Four Screws with Different Thread Pitches
Which screw is hardest to turn and which best holds a load
Four cylindrical screws, threaded in different ways and labelled 1, 2, 3, 4, are drawn across the sheet. Leonardo notes a screw can be made double, triple or of as many starts as one pleases, and that such multi-start screws, through their equal contact with the thing they press, never bend. He then compares them: among screws of equal thickness the one with more threads is harder to turn, more turns of the thread make for easier motion, and a screw whose thread makes fewer turns holds weights more strongly but is harder to move.
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Double and triple screws that never bend
A screw can be made double, triple, or of as many starts as one pleases; by their equal contact with the thing they press, such screws will never bend. Four screws with differing threads are drawn and numbered 1 to 4.
More threads harder to turn; fewer turns hold more
Among screws of equal thickness, the one with more threads is harder for its mover to turn; among equal ones, more turns of the thread give easier motion. A screw whose thread makes fewer turns is stronger at supporting weights but harder to move.