# Prop Shaft Coupling



## casey1999 (Oct 18, 2010)

Could someone explain how a prop shaft coulpling stays connected to the prop shaft?

Specifically I am looking at yanmar prop shaft couplings, but I assume they are generally the same as other suppliers.

Yanmar makes 3 types of prop shaft couplings. A straight, a taper and a spit. Looks like the straight and the taper depend on a set screw to resist the longitudinal forces on the shaft (forces that cause boat to go forward or backward). The split coupling has no set screw and looks to resist longitudinal forces by its clamping action.

All couplings have a key and keyway to resist torsional forces.

I ask this question because I have a split couple and the shaft slid forward on the coupling (I have a V-drive so nothing to stop shaft if it slips) about 3/4 inch until the prop started to rub the shaft strut. My coupling does have a set screw threaded hole- but no set screw was ever installed, and yanmar does not show the set screw hole or screw on their drawings or parts list. Wondering why my coulpling/shaft slipped. Were the clamping bolts not tight enough? I will be having a look at the clamping bolts in next few days, but to date, have not tried to tighten them.

The other question is I have seen many times it is difficult to get a coupling of the shaft and you can use a socket between the trans coupling and shaft coupling to force it off. For one thing, this seems like a good way to damage transmission (with all forces involved), but the question is, how would you get this coupling back onto the shaft? I saw somthing on youtube where they heated the coupling in home oven , the slipped it on to get an interference fit. Is this what you do with a sail boat coupling?

Reason I ask is that in the near future, I will be taking my drive line apart, just trying to figure out how it is supposed to be connected.

Regards


----------



## CalebD (Jan 11, 2008)

Casey,
Using the socket wrench cylinder to push apart the shaft and coupling can damage the output flange on the transmission. This is why I first suggested that you bite the bullet and cut your shaft in half, making removal a relative breeze.
As you know, your shaft should not have been able to move once in the coupling. Either due to corrosion or a bad initial press/interference fit your shaft moved. This suggests that either the coupling or the shaft (or both) are flawed. 
Once you remove the coupling (hopefully the easy way) I'd take it to a good prop shop in Hono and have them check it out. 
When we replaced our drive train I had our prop shop face & fit our old coupling for re-use when they were making up our new shaft. Old couplings are not always re-usable and in your case I'd consider getting a new one that will work better. When I picked up our new shaft & coupling the shop had machined it such that the interference fit in the coupling was fairly easy to achieve without tons of pounding. 
Use of heat (expanding) & cold (shrinking) can also facilitate getting the interference fit more easily.


----------



## casey1999 (Oct 18, 2010)

CalebD said:


> Casey,
> Using the socket wrench cylinder to push apart the shaft and coupling can damage the output flange on the transmission. This is why I first suggested that you bite the bullet and cut your shaft in half, making removal a relative breeze.
> As you know, your shaft should not have been able to move once in the coupling. Either due to corrosion or a bad initial press/interference fit your shaft moved. This suggests that either the coupling or the shaft (or both) are flawed.
> Once you remove the coupling (hopefully the easy way) I'd take it to a good prop shop in Hono and have them check it out.
> ...


I am just trying to figure out what is actually holding the coupling to the shaft. Is it a set screw, interference fit or in the case of a split coupling- clamping force. I think my coupling will come off easily, as it seems to have slipped. One thing I did a couple years ago it to spray wd40 on the shaft coupling to keep it (and bolts) from rusting- could this have penetrated the couple and made it slip? Maybe I can remove the coupling, clean the oil off, and tighten up the clamping bolts to specified torque.

I will be replacing the shaft (and probably coupling), engine mounts, cutlass bearing, packing gland, and prop if required in 6 months when I plan to do haul out- if I can fix the existing shaft slip.


----------



## Alex W (Nov 1, 2012)

It is easy to make a proper removal tool if you have access to a drill press. Do this instead of the socket approach. Here is a photo of one that I made out of 1/4" plate steel:









The 4 bolts are attached (with nuts) to the shaft coupling. The 1/2" bolt in the center is tightened (the plated is threaded for it) and pushes the shaft out of it. This took about 20 minutes to make.


----------



## casey1999 (Oct 18, 2010)

CalebD said:


> When I picked up our new shaft & coupling the shop had machined it such that the interference fit in the coupling


Is that all that is holding your coupling to the shaft? You have no set screw (I would assume you have a key/keyway)?


----------



## casey1999 (Oct 18, 2010)

Look at these couplings:

walter shaft couplings, buck couplings, propeller shaft coupling

Both the split and straight have a set screw. Is this what should be taking the longitudinal forces?


----------



## casey1999 (Oct 18, 2010)

Found some real good information on all types of shaft couplings. Looks like I should tighten the clamp screws and install a grub screw:

Couplings


----------



## tommays (Sep 9, 2008)

In a perfect world it would have a double taper with both ends like the prop which can be removed easy and NEVER SLIP

In this world we put up with the 1/2 azzed light press and lock screw and safety wire


----------



## CalebD (Jan 11, 2008)

casey1999 said:


> Is that all that is holding your coupling to the shaft? You have no set screw (I would assume you have a key/keyway)?


I have a solid coupling (not split) and yes, there is a set screw which has to be wired so it does not decide to pull out. So my shaft is held in place at the coupling by the press fit and the set screw.
The key/keyway is on the prop end of my shaft.

See if you can torque down the split sides together. It may work.

Obviously I've never used this shop:

Marine Propellers Hawaii
(808) 537-6772
1100 Bishop St, Honolulu, HI 96813

but it would seem near the Alu Wai and perhaps convenient.


----------



## Maine Sail (Jan 6, 2003)

A split coupling is held in place by the clamping action and a proper "fit". Good ones will also have a set screw which should always be "spotted" or dimpled into the shaft. 

With split couplings it is very important to tighten the clamping bolts evenly, like lug nuts, or you can create an alignment issue. Contrary to popular misconceptions about "splits" they still require a fit & face when installed.


----------



## casey1999 (Oct 18, 2010)

Maine Sail said:


> A split coupling is held in place by the clamping action and a proper "fit". Good ones will also have a set screw which should always be "spotted" or dimpled into the shaft.
> 
> With split couplings it is very important to tighten the clamping bolts evenly, like lug nuts, or you can create an alignment issue. Contrary to popular misconceptions about "splits" they still require a fit & face when installed.


I picked up some different size set screws (look more like squaure head bolts) at hard ware store. Will see how they fit. All the ones I got are american thread (not metric as they did not have). I was told the set screws are normally american threads. From what I see on Yanmar parts manual, the yanmar coupling does not have set screws. Maybe mine is a after market coupling. Plan to loosen all the bolts. Slide prop shaft to its proper location. Tighten up the clamp bolts and install the two set screws. If she runs good, will run that way to November or so, then hual out and go through the entire drive train.

When you say spot or dippled to the shaft, does that mean I should drill a dimple into the shaft prior to installing set screw, or just torque down on the set screw to "set" it into the shaft?

Regards


----------



## casey1999 (Oct 18, 2010)

CalebD said:


> I have a solid coupling (not split) and yes, there is a set screw which has to be wired so it does not decide to pull out. So my shaft is held in place at the coupling by the press fit and the set screw.
> The key/keyway is on the prop end of my shaft.
> 
> See if you can torque down the split sides together. It may work.
> ...


My boat is actually kept on North Shore Haleiwa. That is why haul out is such a problem. I can pay to have someone pull it out with a trailer at the harbor with a cost of well over $1,000. Or sail 60 miles off shore each way to a marine center that charges $400 for haul out and $70/day on the hard.

I will check out Marine props and see if they can check balance on my prop when I do haul out. I also would like to see prop is correct for the boat (over proped)? I had not Seen Marine Propeller advertised in the local boat mags, just gave them a call, they have a shop and can work props and shafts.
Thanks


----------



## CalebD (Jan 11, 2008)

Casey,

The set screw must go into a dimple on the shaft, so yes, you will have to drill one if there is none. Screw pressure on the shaft alone will not do it.

Of course I never knew there was a marina in Haleiwa having only once visited Oahu. Naturally I thought you'd be in the big harbor in Honolulu. Your spot looks pretty idyllic but without a lot of the industrialized amenities of Hono. 
From my limited google research there are probably about 10 places in the city that could do your prop/shaft work. I was just looking for a place that seemed to specialize only in props & shafts. Most of the other places I found seemed to be multi-service boat places.
I suppose you live up near the North Shore also? You lucky dog!


----------



## casey1999 (Oct 18, 2010)

CalebD said:


> Casey,
> 
> The set screw must go into a dimple on the shaft, so yes, you will have to drill one if there is none. Screw pressure on the shaft alone will not do it.
> 
> ...


Had a few minutes last night to look at the coupling. The two clamping bolts were only torqued to a few foot pounds- basically nothing. The clamp bolts do have lock washers but must have losened up over time. I tried to push shaft back by hand, but could not. I have a tool I will use tonight to try to push back the 3/4 inch (gear puller that can be used to push shaft without damage to trans or couplings). Should not be hard as I can see oily slide marks on the shaft where the shaft slid. Plan to replace the clamping bolts. The set srew I got was right size- 3/8 inch american thread- will drill a dimple and install.

If you ever make it to Hawaii again, let me know. We can take her out. I live a mile up the hill from Waimea bay, right in the sunset beach area. Not much marine service in Haleiwa- most down in Honolulu- about an hour drive- but on the otherside of island- so a 12 hour cruise by boat under good conditions.


----------



## casey1999 (Oct 18, 2010)

All the work went well. Hardest part was getting the correct bolt type and length. I used gear puller to gently push the shaft back, estimate only took about 20 lbs of force. The set screw was never installed so I drilled dimple and install new. The 4 bolts holding the coupling to the transmission output flange were little more than finger tight- lucky they did not break off during running. These 4 bolts were also 316 ss and coarse thread. I have read where not to use stainless for these bolts as they are not very strong, and can fatigue easily. Repalced the 4 stainless bolts with grade 8.

The other good thing that came out of this work is my shaft packing began to drip like normal. A few months ago the shaft packing had almost no drip. Could not figure out why it would stop dripping- that must have been at the time the shaft shifted the 3/4 inch and probably a small groove worn in the shaft at the shaft packing. When shaft shifted the groove was no longer at the packing- and it was very tight with larger diameter shaft. Plan to install new prop shaft at next haul out. The existing shaft is bronze- was extremely easy to drill the dimple for set (grog) screw.


----------



## CalebD (Jan 11, 2008)

Yep, our old bronze shaft had a gouge worn into it right at the stuffing box, which is why we decided to replace it.
Bronze is fairly soft as metals go.

Best practices say to wire up the set screw and couplings; so vibration does not allow it to back out.

Good work.


----------



## casey1999 (Oct 18, 2010)

CalebD said:


> Yep, our old bronze shaft had a gouge worn into it right at the stuffing box, which is why we decided to replace it.
> Bronze is fairly soft as metals go.
> 
> Best practices say to wire up the set screw and couplings; so vibration does not allow it to back out.
> ...


Hard to see in pic, but I did wire tie the set screw (it came with a hole in head for the purpose). I did not wire tie the coupling bolts- I am sure good practice but I would have a hard time drilling the Grade 8 bolts. The coupling is super easy to get to, so can keep an eye on it and check before each use.


----------



## katsailor (Jan 6, 2013)

Coupling fit information

An interference fit coupling is the best and also the hardest to remove and install. The coupling bore is sized .001-.002 per inch of shaft diameter smaller than the shaft. For instance if you have a one inch shaft you will turn the end of the shaft to make sure it is perfectly round then bore the coupling shaft diameter - .0015 in. The key way and key on both the coupling and shaft is fitted before the coupling is heated and installed on the shaft. After it cools you are good to go and do not need a setscrew. You need a coupling puller to remove a interference fit coupling.

An sliding fit coupling is bored slightly larger than the shaft +.001 -.002/inch of shaft diameter. Dimple the shaft using a drill bit and install a setscrew in each hole, there should be one hole directly above the key way and one 90 degrees from the key way. Install a second setscrew to keep the inside one from backing out. Do not use a bolt, if a line or something gets near the spinning couple it will make you life very interesting very quickly.

Split couplings are easy to install however split couplings also fail usually at the most inopportune time, they were outlawed in the hoisting industry for anything that supports a load a long time ago.


----------



## katsailor (Jan 6, 2013)

Did you check you engine alignment?


----------



## casey1999 (Oct 18, 2010)

katsailor said:


> Did you check you engine alignment?


When I removed all the coupling bolts, I checked allignment. It was good. I know the cutlass bearing is worn and I will replace that in the near future at haul out, along with all engine mounts, then do complete allingnment.


----------

