Coupling Noise

[Colin BishopModerator @colinbishop34627]

I am having problems in reducing the noise from the couplings on my liner model.

At the moment it is fitted with the red HUCO type with 385 motors driving 35mm props.

The alignment is fine and I have some adjustment in the motor mountings to fine tune to a 'sweet' spot. Current draw on 6v is 0.25 amps on the bench. It all feels smooth with no vibration felt at the top of the prop tubes but there is a loud whine which appears to come from the couplings themselves as they rotate, maybe from their joints?

I have tried using a double coupling to give a bit more flexibility but the noise is still there. Obviously th wooden hull is acting as a sound box to some extent and when in the water the sound level should resuce, especially as only 5v will be needed to drive the model at more than scale speed but it is irritatiing all the same.

This is my second method of mounting the motors, I initially tried my usual pulley drive but the pulley sizes were very similar so there was no gearing advantage and the belts developed a harmonic vibration probably due to climing up and down the V of the pulley.

I think there is probably too much power to use silicone tubing instead of the couplings but it would be nice to have just a quiet whirr!

Anybody got any suggestions?

Colin

[Colin BishopModerator @colinbishop34627]

[Dave MilbournParticipant @davemilbourn48782]

Admiral

At your age whining joints is just another pot-hole in the road of life… Be thankful that your prostate is still in good shape!

I've used these couplings with some success. They're borderline for brushless motors but with nice soft 385's on that low voltage they should be fine. You can line them up using the same method as on the Huntsman kit you just reviewed. **LINK**

SHG also do mini-flex couplings which are the ultimate dog's doo-dahs but eye-wateringly expensive and the P&P is £3.50 extra.

Dave M

[Paul TParticipant @pault84577]

Colin

If the hull is resonating you could glue sound dampening pads to the inside of the hull and build a sound damping box around the couplings.

Paul

[Ray Wood 3Participant @raywood3]

Hi Colin,

I prefer the motor to be totally rubber mounted on wing seating tape in a cradle balsa block with a rubber band over the top on dowels, how good is your archive ? Photo in my broads cruiser Margoletta June 2005 and the Construction Special you edited in 2011 isolates the motor from the hull, good luck.

Regards Ray

[CookieParticipant @cookie15923]

Hi Colin ,I had the same issue on my MTB using 3 brushless motors , my first couplings were the HUCO type , then i tried rubber flex couplings from SHG and Model Boat Bits but still had the same noise . I then tried car fuel hose which made a significant difference but still annoying , then i gave silicone tube a go and the noise completely disappeared however my fears of it splitting at high revs came true so i then tightly rapped thick fuse wire around the silicone tube were it is pushed over the HUCO brass inserts . Been running now for 3 years with no issues..

BTW it was DM,s idea to try it

Regards Dave

[Colin BishopModerator @colinbishop34627]

Thanks for the suggestions. Since posting I had a brainwave and tried some of the spare tubing I have for my hearing aids! It is a soft, silicone like material which seems to grip really well to the 2.3mm motor spindle and 2mm shaft.. In their intended use they do tend to go brittle over a period of several months which is apparently due to exposure to UV and the natural oilsfrom your body. They seem to keep well enough when stored in the packet. Probably worth trying as they cost next to nothing and can be replaced in the boat in a couple of minutes.

Paul's suggestion of boxing in and sound dampening is proably worth a try too. I will see how much immersion of the hull helps first.

On close inspection the 2mm shafts do have a bit of movement at the inboard end within the bearings but that is probably inevitable with a 10 inch 2mm shaft in a 9 inch 4mm OD tube. Precision engineering it aint!

Colin

[Colin BishopModerator @colinbishop34627]

Thanks Dave and Ray as well.

I would have liked to incorporate a flexible mounting but the motor cans are only about 4mm off the hull inner bottom and I wanted to use that for a mounting plate to screw the motor cradles into. The cradles have 1.5mm ply bottoms.

I was going to try soft tubing on the HUCO inserts too but don't have anything suitable at the moment and it looks like the hearing aid tubing may do the job.

As always it is about compromises but I'm hoping that with the hull in the water, slowing the props down from the load and with only 5v needed to drive the model along (less than 1.5 amps in total for both motors) the noise levels will reduce to acceptable levels. Ultimately I have the final sanction of turning my hearing aids off!

Colin

[ChrisBParticipant @chrisb29081]

Hi Colin. I now keep my coupling noise down by substituting the red plastic UJ with nitrile fuel pipe. The brass inserts are already splined, and securely grip the inside of the tube with no glue required to prevent slip.

[S MParticipant @sm83187]

Colin, if you can get a bit of carpet underlay then cut a square to cover the bottom of your tray and hold in place with a couple of dabs of superglue, then cut a strip the same width as the length of your couplings and stick this across the top of your couplings to your box. Leave both ends open for cooling air for your motors.

[ashley needhamParticipant @ashleyneedham69188]

Colin. I purchased some thick wall silicone tubing especially for this sort of issue. It is a good tight fit on the splines of the brass bits, and is thick enough not to twist under load.

Say the word and a jiffy bag with bits in will wing its way to you, even if you don't want to use it for this one you may find it interesting as a stand by. All my landing craft and small boats have had the treatment, and so far it has been fine with 540 motors.

Ashley

[peter ward 9Participant @peterward9]

ive tried using bathroom silon sealant and pressed it into the red huco couplings to take up any

slack between the brass cross shape uj arrangements and the red plastic itsself

a messy job but quietens enery thing down and still allows flexability

peter

[Charles OatesParticipant @charlesoates31738]

While we on the subject, what's the difference between the red and the black couplings, if any.

Chas

[S MParticipant @sm83187]

Generally the black ones are heavier duty, but in reality there is no difference on such small motors.

[Colin BishopModerator @colinbishop34627]

I think the red ones are generic Charles.

Just an update. After a lot more testing I came to the conclusion that the long shafts may be bending slightly within the tubes at high revs. Although not my usual practice I tried filling the tube with grease and that has certainly stopped the noise when using the flexible tube couplings with little if any additional current draw. However, under load, the tubes tend to wind up and pull off. I tried replacing the HUCO type coupling and the noise was a lot less than previously but still some apparently coming from the coupling itself. I then substituted the miniature double coupling I had and that was a lot better. Unfortunately I only have the one, it's a grey nylon type with steel ends made as a single unit which I bought some time back and which no longer seem to be available.

I did order some similar ones from Deans last week but had trouble assembling them and queried it with Deans. They think some parts may have got mixed up as there are two similar types listed on their website so the couplings have gone back to them for inspection. I have probably damaged them by forcing the inserts into the sleeves though. If they can send me the correct parts then they may be the answer.

Colin

[ashley needhamParticipant @ashleyneedham69188]

Just to clarify, the silicone tube I had was as a direct u/j replacement, so you still need the brass bits.

Shaft wobble…is why I don't go over about 8 inches on a 4mm shaft, and would not entertain long runs on anything smaller. What's really needed is a mid-tube bearing/support.

I have also cured horrid vibration type noises by simply changing the prop.

Ashley

Edited By ashley needham on 06/11/2018 14:48:23

[Patrick Matthews 1Participant @patrickmatthews1]

Colin- I may have missed it elsewhere in the replies… but the problem may well be in the use of single cardan joints. Two of the little guys in series will allow for significant misalignment of motor and shaft.

When you use only one, you ask for trouble… for it to work, the two shafts' centerlines MUST intersect exactly at the middle of the joint. A couple thou' misalignment is enough to put stress on the joint and cause vibration. The other way to make it work is sloppy cardans, but that leads to other noises.

Dumas's "dogbone" joint is actually an elegant solution to the problem… its geometry is essentially a double cardan, it's cheap and simple, and relatively short. Only available in 1/8" and 3/16" bores though… 1/8" can be opened to 4mm or more.

[Colin BishopModerator @colinbishop34627]

Thanks for the comments guys, very helpful. I think I may have cracked it at last with a miniature double cardan from Deans Marine plus greasing the tube. Only had a quick test at the moment but all looks good, I will do full tests tomorrow and report back. Previously I have found that greasing a tube with a 4mm shaft creates unacceptable drag but it seems the the much smaller surface area of the 2mm shaft doesn't suffer to the same extent.

Watch this space.

Colin

[Geoff CropperParticipant @geoffcropper73342]

Be discrete and think of others. regards Geoff

[Colin BishopModerator @colinbishop34627]

Problem does seem to have been sorted. I will post more details but am a bit tied up with other stuff at the moment! plus the hull is wrapped up in masking as I have knocked off one of the well deck bulwark sections yet again and repairs are being made!

Colin

[Colin BishopModerator @colinbishop34627]

Here is a photo of the current setup. It appears that one of the double cardan joints in not 100% aligned but this is the position in which noise is minimised and there is no increased current consumption. Not quite sure why!

Both prop tubes now have Castrol CL water resistant grease in them.Current consumption on the bench at 6v is slightly under 0.5 amp each and total consumption in the bath of both motors on load together at 6v is a bit under 1.5 amps so I am happy with that.

Initially one shaft was slightly noisier than the other but this was traced to the inboard end being about 1mm out of true on the starboard side so I sanded the motor base down and now both motors just emit a quiet whirr which I can live with.

Conclusions are that even with a double cardan coupling the alignment is still critical for noise reduction and that 2mm shafts in a 4mm grease filled tube do not result in any appreciable drag unlike a 4mm shaft in a standard tube, presumably as there is proportionately less shaft external area to be affected by grease drag.The grease certainly does damp down the noise and probably helps reduce any wobble in the long shafts. Should stop water ingress too.

I suppose this just goes to prove that all installations are different with their own characteristics and that there are a number of variables which have to be tested to get the optimum setup.

I do agree with Ashley that the shorter your propshaft, the less problems you are likely to encounter.

Colin

Edited By Colin Bishop on 16/11/2018 12:22:55

[ashley needhamParticipant @ashleyneedham69188]

I have had instances where the brass bit was not that accurately drilled resulting in imperceptible wobble giving vibration. Trying another complete coupling is the only way to see if this is the case mind you.

I have a couple of vessels in which the double couplings are well out of alignment (for various reasons) and they run as smooth as, after all it is why you have a double coupling…?

Soft silicone tubing would be good in your case Colin as there is very little power to transmit. The grey stuff I posted is probably a bit too stiff for this one.
Ashley

[Author]

Posts