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[FerryNostalgic]

Hi, has anybody out there experienced problems and come up with a solution for couplings and motors being out of balance and causing vibration?

I'm using MFA-Como 385 motors that have a 2.3 mm shaft which I am coupling to a straight 2 mm propeller shaft. I've already tried the common red "Huco" universal joint couplings and now just tried the "Microflex" rubber couplings from ModelBoatBits.com. Both suffer from the same problem: Although the couplings are of the correct nominal diameter, there is enough play at the motor end that after tightening the grub screw, the coupling has a visible wobble, even when rotated slowly by hand. When powered, this turns into a severe vibration (I have a short video for anyone who wants to see this). The motors are glued to 4 mm rubber pads which are in turn glued to the inside of the hull.

Any advice would be gratefully welcomed. :)

If I've posted this in the wrong forum topic, please let me know.

JP from London

[david48]

Its just a thought could the couplings be imperial and the motor shaft be MM ,try sizing the hole with the plain shank of a drill bit ,to see if that sheds any light on your woes.
David

[CGAux26]

Like David 48 said, get your shaft size very nearly the same as your coupling bore.  Any looseness will cause vibration.  And then make sure the motor is aligned with the prop shaft as near to perfect as you can see from the top and sides.  No matter what "they" say about flexible couplings, the better the alignment the lower the vibration and wear on your bearings.

[FerryNostalgic]

Thanks guys, both the motor and the coupling manufacturers specify the diameter as 2.3 mm. It's easy enough to find a 2.0 mm or a 2.5 mm drill bit, but I don't think there's much chance of finding a 2.3 mm bit to test with. I'll see what I can find on eBay.
I have noticed the both makes of coupling feel a bit loose on the motor shaft before the grub screw is tightened.

[Colin Bishop]

I have been sold couplings at a nominal diameter that has turned out not to be the right one. Also some of them are not very well engineered. I usually prefer pulley drive which is quieter and allows a degree of gearing down between motor and shaft which increases efficiency.

Colin

[Bob K]

but I don't think there's much chance of finding a 2.3 mm bit to test with.

3/32 " is 2.38 mm  if that helps

[roycv]

I have just had a similar problem and my solution for a slightly loose connection on a motor shaft is a bit unconventional!  I increase the diameter of the shaft with solder.
This needs to be done quickly!
I put solder flux on the shaft and lightly tinned it with some solder and a hot iron.  A cold tissue with cold water quickly cooled it down.
Then using a file I put some volts on the motor and let the file take the surface almost down towards the original shaft diameter.  A good solder joint leaves an even amount of solder on the shaft.  This puts a few thou on the diameter and can be adjusted to fit the coupling.
Perhaps a little unconventional but it has worked several times for me.
I use some commercial motors and they often have 'difficult' shaft diameters.
If there is a bigger discrepancy then a small length of brass tube might solve the problem. as long as there is a good fit the grub screw will secure the coupling.
regards Roy

[FerryNostalgic]

Thanks Roy, that's a very interesting and indeed unconventional solution, but it may just work for me. My only problem is my soldering skills (or should I say, lack of them!) I will give it a go though, at the risk of damaging the motor due to the heat. I presume that's why you cool it down quickly with a wet tissue?

The difference in diameter is probably of the order of 0.1 mm or so as the coupling inserts do fit the shafts, but they're not as snug as one would expect. No brass tubing would be thin enough in this case.

I'll give your solution a try and let you know. Thanks again for your advice.

JP

[Colin Bishop]

There is an alternative solution which is to wind 5 amp fuse wire tightly round the shaft and then solder it into position. Make sure you abrade the shaft first. Then try the coupling for fit. If it doesn't then run the motor with a flat file held lightly against the shaft checking every few moments whether the coupling fits. You can get a snug fit that way although getting the correct diameter coupling in the first place is better.

Colin

[Vintage]

The motors are glued to 4 mm rubber pads which are in turn glued to the inside of the hull.

Just a thought - are the "rubber pads" amplifying the problem?

Obviously the coupling should be as tight fitting as possible but if it's slightly out then the rubber mount, if it's soft enough, might tend to magnify the issue by allowing the motor to shake...

Mark

[Martin (Admin)]

1. I saw some very nice coupling at the Deans Marine & Simon Higgins, Proteus Design ( ex Prop Shop ), at the Doncaster
    CADMA show today, might be worth getting contact with them.

2. Similar to Roy and Colin, I've sleeved up a motor shaft with K&S brass tubing in the past.
      ( spun up the motor, roughed up the shaft with a needle file, same to the tube, stuck tube on with epoxy. You
          can then use the motor as it's own lathe and true up the shaft! )

3. If no great load on the shaft, i.e. not a high speed board, Graupner & Robbe both used to do a tight push on rubber tube
          that acted as a coupling.

             

[Martin (Admin)]

Also see:
http://www.modelboatmayhem.co.uk/forum/index.php/topic,33847.0.html
http://www.modelboatmayhem.co.uk/forum/index.php/topic,53396.0.html
http://www.modelboatmayhem.co.uk/forum/index.php?topic=52087.0

https://www.rcgroups.com/forums/showthread.php?1169530-Universal-Joint-for-2-3mm-dia-shaft

[chas]

The silicon tube method is an often overlooked and sometimes dismissed method, but one I have a high regard of. As an example, I've used this method in a Revell snowberry with a 385 motor for over 10 years, worn out one motor but never a problem with the coupling. As Martin says, in the right installation, it's excellent, cheap and silent. Good alignment is important as any other coupling.
Chas

[roycv]

Hi again, I never throw away the U/Js when the plastic centres fail, happens more often with the smaller size, I keep the brass parts and just fit plastic tubing between the brass bushes.  Usually works very well.
I used the solder method on a boat I am doing at the moment, it is not perfect but a great improvement.

regards Roy

[Stan]

I have fitted in my elco  conversation three 385 motors  and 2mm prop shafts no problems with vibration on 7.2 volts. Problems with these coupling's when I changed to 11. 1 volts to noisy. WILL POST MORE PICTURES IF NEEDED

Stan

[chas]

It's difficult to be sure from the picture, but the outer motors seem to have a lot of unsupported shaft. If that's correct, try disconnecting the outers and checking vibration again.
Chas.

[Stan]

Hi chas I have no problems with my model .The outer shafts have never been a problem the only problem was with the huco fittings when run above 7.2volts give lot of vibration problems. Couplings removed and Robbe rubber tube now used to connect motor/props no problems running on 11.1 volts. I was responding to the first post  by a fellow member.




Stan

[Stan]

Picture showing rubber tubes fitted to reduce vibration when running at 1.1 volts.

Stan.

[FerryNostalgic]

A big "thank you" to everyone who kindly responded to my post. The response has been phenomenal and I've now got lots of ideas and suggestions to try and will surely find a solution that works for me.

If anyone is interested in following the build of my boat, I've created a Facebook group for it here:
www.facebook.com/groups/jp.model.boats

JP

[RAAArtyGunner]

You may not be aware but there are a lot of us here who shun facebook because of it's inherent problems.

Hence why the requests to post photos here on the forum as it helps you with your enquiries.

So why not also post your build here for all the members, a very large number, to see.

[roycv]

Hi I echo those thoughts as well, I decided not to join the 'media community'.  I am not trying to convert but the thread started on Mayhem, why take it anywhere else?
regards Roy (ex computer engineer)

[FerryNostalgic]

In short RAAArtyGunner and roycv, it's a personal choice.

I've been on Facebook for many years (far longer than on Mayhem) and most of my modeller friends are on there too, so it was a natural choice at the time. I'm keen not alter the subject of this thread into "Mayhem vs Facebook" but I take your point and will consider documenting my builds on here in the future.

Regards,

JP

[roycv]

Hi my main problem with facebook, et al, is that I find it difficult to read the small text and likewise to type a message, I do appreciate it has many virtues.  My cheapo basic phone is bad enough!
regards Roy

[Martin (Admin)]

(For me it's that Facebook doesn't display posts  chronologically! ) 

[FerryNostalgic]

Martin, that's a gripe I have with FB too, but as far as i know, it only applies to notifications or possibly posts on personal profile pages. Those in FB groups (such as mine) do seem appear chronologically (as far as I can tell)...
JP