Material for a bush on rudder

alexrunic

alexrunic

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Hi,
I'm trying to select the correct material for a bush that is bellow the water line. I had PEEK suggested to me but it's too expensive. I have also been told Tufnol is not good as it absorbs water and so will Nylon, is this correct?

How about PTFE (I'm not sure about it's water absorption properties) the only problem it can't be bonded easily, this bush has to line an existing hole and could really do with being glued in place as well as some sort of screw arrangment. If any one can suggest a way of bonding PTFE to GRP this might be the answer.

or how about this UHMWPE http://www.directplasticsonline.co.uk/UHMWPERod/

thanks

alex
 
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I always thought that Delrin was the stuff to use. When we bought our boat the lower rudder bearing was worn and I'm sure the our surveyor called for a new Delrin bush to be fitted. It's still fine after 13 years.
 
alexrunic said:
Hi,
I'm trying to select the correct material for a bush that is bellow the water line. I had PEEK suggested to me but it's too expensive. I have also been told Tufnol is not good as it absorbs water and so will Nylon, is this correct?

How about PTFE (I'm not sure about it's water absorption properties) the only problem it can't be bonded easily, this bush has to line an existing hole and could really do with being glued in place as well as some sort of screw arrangment. If any one can suggest a way of bonding PTFE to GRP this might be the answer.

or how about this UHMWPE http://www.directplasticsonline.co.uk/UHMWPERod/

thanks

alex
Click to expand...

RS Components sell a wide assortment of materials although I see what you mean about the price of PEEK!

They sell Acetal aka Delrin which is a lot cheaper..

http://uk.rs-online.com/web/search/...Section&Ne=4294957561+4294965430&N=4294962344
 
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parsifal said:
I always thought that Delrin was the stuff to use. When we bought our boat the lower rudder bearing was worn and I'm sure the our surveyor called for a new Delrin bush to be fitted. It's still fine after 13 years.
Click to expand...

I'd second Delrin. We use it quite extensively as bushings in Sub Sea tooling offshore - Tools are used at water depths upto 10 000 feet and in a wide temperature range (from Seawater to Drilling Mud @ 60 deg C)
 
Yes, Delrin is the correct material. Have it machined to be a press fit into the carrier and a clearance for the shaft. Sikaflex could be used in the carrier if there are concerns about it coming out. Provided the stock is correctly aligned, Delrin should last just about forever as it is water lubricated.
 
The PEEK seems to be thestuff though and there is even a bearing grade. How much is the materials compared to the whole job. How old is your boat? How long will you keep her for?


http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0514691

Put up some pics when the job is being done.

For what it is worth my rudde bearing have clearly been lined with a cream coloured polymer I would guess, nylon or acetal. But the top and bottom bearings are bronze.
 
Rudder bushing material

When my rudder bushings had worn out I used one of the newer plastic materials from
RS components -Nylon which had molybdenum disulphide lubricant incorporated. This
has worked well on my boat, but it was a pig to turn on a lathe as it kept jumping out
of the chuck. I made the mistake of making the hole through the middle too close tolerance,
and then found that I had to ream out the bearings when they were in place. Good material though-it has lasted well. I have probably got a bit left in the garage if you are interested.
 
Nylon is a possible solution! I made a prop shaft bearing from Nylon 66 three years ago. It is still running fine with no sign of wear. I allowed 5 thou clearance per inch of shaft diameter. It is grease lubricated. Whatever plastic material you use will give you a problem finding an adhesive to secure your new bearing in its holder - I machined mine oversize and pressed it in with a vise. I also fitted a couple of grub screws, just in case. The bore was machined to size last of all.

neil
 
A number of people who used nylon for submerged bearings found it expanded in water and locked the rudder tight within 12 months. It was then a total pig to remove!

I succesfully used PTFE on my rudder, and it seems to be working well. I think I would have used Delrin had I known about it! :)
 
oldharry said:
A number of people who used nylon for submerged bearings found it expanded in water and locked the rudder tight within 12 months. It was then a total pig to remove!

I succesfully used PTFE on my rudder, and it seems to be working well. I think I would have used Delrin had I known about it! :)
Click to expand...

Nylon 12 and 66 have a swell rate of about 3% in water, as you found out this can be a problem.

There's a different problem with PTFE. Many of the grades sold appear white because they are filled with powdered glass to improve mechanical properties. This leads to severe wear, especially of 300 series stainless steels. We investigated glass-filled PTFE as trim for very large ball valves, stainless ball, but abandoned it as the ball material was wearing very heavily. The PTFE was hardly worn at all.
 
Tufnol

alexrunic said:
Hi,
I'm trying to select the correct material for a bush that is bellow the water line. I had PEEK suggested to me but it's too expensive. I have also been told Tufnol is not good as it absorbs water and so will Nylon, is this correct?

How about PTFE (I'm not sure about it's water absorption properties) the only problem it can't be bonded easily, this bush has to line an existing hole and could really do with being glued in place as well as some sort of screw arrangment. If any one can suggest a way of bonding PTFE to GRP this might be the answer.

or how about this UHMWPE http://www.directplasticsonline.co.uk/UHMWPERod/

thanks

alex
Click to expand...

I use a Tufnol product and it's brill ~ lasts forever but I can't remember the trade name that Tufnol use. I fitted new bearings in my boat 11 years ago and it's still as good as the day I fitted them

Why not contact them here

http://www.tufnol.com/tufnol/default.asp?id=70

Peter.
 
There are a huge range of brands and trade names in plastic bearing materials. For liesure yachts you probably won't come near the limits of some of the cheaper options but you do have to take account of thermal and moisture related bore closure when machining the finished ID. Suppliers should provide an advised clearance for a range of carrier and shaft sizes. The bore closure changes for different bearing wall thicknesses.

Vesconite quote moisture absorption of 0.5% and thermal expansion of 6x10 -5 mm/deg.
http://www.vesconite.com/charts/specs.htm#Vesconite Hilube - Specifications
It is also a metarial that will soften with heat, it melts.

The material we work with has moisture absorption of 0.13% but more significantly thermal expansion of 2.4x10 -5 mm/deg to 22 deg C and only 3.3 to 100deg it does not melt so does not soften with increased heat from friction but it is not the cheapest.

PTFE whilst having low surface friction properties is not a strong material, in its raw state it is much the same as plasticine, so additives are used to improve structural strength. A bearing needs structural rigidity, good surface properties over a wide temperature range with low values for thermal and moisture related expansion when used in the marine environment.

Many plastics for bearings now have low water absorption but their expansion characteristics from heat are more significant, even in UK waters friction and a tight bearing can raise the temperature.

Delrin will work and for less demanding applications it can be an option, it depends what you are looking for. There is a new material that has high load properties with low friction whilst also having a surface chemistry that resists marine growth, more expensive than delrin but you pays your money...
 
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