Prop shaft bearing

[Guest 000]

I am replacing the tailshaft bearing on the H28, and the less said about getting the old one out the better!

The bearing is made of Tufnol, 85mm long, and being an old imperial lady has a 1 inch shaft and a 1.5 inch stern tube. I have ordered a new bearing which should arrive tomorrow. Do they come ready to fit, no machining required? Does Tufnol swell in water, and if so,does the bore have the necessary clearance to allow for this. It seemed to be a standard off the shelf part and the shopman was a bit vague on the finer details.

Also, the shaft is only 700mm long so it only had one bearing at the prop end, so the flange end has a bit of movement. This would imply that alignment is not as critical as it would be on a multi bearing shaft?

Thanks.

[Guest 000]

Sorry, should have added the new bearing is 4 inches long,I inch I/d and 1.5 inch o/d.

[idlerboat 116]

Surprised its tufnol for starters....any reason for this ?

Any modern plastic shaft bearing material should have very limited swell due to water.

It is normal to do the bearing calculations on shaft size...housing size.....and the method of location. (retaining)

An interference fit usually involves cooling the bearing in dry ice or liquid nitrogen to shrink it and then quickly installing. The design measurements work around this shrinkage.

More common is installation with expanding glue.

This stuff is amazing and actually works by the exclusion of air...

And it comes in a plastic bottle...( I will leave that magic trick for the moment but it is very clever).  A couple of brands make it, but there is variations so chase down the right one...

(Plastic to metal...corrosion resistant...full water immersion...etc)

 

Machining design measurements makes no allowance for expansion or contraction in this case.

This is important ...make sure that your bearing is a "tight sliding fit" before gluing.

If it ends up a forced fit, it will close down your shaft to bearing gap...and may overheat your bearing.

ALL plastic bearings require a fit that is surprisingly lose between shaft and bearing. They also need to be grooved.

It is water that actually  provides the running surface to the shaft at speed.

(My good friend wheels will give you a run down on water "hardness"..)

Try a search for Vesconite bearing calculator on line..and then go to shaft bearings.

I am not wanting to be brand specific here..there is a few options, but this calculator gives you a good idea of what you should be heading for..

As a final thought..

A physical retainer is a good idea. It can be as simple as a small taped thread in the outboard face of the shaft log (stern tube), with a screw and washer. (much easier than drilling right through like is often done)....

[Guest 000]

Thanks for your reply.

I don't have the bearing yet, should be delivered today. When I ordered it from the supplier, I told the shopman that the old one was tufnol so am assuming that is what the new one will be as well. It was a grooved (three) bearing. The stern tube protrudes 50mm or so beyond the deadwood and has a clamp-type arrangement as well as a grub screw for retaining the bearing in place. I would hope though that  any bearing supplied for a shaft would have the required amount of clearance built in to allow for any expansion specific to the bearing material used?

[Steve Pope 243]

Hi Chris, had I seen this I would have recommended "Vesconite" Chatfield are the agents and can / will machine to your specs. No swell with it.