A known type of toothed pulley comprises a plurality of grooves alternating with a plurality of teeth wherein each groove is joined to the teeth by a connection in the form of a circular arc. The toothed belt required to mesh with the pulleys in a synchronous transmission system comprises an elastomeric annular body delimited by two faces, from at least one of which projects a plurality of teeth alternating with a plurality of grooves. Each tooth is also joined to the adjacent grooves by means of a connection in the form of a circular arc.
It is usual in the above type of transmissions for the connections of the pulley and the belt engaging with the pulleys to have circular arcs whose radii are substantially identical or in any event of comparable values.
During the transmission of motion, a state of stress is generated by the relative mechanical thrusts between the belt teeth, which are made of elastically deformable material, and the pulley teeth, which are made of rigid material. This stress is distributed over the body of the belt tooth.
Practically speaking, the belt tooth, which is anchored along its base to inextensible longitudinal strands, behaves in the same way as a beam restrained at one end subjected to a vertical load at the opposite end. In the proximity of the base of the tooth, therefore, the elastomeric material receives the thrust transmitted by the pulley tooth and transfers it to the longitudinal strands. Undesirably, the state of stress at the base of the belt tooth is unevenly distributed, and there is an accentuated localization of the forces acting on the rubber in the area of connection between the flank of the loaded belt tooth and the adjacent groove. This localization of forces, and particularly their intensity, may over the course of time result in irreparable impairment of the correct operation of the transmission and the working life of the belt itself.
Attempts have been made to overcome the above drawback by creating circular connection arcs at the base of the belt and on the head of the pulleys with different radii, giving the radius of the pulley head connection a greater value than that of the belt connection with a view to preventing the pulley head connection from compressing the base connection of the belt tooth in a confined area. This solution, however, may prove insufficient, since when very heavy loads are transmitted by the flank of the pulley tooth to the flank of the belt tooth there is a tendency for the connection zone in elastomeric material at the base of the belt to divaricate. In particular, even if the preceding solution is adopted, there is still a risk of extremely localized pressures on the base of the belt tooth and hence an irregular distribution of forces along the base of the belt. It must also be added that resorting to this solution inevitably results in the transfer of loads at a higher point on the belt tooth. Unfortunately, this circumstance has an adverse effect, since it can readily be understood that the belt tooth is increasingly subject to deformation in flexion the further the load is applied away from its base.
It should also be noted that circular arc connections between grooves and teeth in a pulley could be unsuitable for employment with toothed belts such as those disclosed in U.S. Ser. No. 433,789, filed Nov. 9, 1989 (incorporated by reference herein) also filed by the present Applicant. Briefly, it may be stated that the belts described in that application comprise a base connection formed of a curvilinear segment whose length is greater than that corresponding to a circular arc. In essence, according to the solution referred to, the curvilinear connection segments at the base of the tooth could, for example, be formed with a parabolic arc so as to permit the build-up of a greater quantity of elastomeric material compared with a circular arc without altering the dimensions of the tooth.
Even though adoption of the toothed belt referred to in a transmission system composed of pulleys with a conventional profile might continue to be satisfactory, account must be taken of the fact that under particular operating conditions the belt tooth could rise up from its correct position of engagement with the pulley. Since the toothed belt connection, in the form of a parabolic arc, for example, would in fact be closer to the circular arc connection of the pulley tooth, there would be a consequent reduction of the free space in the proximity of the opposed connections when the belt tooth is inside the pulley groove. Subsequently, due to load variations occurring during the transmission of motion, the flat head of the pulley tooth could shift with respect to the positions where it is centered with the central axis of the groove between the belt teeth with the result that the circular arc of the pulley tooth connection by striking the parabolic connection of the belt tooth would exert a mechanical thrust, possibly followed by shifting of the tooth form its correct meshing position.
Furthermore and in general terms, it has been observed that in known pulleys there may be interference between the summit of the belt tooth and that of the pulley tooth in the transient access phase under a particularly heavy load. This interference is generated by elongation of the pitch between the belt teeth resulting in unwanted approximation of the pulley tooth and the belt tooth that has to mesh.