Basic Screw & Thread Terms

screw threads

1

Screw: A fastener with a helical ridge (thread) that allows it to be driven into materials to hold them together.

2

Thread: The helical structure that wraps around the screw or bolt, allowing for the conversion of rotational motion into linear motion.

3

External Thread: The thread that is formed on the outside of a screw or bolt.

4

Internal Thread: The thread that is cut into the interior of a nut or threaded hole.

5

Major Diameter: The largest diameter of the external thread.

6

Minor Diameter: The smallest diameter of the internal thread.

7

Pitch: The distance between two adjacent thread peaks, indicating the spacing of the threads. It can be expressed in millimeters or threads per inch.

8

Lead: The distance a screw travels axially in one complete turn. For single-thread screws, the lead is equal to the pitch.

9

Thread Angle: The angle between the sides of the thread, typically measured in degrees, which influences the thread's strength and fit.

10

Root: The bottom of the thread groove, where the thread's depth is measured.

11

Flank: The flat surface on the side of the thread that connects the crest and the root.

12

Crest: The crest is the height of the rise of an external thread or the depth of the setback of an internal thread.

13

Thread Pitch Gauge: A device that compares a thread's pitch to a set of blades with known pitches in order to determine the thread's pitch.

14

Torque: A screw or bolt's rotational force, which influences how firmly it is secured.

15

Preload: The initial tension applied to a fastener before any external load is introduced, which helps maintain joint integrity.