Thread Theory

Basic Definitions and Terminology

What is a thread?

A thread may be defined as a helical ridge of uniform section formed on the inside or outside of a cylinder or cone.

What are the uses of threads?

1. To fasten devices such as screws, bolts, studs and nuts.

2. To provide accurate measurement as in a micrometer.

3. To transmit motion.

4. To increase force.

Thread Terminology

1. External Thread

A thread on an external (outside) surface.

2. Internal thread

A thread on the inside surface of a cylinder.

3. Major diameter

The largest diameter of an external or internal thread.

4. Minor diameter

The smallest diameter of an external or internal thread.

5. Pitch diameter

The diameter of an imaginary cylinder which passes through the thread a point where the thread groove and the thread widths are equal.

6. Pitch

The distance from a point of one thread to the corresponding point on the next thread, measured parallel to the axis.

7. Lead

The lead of a thread is the distance a screw thread advances in one revolution. On a single start thread the lead and pitch are equal.

8. Root, Crest and Flank

The root of the thread is the bottom surface joining the sides of two adjacent threads. The root of an external thread is on the minor diameter.

The crest is at the major diameter of an external thread.

The flank is the thread surface which connects the crest with the root.

9. Depth of thread.

The depth of a thread is the distance from the crest to the root measured perpendicular to the axis.

10. American National Thread

The most common thread form used in the country. It has an included angle of 60°.

11. American National Acme Thread

Has an included angle of 29°.

12. Threads Per Inch ( t.p.i.)

The number of threads per inch. Metric threads are defined by pitch rather than threads per given unit of length.