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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.        

RE: Purely mechanical system to give a continuously variable gear ratio

I recall a positive- drive infinitely variable bicycle transmission, announced maybe 20 years ago in the mechanical fish wrappers.  I think it used radially sliding teeth on the crank sprocket, and a regular chain and hub.
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

Why would it be an FAQ when it rarely comes up.

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

(OP)
Sorry about the lack of links. This is a very specialised subject and I was hoping for some really specialised comment.  We are considering a similar academic exercise to that of BYU. What comment I get may influence whether the study goes ahead or not.  

http://contentdm.lib.byu.edu/ETD/image/etd1878.pdf

 http://contentdm.lib.byu.edu/cgi-bin/showfile.exe?CISOROOT=/ETD&CISOPTR=1683&filename=etd2391.pdf

http://contentdm.lib.byu.edu/ETD/image/etd2635.pdf

RE: Purely mechanical system to give a continuously variable gear ratio

(OP)
"Mechanical fish wrappers" - ??  

RE: Purely mechanical system to give a continuously variable gear ratio

Nemisi4,
"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

Nemisis4,
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

2
How serious are the stipulations positive drive, and purely mechanical?

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Purely mechanical system to give a continuously variable gear ratio

(OP)
GregLocock - these are very serious stipulations. I am only interested in mechanisms of this type. Despite many claims to the contrary I don't think it can be done - but it is very difficult to prove a negative etc. Despite many years of interest in the subject I have never seen anything even approaching a solution. It is one of those things that is either done or is not done - there are no "sort-of" situations. If it can't be done, I would like to be able to define just why it can't be done.  I found one statement on the internet that said that gears are by their nature "digital" and a gear varying system of infinitely small steps in ratio change must be "analogue" - so the two are incompatible. I see the point of this but human design and invention have ways of sidestepping such things.  Any system involving friction or hydraulics introduces an "analogue" factor into the system - so I am talking about systems that do not have "analogue" features.    

RE: Purely mechanical system to give a continuously variable gear ratio

How "continuous" do you want your gear ratios?  A computer monitor uses discrete points of flashing light to make a continuous-looking image.
 

RE: Purely mechanical system to give a continuously variable gear ratio

here's a suggestion for a continuously variable, purely mechanical device, straight off the top of my head:

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

should add:  elegant?  no.  proves that it's not a theoretical impossibility?  yes.

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

I think even ivymike's harebrained suggestion will be found to operate in discrete steps dependent on the pitch of the ratchet involved.

RE: Purely mechanical system to give a continuously variable gear ratio

it doesn't have to give discrete steps.  you can use more than one slider, for example, to keep the velocity positive.  in that instance you'd have an oscillating non-zero output speed.

RE: Purely mechanical system to give a continuously variable gear ratio

(OP)
Ivymike - If you use a uni-directional roller clutch you can achieve infinitely small steps. Oscillating systems that are "rectified" into a variable uni-directional output are one of the main groups of suggested variable gear mechanisms - there are thousands of examples in the world's patent files.

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

All of the "rectifying" mechanisms that I've ever seen discussed, are incapable of dealing with back-torque (i.e. in an automotive application, there is no possibility of using engine braking).

RE: Purely mechanical system to give a continuously variable gear ratio

hmmm. the phrase "tough shit" comes to mind.  did marketing ask for engine braking?  

RE: Purely mechanical system to give a continuously variable gear ratio

No, but the motor vehicle safety standards do.

RE: Purely mechanical system to give a continuously variable gear ratio

(OP)
I think an ideal variable gear system must be able to operate in both directions through the "gearbox" - with the reverse direction being the inverse ratio as with conventional gear sets. Once again with pushbikes, they usually freewheel on the overrun so ratchet-type arrangements would be OK.

RE: Purely mechanical system to give a continuously variable gear ratio

Oh, who made the double-cone tension belt CVT with radially grooved pulleys, and laminar teeth projecting from the belt?  That was in production for a long time, I think.

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 distinction "POSITIVE DRIVE" is a bit of a red herring, there are millions of mechanical drives that rely on friction, there are few mechanical ones that would work in the presence of 'cogging' only.

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Purely mechanical system to give a continuously variable gear ratio

(OP)
GregLocock - could you further explain what you mean in your last posting? Is the "no slip" you refer to the slight amount of sliding (as opposed to rolling action) that occurs when two gear teeth engage?  

RE: Purely mechanical system to give a continuously variable gear ratio

so, just for clarity, this post is all about whether making continuously variable "positively engaged" transmissions for on-highway motorized vehicles in the USA and meeting the current set of regulations for such is a theoretical/physical impossibility?  

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

(OP)
Ivymike - Thank you, that does just about sum up what I was originally asking.
 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

It's true that they may never exist, but I suspect that's because there is no market for them, not because they're impossible to make.
 

RE: Purely mechanical system to give a continuously variable gear ratio

Are the people at BYU needing feedback the authors of the papers?

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

Yes, it does seem to be something of an 'academic exercise'.

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

Is this forum being used for peer review of a student project and if so does it contravene the no student posts policy of the site.  

Regards
Pat
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RE: Purely mechanical system to give a continuously variable gear ratio

So, whose Nemesis are you, and what happened to the other three?

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Purely mechanical system to give a continuously variable gear ratio

(OP)
I have no connection whatsoever with BYU, not being a Mormon and living on a different continent. I was trying to draw out some specialised comment on the variable gear subject from engineers who have worked in the theoretical PECVT field (and there are some). But it doesn't look like it is going to happen - the engineers in question either don't read this forum or are secretive about their work.
 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

I don't think there is a non-friction-reliant drivetrain for a motor vehicle in existence, save for a handful of odd circle track parts that either have splines in the clutch, or have the clutch mounted in the transmission so that top gear bypasses it.



 

RE: Purely mechanical system to give a continuously variable gear ratio

(OP)
GregLocock - On the question of moving away from a standing start - any continuous gear ratio range can be arranged by overall biasing to have an infinitely low (1:0) starting ratio. It can even be arranged so that the ratio range is from negative values (reverse direction that would be), through zero speed and into positive values.
  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

Quote:

... engineers who have worked in the theoretical PECVT field ...

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

How can infinitely high torque break anything if there is zero motion.

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

Does nobody remember the DAF 30, 600 and 55 cars?
They all had "Variomatic" CVTs.
Google will help.

Benta.

RE: Purely mechanical system to give a continuously variable gear ratio

there have been cars with CVTs... the question is about a special kind of CVT in particular.
 

RE: Purely mechanical system to give a continuously variable gear ratio

Doh! I missed the reference to DAF even though I read the thread carefully. Sorry.
 

RE: Purely mechanical system to give a continuously variable gear ratio

"On the question of moving away from a standing start - any continuous gear ratio range can be arranged by overall biasing to have an infinitely low (1:0) starting ratio. It can even be arranged so that the ratio range is from negative values (reverse direction that would be), through zero speed and into positive values.
"

Show me a positive drive mechanism that can do this.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Purely mechanical system to give a continuously variable gear ratio

(OP)
Ivymike - I agree with you to some extent. However a lot (maybe most?) major advances in engineering start off as academic/theoretical "researchy stuff".

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

GregLocock,

"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"? 2thumbsup



It's a Steyr CVT, in case you were wondering.

Regards,
Terry

RE: Purely mechanical system to give a continuously variable gear ratio

Better ask the OP, he's the one who defined "positive drive"!

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Purely mechanical system to give a continuously variable gear ratio

(OP)
Just what do the letters "OP" stand for?

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

OP=original poster.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Purely mechanical system to give a continuously variable gear ratio

Nemesis4,

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:

http://blog.makezine.com/archive/2009/10/orrery_based_on_fergusons_mechanica.html

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'll make myself unpopular here, but-
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

There's a good reason for that, which I might summarise as silly questions get silly answers.

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

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Purely mechanical system to give a continuously variable gear ratio

um, Greg, I believe you meant to say harebrained suggestion.

RE: Purely mechanical system to give a continuously variable gear ratio

I would think that if you changed from 1:0 ratio to say 10,000:1 ratio, the torque would be extremely high and the speed extremely slow. The torque could indeed break things without any flex in the system, however all systems have some flex. If a drive shaft conecting the transmission to the wheels had some apropriately designed torsion bars built in, so long as the change from 1:0 to 1:1 or whatever was gradual enough the torsion bar would absorb the torque as the vehicle accelerated to match engine speed.

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

Incidentally my suggestion won't work, as described, it provides a variable stroke, not a variable output speed of rotation, so still needs a ratchety thing to provide that.

Cheers

Greg Locock

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RE: Purely mechanical system to give a continuously variable gear ratio

Right on with the fluid transfer of power, the typical
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

I recommend a long walk home from the pub before you get too excited, leastways that's when I realised my idea was wrong.
 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Purely mechanical system to give a continuously variable gear ratio

However a lot (maybe most?) major advances in engineering start off as academic/theoretical "researchy stuff".

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

The original post wanted to know about "infinitely variable positive drives".  By definition, such a device is probably not possible.  But let's take a look at what such a device might really imply:

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

if you don't mind me adding to this - putting all the previous points raised aside I don't see what is stopping a conventional CVT from having toothed cones..........there would be wear but I am sure this could be overcome, maybe even not too different to what they already ecounter.......that could possibly be a form of 'positive drive'?

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

A toothed cone doesn't work because it has the same number of teeth at the pointy end as it does at the other end.  If you step the cone into different bands of different numbers of teeth,  then you have the same thing that you have in a normal gearbox -- just a bunch of gears.   

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

Quote:

Does nobody remember the DAF 30, 600 and 55 cars?
They all had "Variomatic" CVTs.
I've driven DAFs. Brilliantly simple design. Driver experience is awful if you are used to stick shift. They feel like the drive shaft is a very large rubber band. The DAF seemed to be the Netherlands answer to the French 2CV and the German VW. Brilliant snow cars. Not much in the way of compression braking.  

TOP
CSWP, BSSE
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"Node news is good news."

RE: Purely mechanical system to give a continuously variable gear ratio

Quote:

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.
This would be the Prius. In production too.

TOP
CSWP, BSSE
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"Node news is good news."

RE: Purely mechanical system to give a continuously variable gear ratio

If you extend the stepped tooth cone idea to a large number of steps, you basically end up with a conical wire brush. ... which could mate to an identical brush pointed the other way via an internally toothed ring with an arbitrary number of teeth, or a similar brush, moved axially to change the ratio.  I wouldn't expect particularly good power density or efficiency or life, but it's theoretically possible.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Purely mechanical system to give a continuously variable gear ratio

I just wonder about the semantics here.

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

I've been lurking around trying to make sense of "most" of this thread...Not doing all that well at it, either.

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

Nah, you just need a really big flywheel.

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

evelrod,

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

I have just come across this very long thread. I am a bit surprised that some people are puzzled by (or don't believe at all) the concept of a zero gear speed changing to very low ratios in either direction. This is just standard gear train manipulation.
 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

well, read the above again, and you'll find that the discussion was (smoewhat secretly) targeted toward cars.

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