Editing Continuously variable transmission (section)

=== Pulley-based ===
{{multiple images
| align = right
| direction = vertical
| width = 180
| image1 = CVT-Fliehkraft.JPG
| caption1 = Belt-driven CVT for a [[Scooter (motorcycle)|motor scooter]]
| image2 = Pivgetriebe.png
| caption2 = A PIV chain drive
| image3 = Claas Mercator 12 August 2020 JM (7).webm
| caption3 = CVT in a [[Claas Mercator]] combine harvester. The pulley's effective diameter is changed by pushing the two conical discs either towards or away from each other.
| total_width = 
| alt1 = 
}}

The most common type of CVT uses a [[V-belt]] which runs between two variable-diameter pulleys.<ref>{{cite journal|last=Fischetti|first=Mark|title=No More Gears|journal=Scientific American|volume=294|issue=1|pages=92–3|date=January 2006 |doi=10.1038/scientificamerican0106-92|pmid=16468439|bibcode=2006SciAm.294a..92F}}</ref> The pulleys consist of two cone-shaped halves that move together and apart. The V-belt runs between these two halves, so the effective diameter of the pulley is dependent on the distance between the two halves of the pulley. The V-shaped cross-section of the belt causes it to ride higher on one pulley and lower on the other; therefore, the gear ratio is adjusted by moving the two [[Sheave#Adjustable pulley|sheaves]] of one pulley closer together and the two sheaves of the other pulley farther apart.<ref name="howstuffworks" />

As the distance between the pulleys and the length of the belt does not change, both pulleys must be adjusted (one bigger, the other smaller) simultaneously to maintain the proper amount of tension on the belt. Simple CVTs combining a centrifugal drive pulley with a spring-loaded driven pulley often use belt tension to effect the conforming adjustments in the driven pulley.<ref name="howstuffworks">{{cite web|url=https://auto.howstuffworks.com/cvt2.htm|title=How CVTs Work |website=howsuffworks.com|access-date=26 August 2020|date=27 April 2005}}</ref> The V-belt needs to be very stiff in the pulley's axial direction to make only short radial movements while sliding in and out of the pulleys.

The radial thickness of the belt is a compromise between the maximum gear ratio and torque. Steel-reinforced V-belts are sufficient for low-mass, low-torque applications like utility vehicles and snowmobiles, but higher-mass and -torque applications such as automobiles require a chain. Each element of the chain must have conical sides that fit the pulley when the belt is running on the outermost radius. As the chain moves into the pulleys the contact area gets smaller. As the contact area is proportional to the number of elements, chain belts require many very small elements.

A belt-driven design offers approximately 88% efficiency,<ref>{{cite web |url=http://www.zeroshift.com/pdf/Seamless%20AMT%20Offers%20Efficient%20Alternative%20To%20CVT.pdf |title=CVT Efficiency |website=zeroshift.com |archive-url=https://web.archive.org/web/20140714215029/http://www.zeroshift.com/pdf/Seamless%20AMT%20Offers%20Efficient%20Alternative%20To%20CVT.pdf |access-date=22 April 2014 |archive-date=14 July 2014 |url-status=dead}}</ref> which, while lower than that of a [[manual transmission]], can be offset by enabling the engine to run at its most efficient RPM regardless of the vehicle's speed. When power is more important than economy, the ratio of the CVT can be changed to allow the engine to turn at the [[Revolutions per minute|RPM]] at which it produces the greatest power.

In a chain-based CVT, numerous chain elements are arranged along multiple steel bands layered over one another, each of which is thin enough to easily [[Bending|bend]]. When part of the belt is wrapped around a pulley, the sides of the elements form a conical surface.<ref>{{Cite web|title=XTRONIC CVT {{!}} Nissan {{!}} Technology|url=https://www.nissan-global.com/EN/TECHNOLOGY/OVERVIEW/cvt.html|url-status=live|access-date=2021-09-20|website=Nissan Motor Corporation Global Website|publisher=[[Nissan|Nissan Motor Co. Ltd.]]|archive-url=https://web.archive.org/web/20110120043856/http://www.nissan-global.com:80/EN/TECHNOLOGY/OVERVIEW/cvt.html |archive-date=20 January 2011 }}</ref><ref>{{Cite web|title=Pushbelt|url=https://www.bosch-mobility-solutions.com/en/solutions/transmission-technology/pushbelt/|url-status=live|access-date=2021-09-20|website=Bosch Mobility Solutions|publisher=[[Robert Bosch GmbH]]|language=en|archive-url=https://web.archive.org/web/20210508172104/https://www.bosch-mobility-solutions.com/en/solutions/transmission-technology/pushbelt/ |archive-date=8 May 2021 }}</ref> In the stack of bands, each band corresponds to a slightly different drive ratio, and thus the bands slide over each other and need sufficient [[lubrication]]. An additional film of lubricant is applied to the pulleys. The film needs to be thick enough to prevent direct contact between the pulley and the chain, but thin enough to not waste power as each chain element enters it.{{Citation needed|date=September 2021}}

Some CVTs transfer power to the output pulley via [[Tension (physics)|tension]] in the belt (a "pulling" force), while others use [[Compression (physics)|compression]] of the chain elements (where the input pulley "pushes" the belt, which in turn pushes the output pulley).<ref>{{cite book |last1=Ambrósio |first1=Jorge A. C. |title=Advances in Computational Multibody Systems |url=https://books.google.com/books?id=Jv6e3oxkI6YC |publisher=Springer |access-date=8 July 2020 |page=271 |language=en |date=5 July 2005|isbn=9781402033926 }}</ref><ref>{{cite book |last1=Pfeiffer |first1=Friedrich |title=Mechanical System Dynamics |date=2008 |publisher=Springer |isbn=978-3-540-79436-3 |url=https://books.google.com/books?id=_bjlZaxm5UwC |page=320 |access-date=8 July 2020 |language=en}}</ref><ref>{{cite web |title=CVT Transaxle Steel Push Belt Construction | date=16 September 2016 |url=https://www.youtube.com/watch?v=PiwRUfFEc5k | archive-url=https://ghostarchive.org/varchive/youtube/20211107/PiwRUfFEc5k| archive-date=2021-11-07 | url-status=live|via=YouTube |publisher=Weber State University |access-date=8 July 2020}}{{cbignore}}</ref>

Positively Infinitely Variable (PIV) chain drives are distinct in that the chain positively interlocks with the conical pulleys.  This is achieved by having a stack of many small rectangular plates in each chain link that can slide independently from side-to-side.  The plates may be quite thin, around a millimeter thick.  The conical pulleys have radial grooves.  A groove on one side of the pulley is met with a ridge on the other side and so the sliding plates are pushed back and forth to conform to the pattern, effectively forming teeth of the correct pitch when squeezed between the pulleys.  Due to the interlocking surfaces, this type of drive can transmit significant torque and so has been widely used in industrial applications.  However, the maximum speed is significantly lower than other pulley-based CVTs.  The sliding plates will slowly wear over years of usage.  Therefore the plates are made longer than is needed, allowing for more wear before the chain must be refurbished or replaced.  Constant lubrication is required and so the housing is usually partially filled with oil.<ref>{{Cite web|title=PIV Vertical Drives – Gayatri Gear|url=http://gayatrigear.com/piv-vertical-drives/|access-date=2020-09-15|language=en-US}}</ref><ref>{{Cite web|title=Positively Infinitely Variable (PIV) Chain|url=https://www.usarollerchain.com/PIV-Chain-s/2117.htm|access-date=2020-09-15|website=usarollerchain.com}}</ref>