CVT Gearbox - How it Works - Explained

Continuously Variable Transmission
I modified this gearbox so i can controll it with air instead of oil, to show you guys how a CVT gearbox works.

If you're looking for cars with an automatic gearbox, you may have seen the term CVT. ... The meaning of the abbreviation CVT is Continuously Variable Transmission, and it's a type of single-speed automatic gearbox. The CVT gearbox has been used in a variety of cars, but you'll usually find it fitted to a small car.

The most common type of CVT uses a V-belt which runs between two variable diameter pulleys. 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 sheaves of one pulley closer together and the two sheaves of the other pulley farther apart.

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 in order 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. The V-belt needs to be very stiff in the pulley's axial direction in order to make only short radial movements while sliding in and out of the pulleys.

The pulley-radial thickness of the belt is a compromise between 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, 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 the economy, the ratio of the CVT can be changed to allow the engine to turn at the RPM at which it produces the greatest power.

In a chain-based CVT a film of lubricant is applied to the pulleys. It needs to be thick enough so that the pulley and the chain never touch and it must be thin in order not to waste power when each element dives into the lubrication film. Additionally, the chain elements stabilize about 12 steel bands. Each band is thin enough so that it bends easily. If bending, it has a perfect conical surface on its side. In the stack of bands, each band corresponds to a slightly different drive ratio, and thus they slide over each other and need oil between them. Also, the outer bands slide through the stabilizing chain, while the center band can be used as the chain linkage.

Some CVTs transfer power to the output pulley via tension in the belt (a "pulling" force), while others use link element compression where the input pulley "pushes" the belt which in turn pushes the output pulley.

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, these plates may be quite thin, around a millimetre 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.