Traction Drive Designs
Toroidal and Half-Toroidal Designs
The toroidal drive was invented by C.W. Hunt in 1877 (see Figure 6). The toroidal transmission has been used periodically since then, and is still in development today. General Motors and Nissan are two companies that have contributed to the development of the toroidal transmission.
Toroidal drives transfer power from input disk to power rollers and then to an output disk. By tilting the roller axis, the point of contact to the input and output disks can be varied. This causes the circumference of the contact track to be different from input to output, thereby changing the drive ratio.
Figure 6– US Patent 197,472, showing the Hunt toroidal transmission. The tilting transfer wheel (E) tilts to change the rolling contact radii with toroids (B and D). This creates a stepless means of varying the ratio between input and output speed. [Hunt, Charles W., “Counter-shaft for driving machinery.” Patent 197,472. 27 November 1877] (click to enlarge).
There are two main types of toroidal transmissions, the half toroidal and the full toroidal (see Figure 7). The two differ in their geometry, with each version having advantages and disadvantages. A major difference is that the half toroidal unit requires thrust bearings on the power rollers, thus making the shifting mechanism more complex and space-consuming. The full toroidal avoids power roller thrust bearings by virtue of its geometry, but suffers from reduced torque overload margins.
Toroidal transmissions offer the benefits of a continuously variable ratio range, high torque capacity, better efficiency than competing non-traction drive continuously variable technologies (such as belt CVTs), and fast shifting. The drawbacks of this design include the need for a high clamp force and the cost of manufacturing the toroidal components.
The Kopp Variator was invented in the 1940s by Jean Kopp (see A). This was the first commercial introduction of a cone and roller traction drive, and is still in use for controlling industrial machinery. This design is one of the most commercially successful traction drives of the 20th century.
Figure 8– (A) The Kopp variator has been widely used in industrial application (Kopp, Jean, “Stepless variable change-speed gear with roller bodies.” Patent 2,469,653. 10 May 1949.) (B) The Kopp variator uses a tilting planet axis to change drive ratio (click images to enlarge)
The Kopp variator uses a set of planets that are rotating on an axle. The planets contact the outer diameter of a pair of cones attached to the center shaft. One of these cones is attached to the input, and the other to the output. As the input cone is rotated, it drives the planet, which drives the output cone. By tilting the planet axis, the ratio between the input and output is changed due to the difference in contact radii (see Figure 8B).
The Kopp Variator provides a relatively low cost variator with a 9:1 ratio range and stable ratio under static conditions. However, its drawbacks are that it has low torque density and cannot shift under load.
Milner Continuously Variable Transmission
The Milner CVT is a traction drive that utilizes three or more balls (planets) as the rolling elements. The planets are in four-point contact with two inner races and two outer races. The power is transmitted to the planets through the center shaft and one of the inner races which is fixed to the shaft. The planets orbit the inner race and transmit power to an output carrier that includes follower rollers between the planets. One of the outer races is fixed to ground. The other outer race is moved axially to force the planets in or allow them to move out. The second inner race moves in reaction to the movement of the planets due to the actuation of the outer control race (see Figure 9). These four races keep the planet fully supported. As the planets move radially in or out with the outer control race, the location of four contact points on the planet change, thereby changing the ratio between input and output speed.
Figure 9– Milner design uses 4-point contact on a planet to enable ratio changes (click images to enlarge).