This invention relates to a toothed belt power transmission device comprising a combination of a toothed belt and a pulley. More particularly, it is concerned with a combination, in a predetermined dimensional relationship, of a pulley having a groove and a tooth which are trapezoidal in cross section, and a toothed belt having a tooth defined by a combination of straight lines and arcs.
A toothed belt enables slipless, positive driving, as opposed to a flat or V belt. It also has a number of advantages over a gear and a chain. For example, it does not require any lubrication. For these reasons, the use of toothed belts has recently sharply increased.
The transmission of power from a toothed belt is accomplished by engagement of its tooth with the grooves of a pulley. The belt is likely to make an abnormal noise when engaging or disengaging the pulley, depending, for example, on the tooth shape or the engaged relationship between the belt and the pulley. Moreover, it is likely that the belt may have a worn groove surface, or a damaged tooth.
There is known a power transmission system which comprises a belt having teeth of trapezoidal cross section, and a pulley having a contour which is complementary to the teeth on the belt, and engageable therewith, as specified, for example, by RMA (Rubber Manufactures Association) Ip-24 (1978). The driven slant surfaces of the grooves in the belt are adapted to intimately contact associated drive slant surface of the teeth on the pulley, while there is a clearance space between the crest of the tooth on the belt and the bottom of the groove in the pulley. The belt groove is hard, since it has an external canvas layer and a hard body of high tensile strength positioned adjacent thereto. The tooth on the pulley is also made of a hard material, such as metal or resin. Therefore, the engagement between the belt groove and the pulley tooth causes vibration of the belt. The frequency f of this vibration is usually expressed by the following equation: ##EQU1## Resonance occurs and an abnormal sound is generated in the event that a driving shaft, a driven shaft, an idler shaft, or any other element surrounding the pulley has a natural frequency f' which is equal to nf or f/n (n is natural number). When this device is employed in an automobile, or like engine, it is particularly true that resonance and unpleasant noise often occur in some position of the widely variable rotating speed of the engine.
Besides the noise caused by vibration, when such hard substances strike against each other there occurs a noise having a relatively high frequency. A belt groove is very likely to a pulley tooth with a resultant shear of a belt tooth.
In order to improve these disadvantages, there has been developed a driving system in which a belt tooth has a height greater than the depth of a pulley groove, and is adapted to be compressively engaged therein, as disclosed in U.S. Pat. No. 4,037,485. According to this system, a belt tooth is compressively deformable in a pulley groove to decrease in height, whereby the wear of a belt groove contacting a pulley tooth is prevented. A belt tooth is designed to be substantially complementary to a pulley groove and contacts the whole surface thereof, the drive/driven slant surfaces of a belt tooth contacting the corresponding driven/drive slant surfaces of a pulley groove over a maximum surface area possible so that a low contact pressure may be maintained therebetween to reduce tooth deformation. Accordingly, a belt tooth has only a limited degree of freedom for compressive deformation in a pulley groove. Such a belt tooth fails to provide a satisfactory function of absorbing impact, and there inevitably occurs an undesirable noise when a belt tooth engages a pulley groove.
In view of these problems, it is an object of this invention to improve the shape of a tooth on a belt and the dimensional relationship between the belt and a pulley in their statically engaged position, and provide an improved toothed belt power transimmion device which (1) generates less noise upon engagement between a belt groove and a pulley tooth, and between a belt tooth and a pulley groove, (2) prevents any abnormal sound from arising due to vibration upon engagement between a belt groove and a pulley tooth, (3) protects the surface of a belt groove against wear during the transmission of power at high loads, and (4) ensures a prolonged belt life.