 

[Maine Sail]

Guys it is a light interference fit not a real tight one like a cutlass. If there is shaft length room in the boat a wood or lead mallet can tap it on to the shaft. If there is no room you can make a wood spacer block to tap the shaft into the coupling from outside to in. With two people this is easy, just did it on Friday for about the umpteenth time. I have tried freeze & heat and find it only works marginally well in the real world of a bilge. You'll want to watch the set screw holes carefully and only tap until they are centered in the holes/hole. This is much easier with two people a tapper and a watcher but can be done solo with many trips up and down the ladder.

There is no need to try and recondition a coupling when a brand new one only runs about $50.00 - $65.00. I will post a pic of the one I replaced and you'll see why this is often a very moot point. No amount or re-plating would have saved this one. The split I used on this boat ran $64.00. I doubt I could get one re-plated for that.

 

[Bob S]

Then hone the nickel plate to the fit size you want. You might want to also consider using a galvanic primer on the shaft before installation. In a shrink fit most of the primer would stay on during assembly and reduce the corrosion of the interface of the two dissimalr metals.

Maine/Dave/RichH,

Questions:

• How often in the life of a boat does this need to be done?
• If corrosion between two dismaler metals are a problem then why not have the coupling made from SST? Just curious!

If you heat and freeze the shaft and coupling before installing you better line up the keyway on the first try or you'll probably be using that lead hammer!

Great post Maine! I love your visuals and appreciate the time you take to do them.

 

[Maine Sail]

Maine/Dave/RichH,

Questions:

• How often in the life of a boat does this need to be done?

Usually only when there is an issue, cutlass bearing replacement in a dead wood or one that can't be pressed out, switch to dripless seal or when you need to replace the stuffing box hose.

• If corrosion between two dismaler metals are a problem then why not have the coupling made from SST? Just curious!

Cost and no one that I know of makes them pre-made due to cost. You could always have one machined.

If you heat and freeze the shaft and coupling before installing you better line up the keyway on the first try or you'll probably be using that lead hammer!

The heat trick has never once worked for me on a properly fit coupling. Even on cutlass bearing you only get about the first cm+/- before it stops.. It's nice wishful thinking for a coupling but when upside down in bilge, not such a sure fire thing.