Odd-toothed and half-toothed wheels for controlled turning
Why gear-teeth must be odd, like the clock wheel, and half-toothed wheels tuned for a full or a quarter turn
Several face-on and perspective wheels demonstrate the tooth-counting rules for intermittent gearing. Leonardo insists the toothed 'posts' must always be odd — here 5, never 6 — after the manner of the wheel that drives the time in a clock, so that as one tooth leaves the spindle at o the next is at once engaged at t. A half-toothed wheel, one part toothed and one plain, can be tuned to give the driven pinion a chosen fraction of a turn: 8 spindles with 8 teeth per post yield a full turn, 2 teeth per post a quarter, running equal and firm.
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Why the toothed 'posts' must be odd, like the clock wheel
In this wheel all the toothed parts must be odd — here 5; six would go badly — so make 1, 3, 5, 7 and so on, after the manner of the wheel that drives the time in the clock. As soon as the tooth above leaves the spindle at o, the next spindle is at once taken up below at t.
Half-toothed wheel tuned for a full or a quarter turn
The upper wheel is divided into two equal parts, one toothed and one plain, so the driven pinion can give any fraction of a turn. For a full turn, 8 spindles on the pinion and 8 teeth per post; for a quarter turn, 8 spindles and 2 teeth per post. It runs very equal and firm.