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Improved CVT
Pulley based continually variable transmission. | |
Please look at the pulley based CVT in the link (and maybe read it).
The CVT that I am imagining would work basically the same way as a regular CVT, but it would have differently shaped parts. Normal CVTs have 20-degree cones, but the one I am imagining would have approximately 70-degree cones, more
like this “><” rather than this “X”. The cones would be intermeshing, allowing them to slide “through” each other along a shaft. The belt connecting the input shaft to the output shaft would be a mild “V” shape.
It would be electronically controlled via linear actuators and sensors probably.
This design would increase the contacting area of the belt and reduce wear.
How CVTs Work
http://auto.howstuffworks.com/cvt.htm
Scroll down to the pulley based CVT. [BJS, Jun 15 2006]
variable belt drive
http://www.forix.co...55-transmission.jpg
as used by Daf [xenzag, Jun 15 2006]
Jet Lathe
http://www.jettools.../07_woodturning.pdf
See Picture on Page 2 upper right [jhomrighaus, Jun 15 2006]
Another Lathe
http://www.wmhtoolg...il&breakdown_ID=431
[jhomrighaus, Jun 15 2006]
patent number WO 97/05402
http://www.wipo.int...PAGE=0&DOC_TYPE=PCT
look at drawings on page 19 onwards... [xaviergisz, Jun 17 2006]
[link]
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Daf cars used to have a continuously
variable sliding belt transmission, but
maybe I'm not understanding your idea
properly. |
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I once rebuilt one of these Dafs. Bloody
hard work! (I didn't leave that bone by the
way) |
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The belt doesn't slide, but the cones move towards each other on one shaft, and appart on the other shaft. |
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One difficulty with CVTs is that if the driving and driven elements are in contact at more than a single point, either there will be some sliding friction between them or else one or the other will be stretched or compressed during operation. |
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There is a Yellow Daf for sale in dolgeville, NY at the antique store. neat cars. |
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As to this idea this is already used in many common wood lathes(like Jet 40")and it is constructed exactly as you have described. I say Baked. |
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From the links jhomrighaus has provided, it appears that the transmission on the lathe does mesh together somewhat, however, it isn't even near the 70-degree angle I described. |
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So it's not baked (for cars anyway...). |
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The lathe transmission isn't at a shallow angle, true. But the intermeshing idea is out there, and it could easily be applied to automobiles. Shallow-V pulleys are also fairly obvious, but they don't work. |
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This is another case where there are a lot of issues to be dealt with before the wished-for idea can be implemented. First, a belt running over the gaps in an intermeshing pulley would get chewed the heck up--the lathe transmission avoids that. Second, a shallow-V pulley would have big problems with the belt climbing out of the groove--an inverted shallow-V pulley is in common use on old machinery, as a flat belt will self-center on an inverted V. |
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BJS, you have a habit of suggesting something fairly obvious in highly-developed and well-researched areas. Do some work before you post, and consider that some things aren't being done because they don't work, not because you are the first person ever to think of them. |
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So what's wrong with this idea? |
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It's pretty much impossible to think of anything that nobody has ever thought of before. |
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Increasing surface area does not decrease wear. Wear is not a function of surface area. Wear occurs when ever there is a difference in speed between moving parts. a Large flat belt could experience much greater wear than a narrow one depending on how they are used. I say this to make the point that existing designs do not suffer from any design flaw that makes them more prone to wear. The biggest improvements that can be made to a CVT would be heat dissapation and decreasing the rotational mass of the various parts(which decreases the forces needed to change thier velocity) |
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On further reflection i think your design might actually increase friction under normal operation compared to a narrow belt. I say this because the wide contact patch that has to be pulled off the pully as the belt travels to the other pully would actually increase drag on the system. The one circumstance where this system would reduce wear is that caused by slipping of the belt do to overload. |
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A second point that needs to be considered is the use of the transmission output. For cars you need to go through a fairly drastic change in the gear ratio from starting out to cruising speed. This more than anything will dictate the pitch of your pully faces. Shallow angle pullys will have to be prohibitively large to accomodate the needed gear ratio changes compared to a relatively steep profile. |
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In any case the current designs are without a doubt a balance between economy and performance. What would be really neat and I am sure it is being researched would be a varible pitch CVT system(a CVVT) that could be adjusted for different speed and torque demands(broad flat pitch, high surface area for high torque low speed applications and tall skinny minimal contact area for high speeds) |
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The steep "V" found in existing designs only allows the belt to contact the pulley along the edge. Getting much contact area requires using a thick belt, and thick belts have more friction than thinner ones. Having shallower pulleys would be good if no other problems resulted from doing so. |
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It would be possible to construct pulleys of arc-shaped pieces that could pivot to make the effective diameter expand and shrink. Unfortunately, such pulleys would not be quite "round", and thus their operation wouldn't be quite smooth. |
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It would be possible to achieve a smooth power transfer from the input to the output if there were two linked off-round pulleys, and power was transferred from the source to the first, and from the second to the load. In that case, the mechanical advantages of the two pulleys would vary in sync, so the variations would cancel each other out. The pulleys themselves, however, would be subject to vibration since they would speed up and slow down during the rotation cycle. |
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Note that for the changes in mechanical advantage to cancel each other out, it would be necessary to have an idler pulley on the power source and/or load to ensure that the ratio of belt length to segment size was the same on both the source and load side, and that the phase angles of the variation were matched. |
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jhomrighaus, I didn't say that increasing the surface area would reduce wear, and I never said anything about having a wider belt. And what does the W stand for in CWT? |
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I don't think it will work (if I'm visualising it correctly). |
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The two conical cages interesect forming a v-shaped cradle. But this intersection is always between sections of the cones of the same diameter. This means a) there will be no 'gearing' effect between cones and b) (less importantly) a compensating element to keep the belt taut will be needed. |
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could you illustrate your idea to clarify? |
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xaviergisz, There will be one input shaft which would be parallel to an ouput shaft, along each shaft are two cones facing each other. A belt goes around the narrowest part on each shaft. As one shaft's cones go toward each other, the other shaft's cones simultaneously separate in the equal and opposit direction of the other shaft, creating equaly opposit diameters. |
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//Does that make sense?// no (and I don't think I'm the only one). you'll have to illustrate it. |
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**edit** OK the penny has dropped - I get it now (and I like it [+]). |
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//I didn't say that increasing the surface area would reduce wear, // |
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//This design would increase the surface area of the contacting point and reduce wear.// |
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Did I missunderstand what you said? |
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//and I never said anything about having a wider belt.//
you are right however, the wider belt is dictated by the mechanical design requirements of the belt and the pullies. Also this was a followup to not just your idea but the other annos as well. |
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YOu could interchangeably sustiute the words "area of contact" for Belt in this scenario. |
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You are also grossly oversimplfying the "belt" in a CVT, it is actually a very complex stack of metal plates that are shaped to a very high tollerance to mesh perfectly with the face of the cones. Changing the angle of the cones means a new shape of belt, a shallower angle will dictate a wider plate in order to achieve the proper area of contact. |
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I think you need to take the earlier suggestion and do a little more research and be a little less quick to dismiss comments and critisisms. |
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//And what does the W stand for in CWT?// |
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If you look close it actually says CVVT not CWT, or rather C.V.V.T. not C.W.T. this stood for Constant Variable Variable Transmission. |
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[xaviergisz], [BJS] is correct in the operation of this type of setup. Look at the lathe pictures to interpret this, one set of pullys is lever controled the other is spring loaded and will adjust accordingly. |
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Makes sense to me. I'm not certain the belt will last long enough, but then again, if all you have to replace is a belt, then perhaps it could become a service item. |
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I suspect the NVH concerns for this transmission would be disastrous. It would run terribly rough, due to the necessarilyy 'bumpy' profile the belt runs on. |
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Rather imaginitive bit of thinking though. |
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Modern CVT's in cars use 'pusher' belts instead of the normal pull style, and so I don't think your wide-angle V would generate enough friction. It's doubtful it could even work in a pull configuration, with such contact gaps as would be necessary. |
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I was thinking of some type of rubber belt with fibers in it, but maybe some kind of metal one would work. And also the edges of the metal where the belt touches would be slightly rounded so that the belt won't wear out as quickly. |
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//So what's wrong with this idea? // |
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Several things, which I covered in my earlier annotation, but which BJS doesn't seem to be able to understand. |
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First, the shallow-V part. Back in the good old days, before electric motors were common, many workshops drove all their machinery with belts from a central shaft. They used flat belts, and crowned pulleys, <>, which are inverted shallow-Vs, with no sides to them. The belts were self-centering, as a belt is always trying to climb up the slope of a pulley. When V-belts were invented, largely to fit in the crowded spaces of an engine, they were modified from round belts, another highly-developed area of technology. The grooves of a v-pulley are steep for a reason--to keep the belt from climbing out. Putting a shallow-V belt and pulley on something will not work--there are nearly two centuries of technological experience behind belt drives, and plenty of literature available. Which is why nobody does shallow-V pulleys and belts--to keep the belt from climbing out, or wriggling side-to-side. |
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Speaking of shallow angles, as [jhomrighaus] points out, a set of cones that are at shallow angles will have to slide through each other for a long way before there is much change in diameter of the driving pulley. That adds up to a lot of weight and space, plus longer actuators. Again, steep-sided cones would be better, shorter and lighter to build and operate, which is why standard practice is to use them. |
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Which brings up another point. The shallow cones will have a lot more surface area that will have to be made smooth, than steep-sided cones would have. Again, that is costly to make. (The internal structure of the cones will have to be stronger, too.) The "edges of the metal where the belt touches would be slightly rounded so that the belt won't wear out as quickly" which would be much more costly on shallow cones than on steep cones, as there is more surface area to them. |
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Back to the intemeshing aspect: Smoothing those edges would add incredibly to the costs of manufacture and maintenance, but would still leave the belt thumping and vibrating, which is bad. The intermeshing aspect of this idea is bad, very bad. The CVTs that are currently made do some intermeshing of the actuators, but not of the friction faces of the pulleys, for very good reasons. |
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//It's pretty much impossible to think of anything that nobody has ever thought of before. // |
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For BJS, maybe. Look, combining two old or unusual ideas is a perfectly good way to come up with something that nobody has thought of before. Sometimes the two ideas combine in a synergy that is truly beautiful, or is at least halfbaked. |
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In this case, the shallow-V idea and the intermeshing cone idea combine in a way that is truly wrong. It has no advantages at all, and goes against years of experience and technology for no reason other than that somebody who seems to know nothing about belt drives thinks it might work. It won't. |
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go easy [baconbrain]. I think think this is some pretty good halfbaking (although it took me a while to figure it out). [BJS] has looked at widely known CVTs and thought up a variation/improvement. Although this might be baked or un-bakeable - neither of these conclusions are immediately obvious. |
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The essence of HB is to share your non-obvious insights with everyone - this way you a) give a little joyful "ah ha" to everyone; b) teach people about a topic they weren't aware of and/or c) learn something yourself. |
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I don't think we should be discouraging the speculative nature of halfbaking with heavy handed experts-wouldn't-do-this attitude. So although everything that has been said might be technically correct - there is a polite and light-hearted manner of stating it. |
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Well said, and fully agreed. BJS is a special case, as he seems to understand nothing less than a hit over the head. I shall endeavor to be more polite. Thanks, X. |
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baconbrain, I understand what you are telling me, but I need information from more than just you. |
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As concerns my authority as an authority on belts and CVTs, I'll repeat what I wrote on another bad BJS idea. I installed a Comet brand CVT on a front-wheel-drive motorcycle, and watched it work for many a mile as part of the testing program. I helped design and manufacture the multi-belt system that we preferred to using a CVT. I spent five years in the R&D department of a manufacturer of belt-driven fans. I've even helped run a corn sheller with a flat belt and an old John Deere. |
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I think the key points being made here is that this thought is a) not original(see links) B) not innovative, and C) most of all not an improvement. I think this is what bacon is try to say, and is supported by the evidence. |
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Hey, none of my ideas are "bad", they just might not be "good", or "new". |
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Actually it is original, since I didn't know that it already existed. |
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I dont think you should delete it, but I dont think its a very good or ineresting idea. Personally I think my variation on your idea is much better, at least its unique(albiet stupid and impractical) try comming up with something new. |
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Okay, I won't delete it then. |
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