Traction Drive Applications
Traction drives can be used for both fixed ratio drives as well as variable ratio drives. Most frequently, they are used as variable ratio drives, or continuously variable transmissions (CVTs). This application utilizes one of the primary advantages of traction drives, their ability to provide seamless ratio changes. Because they avoid the stepped ratio changes inherent in a geared transmission, they allow more control of input or output speeds. Depending on the needs of the application, this can allow the driving source to operate more efficiently, or it can provide more refined output speed control. Some of the ways that CVTs can be controlled are described below.
Constant Input Speed, Variable Output Speed
A CVT can vary the output speed of a device while accepting a constant input speed. For example, in an automotive transmission, a CVT can allow the engine to run at its most efficient operating speed, while the CVT ratio can be varied to generate the desired vehicle speed. In a traditional geared transmission, the engine speed must increase proportionally to the change in wheel speed when between gear changes. The same is true with an electric motor driving a variable speed component. For example, in an application requiring a variable pump, the electric motor driving the pump can be kept at its most efficient speed while the CVT varies the speed of the pump.
Variable Input Speed, Constant Output Speed
A CVT can also accept a variable input speed and output a constant speed. Some automotive components operate at peak performance or efficiency at a constant speed, for instance an alternator. However, these devices are typically driven at a fixed ratio to engine speed, so their speed varies from idling to peak engines speeds. A CVT can increase the accessory input speed when the engine speed is low and additional performance is required, and can decrease the accessory input speed when performance is not required to offer better fuel efficiency.
Refined Output Speed Control
The stepless ratio changes provided by a traction drive can offer very fine control of speed. This can be a significant advantage in certain applications. For example, a forklift requires a very fine level of speed control to smoothly and accurately position the vehicle in relation to the load. A CVT can aid in allowing the driver to smoothly decelerate into the proper position without the need for braking.
Disadvantages of Traction Drives
Major disadvantages that have kept traction drives from being more widely used include the cost of manufacturing, the sensitivity to temperature, and the high clamp loads required. Manufacturing costs are driven by the need for high quality bearing steels and the need for precision surfaces for the rolling contact elements. New traction drive designs and technology advances are working to overcome these issues. These advances have led to a renewed interest in this technology.