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In lead screws, there are multiple threads running up
the screw. This causes the lead and pitch to not be the same.
Lead is the distance the nut travels with one revolution of the screw. Pitch is the distance between two lands on the screw thread
For single start screw, the lead = pitch. For double start, the lead = 2 times the pitch. For four start, the lead = 4 times the pitch.
In the following animation the screws are all turned one revolution. These screws all have the same pitch but different leads. The arrow moves along the screw as a nut would move when the screws are turned. Notice how the double start moves the arrow twice the distance of the single start. And the four start travels four times the distance of the single start.