| 摘要 |
The drive shaft 30 of a printer 10 engages a tractor 16 which, in turn, moves a paper web 12 across a print mechanism 14. Located on the drive shaft 30 is a drive shaft advance cam 78 which is fixed to drive the shaft 30 and rotate therewith. Additionally, located upon the drive shaft 30 for limited rotational movement with respect to the drive shaft advance cam 78 is a paper advance cam 90. The shape of the drive shaft cam 78 is similar to the shape of the paper advance cam 90. The paper advance cam 90 carries an O-ring 102 defining a high frictional surface for engaging the paper web 12. The door 50 of the tractor 16 carries a paper pressure roller 104 having a circumferential groove 106 positioned for cooperation with the O-ring 102. During normal paper web 12 drive, the cams 78, 90 are out of registration and rotate in unison. In response to a paper web jam condition, the paper drive cam 90 driven by the paper web 12 momentarily stops and the cam falls 78, 90 are forced into registration resulting in the actuation of a switch 60.
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