thread

I. Brief introduction to threads

    A thread is a shape with uniform helical protrusions on the cross-section of the outer or inner surface of a solid object.

    Based on their structural characteristics and purposes, they can be divided into three major categories: 

(1) Common threads: The thread profile is triangular and used for connecting or fastening parts. Common threads are divided into coarse and fine threads according to pitch, with fine threads having higher connection strength.(II) Transmission thread: The thread profile can be trapezoidal, rectangular, sawtooth, triangular, etc.

 (III) Sealing threads: used for sealing connections, mainly including pipe threads, taper threads, and taper pipe threads.

Continuous protrusions in the shape of a spiral line with a specific cross-section are formed on the surface of a cylindrical or conical base. Threads are classified according to their base shape into cylindrical threads and conical threads; according to their position on the base, they are divided into external threads and internal threads. According to their cross-sectional shape (thread form), they are classified into triangular threads, rectangular threads, trapezoidal threads, serrated threads, and other special-shaped threads. Triangular threads are mainly used for connection, while rectangular, trapezoidal, and serrated threads are mainly used for transmission. According to the direction of the spiral line, they are divided into left-hand threads and right-hand threads, with right-hand threads being commonly used. According to the number of spiral lines, they are classified into single-thread threads, double-thread threads, and multi-thread threads. Single-thread threads are mostly used for connection, while double-thread or multi-thread threads are used for transmission. According to the size of the threads, they are divided into coarse threads and fine threads. Depending on the application and function, they can be classified into fastening threads, pipe threads, transmission threads, special-purpose threads, etc

II. Main geometric parameters of cylindrical threads

① Major diameter (MD): The diameter of an imaginary cylinder that coincides with the crest of an external thread or the root of an internal thread. The nominal diameter of a thread is equivalent to its major diameter.

② Inner diameter (minor diameter), the diameter of an imaginary cylinder that coincides with the root of an external thread or the crest of an internal thread.

③ Middle diameter, the diameter of an imaginary cylinder through which the width of the protrusion and groove on the tooth profile are equal.

④ Thread pitch refers to the axial distance between two corresponding points on the radius line of adjacent teeth.

⑤ Lead, the axial distance between two corresponding points on the pitch line of adjacent teeth on the same helical line.

⑥ Thread angle, the angle between the adjacent two threads on the thread profile.

⑦ Thread lead angle, the angle between the tangent of the helical line on the pitch cylinder and the plane perpendicular to the thread axis.

⑧ Working height, the distance between the overlapping parts of the thread profile in two mating threads in the direction perpendicular to the thread axis, etc. For the nominal diameter of threads, except for pipe threads, which use the inner diameter of the pipe as the nominal diameter, all others use the outer diameter as the nominal diameter. Threads have been standardized into metric (metric system) and imperial systems. The international standard adopts the metric system, and China also adopts the metric system.

In cylindrical threads, triangular threads exhibit excellent self-locking performance. They are categorized into coarse and fine threads, with coarse threads being commonly used for general connections. Fine threads, characterized by a smaller pitch and smaller lead angle, exhibit superior self-locking performance and are often employed in thin-walled tubes for small parts, connections subject to vibration or variable loads, and fine adjustment devices. Pipe threads are used for tight connections between pipe fittings. Rectangular threads are efficient, but they are often replaced by trapezoidal threads due to their difficulty in grinding and the challenges in centering the mating threads during assembly. Serrated threads, with working edges resembling rectangular straight edges, are primarily used to withstand unidirectional axial forces.

The thread form of a taper thread is triangular, and the tightness of the thread pair is mainly ensured by the deformation of the threads. It is commonly used in pipe fittings.

III. Measurement of threads

For general standard threads, thread ring gauges or plug gauges are used for measurement.

2. The thread micrometer is used to measure the pitch diameter of threads. The method of measuring the pitch diameter of threads using a measuring needle is also known as the three-needle method.

3. The vernier caliper is used to measure the pitch diameter and tooth thickness of trapezoidal threads, as well as the pitch diameter and tooth thickness of worm gears.

4. Special measuring tools and instruments are used for measuring other parameters.

IV. Common thread standards in China

Category: Standard Name, Standard Number, Relationship with International Standards

Common thread Common thread Basic tooth profile GB/T192 —1981 Equivalent to ISO68

Common thread - Diameter and pitch series GB/T193 - 1981 is equivalent to ISO261

General purpose metric screw threads - Basic dimensions - GB/T196-1981 is equivalent to ISO724

General Purpose Metric Screw Threads - Tolerances and Fits GB/T197-1981 is equivalent to ISO965/1

Common thread deviation table GB/T2516-1981 is equivalent to ISO965/3

The common thread series for commodity fasteners, JB/T7912-1999, is equivalent to ISO262

The limit dimensions for medium-precision common threads of commercial fasteners, as specified in GB/T9145-1988, are equivalent to those in ISO965/2

Optical threads, optical instruments, special fine thread, ZBN30006-1988

Optical instruments - Eyepiece threads - JB/T8204—1995

Short thread for optical instruments JB/T5450-1991

Tight fit thread, transition fit thread, thread GB/T1167—1996

Interference fit thread GB/T1181-1998

Small thread, small thread profile, GB/T1505 4.1—1994, equivalent to ISO1501

Small thread diameter and pitch series GB/T1505 4.2—1994

Basic dimensions of small threads GB/T1505 4.3—1994

Small thread tolerance GB/T15054.4-1994

Limit dimensions of small threads GB/T15054.5-1994

MJ thread, basic tooth profile of MJ thread, GJB/T3.1 —1982, ISO5855

Dimensions and tolerances of MJ threaded bolts and nuts GJB/T3.2 —1982

MJ threaded pipe - Dimensions and tolerances of thread for pipeline fittings - GJB/T3.3 —1985

MJ threaded joint - Dimensions and tolerances of components GJB/T3.4 —1985

MJ thread calculation formula GJB/T3.5 —1985

MJ thread end-to-end GJB52-1985

Trapezoidal thread, Trapezoidal thread, Tooth form, GB/T5796.1—1986, equivalent to ISO2901

Trapezoidal thread - Diameter and pitch series GB/T5796.2-1986 is equivalent to ISO2902

Trapezoidal thread - Basic dimensions - GB/T5796.3-1986 is equivalent to ISO2904

Trapezoidal thread tolerance GB/T5796.4—1986 is equivalent to ISO2903

Trapezoidal thread - Limit dimensions GB/T12359—1990

Technical conditions for machine tool trapezoidal thread screw and nut JB/T2886-1992

Forged steel valve with short trapezoidal thread JB/TQ374—1985

Serrated thread Serrated (3°, 30°) thread profile GB/T13576.1—1992

Serrated (3°, 30°) thread diameter and pitch series GB/T13576.2—1992

Basic dimensions of sawtooth (3°, 30°) threads GB/T13576.3—1992

Serrated (3°, 30°) thread tolerance GB/T13576.4—1992

Hydraulic press 45° serrated thread profile and basic dimensions JB2076-1984

Pipe threads - Pipe threads with thread seal GB/T7306-1987 is equivalent to ISO7/1

Pipe threads without thread seal GB/T7307-1987 is equivalent to ISO228/1

60° taper pipe thread GB/T12716-1991

Metric taper thread GB/T1415-1992

Pipe thread connection end adopts the common thread size series GB/T1414-1978

Special thread for gas cylinders GB/T8335-1998

General standard: Thread terminology GB/T1479 1—1993 is equivalent to ISO5408. General standard: Thread terminology GB/T1479 1—1993 is equivalent to ISO5408

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