Gears 1 and 2 rotate about fixed axes A and B. Driving gear 1 transmits rotation to driven gear 2 with two long dwells per revolution of gear 2. To avoid impacts gear 2 is brought into and taken out of engagement by means of pins d of gear 1 which enter slots c of gear 2. Concentric locking surface a engages concave surfaces b to prevent unintentional rotation of gear 2 during its dwell periods.
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