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.