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## Definition of Screw Lead Angle

The lead angle (φ) measures the inclination of a screw helix from a plane that is perpendicular to the screw’s axis. In more simple words, it is the proportion between the screw’s lead and its diameter.

As seen in this chart, a screw with a fine pitch and a large diameter will have a small lead angle, whereas a screw with a coarse pitch and a small diameter will have a large lead angle.

The above chart is for single start threads where the lead equals the pitch (L=D). The lead of the screw equals its pitch times the number of starts (L=nXP). Therefore Multi start threads have large lead angles.

The lead angle is measured at the pitch diameter. It can be represented by a right triangle whose base is the circumference of the screw, and its height is the lead of the screw.

$$\large\begin{array}{l} L = n\,\times\,P \\ C = \pi\,\times\,d_2 \\ \varphi = \arctan \left ( \frac{L}{C} \right ) \end{array}$$

Formula symbols:

• n – Number of starts
• P – Pitch of the thread (Inch / mm)
• d2Pitch Diameter of the thread (Inch / mm)