The Threads of Fasteners (3)

1.3.4 Thread Diameter

Thread diameters include the major diameter (D or d), minor diameter (D₁ or d₁), and pitch diameter. The major diameter of general-purpose threads and trapezoidal threads is also referred to as the nominal diameter. The crest diameter is the diameter of an imaginary cylinder or cone that is tangent to the crests of an external or internal thread, i.e., the major diameter of an external thread or the minor diameter of an internal thread. The root diameter is the diameter of an imaginary cylinder or cone that is tangent to the roots of an external or internal thread, i.e., the minor diameter of an external thread or the major diameter of an internal thread.

1.3.5 Thread Handedness

Threads can be either left-hand or right-hand. A thread that advances when turned clockwise is called a right-hand thread, while one that advances when turned counterclockwise is called a left-hand thread. Right-hand threads are commonly used in engineering.

1.3.6 Number of Thread Starts

Threads can be single-start or multi-start. A thread formed along a single helix is called a single-start thread, while one formed along two or more helices is called a multi-start thread.

Most fastening threads are single-start.

1.3.7 Pitch and Lead

The axial distance between corresponding points on adjacent threads measured along the pitch diameter is called the pitch. The axial distance traveled in one full rotation along the same helix is called the lead. For a single-start thread, the lead is equal to the pitch; for a multi-start thread, the lead is equal to the pitch multiplied by the number of starts.

1.3.8 Marking Method

Example: M 10×1 LH --7H-L
◆ M indicates the feature code for general-purpose threads;
◆ 10×1 indicates nominal diameter × pitch (pitch is not marked for coarse threads);
◆ LH indicates the code for left-hand threads (right-hand threads are not marked with a handedness code);
◆ 7H indicates the tolerance zone code;
◆ L indicates the code for the thread engagement length group. Medium length is not marked with a group code; specific length values are marked only when specially required.