Backstitch sewing machine with upper and lower transmission shafts linked by timing belt, includes mechanism varying belt strand length ratios dynamically, to alter rotary phase between shafts

摘要

The upper and lower shaft pulleys (28, 35) are linked by a transmission belt (36), especially a timing belt. Lengths of the belt strands (36, 36') on each side of the pulleys can be varied, increasing one length, with corresponding decrease in the other, and vice versa. This is achieved by a multiple pulley mechanism (41, 42) which deflects and returns the belt transversely, on each side of each main strand (36, 36'). Note: numbering and meanings, are consistent with Fig 1 and operation of the mechanism as drawn. The deflection pulleys (41, 42) are held at constant mutual spacing on a carriage. The carriage includes a slot and peg arrangement, permitting carriage (and hence pulley) movement in a direction transverse to the strands (36, 36'). By this movement, the relative strand lengths are changed. The movement is caused by a peg operating in a spiral slot in a scroll wheel (45). This is driven by the motor (44). A drive controller is included to operate the mechanism, in harmony with the main sewing drive. Varying the strand lengths, varies the relative phase of the upper and lower shafts (33, 26). The controller is operated in accordance with the thickness of the material being sewn.