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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 06-29-2008
johnr
 
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Shaft coupler woes

Just repowered the boat. Got my new shaft coupler for my 1.25" shaft, and I started to install it. It was a very tight fit. Tried greasing the shaft, applying a little heat to the coupler, and just generally tapping and banging until the coupler finally slid all the way on. It was then that I noticed that my aggressive efforts had distorted the coupler. The step around the hub was mashed down and would no longer fit into the recess on the other flange. The outer flange had a slight warping to it. Esentially, I had ruined the coupler trying to get it onto the shaft. Getting it off was even worse, and now the coupler is just a beaten lump of steel.

My question is, how tight of a fit is it supposed to be? Shouldn't the coupler just slide onto the shaft with a few light taps? It has a keyway with two set screws. Would it be ok to gently sand the shaft until it fit a little easier?
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Old 06-29-2008
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It is supposed to fit with a tight tolerance! A little TefGel goes a long way.. Next time install the coupling to the tranny and insert & get the shaft started. Next go outside the boat and using a maple or white oak block, on the end of the shaft, and a WOODEN mallet to hit the block drive the shaft home stopping to check when you are TDC over the set screw dimples in the shaft!

Never use METAL to METAL when seating machined pieces.. Unfortunately, you learned this the hard way...

ANY play, even .001 of an inch of slop, will eventually ruin both the shaft and the coupling if the motor is the slightest bit out of alignment. They fit tightly for a reason..

Do not sand the shaft! If your prop shop has properly "fit and faced" the coupling to the shaft that is the proper fit. If you just bought any old coupling and slid it on your shaft then it needs a "fit and face" at a machine or prop shop..

ALL coupling MUST be fitted and faced to the shaft they are going on!!!!!!
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Last edited by Maine Sail; 06-29-2008 at 06:14 PM.
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Old 06-29-2008
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Generally, if something isn't fitting properly, beating on it isn't a great idea. You should probably have called the shop that supplied it and asked how to install it. Now, you're going to have to buy another one...
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Old 06-29-2008
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Yes, yes, live and learn. Se la vi. Upon closer inspection, I found that the region of the shaft where the set screw holes had been drilled by p.o. contained signifigant burs. Upon checking the inside of the coupling I was trying to install, I found major scoring due to this. I removed the burs and the old coupling goes on much easier now.

Is it possible to get a true fit, or at least an acceptably true fit, without having the coupler machined to the shaft? I realize that would be ideal, but it's not very practicle for me right now.

Now, to find that wooden hammer...
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Old 06-29-2008
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Rockter will become famous soon enough
Mine is very tight, and needs heating and hammering off there.
After cleaning it up, it tends to go on a bit easier.

The manufacturer is anxious that it not be loose.

A sliding fit, by definition must have a clearance in there, and clearances fret and enlarge with time. You don't see sliding fits on prop couplings often. You really do not want it to slide on there easily. That's not the idea. You really don't want any "rock" in there.

By contrast, an interference fit needing heat and/or hammering and/or pressing to get it on there does not have a clearance when cold, and so tends to stay on, but it needs force (or heat) to get it on there. It won't slide on cold, but when you get it on there, it does not rock, and that is what the designers want!!!.
Still use the key and the set screws though... they still help a lot for security. The key stops it turning, and the lock screws stop it backing off. Check them occasionally too.

Bearings on a motorcycle crankshaft are like that. They need to be pressed on (you can't get a press in that bilge right now!). Once pressed on, they don't move, and they won't rock off.

It is a pain when you try to get the coupling off 10 years later, but it is necessary for it to be tight like that.

When you fit the new one, put the new coupling in the oven (or blow-torch it), and lay an ice bag over the end of the shaft (and key) for an hour or so. Spray some WD40 on the end of the shaft quickly to displace the moisture or it will corrode between the surfaces when you get it on!!!, and hammer the hot coupling on there, with caution. The blow torch will make a big difference, the ice bag not much difference (but it helps).

You could even use some dry ice (CO2 solid) on the shaft, but it's very cold indeed, perhaps -110 degF, and I would be wary of hammering it much.

Wear gloves for hot and cold.

I know little about split couplings, but they probably go on easily, then need to be tighened to gain the interference they need. Keep checking the set screws though... it looks like death if they back out.

Rockter.

Last edited by Rockter; 06-29-2008 at 09:48 PM.
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Old 06-29-2008
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My opinion is that if you have to heat/cool the coupler/shaft to get it on, its too tight, and you will have the devil of a time getting it off. As described above, you should be able to drive it on with the coupler connected to the transmission flange by wacking on the other end of the prop shaft--the weapon of choice has to be fairly heavy to move the shaft, and I generally use a soft 2x4 or 4x4. Use a little grease or tefgel to keep things from galling, and be sure to safety wire the set screws so they can't back out for a while...after a few years you wlll need a good puller to get things apart.
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Old 06-30-2008
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Don..

Heating was to assist putting it on there. He's already trashed one coupling trying to hammer it on. The degree of interference varies from machine shop to machine shop, and sometimes it can be very tight.

I don't reckon it's a good idea to whack the end of the prop shaft as the shock load, far higher than anything in service, runs right to the gearbox coupling and final drive.
To each his own, I suppose.

Even the heat of an infrared lamp would help. Heat is welcome in these applications. Even heating a 1.25" coupling by 50 degF will save you about 0.43 thou" in interference. Boiling it in water ( 212 degF from say, 68 degF) should save you more than a thou". That's a long way in interference fits, and saves a lot of hammering.

Once you get it on, align it quickly and set the lock screws. It's about to cool down and regain its full interference.

Rockter.
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Old 06-30-2008
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Quote:
Originally Posted by Rockter View Post
Don..

Heating was to assist putting it on there. He's already trashed one coupling trying to hammer it on. The degree of interference varies from machine shop to machine shop, and sometimes it can be very tight.
If he had burrs on the shaft, from the dimpling of screws, this shaft and coupling were never fitted and faced by a machine shop hence the overly tight and most likely improper fit. Not all shafting is the exact same OD and not all couplings have the same exact ID..

I'll say it again, you CAN'T just buy a shaft and then buy a coupling and mate the two together!! No "good" machine shop would ever leave burrs on the shaft if they actually did a fit and face. It can't be done as the OP found out!!

The coupling and shaft need to be properly fitted and faced and a good machine shop will charges less than $35.00 to do so.

Fitting = Shaft to the inside of the coupling

Facing = Means the face of the coupling is spinning in exact relation to the rotation of the shaft.

I've seen coupling faces out by as much as .01 when installed on a shaft and this MUST be faced. Like a cars brake rotors you need to have the shaft spin in perfect alignment to the face of the coupling or you'll ALWAYS have alignment issues and vibration..
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Old 06-30-2008
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Heat is welcome in these applications. Even heating a 1.25" coupling by 50 degF will save you about 0.43 thou" in interference.
Rock,

.43 is roughly 7/16 of an inch....

You can heat it in a kiln and still not get it to grow 7/16"..

One thousandth of an inch = .001

One ten thousandth of an inch = .0001
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Old 06-30-2008
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Quote:
Originally Posted by halekai36 View Post
Like a cars brake rotors you need to have the shaft spin in perfect alignment to the face of the coupling or you'll ALWAYS have alignment issues and vibration..
Is it possible to get a true fit, or at least an acceptably true fit, without having the coupler machined to the shaft? I realize that would be ideal, but it's not very practicle for me right now.
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