Gear-tooth wear and a one-way ratchet clutch
Why a pinion's teeth wear ten times faster than the wheel's, and a spring pawl that frees the pinion
Leonardo warns to remember the wearing away of gear teeth by friction: a pinion of 8 teeth meshing with a wheel of 80 turns ten times for each turn of the wheel, so, being of equal material, its teeth wear out ten times faster. The sheet also details a one-way ratchet: a piece of well-tempered steel, pivoted like a door and pressed outward by a spring, catches the teeth n inside the hollow of a pinion, so the pinion locks to its pole m when turned one way and runs free when turned the other. A further note explains that the pinion f, though driven forward by the wheel's teeth, can let its pole turn back. Gear, rack, ratchet-disc and lantern-pinion figures illustrate the mechanisms.
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Pinion teeth wear ten times faster by friction
If a wheel has 80 teeth and its pinion 8, then, being of equal material, the pinion's teeth wear out ten times faster than the wheel's. For when the wheel makes one turn its pinion makes exactly ten, and every tooth is touched ten times.
Spring pawl a making a one-way clutch on the pinion
The part a is a piece of well-tempered, unannealed steel, set in pivots like a door and pushed outward by an inner spring against the teeth n within the hollow of the pinion. Turned one way the pinion is united with its pole m; turned the other, it runs free and detached from that pole.
Pinion f whose pole can turn back
This is the front, or head, of the pinion f, which also has pinion g. Although the pinion is driven forward by the teeth of the wheel that moves it, its pole where it is pivoted can nevertheless turn back, as shown.