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[Corposant]

What are the pros and cons of using a propshaft with ptfe bushes? I imagine that no lubricant is needed but that they would need to be a snug fit to prevent water ingress. Are they likely to slip out of the tube? I'm planning to use a low-powered motor so free turning will be important (the model is "a plodder").

The current shaft (which came with the kit) is about to be removed as it has gone rusty after only three trips in the bath!

Mike

[Fifie]

Be carefull some forms of PTFE absorb water
For example Nylon 66 absrobs a significant amount ( I have forgotten the percentage) of water in 24 Hrs
So any clearances are nullified
Fifie

[Corposant]

Thanks for the warning! I knew about Nylon but I'm very surprised about PTFE.

Mike

[Fifie]

Just checked

What you require is Teflon rod

PTFE( Teflon) is also water repellent and is often used in the manufacture of modern high performing, water repellent and breathable clothing.

PTFE is excellent for low friction bearings, bushes, rollers and pulleys. PTFE is also almost exclusively used in cryogenic components due to its ultra low operating temperatures. PTFE Rod is regularly used for seals. PTFE Rod has become a very important engineering plastic used within the aerospace industry and aeronautics. PTFE is often used within the food industry companies. Another useful application over the years has been in the use of product or component handles due to its resistance to heat and heat transfer. When an application for electrical resistance becomes too high for other materials, PTFE Rod can fill a very important gap.

Because PTFE has such a low co-efficient of friction it offers a material that when used will reduce load and drag in bearings to improve efficiency and reduce frictional losses and PTFE will outlast other materials.
Hope this helps
Fifie

[Ian Robins]

hi.
I machine plastics for my job. i would strongly advise against it unless the loads are very low.
The prop will make the bushes out of round. PTFE is very soft.
It is also very dangerous to inhale the fumes, especially if the ptfe is burning.
some shaft manufacturers use delrin(or acetal--trade name). this is easier to machine, it will last longer and it is recommended for use under water.
it will make better bbearings and will last a long time.

hope this helps

pm if you want a material offcut

ro88o0

[chingdevil]

Just as ro880 has said PTFE is quite soft, I would have thought that any type of rotational load would make it become oval. Delrin is good but does work harden as you turn it so speeds and feeds have to be controlled. I personnaly would go for bronze or brass for a prop tube bearing, the close tolerances that you can get with these materials means water ingress is minimal and can be eliminated with an oiler fitted to the tube..

Brian

[HS93 (RIP)]

just go to SHG they sell them for 4 and 5 mm shafts two for £1 not worth making them if you only want a few.

peter

[Corposant]

Fifie,

Thanks for the extra info. It seems, however, from the other replies that PTFE is not a good idea for reasons other than the effects of water!

Ro88oO,

Thanks too for your info. and offer of an off-cut. I'm actually planning to replace the shaft and tube together rather than just re-bush the existing.

Brian & Peter,

Thanks for your advice. SHG's product range and prices look good. I'll probably go for the cheaper Delrin option as I can't see the puffer needing a high performance jobbie!

Many thanks to you all - just the help I was looking for!

Mike

[HS93 (RIP)]

hear is a simple mod I did some time ago for shg shafts

http://www.modelboatmayhem.co.uk/forum/index.php?topic=6401.0

Peter

[RaaArtyGunner]

hear is a simple mod I did some time ago for shg shafts

http://www.modelboatmayhem.co.uk/forum/index.php?topic=6401.0

Peter

Peter,
Couldn't be any simpler thank you

[Corposant]

Thanks for that, Peter. Very neat! Hush Puppies - that's a blast from the past! "Brush hard, they like it" (I still have a pair I use as gardening shoes!)

Mike

[CGAux26]

PTFE is a common material used in industrial sump pumps as bearings.  The shaft is submerged in water (often dirty), so the bearings are water lubricated.  I have seen them last several years in continuous service.  Many times longer than any model boat will ever run.  Carbon filled PTFE is even better.

[Corposant]

I imagine that the bearings in an industrial sized pump would be housed in such a way as to contain the "flowing" properties of PTFE. They would also be a relatively tight fit on the shaft. Having said that, I take your point about the running time of a model boat. Carbon filled PTFE would seem to be the perfect solution!

Many thanks for your interesting reply.

Mike

[Ian Robins]

hi all,
if we really want the dogs. then ptfe filled acetal (delrin) is superb. but how much do we want to pay for shaft bearings.

ro88o0