What is a Lead Screw Pitch?

By Shizu Yamaguchi

So, you want to know what a lead screw pitch is. We’ll dive into that here and define a few additional terms related to lead screws.

Pitch

The pitch of a lead screw is essentially the distance between threads, measured from the top of one screw thread to the top of the next screw thread.

Pitch is measured in millimeters. For inch-sized ACME screws, the measurement metric for pitch is threads per inch.

Starts

A screw start is the number of independently running threads on the screw shaft. One can tell how many starts there are on a given lead screw by examining the end of the lead screw.

Single start means that there is only one thread or ridge running along the length of the screw shaft. Every time the screw turns 360 degrees, the system move forward axially by one thread width.

Photo: “Helical screw single double triple quadruple start” by Jahobr, licensed under Creative Commons CC0 1.0 Universal Public Domain Dedication

Lead

The screw lead is the axial distance covered by a 360 degree rotation of the screw. This term is often used in discussions involving ball screws.

For screws that have a single start, the lead and pitch are equal to each other. That is, the distance between threads (pitch) is equal to the axial distance covered by a full turn of the screw (lead). Because most screws are single start screws, the terms pitch and lead are often mistaken for one another.

For multiple starts, the lead is equal to the number of starts multiplied by the pitch. For example, a 3 start lead screw has a lead 3 times its pitch.

Photo: “Lead_and_pitch” by Lambiam, licensed under CC BY-SA 3.0

Other terms related to thread forms:

Major diameter

The major diameter or land diameter is the outermost diameter of a given screw.

Minor diameter

The minor diameter or root diameter is the diameter at the bottom of the thread, where the threads touch the shaft of the lead screw.

Pitch diameter

The pitch diameter on an acme screw is the diameter measure about halfway between the major and minor diameters. The diameter is where the space between threads is equal to the thread thickness.

References:

https://www.thomsonlinear.com/en/support/tips/difference-between-screw-pitch-and-lead

https://www.helixlinear.com/media/10913/engineeers-guide-to-identifying-lead-screw-threads.pdf

https://spool3d.ca/news/lead-vs-pitch/

https://everhardwarestore.com/screw-pitch-different-lead-pitch.html