Springs for locks: weak-and-wide versus strong-and-short
Comparing two springs, a tapering spring like half a crossbow, and how to fix pivots in wood
Folio 79v studies springs and pivots for mechanisms. Drawing two springs, a and b, Leonardo asks the reader to judge their quality and power, one weak with a large movement, the other strong with a small one, and reports that locksmiths repairing locks discard weak springs of great motion and fit strong springs of small motion. He argues a spring should begin thick and taper toward its end, like half a crossbow, and poses the problem of making a spring half as short yet equal in first motion, concluding it is impossible for two springs of different length to have equal force at equal motion. A further note gives a method of fixing pivots in their wooden bearings with strengths suited to their loads. The lower drawings of a vaned wheel and pointed pivot-pins, together with a signature-like inscription beneath the wheel, carry further untranscribed annotation.
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Which spring for a lock?
Comparing two springs, one weak with great movement, the other strong with small movement, Leonardo would choose the small-motion spring: masters repairing locks remove weak springs of great motion and fit strong ones of small motion. The motion is taken to drive the levers at their upper parts, at a and at b.
A tapering spring, like half a crossbow
A spring should begin thick and taper toward its tip, after the manner of half a crossbow. Leonardo wants a spring half as short yet equal to the long one in its first motion and asks what thickness and width it needs, concluding it is impossible for two springs of different length to be of equal force at equal motion.
Fixing pivots in their wood
A note gives a method of fixing pivots (poli) in their wooden bearings with various strengths, according to the variety of stresses they must bear. Pointed pin forms drawn at the lower right correspond to these differently seated pivots.