Model Boat Mayhem
Technical, Techniques, Hints, and Tips => Engineering Techniques and Materials. => Topic started by: Tom Eccles on September 28, 2007, 10:08:39 pm
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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
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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!
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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. :D
aye
John e
Bluebird
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Good explanation John. Would it be practical a modelling scales do you think?
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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
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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
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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
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you guys arethinking the same as me ,i have a moped in the garsge which has that system for the drive
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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.
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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
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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.
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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
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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
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WOW...that sounds impressive, any further info on that?
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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
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I like the idea of that marx vario prop ;D 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?
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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
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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
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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.
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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
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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
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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 O0 never worked again :embarrassed:
Cheers Tom
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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
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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
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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,
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Hey Sweeper,
Thanks for the address, there is some very interesting "gear" on there but I think it is rather larger than the variable speed g/box I have lurking somewhere in my tiny mind.
So far (like many a good designer) I know where I am (input shaft) and I know where I want to get to (output shaft) I just need a map to get me there in the most power efficient way.
I just have this idea that the motor would be most efficient running at rated revs. As you will have seen from the thread there are some terrific ideas out there, every reply has had me thinking again.
Great stuff!
Cheers
Clegg
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Probably the reason that a set up using an electric motor will normally have just an electric controller comes down to cost and reliability.
In many full size applications involving large deisels, rather than have a gearbox and clutch, there is a generator and electric motor. Making a mechanical arrangement to allow the motor to just run at its peak efficiency has probably never really been feasible on a cost basis, bearing in mind that most electric motors (talking conventional permanent magnet, brushed) do have a wide spread of power/rpm, probably much wider than IC.
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Hi there Clegg,
I've been sitting here contemplating your "design" parameters. Using methods from way back in the grey cell area (i.e. as a young student), if you want to keep your motor running at the rated revs but be able to control the prop, how about the idea of a magnetic clutch?
By varying the excitation of the magnet you are able to allow slippage in the drive ,without doing any nasties to the clutch, as there are no clutch plates.
A simple version would be easy enough to construct with the connections to the magnet winding done through two slip rings.
I understand that some of the submarine guys use a similar idea on their models to avoid punching holes through the hull (but use it locked motor to prop).
Just a random thought on the subject.
Regards,
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Right Clegg,
Now that we have got all the ideas out of the way, whether they be simple or complicated, all are going to cost either in time to make or commercial purchase costs more than the cost of a good speed controller.
If you are just out for a good efficient system that will allow you to run for about four to five hours from a reasonable sized battery, forget about the commercial motors that are available. People seem to think that these are the only ones available and like blind sheep they buy them from the local model shop thinking these are the only ones available and then complain when they get only an hours running from an 8AH battery with their tug or warship etc running round like a fast electric.
You should be looking at more efficient motors, and these aren't expensive, a lot of the time they can be free. I have used 850 size motors that can turn a 4" scale prop with direct drive and pull less than 500MA on full load, that will give me 6 hours running at full load from a 12volt 3AH gel cell. on a very large model. Almost all the motors I use are 24volt running on 12volt, and for years our club has been using motors like this, and they all sail for a full session, or a few sessions on one charge of a small gel cell, and no one has ever complained about not being able to sail at scale speed or above.
So look at your motor in the first instance, is it efficient.
John
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Hi sweeper,
I think we have a touch of the "great minds thinking alike" syndrome.
I have been thinking how to transfer (controllable) power without a mechanical clutch and magnetics was something that wandered through my mind.
It seems that all we need now is an electronics genius and we may be heading somewhere!
We have a couple of submariners in our club, and an AGM next Monday. I will do a bit of brain picking.
Thinks........I wonder if Floating Wombat and/or FLJ are reading this........
Thank you very much for your input
Cheers
Clegg
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Hi there bogstandard,
Thanks for your input.
I probably didn't make clear what I am trying to acheive here. I am NOT knocking an ESC (god knows I rely on them enough!) I am having a sort of brains trust about alternatives to the ESC controlling the motor as it implies in the title of the thread.
I am trying to think outside the accepted box and pick the brains of all who have been kind enough to contribute to the thread. You never know, something may come of this and, if nothing else, it takes minds away from the more accepted ways of doing things.
Thanks again for your input, EVERY view is welcome.
Cheers
Clegg
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Hi Clegg,
Would respectfully suggest that it isn't an electronics whizz you need, the solution to this problem is to do some slide rule bending to evaluate the field strength required, the number of poles and then manufacture two discs very accurately - one holding the field coils and the other holding the ferrous pieces. You would need to be able to produce these with some very close tollerences (air gap of a couple of mm max). The closer you can get the discs, the more efficiently the magnetic flux can be transmitted. Large airgaps are a big no-no in any magnetic system. I suppose that, for comparison of operation, the nearest thing I can think of is the operation of a synchronous motor (interaction between stator field and rotor).
I've just been rooting around through some old (and I mean old) textbooks. Yes, there is an illustration of such an animal but the application is for operation in an engaged/disengaged mode.(The book is circa 1937).
Looking at the practical considerations of the whole idea, I would think that the biggest problem would be the arrangement of weakening the field to obtain slip between the two plates of the clutch while maintaining sufficient field strength to continue drive.
An interesting little problem.
Regards,
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The mechanical arrangements are fine for use with a mechanical power source, such as an I/C engine, where self starting from a stop is a problem, as is reversing.
With electric motors, there is a vast speed range which is controlled very efficiently and effectively by modern PWM controllers. When you consider how much operating time you get off one set of batteries, any alternative has to provide an improvement in running time at the same performance from the same battery, otherwise it's just an interesting oddity. All the proposals so far use power to lose power whereas a PWM controller just applies as much power as is required to do the job.
This one reason why railways tend to use a generator/electric motor arrangement instead of a mechanical arrangement of a clutch/gearbox/hydraulic torque converter etc when they use diesel engines. Some designs may have a gearbox to ease starting/low speed control, but a boat has a built in torque converter/clutch in the for of a propeller in water.
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The earlier pulley discussion focussed on the belt system. That system is also used on the Boxford variomatic lathe drive. More common (for modellers) systems used friction wheels in 2 arrangments. These have been used to give wide speed control for IC engines (especially diesel) before good throttleable glow engines were designed. A 'famous' one is the Model Engieer's 060 Loco desingned by Edgar Westbury in the '40s. Strangely it used a bronze cylinder casting instead of aluminium because there was a war on. I digress.
The above used a large diameter wheel on the engine shaft. A slotted shaft at right angles crossed the face of this wheel. A friction wheel keyed to this shaft can slide along the shaft and makes contact at a variable diameter of the disc. When passed over the center of the disc it reverses. Input and output can be reversed, but it is potentially inefficient.
Method 2 has a long cone on the motor shaft. The output shaft has a similar cone, reversed, so the two cones run parallel with a gap between them, narrow diameter of one next to large diametor of the other. A friction wheel is interposed to transfer drive from one to another and moved to vary the ratio.
Two more methods are seemingly more awkward but are the two actually marketed commercially nowadays in medioum power applications up to 200HP
The first is I think German. Take a shaft with an eccentric on it, with follower that produces an oscillating liniar motion. This motion is used to drive a pawl that engages a toothed wheel and drives it round one notch at a time. (you may know some winches work like this) The toothed wheel moves round slowly but in jerks. If levers are introduced that make the oscillation amplitude variable more than one tooth can be engaged making the output turn faster. Replace the pawl/wheel witha roller clutch giving an 'infinitely toothed wheel' and you get smooth rotation.
The second is deliberately lossy. Imagine a car differential but use one wheel as the input shaft, lock the normal input , and the result is that the second wheel rotates at the same speed as the input but in the opposite direction. If you allow the 'normal input' shaft to rotate controlled by a brake then the output shaft slows down and with the brake right off the output rotates in the same direction as the input ( I think, brain starting to hurt)
Both these could be made in meccano to experiment.
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Just remembered another one, proposed rather unsuccessfully for bicycles.
The large chainwheel of the bike (the one the pedals are on) is replaces by a disc with several little toothed wheels, say 6, round the outside. The chain goes round the outside of these little wheels so is sort of going round a hexagon and must give an uneven drive. Anyway a mechanism allows the small gears to move in towards the center of the disc so the effective diameter the chain goes round reduces. You need a system like the bike derailier to take up the slack in the chain. Didn't take off.
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Maybe you could try a fluid coupling, it is a type of slipping clutch as used on auto transmission on cars. It is a very simple system using two paddled rotors in a casing separated by a fluid.
There are fluids out now that change properties when a current is passed thru them, the more current, the thicker the liquid goes, the more power is transmitted as the fluid thickens. Infinitely variable just by varying the current.
A very basic system of this was used in old cassette decks, when you opened the lid it opened very slowly. It was just a paddled rotor running in a very thick liquid.
John