Why a pivot wears unevenly: the knife-grinder's wheel
A pole wears most where its driver applies the greatest force
Leonardo argues that a pivot (polo) wears away most on the side where its mover exerts the greatest force, citing the itinerant knife-grinders who turn their wheels with a foot pedal. He reasons that the downward thrust of the foot begins the force and drives a whole turn, but as the motion moves away from its cause it slows and wears the support less; each new foot-thrust renews both the force and the wear, so the pivot is consumed unequally. The page is entirely text, with no diagram.
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A pivot wears most where the force is greatest
Leonardo holds that a turning pivot is worn away more on the side from which its mover exerts the greater force. As the motion of a turn moves farther from the moment force was applied, it slows and wears its support less; the next thrust renews the force and the wear, so the pivot is consumed unequally around its circuit.
The knife-grinder's foot-driven grinding wheel
For evidence Leonardo points to the knife-grinders who travel the cities, turning their grinding wheel with the foot. Where the grinder thrusts his foot down he begins the force that generates a whole revolution of the wheel.