Traction drives are fixed-ratio mechanical drives. They transmit power
through the traction forces on metal rollers that are arranged on loaded
in a planetary configuration. These rollers can be cones, cylinders,
discs, rings, spheres, or toroids. The speed ratio is determined by the
radius of rotation of the driver roller on the driven member. A torque responsive roller loading mechanism adjusts roller normal load to applied torque, to maximize drive efficiency and life. Traction drive depends upon friction between a speed adjusting mechanism and specially shaped input and output plates to achieve adjustable speed with relatively high efficiency.
Types
Traction drives are of two types:
Dry Traction Drive - These drives eliminate the need for lubricant and allow nearly 100% efficiency in power transmission. Slippage between driving and driven members is presented by a spring-loaded system.
Lubricated Traction Drives - These drives use synthetic fluids. Traction fluids reduce the contact pressure between the rolling members to increase life and power capacities. Under high pressure, viscosity of the fluid increases dramatically so the fluid behaves more like a plastic material. This plastic-like material enables the drive to transmit power without appreciable metal to metal contact.
Application
Traction drive has not made a major impact on the power transmission industry. This is because of the lack of confidence in the fact that two smooth rollers can transmit force through a fluid. With the advanced technology, there are significant changes brought in the traction drives. These drives today, are the ideal for engineering applications where high efficiency and quiet operation is required.
Traction drive has their applications where compactness, ruggedness, and speed control accuracy are primary concerns. The micrometer speed-setting dial on most traction drives attests to their inherent accuracy in maintaining a specific output speed. They provide extra torque at reduced speeds.
Traction drives provide an attractive alternative for gear drives in planetary vehicles. These drives are composed of rollers that form a contact geometry resulting in low wear to the solid lubricant.