The present invention relates to a belt for a continuously variable transmission comprising a large number of metal elements supported on metal ring assemblies each of which is comprised of a plurality of endless metal rings laminated one on another.
As shown in FIG. 3, if a metal element 32 supported with each of ring slots 35 thereof fitted over a metal ring assembly 31 is inclined with respect to the metal ring assembly 31, edges a and b of the ring slot 35 abut against an inner periphery of the metal ring assembly 31. For this reason, there is a problem that a large stress is produced in such abutment portion to exert an adverse influence to the durability of the metal ring assembly 31. The inclination (pitching) of the metal element 32 is caused when a friction force by an urging force acting between the metal elements cannot resist against an angular moment by a tangential friction force received by the metal element 32 on its surface in contact with a pulley. This tendency is particularly significant in an outlet portion of a driven pulley. The reason will be described below.
It is known that as shown in FIG. 4A, the tangential friction force FV received by the metal element 32 (see FIG. 3) from a drive pulley 6 or a driven pulley 11 is larger in the outlet portion of the pulley 6 or 11, and the value of the friction force FV reaches four times a value generated when the tangential friction force FV is averagely distributed over the entire area of the pulley 6 or 11 around which the metal element is wound, for the reason that the pulley 6 or 11 is deformed to cause a concentration of the axial thrust. As can be seen from FIG. 3, the tangential friction force FV acts on the metal element 32 so as to fall the metal element 32 in a counterclockwise direction about the center 44 of swinging movement.
Furthermore, as shown in FIG. 4B, the urging force E acting between the metal elements 32 to inhibit the inclination of the metal elements 32 has a large value in the outlet portion of the drive pulley 6, but is 0 (zero) in the outlet portion of the driven pulley 11. As can be seen from FIG. 3, radial friction forces E1 are applied to front and rear surfaces of the metal element 32 by the urging force E so as to fall the metal element 32 in a clockwise direction about the center 44 of swinging movement, namely, to oppose a counterclockwise moment generated by the tangential friction force FV. Therefore, the metal element 32 is liable to be inclined to the largest extent in a position in which the tangential friction force FV acting the metal element 32 to incline the metal element 32 is the maximum and the urging force E inhibiting the inclination of the metal element 32 is 0 (zero), i.e., in the outlet portion of the driven pulley 11.
A belt for a continuously variable transmission has been proposed in Japanese Patent Application Laid-open No.6-10993, in which a projection is formed on a saddle surface of a ring slot in a metal element (on a face against which an inner periphery of a metal ring assembly abuts), the projection being offset forwards in a direction of movement of the metal element from a thickness-wise central portion of the metal element, so that a moment generated by the tangential friction force is countervailed by a load for urging the projection by the metal ring assembly, thereby to prevent the inclination of the metal element.
However, if the projection is formed on the saddle surface of the ring slot in the metal element, it is difficult to form the metal element only by pressing. This causes not only a problem that it is necessary to carry out the machining or cutting after the pressing to bring about an increase in processing cost, but also a problem that a local compressive stress is generated in the inner periphery of the metal ring assembly abutting against the projection on the saddle surface, this causing the amplitude of a stress inside the innermost metal ring to increase to thereby reduce the durability. Further, in the art disclosed in Japanese Patent Application Laid-open No.6-10993, it is difficult to effectively prevent a moment acting so as to fall the metal element in the advancing direction.
The present invention has been accomplished with the above circumstances in view, and it is an object of the present invention to ensure that the inclination of the metal element is prevented without increasing the processing cost of the metal element to enhance the durability of the metal ring assembly.
To achieve the above object, according to the present invention, there is provided a belt for a continuously variable transmission, comprising metal ring assemblies each of which is comprised of a plurality of endless metal rings laminated one on another, and a large number of metal elements supported along the metal ring assemblies, the belt being wound around a drive pulley and a driven pulley to transmit a driving force, the metal element comprising ring slots into which the metal ring assemblies are fitted, pulley abutment surfaces provided radially inside the ring slots to abut against the drive pulley and the driven pulley, a rocking edge which is provided at a radially inner end of a main surface for transmitting an urging force between said metal elements, and which serves as a fulcrum for the pitching of the metal elements, characterized in that a relation, L/txe2x89xa6FL/2FV is established, wherein FV represents a tangential friction force received by the pulley abutment surface from the drive pulley and the driven pulley; L represents a radial distance from a point of action of the tangential friction force to the rocking edge located radially outside the friction force-acting point; FL represents a radially inward load received by the metal element from the metal ring assemblies; and t represents a thickness of the metal element.
With the above arrangement, when the tangential friction force received by the pulley abutment surface from the drive pulley and the driven pulley is represented by FV; the radial distance from the point of action of the tangential friction force to the rocking edge located radially outside the friction force-acting point is represented by L; the radially inward load received by the metal element from the metal ring assembly is represented by FL; and the thickness of the metal element is represented by t, the relation, L/txe2x89xa6FL/2FV is established. Therefore, even if a moment is produced to fall the metal element forwards by the backward tangential friction force acting on the pulley abutment surface of the metal element in the outlet of the driven pulley, a load for urging the rear edge of the ring slot radially inwards by the metal ring assembly cancels the moment to prevent the inclination of the metal element. As a result, the angle formed by the metal ring assembly and the ring slot of the metal element is maintained at a right angle and hence, it is possible to prevent a local load from being applied to the inner periphery of the metal ring assembly by the edge of the ring slot and to inhibit the increase in amplitude of a stress inside the innermost metal ring, thereby enhancing the durability of the metal ring assembly. Moreover, the pulley abutment surface of the metal element can be accurately brought into surface contact with the pulley to avoid the generation of an abnormal wearing. Further, the outer periphery of the metal ring assembly is prevented from interfering with the metal element and hence, it is possible to inhibit the increase in amplitude of the stress in the outermost metal ring and to prevent the local load inside the innermost metal ring from being increased by a reaction force from the interference. In addition, it is unnecessary to provide a special shape in a thickness-wise direction to the saddle surface of the ring slot in the metal element, against which the metal ring belt assembly abuts and hence, there is not a possibility that the processing cost for the metal element may be increased.