Background

Definition of CVT

A continuously variable transmission (CVT) is a transmission that can change effortlessly through a continuous range of effective gear ratios. Unlike traditional mechanical transmissions that has a discrete number of gear ratios. A CVT with suitable control allows the input shaft to maintain a constant angular velocity even as the output speed varies.

Existing CVT Design

There are already many existing CVT designs on the market today. Some of these designs are listed below.

One of the designs is the NuVinci hub CVT design by Fallbrook. This engineering design utilizes an internal mechanism composed of a sun gear (input to the transmission) and planetary gears that vary their geometry in contact with the sun gear and outer hub in order to vary the output drive ratio. These planetary gears have a spherical shape which allows a varying gear ratio by varying the tilt of their axis of rotation. Such variation on axis tilt will vary the point in contact the sun gear and output hub have with the sphere surfaces. This causes a change in gear ratio due to the relation of the point of contact to the radial dimension to the axis of rotation of the spheres.

The eDrive is a lightweight and small, continuously variable transmission that is tested on a bicycle The design is based on a sprocket located at the rear wheel of abicycle that can vary in diameter, which in turn varies the gear ratio. The ratio is changed when two plates that share a centeraxis rotate relative to each other, forcing pins to be concentrically pushed from the center of plate. A belt wraps around thesepins, and drives a pulley which in turn drives a second belt that transmits power to the wheel.

The most common category of CVT designs that has been utilized on vehicles is the belt type CVT design. A typical example of the belt design can be found in US Patent No. 4881925A by Honda Motors. The core components of the belt CVT are the two V-pulleys. One of them is the drive V-pulley which accepts the input torque from the vehicle engine, and the other one is the driven V-pulley which outputs the output torque onto the wheels of the automobile. Both V-pulleys are comprised of stationary pulleys half mounted on the shaft, and movable pulleys half slidably connected to the shaft. Through varying the distances between the two sets of V-pulleys, their effective radii changes, thus changing the gear ratio of the transmission.

Despite there are already several commercially available CVT designs on the market today, they are by no means perfect. Since belt systems have high slip (inefficiency) for low RPM high torque applications, since variable sprocket size designs struggle with a narrow gear range and non-smooth shifting action, and since planetary systems are complex and could experience slip inefficiencies if drive transmitted by contact of smooth surfaces, our team has come up with a CVT design that is novel, practical, simple and inexpensive.