Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in

Purely mechanical system to give a continuously variable gear ratio 3

Status
Not open for further replies.
Nemesis4

Nemesis4

Mechanical
Jul 8, 2009
31
0
0
AU
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.
 
Replies continue below

Recommended for you

Sort by date Sort by votes
"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 for tips on how to make the best use of Eng-Tips.
 
Upvote 0 Downvote
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?).
 
Upvote 0 Downvote
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]

p41.gif


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

Regards,
Terry
 
Upvote 0 Downvote
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 for tips on how to make the best use of Eng-Tips.
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
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:


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
 
Upvote 0 Downvote
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 for tips on how to make the best use of Eng-Tips.
 
Upvote 0 Downvote
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
See FAQ731-376 for tips on use of eng-tips by professional engineers &
for site rules
 
Upvote 0 Downvote
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

SIG:please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
Upvote 0 Downvote
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?
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
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
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
Does nobody remember the DAF 30, 600 and 55 cars?
They all had "Variomatic" CVTs.
Click to expand...
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

"Node news is good news."
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

deepnsteep
  • Locked
  • Question
Gear Ratios and BSFC
Replies
9
Views
28
BrianPetersen
BrianPetersen
Uncia_Uncia
  • Locked
  • Question
Air to Boil test in diesel engine cooling system
Replies
3
Views
25
Uncia_Uncia
Uncia_Uncia
Charlie144
  • Locked
  • Question
help to finding starter ring gear tooth profile
Replies
3
Views
21
GregLocock
GregLocock
Uncia_Uncia
  • Locked
  • Question
Governor system in off road diesel engines
Replies
3
Views
17
TugboatEng
TugboatEng
Joel_Lapointe
  • Locked
  • Question
Pressure angle for helical gears, with tangential, axial and radial forces.
Replies
2
Views
18
pmbrunelle
pmbrunelle
Back
Top