Screw Thread Terminology

A screw thread is a helical groove formed on an internal or external diametrical surface and is described by various terms. These terms are described here:

Thread Axis: An imaginary line running through the centre of the thread screw.

Major Diameter, d_M: The diameter of an imaginary cylinder (concentric to the thread axis) that envelopes and touches the crest of the external threads or the root of internal threads.

Minor Diameter, dm: Also called the root diameter, this is the diameter of an imaginary cylinder (concentric to the thread axis) who’s surface touches the thread roots.

Pitch Diameter, d_P: The diameter of the imaginary cylinder (concentric to the thread axis) that intersects the surface at the point where the thread thickness equals the space between the threads or the diameter at which the tooth thickness = pitch/2. Refer to the graphic further down this page. On a sharp V thread form as per the image below, the pitch diameter would be located equidistant between the sharp major and minor diameters. However, few threads are actually formed like.

Thread Angle: The angle between the thread flanks.

Pitch, P: The axial distance between equivalent points on adjacent threads.

Crest: The top most point or surface of a thread.

Root: The bottom of the groove.

Lead: The axial distance travelled by the thread during 360° revolution of the screw. The smaller the lead, the higher the mechanical advantage.

Pitch Surface: The imaginary cylinder formed at the average of the crest and root height of each tooth. The diameter of the pitch surface equals the pitch diameter.

Pitch Angle: The angle between the pitch surface and the gear axis.

Multi Start Threads: If self-locking is not a requirement of a system but a high speed of translation along the thread axis is then a multi start thread may be used. This implies that multiple thread forms are created on the screw shaft.

Mechanical Advantage: A very important property of a screw thread is that it can be used to amplify force - hence delivering the application in a screw jack or screw jack system. A small torque applied to a screw can exert a large axial force on a mass. Therefore, a threaded component is said to produce a Mechanical Advantage. Mechanical advantage is a maximum when Lead is minimised. Therefore, due to the steeper helix angles of multi-start threads, mechanical advantage reduces.