Solid or Flexible Coupling

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Solid or Flexible Coupling

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23 November 2012 at 14:08 #37754
Neville Andrews
Participant
@nevilleandrews23198
Hello all.

As you will soon gather I am new to model boats. I usually frequent another forum, you know the one where they talk about aeroplanes, there will be a link at the bottom of the page. Very helpful lot over there so I am hoping you will be too as I have a few newbie type questions.

I’m building a Higgins Hellcat, I’m sure you have seen it but want to substitute a brushless motor in a direct drive configuration for the original brushed motor and geared set up. The problem being space is tight, so my first question is do I need a flexible coupling between motor and shaft or can I just use a simple solid coupling. A solid coupling would be much shorter and so installation would be much simpler. The original set up used a Speed 500 motor and a 2:1 reduction, I would choose a motor to give a similar end result so not a massive amount of power being transmitted.

I realise I haven’t given you all the details but didn’t want to write a war and peace for my opening post. Thanks in advance for helping, all advice and comments are welcome as I’m on a bit of a learning curve.

Nev.
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23 November 2012 at 14:08 #2206
Neville Andrews
Participant
@nevilleandrews23198
23 November 2012 at 17:37 #37759
shipwright
Participant
@shipwright
Hello Neville,

Inadvisable to use a rigid coupling. Virtually impossible to get perfect alignment between motor shaft and propshaft. A rigid coupling with slight misalignment will be noisy, inefficient and put a strain on bearings and cause wear in the bearings.

Ian
23 November 2012 at 19:36 #37765
Neville Andrews
Participant
@nevilleandrews23198
OK Ian, thanks.

I will need to do some careful measuring. I have seen some flexible couplings today that look to have metal connectors for the shafts, joined together with a rubber bush. These are a bit shorter than the universal joint type, I assume these would be OK.

Edited By Neville Andrews on 23/11/2012 19:38:07
23 November 2012 at 20:10 #37767
shipwright
Participant
@shipwright
Neville,

Although I have only used a universal joint coupling I expect that the type with a rubber bush should work just as well. Hope your model boat is a success.

Ian
23 November 2012 at 21:09 #37771
Neville Andrews
Participant
@nevilleandrews23198
Thanks for the advice, I’ll post a picture when I get it put together.

Nev.
24 November 2012 at 08:40 #37774
ashley needham
Participant
@ashleyneedham69188
I thinbk trhe rubber bush ones may be for i/c use, as these engines are VERY powerful, and need a stronger flexible copling. They are a bit stiff, so would help but not much. An alternative is the ball and socket style in all metal (I think in most cases). These are very short.

Ashley
24 November 2012 at 09:06 #37776
Neville Andrews
Participant
@nevilleandrews23198
Thanks Ashley
You couldn’t point me in the right direction with an example could you. I’ve had a quick serch but just keep coming up with leg joints !
24 November 2012 at 09:48 #37777
The Long Build
Participant
@thelongbuild
The only Person/Trader that springs to my mind for flexible couplings is Mark @ marksmodelbits

http://www.marksmodelbits.com/

I think the pictures show the heavy duty ones but he does do others, if you cant see what your after ask him as he is very good at bespoke stuff.

Edited By The Long Build on 24/11/2012 09:51:00
24 November 2012 at 12:11 #37789
shipwright
Participant
@shipwright
Neville,

Have a look at Robbe R1595 Navy Direkt Flange 18/4 available from **LINK**

Basically the 2 metal discs with pins engage with a (flexible) plastic disc with holes. As there are no bushes/collars the overall length will be not much more than the thickness of 2 metal discs plus the plastic disc. The website is not clear whether each metal disc (available for 1/8 inch, 4mm and 5mm shaft diameters) comes with a matching plastic disc so you would need to contact Cornwall Model Boats for clarification – also check that each disc comes complete with grubscrews. Finally ask them about the thickness of the discs so that you can be sure that the overall length is within your specification. Hope this helps.

Ian
24 November 2012 at 16:49 #37797
Neville Andrews
Participant
@nevilleandrews23198
Hi Ian yes they look like the might be the way to go, I had seen them before but assumed they were a bit heavy duty and not given them a proper look . I had been looking at the cantering coupling but wasn’t sure how long it would last. Haven’t had time to phone Cornwall yet but the pin type looks like it would last forever
24 March 2013 at 20:07 #39881
Neville Andrews
Participant
@nevilleandrews23198
Well I tried the Robbe flange coupling.

It looked just the job.

When I built it into the boat however and spun the motor I found quite a bit of vibration, a lot more than I was expecting. Spinning the shaft slowly you could see that the 2 flanges were slightly off centre causing the vibration. I am not sure which bit isn’t true it could be either of the flanges or the rubber coupling.

So I ended up using the cantered coupling instead. This is a lot more flexible in the joint and as it happens about an 1/8” shorter. Just a slight hint of vibration when running at full speed but I think that will disappear when I lubricate the shaft. A bit more building to do first. Nev.
25 March 2013 at 07:15 #39886
Paul T
Participant
@pault84577
Hi Nev

The slight vibration that you are witnessing is probably being caused by centripetal force from the motor that is being amplified by the partially unrestrained plywood bulkhead.

You will probably find that the vibration will disappear when the top edge of the bulkhead is connected to the rest of the hull however in the meantime I would recommend the addition of penny washers on the motor mounts fitted between the nuts and plywood.

These large washers (that you might have to trim to fit) will increase the stiffness of the bulkhead in the area of the motor and should effect an immediately noticeable decrease in vibration.

I hope that this helps

Paul
25 March 2013 at 09:07 #39889
Neville Andrews
Participant
@nevilleandrews23198
Good Idea Paul.

It was just a quick test to make sure everything was lined up ok before it all got boxed in and a lot more fiddly to adjust. I will add some washers when I fix everything in permanently. I’m sure your right about the vibration reducing when the model is complete, its only slight.

Nev.
25 March 2013 at 18:57 #39900
Paul T
Participant
@pault84577
Hello Nev

Let us know how you get on with the build and if there is any vibration when you test the motor under full wet load.

Paul
25 March 2013 at 22:34 #39906
ashley needham
Participant
@ashleyneedham69188
I think that you will have to fit the sides and then look at the vibration. Fitting the sides will alter the whole dynamic of the thing. As it is, the bulkhead is loose and free to wobble, even by just a tiny bit, and that will give your vibration.

I also think that the coupling you have is simply a shock absorber, and NOT a misalignment assistor (like the u/j type, or a "Marks models" rubber tube coupler) and will not help you if there is the slightest trace of misalignment. NOT that i am suggesting there is, but. it is tricky getting everything 100%. Movement within the hull structure and assorted bending motions when on the move may cause vibration, or lead to vibration periods at certan revs.

As paul suggests, the real test is in the water, as this will dampen (pun intended) everything out.

Mind you, I have been 200% careful in the past and still suffered. Props are a good cause of vibration on their own, let alone anything else. I have also had vibration from couplings that have not been completely accurately machined.

It is a bit of sods law. On the pond, there are several boats which perform whisper quiet, and others that buzz at a certain engine speeds, despite thier constructors taking all precautions.

Ashley.
30 March 2013 at 18:02 #39998
ashley needham
Participant
@ashleyneedham69188
As an afterthought, if you have not glued the shaft in yet, just get a shorter one and use a normal u/j. No issues then.

Ashley
30 March 2013 at 22:16 #40005
Neville Andrews
Participant
@nevilleandrews23198
Thanks for the suggestions but the shaft is well and truly glued in.

As a beginner I didn’t want to start altering the plans too much, so didn’t go for the shorter shaft option, as I had no idea what effect it would have on the finished model. I assumed a shorter shaft would have hade to increase the angle of the shaft, so was a bit wary of this, not knowing if there was an optimum angle etc.

Anyway building has progressed so we will just have to see now how it turns out, I don’t think it will be a problem. Nev.
25 May 2013 at 22:15 #41283
Neville Andrews
Participant
@nevilleandrews23198
Well this is how it turned out and as you predicted is nice and quiet on the water.

A quick test in the bath showed about 130 watts using a 2 cell LiPo, more than enough. A bit too much for me in the canal so I set a 70% limit on the radio and was comfortably sailing around at about 50% so power aplenty.

Thanks for your input I’m happy with this one. Nev.
27 May 2013 at 10:03 #41294
ashley needham
Participant
@ashleyneedham69188
Nev. Good show! I think, after all that prevaricating, that the boat would have performed just as well with any of the couplings at the end of the day.

Ashley
28 May 2013 at 07:35 #41298
Paul T
Participant
@pault84577
Hello Nev

The model is lovely and looks superb on the water.

Paul
28 May 2013 at 17:22 #41303
Neville Andrews
Participant
@nevilleandrews23198
Thanks Gents, it was a lovely afternoon. It looks even better under way but it’s a bit difficult juggling the camera and radio at the same time as I’m sure you already know.

May have to experiment a bit with some lead in the bottom to try and stop it rolling quite so much, I’m sure I had the prop out of the water in a couple of turns. A bit unnerving for a novice.

Nev.
28 May 2013 at 21:37 #41311
ashley needham
Participant
@ashleyneedham69188
Nev. No no no lead. Lead is for fishing boats. I suggest a couple of strakes underneath if you must, perhaps even at the sides (underneath), so they dig in a bit when turning, a bit like bilge keels. A boat like this needs to be light or you will ruin the performance. OR, slightly deepening the keel might be benificial in dampening the rolling a bit without compromising the speed and so on.I like hot glue guns. They are made for this sort of thing. Use hot glue to fit some bits underneath and see what happens. when you are satisfied you have made an improvement, fix the bits on permanently.

Trim should be effected by shifting the battery a bit if required or even adding a larger battery, but I dont think you need to go there are the boat looks to be at a good angle on the water.

Ashley.
28 May 2013 at 21:40 #41312
ashley needham
Participant
@ashleyneedham69188
Nev. No no no lead. Lead is for fishing boats. I suggest a couple of strakes underneath if you must, perhaps even at the sides (underneath), so they dig in a bit when turning, a bit like bilge keels. A boat like this needs to be light or you will ruin the performance. OR, slightly deepening the keel might be benificial in dampening the rolling a bit without compromising the speed and so on.I like hot glue guns. They are made for this sort of thing. Use hot glue to fit some bits underneath and see what happens. when you are satisfied you have made an improvement, fix the bits on permanently.

Trim should be effected by shifting the battery a bit if required or even adding a larger battery, but I dont think you need to go there as the boat looks to be at a good angle on the water.

Ashley.
29 May 2013 at 00:01 #41314
Telstar
Participant
@telstar
Hi Neville If the boat is only rolling into turns, you my want to check the rudder, if it is very deep, ie well down below the prop, it can cause the boat to lift on to its side on hard turns. While a big rudder can be beneficial at low speed, it can introduce instabilities at high speed.

cheers Tom
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