In U transmissions, force transmission is achieved over belts, bands, and chains which are frictionally clamped between disk pairs. By varying the rolling radii on the disks, the transmission ratio can be varied infinitely. As a result, the disk pairs along with the accompanying adjusting mechanism are also called variator. For the force transmission that is in the center of development work, so far different belt, band, and chain concepts have been proposed. Figure shows the present systems used for force transmission.
In contrast to rubber belts used in variomatic transmissions in the 50s by Van Doome, Holland, the transferable torque (up to 210 Nm) and efficiency were, in particular, significantly improved by changing over to a steel-link band in current CVTs. The transmission element, also called thrust link band due to the kind of loading, consists of approx. three hundred steel segments that are held together on both sides by steel band packages (band size 0.1 mm) piled up onto each other.
In contrast to the thrust link band, transmissions manufactured by PIV-Antriebe Werner Reimers KG, a chain loaded by tension is used for force transmission. The individual links of the chain are fastened to each other using cradle pressure elements. Force transmission takes place as a result of friction between the contact areas of the cradle pressure elements and disk wheels. U-shaped metallic clips that enclose the shackle band from the outside are responsible for chain guidance. The whistle otherwise typical for chain drives which occurs when the link comes into contact with the disk wheels, can be more or less avoided by an appropriately designed chain.
In order to keep the centrifugal load on a U-band as low as possible, Gates Rubber, USA, developed a rubber belt called Power-Trac which is reinforced by kevlar fibers. In order to increase the transverse loading capacity, the rubber band is provided with an integrated metallic structure. Oil lubrication which is required in the case of steel-steel friction pairs, is not necessary. This allows for a low-weight, open transmission design.
Kumm Industries, USA, proposes the fourth transmission system shown in the figure. In contrast to former U transmissions in which the force transmission element is clamped between two conical disk pairs, the Kumm CVT includes a rubber band which is also reinforced by kevlar (Gates-development), and runs at each disk end on studs. The studs can be shifted in spirally-shaped grooves. No lubrication is required here as well.