Toric-drive Continuously Variable Transmissions (hereinafter generically referred to as “CVT”) are believed known in the art. The operation of such a CVT will therefore only be briefly discussed herein.
Generally stated, a CVT is provided with a drive disk having a toroidal surface, a driven disk also having a toroidal surface and facing the toroidal surface of the drive disk, both disks being linked by rollers in contact with their respective toroidal surfaces. The angle of the rollers with respect to the drive and driven disks dictates the speed ratio between the driven and drive disks since this angle dictates the radial position at which the rollers contact the toroidal surfaces.
Such a CVT requires some kind of preloading mechanism to force the drive and driven disks towards each other to provide a predetermined minimal friction between the disks and the rollers. A pressure applying mechanism, also known as a clamping mechanism, is generally also provided to increase the pressure compressing the disks towards each other, therefore increasing the pressure between the disks and the rollers, when the CVT is in use.
It has been found that in a CVT configuration, the clamping forces have an impact on the lifespan of the CVT components, particularly on the disks. In other words, over clamping may be detrimental to the lifespan of the CVT.
It also has been found that to prevent slippage, a sufficient clamping force has to be applied to ensure that an adequate friction exists between the rollers and the disks. The slip amount is generally determined as the difference in linear speed between the driving surface and the driven surface.
The spin is defined as a pivoting or rotating motion of two surfaces in contact relative to each other. When two curved or rounded bodies are compressed together, an ellipse is formed at the point of contact. Because the two bodies do not rotate about the same axis, and since the plane of contact can be in various positions relative to the axis of rotation of the two bodies, the ellipse of contact of the first body will be in a rotating or pivoting motion relative to the ellipse of contact of the second body. This relative motion of rotation of two ellipses in contact is referred to as spin.
It has been found that by providing a slip/spin ratio of about 0.8 the contact has the most efficient power transmission. In each contact, there is a loss of speed due to slip, and a loss of torque due to spin. At a ratio of slip to spin of about 0.8, the proportion of loss of each is minimised yielding the best efficiency. At greater rates of spin, torque loss increases at a greater rate than slip loss decreases, and subsequently decreases efficiency. At greater rates of slip, it is the opposite, the power loss due to slip is greater than the decrease in torque loss due to decreased spin.