With advance of science and technology and maturity of fabric industry, various new fabrics having the quality distinct from that of the traditional fabrics, have been developed. Further, with economic promotion, people pay more attention to dressing and pursue style. Therefore, to meet diversified dressing designs, sewing method has to advance further.
In earlier times, the translation gear of a single-translation-gear sewing machine was only to feed fabric into the sewing position. Under constant rotation speed of the motor, if the number of the needle reciprocation was also constant, changing the sewing thread pitch would be achieved via only changing the travel range of the translation gear, which determines the fabric feeding quantity. However, confronting current new-type fabrics (such as an elastic fabric) and various dressing designs (such as a dress with wrinkles intentionally formed on the sewing thread), the above mentioned single-translation-gear sewing machine cannot satisfy the demand obviously. Therefore, double-translation-gear sewing machine is adopted in most cases nowadays. The function of the rear translation gear thereof is equivalent to the above mentioned single translation gear, and the sewing thread pitch can be adjusted via changing the travel range of the rear translation gear. The function of the front translation gear is to determine the ratio between the delivered-out fabric amount and the feed-in fabric amount to form different patterns of the fabric neighboring the sewing thread, such as plane, tension or wrinkled pattern, i.e. so-called differential ratio, via that the front translation gear translates the fabric in speed equal to, higher or less than the feeding speed of the rear translation gear. Thus, the double-translation-gear sewing machine can meet the demand of current elastic fabrics and diversified dressing designs.
Nevertheless, none of the current transverse shuttle-type sewing machines can separately adjust the travel ranges of the front and rear translation gears, i.e. the sewing thread pitch and the differential ratio, for example, a R.O.C. Patent Publication No. 541377 of the inventor, entitled “Sewing Machine with an Adjust Mechanism of the Front Differential Feeding”, which uses an adjust mechanism of the front differential feeding to adjust the speed of the front translation gear and comprises: a swing element, four linking-up elements, an adjust mechanism, and a transmission shaft. In this patent, the adjust mechanism of the front differential feeding is actuated by the transmission mechanism of the rear differential feeding and drives the front translation gear to move to and fro. The adjust mechanism of the front differential feeding has an adjust mechanism, which can change the ranges of the planar to-and-fro feeding travel of the front translation gear. When the user shifts the adjust mechanism during operation, he can modify the travel range of the front translation gear. Although the travel range of the front translation gear can be adjusted in this invention, the travel range of the rear translation gear cannot be adjusted but is determined by the front translation gear. Therefore, the travel ranges of the front and rear translation gears cannot be separately adjusted. Thus, the sewing thread pitch and the differential ratio cannot be adjusted separately and arbitrarily as the user demands. Accordingly, it is a problem the sewing machine manufacturers desire to solve to provide a sewing machine wherein the user can arbitrarily and separately adjust the sewing thread pitch and the differential ratio to meet the cases that a special fabric is used, special sewing thread pitch is needed or wrinkles are demanded.