Lead: the axial distance between two corresponding points on a helix. When a threaded part is rotated about its axis with respect to a fixed mating thread, the lead is the axial distance moved by the part in relation to the amount of angular rotation. On a double lead thread (double start), the lead is equal to twice the pitch. It is necessary to distinguish measurement of lead from measurement of pitch, as uniformity of pitch does not assure uniformity of lead.
Additional Discussion: Single-start threads are most common, and are more useful for general fastening needs. They offer more resistance to loosening than multi-start threads, due to the lower helix angle of the thread. Multi-start, aka: multiple-lead, is a thread that offers quick assembly and disassembly.
Thread lead can be described as the linear screw travel accomplished in one revolution. Thread pitch is the distance from any point on a screw thread to the corresponding point on the next thread, measured parallel to the axis. With a single-start thread, lead and pitch are equal. A double-start thread's lead is twice the pitch measurement.
Using 20 thread-per-inch thread as an example, the screw must turn 20 revolutions to advance 1” in the part. A 1/4-20 double-start thread has the same number of threads per inch, but with two starting points, 180° apart, needs just ten revolutions to advance 1”. A triple-start thread, with starts 120° apart, will advance 3 times faster than a single lead thread. The result continues to change appropriately as additional starts are added.
Original Posting: 3/20/2012
Last Revision: 10/19/2017
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