Hauling weights on wheels: line of traction and a twin-nut screw
Where oxen should pull a loaded cart, with a detail of a screw and its two nuts
Leonardo argues that any weight moved on wheels should be drawn along the line of the wheels' centres, so the oxen's yoke stands as high as the wheel centre — better still, that the pull reach the centre of the effort the oxen exert as they lean into the load. A large loading rig with a long inclined beam and a ribbed roller-cart occupies the upper sheet. At the lower left a screw mechanism is detailed, whose two nuts a and m — the lower movable, the upper fixed — receive varying force from the varied screw passing through them.
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The correct line of traction on wheels
Any weight hauled on wheels should be pulled along the line of the wheels' centre, so the oxen's yoke is as high as the wheel centre; better still, the pull should reach the centre of the weight the oxen exert as they lean down to draw the load.
The two nuts a and m of the screw
a and m are the two nuts of the screw; the lower one is movable and the other is fixed. They receive varying powers from the varying screw that passes within them.