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[Tom Eccles]

Having posted on another thread > motors gearing pulleys etc<. I spent a great deal of a sleepless night and a lot of today thinking about what I had said and wondering if it was feasible.

Having learned from Colin's post about electric motors being happiest when they are running at rated r.p.m. and realising that on our club pond very few of us run at max rpm for very long I wondered if perhaps a change of method is in order.

Instead of an ESC controlling motor speed; would it be possible to have a constant speed motor and a variable speed gearbox controlled by the ESC?

Please shoot me down, tell me it has been done before and I am looking to reinvent the wheel. I need some sleep.

Clegg

[Colin Bishop]

That is certainly an interesting concept. Not sure hope you would implement it though. As said on the other thread it's a bit like the continuously variable automatic transmission used on some cars.

What I'm not syre about is how "efficient" ESCs are. After all, they do apply full voltage to the motor but in varying pulses. I don't know enough about the interacting electronic factors to answer that one!

[Bluebird v2]

Hi there Clegg,

There is a method of variable speed controlling using the pulleys and flat belts.   Ive have done a scribble to try and explain.

On this drawing, the drive pulley is fixed on the motor and on the output shaft one side of the V drive pulley is fixed.  The other opposite side of the pulley is allowed to float in and out along the shaft.

Method of working: The closer the pulley sides come together on the bottom pulley, the higher the belt will ride up the diameter of the pulley.  Therefore, the shaft will turn slower (larger diameter).  The further away the pulley sides separate, the belt goes towards the centre of the shaft turning the shaft faster.

The operation of this pulley could be done using a servo.

Now my friend, if you can understand this explanation of the above, you are a better man than me...so off you go and have a good night's sleep. 

aye
John e
Bluebird

[Colin Bishop]

Good explanation John. Would it be practical a modelling scales do you think?

[HS93 (RIP)]

The ones I've seen Both cones go in and out as one goes in the other goes out , that way the belt is held at a constant tension.

Peter


http://auto.howstuffworks.com/cvt2.htm

[Bluebird v2]

Hi all, Yes Peter you are correct, there are versions where the two cones move in and out to keep constant tension on the belt and these tend to be used in heavy industry.

There is a lighter, smaller version which this was based on, obviously for lighter applications which has a spring loaded tension roller which runs against the belt.   I did not include this in the drawing a)  :) because I forgot and b)  have you seen the time of night.

Colin - if we had Peter's skills in model building and machining I dont think this would be too much of a problem.

The only thing I could honestly foresee would be getting a belt which is stiff enough across the width to withstand the squeezing of the cones as they move in and out.

Food for thought though isnt it.

Aye
john e
bluebird

[J.beazley]

From experiance the best thing to relate to with this method is 50cc and 125cc scooters twist and go and use the exact same method to power them along.

I would suggest getting yourself down to a local motorcycle dealer and see if they have one with the side casing off so you can see
(but then im sure there are plenty of pics on google)

scaleing it down wouldnt be too hard either

 O0Jay

[portside II]

you guys arethinking the same as me ,i have a moped in the garsge which has that system for the drive

[Colin Bishop]

It would be interesting to see if somebody could come up with a model size prototype. I'm no engineer I'm afraid. It might take up a bit of room in the hull but I reckon that if you got it working it could be quite efficient.

[HS93 (RIP)]

If you just Goggle C V T you will see that they are used everywhere   We had them on a stand drill some years ago and there where some 1/8 cars that tried them before going to the system they use now. I first saw them in the DAF55 I think they made a good rally cross car..
Peter

[tobyker]

You are re-inventing the Zenith Gradua Motorcycle gearbox of the 1920s. In that case only one pulley changed its diamater, so to maintain belt tension the rear wheel moved backwards and forwards (simple, really). It was manually controlled, by the "coffee grinder" handle on the side of the tank. It was so effective the Zenith was banned from hillclimb competitions and adopted the "barred" motif. My uncle Jack had a Zenith.

[Tom Eccles]

As I mentioned on the other thread I think the gearbox on the Daf 55 was a "Torsten" I once met a designer in Blackburn who was working on a newer design variant for a major car manufacturer.

As I understood the design concept there was always an element of mechanical inefficiency which was the major bugbear of the original design. There was slippage due to distortion of the belt(s) as the pulleys varied diameter. (I had a friend who had a "55" and riding in it one always felt as though the (nonexistent) clutch was slipping). The design modification I saw used a many segmented metal belt to overcome this problem.

I am aware that this is not the most mechanically efficient form of power transmission BUT with a fixed motor r.p.m. and a variable output shaft speed (perhaps a variable pitch prop would be over egging the pudding) it would still be of some interest as a concept.

Bu**er it! I was on my way to bed and now I have a great deal more to think about.

Good 'ere innit?

Thanks all of you
Clegg

[Shipmate60]

Or go for a variable pitch prop.
Motor spins at constant speed, prop blades "rotate" for ahead and astern.
They are produced commercially.
They are especially good on steam engines.
Throttle servo controls the pitch.

Bob

[Voyager]

WOW...that sounds impressive, any further info on that?

[bogstandard]

Hi all,
I see you are discussing variable speed with constant motor speed.
I have a couple of commercial items that explain things rather well.
The first is a variable pitch prop, made by Marx many moons ago. They say it can handle a 10cc glow motor, but I doubt whether the coupling they suggested would handle much above a standard 540. The pitch is controlled by a servo that allows fwd and reverse (difficult on an ic engine). It is fully ballraced up the tube, and is a well made item.
The second item is a thing I picked up many years ago and have still to find a good use for it. It is an infinitely variable gearbox, from 0 to 1 to 1, the first pic shows it next to a 380 sized motor. The ratio is adjusted on the move by turning the little gearwheel on the side.
I have never had the guts to strip it down and see how it works, I would most probably and up with a bag of leftover bits. Something like this this must have been very expensive in its day, and even more expensive now.

John

[Voyager]

I like the idea of that marx vario prop Do you know if they are still being made? I know you can still buy the marx motors, i never knew the smae company made vario props?

[HS93 (RIP)]

I got a couple of these a few years ago for a steam project, still not used them as they where a bit big for what I wanted I also got
one made by Simon of prop shop just after he started it took a long time and he said he would never make one again.

Peter

[Tom Eccles]

I see the variable pitch prop illustrated is a (comparatively) simple one. At the sort of r.p.m. we are talking about for steam then having the pitch set by a servo operated lever would seem to be o.k.

However, if we are talking about circa 5000r.p.m. then whacking this prop into coarse pitch at those speeds could have some interesting effects on the drive train; you can be certain that if it can be done, even by accident some one would do it!. A very small therefore very intricate therefore VERY costly governor may well be required.

On the other hand the variable speed gearbox looks interesting, are there any dimensions and weight for it please?

Cheers
Clegg

[malcolmfrary]

I saw an article in an engineering mag that fell into my hands a couple of years ago detailing an infinitely variable gear that relied on friction between shaft mounted balls (can I say that?) arranged so that a control arm varied the contact position and thus the diameter ratio.  It was specified that it was only suitable for low power.

[bogstandard]

Clegg,
The only reason that I showed this variable gearbox was to explain that things are available.
As can be seen from the pic it is about twice the length of a 380 motor and about twice the weight.
I picked this up about 15 years ago, and it was in this condition then, so maybe 25 to 30 years old. It looks like it came out of a bit of american machinery. If they are still made today I would think that the cost would be very prohibitive to fit into a model.
When I was doing a show at Nuremberg toy fair a few years back, I noticed that Marx had a very small and neat one servo selectable gearbox with a range of gears incl. fwd/rev. on show, I don't know if that ever went into production, but it might be worth a try.

John

[Shipmate60]

Clegg,
Reference the Variable Pitch Prop.
From the ones I have seen fitted, (not that many), they performed well.
Their biggest advantage is that you cant just go from full ahead to astern that quickly to allow the loads you are thinking of.
The pitch can only be changed at the speed that the servo arm can travel.
Plus as the pitch is variable there shouldn't be any shock loadings, just a gradual increase in load.
with the max pitch set at about 1:1 (45 degrees) and the large blade area means that yo don't need to use full pitch at 5000 revs, so you can use a slower reving, less power hungry motor.

Bob 

[Telstar]

Hi Malcome

detailing an infinitely variable gear that relied on friction between shaft mounted balls

Think the device your referring to is a Kopp variator. Used to have these on conveyor drives where I used to work, they had steel balls and tapered plates. If you tried to change speed ratio when they were stopped you "killed" them. We dismantled one once never worked again
Cheers Tom

[Tom Eccles]

Shipmate60,

Your info sounds good but my (VERY) limited knowledge of electric motors has me understanding that they rev pretty quickly. Wouldn't that have an effect?

I suppose with something like a tug a geared down motor would give lower revs and much higher torque which a variable pitch prop could absorb but there is something at the back of my mind about variable pitch props on aircraft that won't come to the front - something about the prop needing to absorb the power from the engine to give greater efficiency, I know air and water are slightly different mediums but in fluid mechanic terms I think they (the props) are doing the same job.

Maybe someone out there can jog my 30 odd year old memory of working on the old DeHaviland Beaver which had a Hamilton V.P. prop.

Thanks for your input

Clegg

[Shipmate60]

Clegg,
Model Motors Direct can sell motors on direct drive from about 150rpm to 5000 plus.
These are up to 11 pole motors which have very low current consumption.
But the trick is to guess the revs right first time!!!

Bob

[sweeper]

It may be of interest to you that drawings and parts lists for the construction of a number of gearboxes and clutch units are available.
The gentleman responsible for them is I. Cerjak, they may be viewed at  home.tiscali.nl/iliyacerjak/Bradley.htm

Hope this is of use to you.
Regards,