Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length.

[Tug Fanatic]

A simple technique that works exceedingly well is to solidly couple the motor to the shaft, but have the inner bearing in the stern tube flexibly mounted with a piece of silicone tube.

I don't really understand what you are doing. Where exactly does the silicon tube go?

[Treble]

Klunk , Have you checked out the various uj offerings from Prestwich Model Boats ? Trevor.

[Andyn]

Thing is (as I’ve said countless times on this forum), electric motors do not need a flexible coupling. The model should be built so that the motor and shaft are in perfect alignment to enable the use of a solid coupling

[Circlip]

Didn't even mention the I/C engine version with the pins inside the flywheel and the shaft coupler slotted (Like a BIG ball and socket) John. There were heavy gauge Neoprene sleeve type couplings with the straight knurled brass fittings for motor and prop shafts back in the fifties. ABSOLUTELY useless when one end connected to an ED Racer but totally solved with the ones supplied by Max Coote.


  Alignment and solid couplings, same beast in   my book but don't solve the end thrust on the motor unless shaft end float carefully adjusted to eliminate motor float limits being exceeded.


  Regards  Ian.


 Forgot the fibre disc and two white metal castings supplied in the Adamcraft Sea Plane Tender John.


 'Search' facility should highlight how many times this subject as been beaten to death.

[Subculture]

Brushless motors nearly always have ballraced shafts so can deal with axial thrust well enough I find.


I think the main snag with a solid coupling is that parts aren’t always made accurately, and you can easily end up with some runout that is a little too much for a smooth result. So you have to be careful where you source parts or your own fabrication if you make them yourself.

[JimG]

I don't really understand what you are doing. Where exactly does the silicon tube go?

Basically the silicon tube replaces the inner bearing so the shaft can move around in case the coupling is not completely in line. The shaft for the old Sloopy fast electric only used an outer bearing on the tube with a solid coupling on the motor. The inner end of the tube was open and allowed for slight misalignment of the motor and shaft. Worked well using a narrower shaft than normal, I think 2mm  with an adaptor fitted to the end to take 4mm threaded props.
Jim

[Subculture]

It’s not quite like that. The silicone tubing doesn’t replace the bearing, it acts as a flexible attachment for the bearing, so that it’s not rigidly pressed into the stern tube. Effectively the metal bearing becomes self-aligning through compliance of the silicone tubing.

[Martin (Admin)]

On a side note .......  Glynn Guest provides a definitive guide to installing motors and running gear.

 http://www.modelboats.co.uk/news/article/drivelines/27166

[ChrisF]

Interesting point that Glynn makes about the alignment of parts.

I'm just completing the installation in my Fairey River Cruiser and I'm using a Raboesch prop shaft with a ballrace bearing at the motor end. I'm using a Huco type coupling, also Raboesch, made from what looks like Nylon and well made.

This model is only going to be used at moderate speeds so I have no concerns about the coupling. The motor was installed, connected using a solid alignment connecter and then the prop tube epoxied in place, so I have used my best efforts to ensure alignment is as good as I could get and the motor was left in place.

It could be argued that the exposed prop shaft is a little long at around 50mm but I don't think it excessive. Anyway, once the flexible connecter was fitted and the motor run you could see it running out of true slightly where the prop shaft goes into the connecter.  Whether it's the connecter being slightly out of round or the prop shaft not being quite straight I don't know.

But it turns nicely and no excessive noise or vibration so I'm not concerned.

I've used the same method in all my builds and did wonder about just leaving the solid alignment couplers fitted but decided not to.

Chris

[tonyH]

It's always possible for the alignment coupling to be slightly out and yes it has happened to me

[Martin (Admin)]

Fair enough, quite a few people have complained out these Huco 'Red' coupling, but personally, I never had an issue with them, including on an
OS 40FSR powered offshore race boat!   Well, actually, one did explode on that boat .... but only after something broke, it flew up the bank 10 feet and got caught in the long grass.... the coupling didn't like that much!   
 
https://www.modelboatmayhem.co.uk/My_models/05%20raving%20mad.htm

Another lesson this morning in why huco couplings are garbage…

[ChrisF]

You're a brave chap and lucky Martin!

We spend quite a bit on our boats so why risk it with cheap connecters? Ok on cheap and or slow boats where if they do fail not much damage would be done.

Chris

[Klunk]

Klunk - what boat and motor do you have which is eating couplings!

I've got some of the better quality Huco type for use in low powered/slow models and wouldn't touch those red plastic ones with the proverbial.

I've pretty much standardised on the alloy/rubber/alloy sandwich ones, which are supposed to be heavy duty so I hope I dont have any problems.

Chris

German e boat. Cannot remember the motor size off hand. But a 540 brushless equivalent with a 120 esc (what I had to hand)
At full speed on 7.4v lipo she develops a nasty torque roll.

[Klunk]

Are these any good?
Are they robust enough?

eBay item number:283138679258
eBay item number:283783204646
eBay item number:284997911742


       

Left pic used ok but problematic on larger brushless
Middle used 15 seconds of life
Right pic not used so will get one to try

[Subculture]

What RPM are you running at? Balancing becomes very important at higher RPM’s, and a lot of these couplings aren’t with those offset grub screw fixings.

[Klunk]

Roughly 17000rpm

[Martin (Admin)]

From M-Jet RC Jetboats ( Facebook )
 

   

[roycv]

Hi Pete, are you sure you are spinning a propellor at that speed? 
That is 280 turns per second!  I did have a small boat with a high speed engine and it just sat in the water, the prop was like a mixer, nothing happened and then you give it a push and it was off like a rocket.
My old Steamlinia when converted to electric planed very nicely, the 45mm. prop was running at 2000 rpm and the motor consuming 30 watts.  All measured and calculated, Malcolm will bear me out as we were first of all trying to get a steam driven boat to plane.
How many watts are going into the motor?
All the best
Roy

[roycv]

Hi all, in post No. 33 Chris F mentions the coupling end running out of true. 
I have a PT boat and the builder being an aeroplane man had not noticed that the thread at the prop end was actually running in the bearing of the prop shaft.
I decided to replace the bearing and shaft from stock.  This did not work as my calipers were measuring several different shaft diameters among my prop shafts, also the prop tubes had different wall thicknesses.
 
 The threads all fitted onto the Huco coupling but some were a bit slack or floppy.  A good fitting nut then a couple of tight fitting washers and then the coupling did help straighten out the coupling connection.
The most likely cause is the 'looseness' of the thread assuming it is screwed in place and the same would be true for a plain end where the brass part would screw onto the shaft.

As most of you will know it is easy to knock out the bearing from the shaft but do not do anything else until you have a bearing that is the correct diameter and is a pressure fit into the tube as a replacement.   I wondered if the different diameters were due to BA and metric sizes but none reached 4mm diameter

I must have spent a day on the combination of shaft length to diameter and the right coupling, most frustrating.  It ended up a compromise but we shall see, and I am not looking for high speed.

Lately I have used the brass parts of the couplings and fitted a thick walled tube between them, it has to be in line but is very cheap and best of all, it works.

In a previous thread on this forum there was an analysis of the speed of the driven end of the Huco or any other U/J joint.  Briefly if you are running at an angle although the rpm is the same the angular velocity is different the whole time, i.e. it jerks while rotating! 
To test this hold the U/J in your hand and bend to 50 degrees or so and you can see that the rotation rate is different as parts of the coupling change their relative angles.  I have found that the old half inch water pipe copper tubing is a close fit over a Huco coupling and might be used to align the motor to the shaft.
Regards
Roy

[Klunk]

https://www.technobotsonline.com/mfa-motor-re-380.html
Mines a brushless motor running on 7.4v lipo. Above is s standard 380 size motor. Rpm is stated, but drops under load and weight, by 5 to 20% depending on hull performance etc

[Subculture]

You could only drive a small prop with that motor spec.

What boat and prop size are you trying to run?

[Klunk]

It's a German s39 eboat. Runs on a 25mm prop. I don't trust huco couplings on brushless as have seen several blow, including ones meant for brushless.

[Geoff]

I think some of the problems are centred around a few things:


- The standard Huco coupling will only work with a very slight miss alignment in both planes - you need two in line to make it a real constant velocity joint. Other metal joints with discs have the ability to flex in both directions and as such are better if more expensive. This is important for smooth running.


- At high rotational speeds manufacturing tolerances come into play along with grub crew imbalances which cause vibration and noise and eventually joint failure. Brushless motors get to much higher revs than the original couplings were ever designed to take and at those speeds the centrifugal forces on the couplings can make them pop open.


- Where does the thrust go - I've seen lots of pictures of Huco couplings (and others) with the brass inserts fully inserted - this is not good practice and you should allow for 1mm of float so the in line thrust is not transmitted through the coupling. The thrust needs to be taken either with a separate internal bearing (not often done albeit it is typically full size practice) or at the end of the prop shaft with a collet and a couple of small washers so the prop thrust is pushed against the prop shaft casing and hence to the hull.


- If the thrust is pushed against the engine via the coupling it will put a strain on the coupling and can cause a Huco to burst. It will also put a strain on the engine bearings. It is also possible that if the engine is mounted in a typical "L" shaped metal bracket that the bracket will deflect under load putting more strain on the coupling and premature failure and increased current consumption because of friction.


- Solid couplings also need to take this into consideration - the thrust should be imparted to the prop shaft casing.


However having said all this we are making models which are massively stronger than their real life counterparts so we can typically get away with things that would fail in real life.


Cheers


Geoff

[Circlip]

Yep, a lot to be said for the OLD Rip Max ball and socket U/Js. Better if aligned but NO thrust transmission problems.
Huco have promoted laziness.


 Regards  Ian.

[Subculture]

Brushless motors are almost always equipped with ballraces, which will absorb axial loads effectively for the kind of load we're talking about.

When using the motors in model aeroplanes the motors take the axial force from the airscrew, so is there any difference with a boat?

Fullsize Schnellboats hit about 50mph at full crank, so a model will need a good turn of speed, but scale speed will depend on the scale of the model. Also useful to know prop pitch.

To find out scale speed, calculate the square of the models scale, then divide with the speed of the fullsize boat.

e.g for 1:1 50mph, for a 1/32 model-

Square of 1/32 is 5.7
50 divided by 5.7= 8.8mph

Assuming the motor can wind within 95% of its unloaded RPM, e.g. 16150RPM, the pitch is equal to the diameter (a "square" prop), then you're looking at around 15mph with zero slip, which is impossible. In practice prop slip, plus drag from the hull etc. means you can usually halve that figure.

Few commercial model props are "square", most designed for submerged running tend to be about 1.2-1.4 times diameter, some designed for steam power tend to have a very high pitch to compensate for the low RPM/high torque of many steam engines.

In practice most speeds usually need to be increased a bit over scale for very slow boats, else a model will dribble along. Not the case with something like a Schnellboat

Does that help or are you now thoroughly bamboozled?