1. Field of the Invention
The present invention relates to a belt for a continuously variable transmission, which comprises a metal ring assembly formed of a plurality of endless metal rings laminated one on another, and a large number of metal elements each having a ring slot into which said metal ring assembly is fitted, the belt being wound around a drive pulley and a driven pulley for transmitting a driving force between both the pulleys.
2. Description of the Related Art
There are metal belts for continuously variable transmissions known from Japanese Patent Publication Nos. 57-23820, 3-1537 and 5-48364, in which a saddle face of each of metal elements supporting an inner peripheral surface of an endless metal ring assembly is crowned, and the metal ring assembly is centered to a central portion of the saddle face, thereby preventing a reduction in durability caused by the contact of opposite ends of the metal ring assembly with portions other than the saddle face of the metal element and a V-face of a pulley.
When the metal belt is wound around the pulley, the metal ring assembly is curved into an arcuate shape about a pulley shaft and for this reason, a tensile stress is generated on an outer peripheral surface of each of the metal rings constituting the metal ring assembly. At this time, if the saddle face of the metal element has no crowning, the tensile stress on the outer peripheral surface of the metal ring is uniform in a widthwise direction of the metal ring. However, if the saddle face of the metal element is crowned, the cross-sectional shape of the metal ring is curved along the shape of the saddle face and hence, the tensile stress on the outer peripheral surface of the metal ring is large at the widthwise central portion which is stretched strongly, and small at the widthwise opposite ends which is stretched weakly. In an area where the metal belt is wound around the pulley, the tensile stress on the outer peripheral surface of the metal ring varies in the widthwise direction, and in a straight chord between both the pulleys, the tensile stress on the outer peripheral surface of the metal ring is uniform in the widthwise direction. That is, the tensile stress varies periodically accompanying the rotating travel of the metal belt, which causes a reduction in fatigue life of the metal ring. This tendency is significant as the width of the metal ring is increased and as the crowning height is increased, which becomes a factor to hinder an increase in driving force transmitting capacity of the belt type continuously variable transmission.
The present invention has been accomplished with the above circumstances in view, and it is an object of the present invention to extend the fatigue life of the metal rings by making uniform a tensile stress on an outer peripheral surface of the metal ring in a widthwise direction in a belt including metal elements each having a crowned saddle face.
To achieve the above object, according to a first feature of the present invention, there is provided a belt for a continuously variable transmission, which comprises a metal ring assembly formed of a plurality of endless metal rings laminated one on another, and a large number of metal elements each having a ring slot into which said metal ring assembly is fitted, said belt being wound around a drive pulley and a driven pulley for transmitting a driving force between both the pulleys, wherein the metal rings and a saddle face of the ring slot supporting the innermost metal ring each have a crowning, and a crowning radius of the metal rings is set smaller than a crowning radius of the saddle face.
With the above arrangement, the metal ring assembly can be centered to a central portion of the saddle face by crowning the saddle face of each of the metal elements, whereby the metal ring assembly can be prevented from interfering with the other portions of the metal element and the pulley and thus, the durability of the metal ring assembly can be enhanced. The crowning radius of the metal ring is set smaller than the crowning radius of the saddle face and hence, a difference between a tensile stress at a central portion of the metal ring and a tensile stress at opposite ends of the metal ring can be reduced, to thereby alleviate a reduction in durability of the metal ring assembly. Thus, even if the width of the metal ring is increased, the durability can be secured, which can contribute to an increase in driving force transmitting capacity of the belt type continuously variable transmission.