Keys, latches and screw-jacks for lifting
Why a key struggles to raise a heavy latch, and a threaded key as a better lift
Folio 49r studies keys that turn latches and bolts. The text explains that a key often cannot raise a hard or heavy latch [saliscendi]: standing as n m it presses against the centre f, and turned the opposite way as a n it pulls the latch while still bearing on the pivot, so the motion is hard either way. A further note proposes that a key threaded as a screw is a better way of raising. The sheet is filled with diagrams of levers, pulleys hoisting square weights, and screw-jacks (labels f, a m, n; n, m, f; a, n, m; o, S, f).
On this page
Why a key struggles to raise a heavy latch
A key set as n m makes its force against the centre f; turned the opposite way as a n it pulls the latch but still bears on the pivot. In both positions the load falls badly on the pivot, so the motion of the key is difficult.
A screw-threaded key as a better lifting device
Leonardo notes a preferable way of raising: when the key is made to work as a screw, in the manner figured on the page. Accompanying drawings show screw-jacks and pulley hoists lifting square weights.