Question About Screw Threads
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Question About Screw Threads

Screws and bolts are common threaded fasteners. They are mechanical parts with internal or external threads. They are generally used as a mechanism for fastening to facilitate the combination of multiple components. Unlike welding, the function of the locking mechanism is easy to adjust, easy to disassemble. If a fastener has external threads and is not used with a nut, it is a screw (such as wood screws, self-tapping screws), and if it is used with a nut, it is a bolt (such as flat head bolts, non-slip bolts). Different threads have different performance applications.
Published: May 10, 2022
Question About Screw Threads

What Is a Thread?

A thread is a shape with a uniform helical protrusion on a cross-section of a solid surface or inner surface. The principle of the inclined plane is also applied, and the cross-section is uniformly protruded along the inclined plane. The basic functions of threaded parts are good screw ability and sufficient strength.

Classification of Threads

By the structure and use can be divided into:
1. Ordinary thread (fastening thread): The tooth profile is triangular, which is used to connect and fasten parts. Ordinary threads are divided into three types: coarse, fine and superfine according to the pitch.

2. Sealing thread: used for sealing connection, mainly three kinds of pipe thread, tapered thread and tapered pipe thread.

3. Transmission thread: There are trapezoidal, rectangular, saw-shaped and triangular tooth shapes.
4. Special thread: special purpose thread.

By region can be divided into:
1. Metric thread (metric thread): ISO metric thread consists of symmetrical V-shaped threads, the sides are at an angle of 60° to each other, and the thread depth is 0.614 × pitch. The marking of metric threads starts with the letter M, followed by the outer diameter of the thread (the outer diameter of the thread), in mm. For example, M8 is a metric thread with an outer diameter of 8 mm, and M8×1 is an outer diameter of 8 mm. Rotate 360 ° will advance a metric thread of 1 mm.

2. Inch thread: Inch thread is commonly known as "Wheel thread" and is marked with W*N. Inch thread pitch indicates how many threads per inch, for example, 1/8”-40 (outer diameter=1/8″= 3.175mm, 40 threads per inch), while some are marked as 1/4"x0.50 which are 1/4" OD screws and 0.5" long.

3. American thread: generally marked with a serial number, such as #2-56; #4-40; #6-32; #8-32; #10-24. For example, #8-32 is a size 8 screw with 32 threads per inch.

Thread Terms

External thread: A thread formed on the outer surface of a cylinder or cone.
Internal thread: A thread formed on the inner surface of a cylinder or cone.
Major diameter: The diameter of an imaginary cylinder or cone tangent to the external thread crest or internal thread root.
Minor diameter: The diameter of an imaginary cylinder or cone tangent to the root of the external thread or the crest of the internal thread.
Pitch Diameter: The diameter of an imaginary cylinder or cone whose generatrix passes through the groove on the tooth profile where the width of the protrusion is equal. This imaginary cylinder or cone is referred to as a mid-diameter cylinder or mid-diameter cone.
Right-hand thread: A thread that is screwed in when turned clockwise.
Left-hand thread: A thread that is screwed in when turned counterclockwise.
Tooth profile angle: On the thread profile, the angle between two adjacent flanks.
Pitch: The axial distance between the corresponding two points on the center diameter line of two adjacent teeth.

Marking Recognition of Threads

There are many specifications and types of fasteners. To distinguish them clearly, you must understand their name composition, that is, mark identification.

1. Marking of screws or bolts: name, standard number, nominal diameter × thread diameter × nominal length - material - wrenching form - surface treatment.

2. Marking of nut or washer: name, standard number, nominal diameter x thread pitch - material - surface treatment.

3. Marking of the retaining ring: name, standard number, nominal size.

4. Pin marking: name, standard number, nominal diameter x nominal length - material - surface treatment.

Published: May 10, 2022 Source :SOIC

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