1. Field of the Invention
This invention relates to the field of power transmission chains, and in particular to a friction plate structure of improved durability and frictional surface hardness, for a continuously variable transmission ("CVT") chain.
2. Prior Art
In a friction drive assembly for a continuously variable transmission or CVT, a power transmitting chain defining a tapering lateral contour is routed around a driving pulley and a driven pulley, at least one of the pulleys having inwardly-facing conical surfaces that can be adjusted in their axial separation to thereby adjust the effective radius at which the chain passes around the pulley. In this manner, the ratio of power transmission between the pulleys is continuously variable between upper and lower limits.
Friction drive elements for CVT chains known in the art can be classified in two categories. One type of chain, as exemplified by the disclosure of Japanese Patent Publication Gazette No. 23820/1982, uses a chain formed using frictional elements which are plate-like. The frictional elements have an I-shaped cross-section, i.e., each frictional element or friction plate has an upper (or outer) transverse beam member and a lower (or inner) beam member. One of the beam members is tapered or V-shaped in cross-section and spans the distance between the spaced sheaves of the pulley. This V-shaped taper on the beam member of the I-shaped link or friction plate bears inward forces exerted on the chain by the sheaves of the pulleys in the frictional engagement of the chain and the pulleys when transmitting power. Referring to FIG. 2 hereof, a V-shaped, pulley-engaging frictional drive surface is defined by the surface F" of the lower or inner beam member of the I-shaped friction drive element F.
A second type of known chain, as exemplified by Japanese Patent Application No. 258240/1987 (now laid open), is formed by frictional elements which are block-like, having a V-shaped cross-section. The force and friction bearing surfaces on the V-shaped friction block extend all the way up the lateral sides. Moreover, the block has a relatively greater amount of material than an I-shaped block, for bearing laterally inward forces.
In the case of the former, I-shaped type of chain, the surface area of the friction drive surface applied to the pulleys is smaller than that of the latter full V-shaped type. Accordingly, pressure per unit area of pulley contact, due to the pressing force of the surfaces of pulleys, tends to sharply increase. The I-shaped structure substantially increases the frictional force per unit area between the friction plates and the pulleys. At the same time, the I-shaped structure must have sufficient strength to withstand laterally inward pressing forces exerted by the pulleys, resisting bending and/or breakage.
The present invention relates to the I-shaped type of CVT chain frictional element. In conventional I-shaped friction plates, high class steel material having good tenacity and/or toughness has been used. The plates are heat treated, such as by quenching and annealing, to increase strength and improve the service life of the frictional elements notwithstanding the substantially heavy loads on this type of structure as explained above.
However, although strength so as to withstand the pressing force as mentioned above may be imparted to the friction plate by means of quenching and annealing, it is not possible in this manner to give hardness to the friction drive surface of the lower beam member of the friction plate so as to withstand the huge frictional force. If the material of the friction plates is provided with necessary hardness to enable the friction plates to resist friction, the friction plates tend to lose toughness and may be unduly brittle. Consequently, the plates are subject to damage due to the impulse force occurring on each friction plate as the chain winds itself around the pulleys. Moreover, the bending strength against the aforementioned pressing force applied to the lower beam member F' deteriorates.