The perpetual screw and its equality of force in clocks
Why an endless screw drives more weakly far from its center, and how it keeps force even
The sheet studies the 'perpetual screw' (vite perpetua), an endless screw meshing with a toothed wheel, drawn as a fusee-like disc with radial arms and, below, mounted in a frame with a fly. Leonardo observes that such a screw is not of equal force, since it drives the wheel's arm more weakly the farther that arm is from its center, so it is fit only for clocks. He then argues that because the screw always keeps four teeth of the wheel engaged at various distances from its center, it acts with a continual equality of force. A plumb-balance and a twist-spindle are drawn alongside.
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Perpetual (endless) screw for clocks
The perpetual screw is not of equal force, because it moves the arm of the wheel it drives more weakly when that arm is far from its center than when near it. For this reason it is suited only to clocks.
Four teeth engaged for continual equality of force
Because this screw always has four teeth of the wheel it moves engaged at various distances from its center, it works with a continual equality of force, evening out the weakness noted above.