What are Continuously Variable Transmissions CVTs : Car Tires and Maintenance  -- Cars Discussion Forum and Review What are Continuously Variable Transmissions CVTs : Car Tires and Maintenance -- Cars Discussion Forum

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 06-18-2009, 11:55 Post: 163490
kwschumm



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 What are Continuously Variable Transmissions CVTs

My mental picture of a CVT involved a belt driving two piece conical shaped pulleys. The variable ratios were controlled by moving the two pulley halves closer or further apart.

Early CVTs seemed to be used only on the smallest of low powered cars.

Now they are becoming more common and I was astounded to hear that they are being used on heavy equipment. Furthermore, someone here mentioned that JD had a CVT design that was essentially a gear based transmission.

I'd sure like to understand how a gear based CVT would work. You can't vary the teeth on a gear. You can vary the number of gears but you can't have an infinite number of them and it seems they could never be continuously variable in the true sense of the term.

What was the big breakthrough design that allowed CVTs to become more successful?






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 06-18-2009, 17:04 Post: 163493
auerbach



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 What are Continuously Variable Transmissions CVTs

I don't know but they've come a way since the DAF. There are now eight different types being used in cars, including some bigger and heavier brands -- all outlined in Wiki.






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 06-18-2009, 20:17 Post: 163495
hardwood

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 What are Continuously Variable Transmissions CVTs

The CVT design I'm familliar with all goes back to Henry Ford's planetary gear design with a few modifications.
OK, picture in your mind the common planetary gear arangement.

Sun gear in the center.

Planet gears in mesh with teeth on the sun gear and in mesh with teeth on the inside of the ring gear.

Ok, now if the ring gear is held in a fixed position and the sun gear is spinning in the center at a given RPM, the power is being sent out thru a bracket and shaft holding the planet gears in place. The
RPM of the output shaft connected to the planet gears will be less. The "RATIO" of input shaft to output shaft determined by how many teeth are on the sun, planet and ring gears.

Now same design as I described above only now what happens if the ring gear is not held in a fixed position while the gearset is running and can be spun in either direction at varying speeds by means of teeth on the outside of the ring gear? You are varying the ratio of the gearset while in use = CVT. The ring gear is being spun in either direction at varying speeds by a hydraulic motor to get the travel speed, (RATIO), you want.

OK, put two or three of the systems I described in series sending power from the first to second to maybe even a third gearset with the variable speed ring gear, pretty simple basic design.

Some may argue that the hydraulic motors are transmitting some of the power, perhaps they are but it would be minimal compared to the power being transfered thru the gearsets.

Clear as mud. Frank.






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 06-18-2009, 21:50 Post: 163497
kwschumm



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 What are Continuously Variable Transmissions CVTs

Thanks Frank, that makes sense. It's sort of funny, shortly after posting this Subaru's latest newsletter arrived in my inbox. With their 2010 models they are rolling out CVTs on some of their models. It seems to have increased highway mileage by 15% or so over previous models. They included a link to a video and it shows the design that I was familiar with, two two-piece conical pulleys with a belt. There's something about that design I just don't trust.






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 06-19-2009, 07:39 Post: 163500
hardwood

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 What are Continuously Variable Transmissions CVTs

KW, before hydros came along combines used the variable speed belt design you describe. WE never had much trouble with them, but more maintenece was involved than hydros.






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