Technical, Techniques, Hints, and Tips > DC Motors (Brushed) and Speed Controllers

Motor/Coupling Vibration Woes

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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

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