| 摘要 |
Two arms carry two pawls each. One pawl of each arm engages one ratchet wheel and, alternately, the other pawl of each arm engages another ratchet wheel when the arms are moved at the end of the travel of a ribbon. The ratchet wheels carry ribbon spools. One arm carries push pawls. The other carries pull pawls. The push pawls act as no-backs, but they also drive the ratchet wheels. Each ratchet wheel is thus positively driven more or less continuously when it is driven. The ratchet wheels are driven alternately. An over-center transfer mechanism places a desired pair of pawls in position for ratchet wheel engagement and the other pair for disengagement at each end of the ribbon travel.
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