Purely mechanical system to give a continuously variable gear ratio
Purely mechanical system to give a continuously variable gear ratio
(OP)
I would have thought this subject would be a FAQ - but apparently not. I have always wondered if an "infinitely variable gear" is possible. I am not referring to any of the currently commercially available CVTs etc. but a constantly engaged, positive drive, all teeth and chains (or whatever) system. I far as I can tell this has been an unsolved problem for at least 500 years. I have found personally that most engineers (in the transmission business) are reluctant to speculate at all - I think there is more than a touch of "pseudo science" and "wild-eyed inventors" about the subject. What is the general opinion on the subject?
I notice that currently the Mechanical Engineering department at Brigham Young University is making a major effort in this area. I think you would have to regard BYU as distinctly non "wild-eyed". There have been successive MSc. theses from Brian Andersen, Ryan Dalling and Levi Haupt (all these theses can be read on the internet).
This is all very high-powered academic stuff and they seem to think that they have solved the age-old variable gear problem. Needless to say, none of it makes a lot of sense to me and I remain unconvinced. I would also like opinions on these three theses. Thank you in advance.
I notice that currently the Mechanical Engineering department at Brigham Young University is making a major effort in this area. I think you would have to regard BYU as distinctly non "wild-eyed". There have been successive MSc. theses from Brian Andersen, Ryan Dalling and Levi Haupt (all these theses can be read on the internet).
This is all very high-powered academic stuff and they seem to think that they have solved the age-old variable gear problem. Needless to say, none of it makes a lot of sense to me and I remain unconvinced. I would also like opinions on these three theses. Thank you in advance.





RE: Purely mechanical system to give a continuously variable gear ratio
I don't recall ever seeing one in person.
If you want us to download and read something, at least do us the courtesy of providng a link.
Mike Halloran
Pembroke Pines, FL, USA
RE: Purely mechanical system to give a continuously variable gear ratio
You can write up and post an FAQ yourself if you think it is warranted and worth your time.
Mike is correct about the links.
This subject has been discussed in detail several times before. A google site search might find it.
As far as I am aware VRTs have been around alsost as long as the wheel. The problems are bulk and weight vs load carrying capacity.
Belt drive over opposing cones, hydraulic drives like a torque converter, variable pitch propellers or vanes, wheel against the side of a disk are all methods I am aware of off the top of my head. Even a typical automotive friction clutch is a variable ratio drive, all be it.
Regards
Pat
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RE: Purely mechanical system to give a continuously variable gear ratio
http://contentdm.lib.byu.edu/ETD/image/etd1878.pdf
http:
http://contentdm.lib.byu.edu/ETD/image/etd2635.pdf
RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
Regards
Pat
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RE: Purely mechanical system to give a continuously variable gear ratio
"fishwrapper" does indeed refer to newspapers usually tabloids.
The name derives from the using of newspapers by fish sellers to wrap their products.
Pete.
RE: Purely mechanical system to give a continuously variable gear ratio
Napier built an aircraft engine using a series of tapered discs as a variable ratio drive for the supercharger.
The engine was either the Sabre or Nomad if I recall correctly.
This may provide some leads.
Pete.
RE: Purely mechanical system to give a continuously variable gear ratio
Cheers
Greg Locock
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RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
input shaft turns the crank portion of a crank-slider mechanism. Crank throw (radius) is mechanically adjustable to give variable stroke. The slider is a moving rack which drives a pair of fixed pinions, which through ratcheting mechanisms and a spring+flyweel converts the back-and-forth motion into relatively smooth unidirectional rotation.
RE: Purely mechanical system to give a continuously variable gear ratio
you can come up with countless other concepts, if you give it a try. There's the chain-over-conical-sprocket idea, maybe some toroidal gear pairs with variable interface angle, etc... who knows
RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
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RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
MikeHalloran - after reading the various comments, presumably "mechanical fish wrapper" would be a magazine of the "Popular Mechanics" variety (you could only wrap very small fish with PM's page size). I think this comment demonstrates what I wrote in my original statement. Mike is presumably a typical conservative engineer - his immediate reaction seems to be that speculation on subjects like variable gear systems are not worthy of discussion in "proper" engineering publications. And to some extent I don't disagree with this view - there are an awful lot of ridiculous mechanisms proposed by people trying to promote their pet ideas. I think you would discover that the bicycle mechanism that MH mentions would have some fatal "flaw" in its basic idea. Bicycle transmissions are one of the main "hotbeds" of variable gear design. Because they transmit such low amounts of power they can get away with a bit of dodgey design.
Any comments on the BYU papers?
RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
The BYU papers are nicely done. Too bad there wasn't time to actually make something and test it to destruction.
Scrap paper is a less satisfactory measure of intellectual progress than is scrap metal.
Mike Halloran
Pembroke Pines, FL, USA
RE: Purely mechanical system to give a continuously variable gear ratio
The reason for that is that no slip is a bit of a tough call during engagement.
So, the requirement is BS, hence no answer can be satisfactory.
Cheers
Greg Locock
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RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
Seems like the simple answer is no, it's not a physical impossibility. Why the ongoing prodding?
...or were we supposed to read through each of the theses and provide some kind of specific feedback about the approaches they propose?
RE: Purely mechanical system to give a continuously variable gear ratio
However, my simple answer would be that it is highly likely that it is a theoretical/physical impossibility - so we have slightly different points of view.
Positive engagement continuously variable transmissions ("PECVT's" to those in this specialised field) don't exist now, have never existed and may never exist. This is something of a sweeping statement but I suspect it is true.
Yes, I would very much like somebody to read through the theses and provide specific comment and feedback (they are all on a similar theme and develop the idea from one thesis to the next). I realize they are very heavy going to read but I would like to know what people think - and I suspect that the people at BYU would also like to see some feedback etc.
RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
Shouldn't the "committee" and the authors' peers be providing such feedback?
Or is the "committee" requesting feedback from this group, so as to offer some insight to the authors of the papers?
or are we fishing for something to "defend" our thesis?
RE: Purely mechanical system to give a continuously variable gear ratio
How does your theoretical device handle moving off from a standing start while retaining positive engagment? It almost certainly relies on relative motion between a moving part and a stationary part somewhere, otherwise you could call it a Zeno transmission.
Cheers
Greg Locock
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RE: Purely mechanical system to give a continuously variable gear ratio
Regards
Pat
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RE: Purely mechanical system to give a continuously variable gear ratio
Mike Halloran
Pembroke Pines, FL, USA
RE: Purely mechanical system to give a continuously variable gear ratio
It is probably best if I don't reveal whose Nemesis I am - the other three met their demise, I'm afraid.
RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
However I don't think any of the commercially available automotive CVTs are arranged like this though - I think DAF used a centrifugal clutch for starting and others have fluid couplings etc. The infinitely low (1:0) ratio can also produce (theoretically) infinitely large amounts of output torque, so if the output was locked for any reason something would self-destruct. In some circles, the 1:0 ratio is referred to (perhaps somewhat cutely) as an "engaged neutral".
RE: Purely mechanical system to give a continuously variable gear ratio
Perhaps you'd ought to call up the authors of the theses you wrote. I'm sure they've gone on to become engineers. I'm not sure you'll find many "real" engineers working on academic problems. There are always the few that work on researchy stuff in the backwaters of big companies, but the only products that see the light of day are the ones driven by a business case and a list of customer benefits. The EIEIO transmision seems to lack both.
RE: Purely mechanical system to give a continuously variable gear ratio
I think a lot of circle track guys stll use a dog clutch and a push start.
All automotive drive systems come down to friction at the road interface unless you are talking about rack and pinion drives or tyres with spikes for use on ice.
Regards
Pat
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RE: Purely mechanical system to give a continuously variable gear ratio
They all had "Variomatic" CVTs.
Google will help.
Benta.
RE: Purely mechanical system to give a continuously variable gear ratio
Regards
Pat
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RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
"
Show me a positive drive mechanism that can do this.
Cheers
Greg Locock
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RE: Purely mechanical system to give a continuously variable gear ratio
PP - You are right, nothing will break exactly on the 1:0 ratio as there is no motion (this being right on the "neutral" point) - however as soon as the ratio moves even a tiny amount away from 1:0 the ratio becomes something like 500:1 (or whatever) with a slight motion, and this will certainly break things.
I think speedway sprint cars are like this. I also think to save weight, they may even have no dog-clutch or flywheel - the driveline and wheels acting as the flywheel - but I could be wrong.
GregLocock - Do you doubt that this mechanical effect of moving away from an "engaged neutral" can be achieved even in theory, or do you doubt that it is ever used in practice? It certainly works in theory. I will search the net for existing examples of this effect - I have seen some recently. (Does any Forum reader know of an example?).
RE: Purely mechanical system to give a continuously variable gear ratio
"Show me a positive drive mechanism that can do this.
OK, I'll bite. Since a liquid is essentially incompressible, would a split torque "Ferguson" gearset configuration, using a hydrostatic variator on one side, qualify as "positive drive"? Or would you only consider the hydrostatic variator as being "positive displacement"?
It's a Steyr CVT, in case you were wondering.
Regards,
Terry
RE: Purely mechanical system to give a continuously variable gear ratio
My opinion is that the definition or requirement is a red herring, but since you have relative motion at the liquid to vane interface, then by my understanding it is not a positive drive, or if it is then a perbury or rototrak (sp?) is just as much.
Cheers
Greg Locock
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RE: Purely mechanical system to give a continuously variable gear ratio
Tbuelna - Yes, I would regard hydrostatic drives as "positive" (except perhaps for a bit of leakage past the pistons etc). I think some hydro. drives (in fork lifts, lawn tractors etc.) are arranged so that the speed range is from zero upwards - so presumably they would exhibit very high torque at low output speeds (maybe they have pressure relief valves to limit destructive hydraulic pressures).
Although positive drive I have not included hydro. drives in the mechanical/teeth only etc. class as they are distinctly "analogue" in nature.
To explain gear ratio "biasing" further - if an output from an engine (or whatever) is split into two equally-geared parts, one part reversed in rotational direction then fed into the side gears of a differential, then the output of the diff is zero. Varying the input to one of the side gears through a positive drive variable speed mechanism allows the diff to have an output from zero up or down. In practice this is usually done very neatly through an epicyclic gear train.
RE: Purely mechanical system to give a continuously variable gear ratio
Cheers
Greg Locock
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RE: Purely mechanical system to give a continuously variable gear ratio
The hydrostatic variator in that Steyr CVT is a swash-plate piston device. When brand new, that piston hydrostatic pump/motor combination has pretty good efficiency (ie. >92%). And since only about half the power goes thru the variator, the loss amounts to about 4% max. Compare that to a typical "positive drive" power transfer device (like a gear set) that might have 2% or 3% losses, at best. It's pretty close, efficiency wise.
The forward/backward output rotation from these "split torque" transmissions is obtained thru their planetary gearing arrangements:
h
And finally, even gear teeth transfer power thru an EHL oil film contact (when designed correctly). So is a transfer of power through a hydrostatic pump/motor really all that different from a transfer of power through a gear tooth contact?
Best regards,
Terry
RE: Purely mechanical system to give a continuously variable gear ratio
I learned more about CVTs and IVTs from a brief Wikipedia session than I have from this thread...
RE: Purely mechanical system to give a continuously variable gear ratio
Fundamentally the issue seems to me, how can you define positive drive in such a way that allows the inevitable speed mismatch at 0 rpm?
And if you do define it that way, does it still match what the real world means by positive drive (which for the sake of brevity I'll call cogging)?
Well here's my best guess at one, very similar to parts of Ivymike's suggestion.
Take a Stephenson's valve gear, and use it to drive a crank or scotch yoke instead of a slider valve.
Add a second valve gear on the same shafts but 90 degrees out of phase at each end. This stops it locking up at TDC.
It won't be very efficient, but that wasn't a requirement.
Cheers
Greg Locock
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RE: Purely mechanical system to give a continuously variable gear ratio
RE: Purely mechanical system to give a continuously variable gear ratio
I presume a torsion bar would be considered a direct drive
Regards
Pat
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RE: Purely mechanical system to give a continuously variable gear ratio
Cheers
Greg Locock
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RE: Purely mechanical system to give a continuously variable gear ratio
excavator travel system fills the bill here.
vairiable axial piston pumps, and the travel motors.
So were you thinking a zero slippage mechanical system?
I just thought of one, do I need a patent on it?
RE: Purely mechanical system to give a continuously variable gear ratio
Cheers
Greg Locock
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RE: Purely mechanical system to give a continuously variable gear ratio
sure, major advances require research. Most (nearly all) successful new products, however, are incremental improvements of existing ones, or simple expansions of a product line. The researchy ones have to have a clear & substantial benefit ahead of the research to make the research worth funding.
What do you imagine the benefits of your proposed transmission are?
- Will it be nearly lossless? I don't see that as such a big benefit, honestly. Current transmissions do a reasonable job.
- Will it be more fun to drive? I doubt it.
- Sound better? Feel better? Require less skill to operate? Nope.
- Enable some other key technology? Don't know - tell me if it will, I can't think of an example.
RE: Purely mechanical system to give a continuously variable gear ratio
Even a very well designed gear or chain drive does not have perfect mechanical efficiency. And modern push belt or traction CVTs can have input-to-output efficiencies that are competitive with geared automatic transmissions. So if efficiency is what you're concerned with, having a "positive drive" characteristic might not be so essential.
Also, even a seemingly "positive drive" transmission with gears, still likely has a friction clutch or hydrodynamic torque converter device for starting/stopping and/or shift synchronization. And neither of these devices is a "positive drive" mechanism.
Finally, the device that ultimately transmits the engine power to the road surface in order to propel the vehicle forward is also not a "positive drive" mechanism. The tire operates on friction.
So considering all of that, just how important is it that a vehicle transmission be "positive drive" in its speed change function?
Regards,
Terry
RE: Purely mechanical system to give a continuously variable gear ratio
There would be specific technical reasons they chose what they did even if it was a simple case of not having enough research at the time to warrant an implementation of the technology so i'm not going to doubt the rationality or even the question if it is a 'red herring.'
I am presuming a 'positive drive' is desirable in terms of efficiency but i would question if it is a case of 'fine tuning' what is already in manufacture rather than a more radical approach. Then again in the interest of progress I don't think it is an invalid question to ask.
RE: Purely mechanical system to give a continuously variable gear ratio
Engineering is not the science behind building. It is the science behind not building.
RE: Purely mechanical system to give a continuously variable gear ratio
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RE: Purely mechanical system to give a continuously variable gear ratio
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RE: Purely mechanical system to give a continuously variable gear ratio
Mike Halloran
Pembroke Pines, FL, USA
RE: Purely mechanical system to give a continuously variable gear ratio
Infinitely variable implies no upper or lower limits.
I think everything being described has a limit on one end so we really mean seamlessly variable within yada yada yada limits.
Regards
Pat
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RE: Purely mechanical system to give a continuously variable gear ratio
From the OP I get the need for an purely mechanical IVT with zero slippage...Hmmmmm? If I have it right, you want an automotive application whereby the engine is running and the car is standing still with the drive positively engaged with no slip....That brings up some really serious issues, hey?
Rod
RE: Purely mechanical system to give a continuously variable gear ratio
Cheers
Greg Locock
I rarely exceed 1.79 x 10^12 furlongs per fortnight
RE: Purely mechanical system to give a continuously variable gear ratio
Although it is not a "purely mechanical IVT with zero slip", the Torotrak IVT (with its geared neutral architecture) actually can be fully engaged while the vehicle is stopped. It can also creep forward and backward without any shifting.
http://www.torotrak.com/automotive/benauto
Regards,
Terry
RE: Purely mechanical system to give a continuously variable gear ratio
Also surprising are the people who think that there is no practical application for such things. You could probably argue this point when it comes to cars (I don't like CVTs either or even 4 or more speed automatics) but there would be an enormous market for a zero to 1:1 continuous range transmission in the heavy truck industry. If you have driven (or attempted to drive) a truck with a 15 speed Roadranger non-synchro gearbox you may agree. I think that the RR gearbox is one of the most diabolical and awkward (and dangerous for a novice) devices known to mankind.
This is besides the point that such a mechanical variable box is probably not possible let alone one that would survive in a heavy truck.
RE: Purely mechanical system to give a continuously variable gear ratio