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Which prop shaft?

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Wayne Clarke11/06/2021 11:26:16
20 forum posts

I am building a 30 inch Cobra 21 from Rc Modeler plans for a 2000kv Brushless motor. What would be the best brand of prop shaft for this boat? Would a ball bearing shaft be best?

Richard Simpson11/06/2021 12:26:57
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766 forum posts
371 photos

Technically a plain bush should be better as you have better support and the surface of the sintered bronze bush should absorb some lubrication and keep the bearing lubricated.

Unfortunately however far too many of the manufactured plain bush tubes are not machined to close enough tolerances and too frequently they are not even made of sintered bronze. In fact some are just plain brass. You can get away with this on slow turning shafts but performance ones such as you are talking about require the best possible alignement and closest tolerances.

Never having had any experience with performance models I am not in the best position to offer advice however I would say that ball bearings are probably the best chance of getting the closest clearances. They should also be double shielded although there is also a school of thought that the inside face should not be shielded to allow lubrication from the tube to access the balls.

My apologies, I've probably only confused things even more!

Edited By Richard Simpson on 11/06/2021 13:16:58

Ray Wood 211/06/2021 12:52:19
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2546 forum posts
877 photos

Hi Wayne,

J Perkins do a range of different length Brass Stern tubes 6mm diameter O/D with a 4mm threaded stainless steel shaft running in bushes which work fine for me from Cornwall Model Boats, SLEC or similar retailers.

Ball raced bearings probably better suited to the larger ic power models and larger diameter.

Regards Ray

Richard Simpson11/06/2021 13:25:34
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766 forum posts
371 photos

Ray, I'm not so sure it is the size that is the biggest concern but more the speed at which the shaft is rotating. In a performance model with a surface piercing prop where the shaft will be rotating very fast I would not use the same shaft arrangement that I consider suitable for a large diameter brass prop with a big pitch such as you might fit to a tug.

The higher the rotational speed the more critical the balance and alignment of all the propulsion system becomes, hence the quality of the shafts, bearings, couplings and motor etc. are all more of a consideration.

As an example the bog standard splined universal coupling is great for transmitting larger loads to a big prop but they are notoriously poorly balanced and would not be suitable for a high performance model. I have been through three on a particular Club 500 in an attempt to cut down vibration.

Ray Wood 211/06/2021 14:23:19
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2546 forum posts
877 photos

Hi Richard,

I thought Wayne asked which brand would we recommend ? I did

Where is Ashley when we Needham ??

Regards Ray

Edited By Ray Wood 2 on 11/06/2021 14:26:51

ashley needham11/06/2021 17:11:36
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7394 forum posts
159 photos

Here we are Ray!

obviously a ball raced shaft would be best, but not necessarily warranted.

A decent shaft as per J.perkins will/should be ok. How long is the shaft? Personally, anything over about 9 inches or so and I use a 5mm diameter one to prevent shaft-whip, especially at high revs, like 2000Kv and 3s.

As Richard suggests, the standard red plastic u/j are not up to very high revs, and a safer option is to get solid metal u/j types or a specialist type. Or even use two u/j.

My current build is a near 20,000 rpm job and, unable to get a die straight shaft I have used a rubber coupling one end and it runs as smooth as anyfink.

Ashley

Trevor Drabble 113/06/2021 20:28:43
70 forum posts

Wayne , Maybe with giving Mobile Marine Models a ring and discussing your requirements with them since they actually make them at their facility in Lincolnshire. Trevor

Kev.W13/06/2021 23:16:34
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326 forum posts
38 photos

Trevor, with all due respect, Mobile Marine deal with tugs, they are not the best to consult when it comes to revolutions over 5-6k , brushless are not within their remit.

For high reving brushless motors, the better firm to talk to would be Prestwich model boats.

Richard Simpson14/06/2021 10:13:46
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766 forum posts
371 photos

Also worth knowing if you go to Cornwall Model Boats and simply put "shaft" into the search box you will pull up a huge selection of options. They have a wide range from the cheapest brass tubes with nylon bushes, through brass tubes with brass bushes and finally to ball bearing fitted tubes of every length and diameter you can imagine and exposed shafts or fully enclosed shafts, to fit just about every possible application. They also do a wide range of varying different types of couplings.

If you speak to them they should be able to point you towards the most appropriate one for high revs or I would even suggest phoning Raboesch themselves to ask for their recommendation.

Ray Wood 214/06/2021 10:21:50
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2546 forum posts
877 photos

Hi All,

If nothing else we may have scared Wayne off asking a question

Regards Ray

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