Double universal joint coupling

[Dave ReedParticipant @davereed72029]

Hi, after a bit of advice.

 

When using 2 universal joint couplings to create a double UJ, how should they be aligned?

1 – each pair of plastic lugs point in opposite directions (black example)

2 – each pair of plastic lugs point in the same direction (red example)

3 – some other way, eg  lugs not aligned. One end rotated 45 deg from above examples

4 – it doesn’t matter.

 

Ready made double UJ’s I’ve seen seem to all be like the black example but I’ve also seen posts where people have used the red example.

 

Thanks

Confused bloke what don’t know how to setup UJ’s.

[Len Morris 2Participant @lenmorris2]

Hi Dave,

To maintain a constant velocity between the input shaft and the output shaft the joints should be arranged just like your black example.  This is exactly how the joints on a cars prop shaft are arranged to avoid torsional vibration.

A single UJ has a sinusoidal speed difference for each rotation that increases as the angle of misalignment increases and can introduce torsional vibrations.  With two UJs, the speed variation is almost cancelled out.

For model boats?  I don’t think it really matters.  Many models use a single UJ.  Might be important if you’re involved with heavy masses, high speeds and lots of power.

Len. 🙂

[Richard SimpsonParticipant @richardsimpson88330]

I think the main value of using two is the fact that they can allow for parallel but displaced misalignment while a single one can only allow for angular misalignment.

Over the years though I have become more and more disillusioned with the Huco type coupling.  I have frequently found them to be poorly manufactured and anything between sloppy and very tight on the splines and frequently out of balance.  I have also noticed they can be quite a tight fit at the cross piece bearings, which relates to power losses.  There are metal alternatives, which seem to be of a much better quality but, not surprisingly, much more expensive.

What I do think we sometimes forget is the fact that the strength of the Huco coupling monumentally outweighs the power that is being transmitted.  Frequently, especially in scale models, the power can be very easily transmitted with a short length of silicone tubing pushed onto the brass splines.  This will accommodate any type of misalignment, takes no power from the transmission and runs beautifully quiet.  UV joints obviously have their uses with higher power outputs but I still have serious reservations with the plastic Huco type.

I once saw an article that explained precisely how the issues Len refers to happen in a UV joint.  Very interesting and helps to explain some of the challenges with them.

Interestingly only last weekend I witnesses a steam tug at our annual steam day.  Thee were two oscillating engines, connected by two UV joints to a gearbox each, then from the gearbox two more UV joints to the shafts.  A total of eight UV joints in one model!  They were all metal though and, I must admit the whole thing ran very smoothly.  I wish I’d got a picture!

[Dave ReedParticipant @davereed72029]

Thanks both for your advice. I’m initially considering it for a displacement hull about 30″ long where the motor will be under a very low deck. There won’t be a great deal of power needed but I might have to lay the motor flat on the keel introducing an angle between motor and propshaft.

Dave

[Colin BishopModerator @colinbishop34627]

For your hull the silicone tube suggested by Richard should work fine. Otherwise a double coupling. As Richard says, the cheaper HUCO type are very variable in quality, out of balance and with the holes out of true.

Colin

[Tim CooperParticipant @timcooper90034]

<p style=”text-align: left;”>I have used the aluminium couplings with the rubber middle, they seem to work quite well.</p>
Dave Milbourn recommended brass inserts and the silicone tubing, but set up using stiff plastic collar to make it rigid.

Tim

[John W EParticipant @johnwe]

hi there

just a thought; is there any way in which you could lay the motor parallel to the prop shaft?   & use a belt drive – I have done a little sketch to show you what I mean – but – it is food for thought.   It does away with mis-aligning couplings.   I have done a similar layout to this on a semi scale Miss England, which I built recently.   The motor was right up forward and very little room for a coupling.

John

[Dave ReedParticipant @davereed72029]

Hi John,

I hadn’t even considered belt drive, that’s an interesting idea. Presumably your Miss England puts a fair bit of power through it as well. Next problem – where would I find a source of suitable pulleys and belts?

Dave

[John W EParticipant @johnwe]

Hi Dave,

have a look at this web site

John

 

https://www.active-robots.com/components/motion/belt-drive.html

[Colin BishopModerator @colinbishop34627]

Most of my boats employ belt drive, it works fine. Photos below show my Greek Fishing boat and also my 48 inch twin screw Fishery Cruiser Brenda.

You can get toothed belts and pulleys but this is not necessary for smaller boats. Plain pulleys and rubber ‘O’rings of 2.5mm or 2mm cross section are quite adequate. A further advantage of pulley drive is that you can have a smaller pulley on the motor than on the shaft which gears the motor down and makes it more efficient. 2:1 is usually a good ratio. There are plenty of sources but I have used the following. Just google model pulleys.

https://www.technobotsonline.com/

https://ema-models.co.uk/mechanisms/plastic-gears/brass-hubbed-pulleys.html

Until recently there was a large range of red plastic pulleys with brass hubs and a 4mm bore but they seem to have disappeared at the moment. There are push fit versions and some of these are suitable for the 2.3mm shafts of small motors but make sure you get the right size bore as they differ. There seem to be plenty of aluminium pulleys available with hubs and they are not very expensive.

https://www.technobotsonline.com/plastic-model-pulleys.html

Colin

[Richard SimpsonParticipant @richardsimpson88330]

This is the old MFA Como Drills unit, now supplied by Component Shop:

https://www.componentshop.co.uk/2-1-1-belt-reduction-drive-for-800-850-motor-mfa.html

 

[Colin BishopModerator @colinbishop34627]

Nice unit but an 800 motor would be overkill for a 30 inch displacement hull. With the gearing down a 385 would probably do the job, at most a low drain 540.

https://www.componentshop.co.uk/385-dc-motor.html

https://www.componentshop.co.uk/540-standard-dc-motor.html

Colin

[Dave ReedParticipant @davereed72029]

Thanks to all for your suggestions

I got a bit hung up on making the UJ’s work without looking at possible alternatives. Looking now at the belt drive option with motor beside the keel – it’ll be interesting to try something new. A 540 is too big to fit under the deck even beside the keel, a 385 should just about fit. I don’t have a 385 handy but do have a 1000kv brushless that’s the same diameter but only 2/3 the length. RPM at 12v is in the same ball park as the 385 and 540. I can always go down to a lower voltage if it’s a bit ‘hot’.

Dave

[ashley needhamParticipant @ashleyneedham69188]

A 1000kv 28mm brushless will be just great. It will be fine on 2s or a 7.2 stick depending on prop size.

Your prop is how big?

Ashley

[Dave ReedParticipant @davereed72029]

Hi Ashley

On the basis that the prop will be the easiest bit to change, it’ll be trial and error. I’ve got a selection of 2 and 3 blade ones to try. It’ll have clearance for up to 45mm so I should be OK.

It won’t be going across the pond at Mach 2

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