| 摘要 |
To permit formation of a threading element as a closed-loop structure, while being capable of accommodating threading paths of different lengths, the threading element is formed as a sprocket chain which has a long return portion (1) in a return path, coupled to a driven sprocket wheel (2), and connecting portions (3, 4) of a length suitable for the respective threading paths (B, C), and selectively connectable with the return portion (1), to thereby form a closed loop with the selected connecting portion (3 or 4). Pneumatically operated pistons located on path selection switches (5, 6) push the last link of the return portion into engagement with a selected end of the connecting portion, for example by engaging a cross pin (26) in a slotted end link (22, 23; 24, 25) of the connecting portions. The system is entirely automatically controllable, and additional pneumatically operated pistons (7, 10; 8, 11; 9, 12) engage between the links of the link pairs of the respective return and connecting portions of the sprocket chain to hold the respective portions in position during switch-over of the terminal link and, after switch-over, to retain in position that one of the connecting portions which is not connected, for future use. |
