Model Boat Mayhem
Mess Deck: General Section => Model Boating => Topic started by: ukmike on May 24, 2017, 02:24:01 pm
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Hi.
I would like to use exposed prop shafts for my next build, (4mm shafts).
I am thinking about a conventional short shaft tube to just clear the hull bottom but with a full length shaft thru' P brackets,(phosphor bronze bushed), to the props.
How can I prevent the thrust, which will be considerable, from being taken by the couplings/motor prop shafts.
I will be using brushless outrunner motors which I don't think have thrust bearings fitted.
Mike.
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on my boat with an exposed propshaft I have a 4mm collar at each end of the outer shaft, with just enough clearance to run freely (you could even put thrust bearings between the collars and the outer shaft).
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Hi.
I would like to use exposed prop shafts for my next build, (4mm shafts).
I am thinking about a conventional short shaft tube to just clear the hull bottom but with a full length shaft thru' P brackets,(phosphor bronze bushed), to the props.
How can I prevent the thrust, which will be considerable, from being taken by the couplings/motor prop shafts.
I will be using brushless outrunner motors which I don't think have thrust bearings fitted.
Mike.
Doesn't the thrust get taken up against the P bracket?
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The P bracket bearings are there just to support the shaft. The thrust is transmitted to the hull on a model on the outer end of the prop tube using a collar or collet. (in a full size ship there are thrust bearings inside the hull to absorb the thrust).
Colin
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Thanks for the replies but I'm trying to avoid the big brass collet, particularly because the stern tube is going to be only 6mm o.d.
Mike.
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you can get thrust collars that are the same size of the shaft.
I get mine from George Sitek or Steve's Model bits
If you look at the shaft shown here, you can see the thrust collar.
(http://i251.photobucket.com/albums/gg282/gingyer/T12/IMG_0048.jpg) (http://s251.photobucket.com/user/gingyer/media/T12/IMG_0048.jpg.html)
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I use Teflon Drive Shaft Washers (part no. 194000026-0) from Hobbyking.
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I have made them in the past but Gingyers post shows similar. A piece of brass to the diameter of the outer tube, drill centre for shaft diameter and then taper the whole piece so it looks like an ice cream cone. Drill and tap for a grub screw to hold it to the shaft and you have one thrust collar that tapers into the shaft!
Sounds complicated and needs a small lathe so if you can buy them at reasonable cost I'd go for that option.
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Thinking a bit out of the box, if a gear or pulley drive is used, the inner end of the shaft can push against an internal thrust bearing.
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Yes, that's true but it means there won't be the tight seal against the outer end of the prop tube which is so useful in preventing water entering.
Brian's suggestion is the neatest if not the simplest.
Colin
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I have used a lead/teflon bearing in the A frame and a ball bearing race at the coupling end of the prop shaft to take the thrust. There is no bearing at the bottom end of the prop tube and I use an O ring to seal. I suspect you could modify a commercial propshaft assembly. Next time I will use a sealed stainless or ceramic bearing and forget the O ring.
Keith
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I tend to be a bit wary of external collars since they are usually brass fixed with a steel screw. My sailing happens in salt water and I suspect that a brass collar on a stainless steel shaft fixed with a steel screw would rapidly turn into a fixed lump due to electrolysis. Probably different in fresh water.
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Fair point Malcolm, saltwater is certainly corrosive and the corrosion continues after you take the model out of the water.
Colin
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Problem solved.
Thanks to everyone for all the sugestions and advice.
Have ordered 2 micro flanged pillow thrust blocks that will do the job perfectly.
They will be fixed inside the hull just before the couplings.
Can post pictures when they arrive.
Mike.