Helical gears 1 and 2 rotate about fixed axes A and B. When driving gear 1 rotates continuously, driven gear 2 rotates intermittently with dwells. Gear 1 has helical teeth a and concentric projection e. Gear 2 has four symmetrically located grooves b with helical teeth d between them. The number of helical teeth a of driving gear 1 equals the number of helical teeth d between two adjacent grooves of gear 2. When gear 1 rotates continuously, its projection e periodically enters a groove b of gear 2 thereby locking the latter against unintentional rotation during the dwell. For each revolution of gear 1, gear 2 has four dwells, turning through 90° between successive dwells.
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