Screw Thread and its Terminology

**Screw Thread**

A ridge or uniform section in the form of a helix on the external or internal surface of a cylinder, or in the form of a conical spiral on the external or internal surface of a cone.

External Thread

An external thread is a thread on the outside of a member.

Internal Thread

An internal thread is a thread on the inside of a member.

Major Diameter

The largest diameter of the thread of the screw or nut. The term "major diameter" replaces the term "outside diameter" as applied to the thread of a screw and also the term "full diameter" as applied to the thread of a nut.

Minor Diameter

The smallest diameter of the thread of the screw or nut. The term "minor diameter" replaces the term "core diameter" as applied to the thread of a screw and also the term "inside diameter" as applied to the thread of a nut.

Pitch Diameter

On a straight screw thread, the diameter of an imaginary cylinder, the surface of which would pass through the threads at such points as to make equal the width the threads and the width of the spaces cut by the surface of the cylinder. On a taper screw thread, the diameter, at a given distance from a reference plane perpendicular to the axis of an imaginary cone, the surface of which would pass through the threads at such points as to make equal the width the threads and the width of the spaces cut by the surface of the cone.

Pitch

The distance from a point on a screw thread to a corresponding point on the next thread measured parallel to the axis.

The pitch in inches: 1 divided by the Number of threads per inch

Half Angle of Thread

The angle included bewteen a side of the thread and the normal to the axis, measured in an axial plane.

Angle of Thread

The angle included bewteen the sides of the thread measured in an axial plane.

Helix Angle

The angle made by the helix, or conical spiral, of the thread at a pitch diameter with a plane perpendicular to the axis.

Crest

The surface of the thread corresponding to the major diameter of the screw and the minor diameter of the nut.

Lead

The distance a screw thread advances axially in one turn. On a single threaded screw the lead and pitch are identical; on a double threaded screw the lead is two times the pitch; on a triple headed screw the lead is three times the pitch, etc.

Root

The surface of the thread corresponding to the minor diameter of the screw and the major diameter of the nut.

Side or Flank

The surface of the thread which connects the crest with the root.

Axis of a Screw

The longitudinal central line through the screw.

Base of Thread

The bottom section of the thread; the greatest section between the two adjacent roots.

Depth of Thread

The distance between the crest and the base of the thread measured normal to the axis.

Number of Threads

The number of threads in one inch of length.

Length of Engagement

The length of contact bewteen two mated threaded parts measured axially.

Depth of Engagement

The depth of thread contact of two mated parts, measured radially.

Pitch Line

An element of the imaginary cylinder or cone specified under "Pitch Diameter".

Thickness of Thread

The distance between the adjacent sides of the thread measured along or parallel to the pitch line.

Mean Area

The term "mean area of a screw," when used in specifications and for other purposes, designates the cross-sectional area computed from the mean of the basic pitch and minor diameters.