Screw thread terminology

Basic Fastener Thread Terms

 

 

1. Root 

The screw root is the deepest part of these grooves, located between the threads' crests.

2. Crest

The crest is the highest point on these ridges, and it runs parallel to the axis of the screw.

3. Helix Angle

The screw helix angle refers to the angle formed by the helical groove of a screw thread with a plane perpendicular to the axis of the screw. In simpler terms, it is the angle between the threads and the centerline of the screw.

4. Thread Angle 

It refers to the angle between the flanks of the threads on a screw or bolt, the most common thread angle for standard threads is 60 degrees.

5. Flank

The screw thread flank refers to the two sloping surfaces of a thread. Each thread on a screw or bolt has two flanks, which are the surfaces that make contact with the corresponding flanks of the threads in a mating component (e.g., a nut).

6. Pitch

The pitch refers to the distance between corresponding points on adjacent crests in a screw or bolt.

7. Depth

The screw thread depth refers to the distance between the crest and the root of a thread on a screw or bolt. It is the distance from the crest to the root, measured perpendicular to the axis of the screw.

8. Minor Diameter

The screw thread minor diameter refers to the smallest diameter of the threaded section of a screw or bolt. In other words, the minor diameter is the distance between the bottoms of the opposing threads in a screw or bolt.

9. Pitch Diameter

The thread pitch diameter is a theoretical diameter of an imaginary cylinder that passes through the point where the width of the thread groove is equal to half the pitch. Pitch diameter is approximately halfway between the major and minor diameters.
 
10. Major Diameter

The major diameter of a screw or bolt thread represents its largest diameter of the threaded section. For external threads, such as those found on bolts, the major diameter is measured at the outermost points of the threads, while for internal threads, like those in nuts, it is measured at the bottom of the thread grooves. 

11. Axis

The screw thread axis refers to an imaginary line that runs along the center of a screw or bolt, parallel to the direction of the threads. In other words, it is the central line that passes through the midpoint of the threaded portion. The axis provides a reference for the orientation and alignment of the screw.