Three screws and why a screw bends when it presses
A screw keeps straighter pulled than pushed; unequal leverage across the thread at a and m
Three short screw shafts are drawn down the right side, each with a different thread winding, illustrating a study of how a screw behaves under load. Leonardo argues that a screw keeps its straightness better when pulled than when pushed, and that a screw pressing an object only with the outer end of its thread will bend on the side opposite that thread-tip. He grounds this in leverage: in the screw a n m the point a is twice as far from the axis n as m is, so the same power does double work at a, drawing a p together and pushing m q apart; a thread bearing equidistant from its axis (screw c d f) breaks first.
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A screw stays straighter pulled than pushed
Leonardo states that a screw keeps its straightness better when pulled than when pushed or pressed. When it bears on the pressed thing only with the outer end of its thread, the strain makes it bend on the side opposite that pressing thread-tip.
Unequal leverage across the thread at a and m
In the screw a n m the point a is twice as far from the axis n as m is, so one and the same power does double the work at a than at m. Through this excess of force a p draw closer together and m q become more remote; where the thread bears equidistant from the axis, as in screw c d f, the thread breaks first.
Three screws with different thread windings
Three short screw shafts are drawn one above another at the right, each with a differently pitched and shaped winding. Two read as squared cylinders with stepped turns and the lowest as a coarse open spiral.