An off the wall thought re rudder bearings

[Alan ashore]

Alright folks, humour me a moment.

Our Fulmar needs new rudder bearings. Since we bought her a couple of years ago the rudder has been progressively stiffening up. Most likely the present bearings are an unsuitable material. In any case they are doubtless very old. As I'm sure most forumites know, these are plain acetal bearings.

There's no great difficulty in getting new bearings made. She's out of the water for the winter, but unfortunately on concrete. Here's the thought: is there any reason why it would not be feasible to make the lower bearing in two halves, so that it could be fitted without dropping the rudder completely. The rudder could be dropped enough to get the old bearing out and carefully cut it off the shaft.

Yes this is just to try and save some money on cranage. No I don't believe it's feasible to plan on doing the whole thing during an extended hold in the slings on launch day (not least because it is known that Fulmar bearings do not all have the same OD, so we need to measure the old ones).

All views gratefully received

Alan

 

[vyv_cox]

Perfectly feasible. All the crankshaft bearings in your car are made in two halves and work perfectly well. Admittedly they have clearly defined thrust and anti-thrust sides, not so well defined in your rudder, but then loading and speed are considerably lower. It would probably be best if you can come up with a means of preventing the bearing from rotating in the housing, and obviously from falling out, but otherwise I see no problem.

 

[Contest1]

A couple of locating pins should keep them captive when pushed back in

 

[Neil_Y]

Yes, we do it all the time, there is extra cost as you have to split the bearing and then clamp the two halves together before you machine the ID and OD, you can add a very small flange to stop the bearing sliding up into the carrier if there is no lip for it to press against.

The only thing we need are accurate details of the shaft and carrier as we machine a running clearance which is dependent on bearing thickness and over all dimensions. Delrin can creep (deform) under pressure which can give rise to extra friction, it's OK within its limits but you never know quite what they are. We are happy with bearings operating up to 20Mpa which was our test to get Lloyds certification for rudder bearings.

 

[Alan ashore]

Thanks everyone for helpful replies. Neil, I’ll be in touch if we decide to go down this route.
It does strike me though that extracting the lower bearing from the hull with the stock still passing though it may be problematic, given that everything is presently so tight. It does have a flange, which may help.
I’ll keep the forum posted.

 

[bikedaft]

my off the wall thought was borrow/hire a jack hammer, dig up the concrete, dig a pit, fix rudder and re concrete, parts are cheap? or ask yard to move boat to a spot where stern over earth?? (sorry!!)

 

[Daydream believer]

I would be tempted to check if Oilon swells like Nylon - I suspect not as it is oil filled. If not then I would use it.
https://www.directplastics.co.uk/oilon-rod
Machines well, I have made quite a few bearings with it.

 

[Neil_Y]

Looking at the oilon, I'd want to check its moisture absorption rates and its thermal expansion rates. If it is basically Nylon 6 moisture absorption could be quite high at around 8%. Much higher than UHMWPE and composites less than 1%. Some materials claim zero but it is likely they are in the less than 1% area as all the composites and plastics we know of do absorb some moisture.

When we work out clearances we allow for both and in many of the plastic/UHMWPE it is the thermal change that is the biggest factor to allow for. For Nylon 6 though moisture will be the biggest factor.

To calculate the machined size and final clearance you really need to know the materials the bearing is in (shaft and carrier) their thermal expansion rates and the moisture swell and thermal expansion of the bearing. Only then can you be sure the bearing will not run out of clearance. The issue is friction causes heat which causes expansion which causes more friction if you run out of clearance. As an example with a Maritex bearing for shaft of 80mm and carrier 100mm We allow, 0.08 min running clearance, 0.07 moisture swell, 0.10 unscheduled thermal expansion (hot water, extreme loading situation) = total minimum clearance when new of 0.25mm. This takes into account the material changes and the wall thickness of the bearing.

 

[Daydream believer]

I did suggest checking re moisture first. However, being oil filled it may be different to straight nylon.
Do you really think heat is a problem in a boat rudder bearing?
Oilon encounters far more heat in conveyor bearings etc than in any rudder bearing, even the top one exposed to the sun. I think that is no problem whatsoever.
Of course if the OP does manage to get the boat lifted, there is a small chance that it is a standard size.Then he can get roller bearings from Jefa in which case he would be far better off.

 

[fisherman]

google says this boat is bilge keel, can it be tipped on its nose?

 

[testmonkey]

why not remove the rudder when the boat is still in the water? I removed, and later on re-installed my rudder whilst afloat with no problem. Just need some planning and a bit of snorkeling.

 

[Alan ashore]

Fisherman: No it's fin,but thanks for the thought.

Bikedaft: a) We'd certainly be in trouble if we started digging up the concrete! b) There isn't really anywhere they could move her to with the rudder over a dig-able surface, and in any case of course paying for cranage is the thing I'm seeking to avoid. But again, thanks for the thought.

Daydream believer - a) yes we would be better off with roller bearings but I don't think there's any chance that this could be done without a fairly significant piece of grp re-engineering to create the space - the existing bushes fit straight into the steering tube. b) I don't feel any need to experiment with materials that are not tried and tested for the job. But indeed thank you all the same.

Cheers all.

 

[Daydream believer]

Roller bearings only take up about 8mm all round the shaft ( total 16mm from memory of mine) so you would not have to make the overall space any larger. A lining to reduce the diameter perhaps. I am sure a delrin bearing would be every bit as thick so you must have room to get one in.
Jefa has all the details on their site.
However, that would not fit in with your desire not to remove the shaft.
Is a crane that expensive? My bearings were loosened whilst the boat was waiting for the new rudder ( old rudder was dropped 150mm to the hard, top bearing unbolted) & when the boat was launched the old rudder was dropped & the new one put in along with the bearings. So the cost was just one hour for 3 blokes & an extra 1 hour for the cradle . The manouvering time etc was no extra as the boat was being launched anyway.
Obviously your situation may be different but with a bit of planning !!!!!!

 

[seumask]

To get the old bottom bearing off you may need to slide a tube over the shaft from the top. We needed a scaffold pole to knock the old one out on our Fulmar after I had sized it wrongly! I was fortunate to have the boat in a hoist at the time.

 

[Alan ashore]

To get the old bottom bearing off you may need to slide a tube over the shaft from the top. We needed a scaffold pole to knock the old one out on our Fulmar after I had sized it wrongly! I was fortunate to have the boat in a hoist at the time.

That's encouraging. I'd thought a tubular drift might be needed, but feared it would have to be a size more difficult to come by than scaf. pole!

 

[seumask]

It will need to be a different tube to a scaffold pole as our rudder was dropped out while I removed the lower rudder bearing which I couldn’t get a firm grip on at the flange.
At the time I noted internal diameter 2.0” 50.8mm, External diameter2.5” 63.5mm of the top bearing so you need a tube to fit between these tolerances.

 

[Alan ashore]

It will need to be a different tube to a scaffold pole as our rudder was dropped out while I removed the lower rudder bearing which I couldn’t get a firm grip on at the flange.
At the time I noted internal diameter 2.0” 50.8mm, External diameter2.5” 63.5mm of the top bearing so you need a tube to fit between these tolerances.

Ah yes, of course.