The endless screw for safe hoisting; a saw drive
A worm engaging many teeth, a pawl against back-running, and a wheel that raises and lowers a saw.
Folio 17v studies three machine elements. At upper right an endless screw (worm) drives a large toothed wheel that lifts a hanging weight; because the worm's many turns engage many teeth at once, the drive is very safe, whereas a worm taking a single tooth risks great damage if that tooth breaks (text 1). Text 2 accordingly calls for a support or pawl on a single-tooth wheel so that it cannot run backward should a tooth fail, shown in the middle mechanism. The lower device raises and lowers by one same turning of its motor and, Leonardo notes, would serve a saw (texts 3-4).
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Endless screw engaging many teeth for a safe hoist
The pull of the endless screw, through its many turns, takes many teeth of the wheel at once, making the drive very safe; a worm that took only a single strained tooth could cause great damage and ruin if that tooth broke.
Pawl to stop a single-tooth wheel running back
When a screw takes only a single tooth of the wheel, a support (pawl) must be added so that, if that tooth breaks, the toothed wheel does not run backward.
A wheel that rises and falls to work a saw
This lower instrument raises and lowers by one same turning of its motor, and would serve to drive a saw.