Unlike other transmission systems, CVT does not have a set of gears. The common CVT uses a pulley system to transmit rotation. So theoretically, the number of gears a continuous variable transmission system offers is infinity. While riding a vehicle with a CVT system, you would find the transmission to be smooth and you does not feel the gear shift like in automatic and manual systems. There are basically three kinds of CVT systems.
a)Pulley based b) Toroidal and c) Hydrostatic
The pulley-based system
The pulley based system consists of two pulleys, the driving pulley, (also called the driving pulley) and the driven pulley (output pulley). A triangular shaped belt runs around them. The triangular shape is because the pulley groves taper towards the inside and so to have maximum friction.
The distance between the center of the pulleys where the belt makes contact in the groove is known as the pitch radius. When one pulley increases its radius, the other decreases its radius in order to keep the belt tight. So relatively, both the pulleys change their radii resulting in gear shifting. For example, when the pitch radius is small on the driving pulley and large on the driven pulley, the rotational speed of the output pulley decreases and vice-versa. The introduction of metal belts further enhances the performance of the belt. These flexible belts are composed of several (typically nine or 12) thin bands of steel that hold together high-strength, bow-tie-shaped pieces of metal.
Toroidal CVT
Toroidal CVT are very similar to the pulley-based model. There are two discs, one called as the input disc and the other called as the output disc replacing the input and output pulleys of the pulley based system. The discs are cone shaped structures with the tapered ends facing each other, and they have a curved surface. The rollers or wheels are rotating structures fixed to an external axis and are placed between the discs. They act like a gear wheel and transmit rotational force from the input disc to the output disc. The rollers while rotating on the external axis can also tilt themselves between the discs touching different places of the disc offering various speeds of rotation. For example,When the wheels touch the driving disc near the rim, they must contact the driven disc near the center, resulting in an increase in speed and a decrease in torque (i.e., overdrive gear). A simple tilt of the wheels, then, incrementally changes the gear ratio, providing for smooth, nearly instantaneous ratio changes.
Hydrostatic CVTs
Both pulley-based and toroidal models are friction based. But a hydrostatic CVT is a little different because it uses variable-displacement pumps and a hydrostatic motor. The rotational motion of the engine operates a pump and the pump converts this motion into fluid flow. This flow is then converted into rotating motion by a motor on the driven side. This system is usually used along with a planetary gear set as a hybrid system. Hydro-transmissions are good for heavy-performance vehicles which is why they are used in tractors.