Type of thread

Basic element of thread

Thread refers to a spiral line produced on a cylindrical or conical surface, a continuous projection of a specific cross-section. The bumps here are the solid parts on both sides of the thread, also known as teeth. The basic elements of the thread are: pressure type, nominal diameter, number of lines, pitch (lead), and rotation.

Thread classification

According to its use, shape, and direction, the thread can be classified into the following categories:

● According to the shape of the mother is divided into cylindrical thread and taper thread;

● According to the position of the thread in the mother is divided into external thread and internal thread.

● According to the thread section shape, it is divided into triangle thread, rectangular thread, trapezoid thread, serrated thread and other special shape thread.

● According to the direction of the helix, it is divided into left and right screw threads.

● According to sealing, it is divided into sealed thread and non-sealed thread.

● It is divided into connection thread and transmission thread according to use.

Thread designation

Different thread standards have different codes, the following mainly introduces the common several.

International metric standard thread number M

The thread adopted by our national standard. The top of the tooth is flat, easy to turn, and the bottom of the tooth is rounded to increase the strength of the thread. The thread Angle is 60 degrees, and the specification is expressed in M.

American standard thread code NC/NF/NEF

The top and root of the thread are flat, good strength. The thread Angle is 60 degrees and the specification is expressed in the number of teeth per inch. The thread can be divided into coarse teeth (NC); Fine teeth (NF); Extra fine teeth (NEF).

Unified standard thread one code UNCIUNFIUNEF

It is jointly developed by the United States, the United Kingdom and Canada, and is the commonly used British thread. The thread Angle is also 60 degrees, and the specification is expressed by the number of teeth per inch. This thread can be divided into coarse teeth (UNC); Fine teeth (UNF) Extra fine teeth (UNEF).

Wheat-type thread designation W

British National standard used thread. The thread Angle is 55 degrees and the symbol is "W". Suitable for rolling manufacturing.

Round thread number Rd

Standard thread for DIN in Germany. Suitable for the connection of light bulbs and rubber tubes. The symbol is Rd.

Pipe thread designation P.S. / N.P.S. / N.P. T.

A thread used to prevent leakage, often used to join gas or liquid fittings. Thread Angle is 55 degrees, can be divided into straight pipe thread code "P. S. , N. P. S. "and the inclined pipe thread code" N. P.T. The taper is 1:16, that is, 3/4 inch per foot.

Square thread

The transmission efficiency is large, second only to ball threads, and can not be adjusted with nuts after wear, which is its disadvantage. Generally used for vise screw and crane thread.

Trapezoidal thread one code Tr

Also known as Acme thread. The transmission efficiency is slightly smaller than that of square thread, but it can be adjusted by nuts after wear. The thread Angle is 30 degrees for the metric system and 29 degrees for the imperial system. Generally used as a guide screw for lathes. The symbol is "Tr".

The corresponding thread code can be seen in the following figure

Thread fit

Thread fit is the size of the loose or tight screw threads, and the grade of the fit is a specified combination of deviations and tolerances acting on the inside and outside grain.

The tolerance system consists of tolerance grade and tolerance zone position. Tolerance levels are indicated by numbers such as 4, 6, 8. The tolerance zone position is indicated by letters such as day, G, h, g. The selection of tolerance zone generally takes into account the combination of surface coating thickness and thread clearance.

Tolerance zone position

● Internal thread: G represents the basic deviation is positive; Day represents a basic deviation of zero.

●External thread: a, b, C, d, e, f, g indicates that the basic deviation is negative. h means zero base deviation. Internal thread tolerance zone location diagram is as follows:

Practical application of tolerance zone

H is a commonly used tolerance zone position for internal threads, and is generally not used as a surface coating, or with an extremely thin phosphating layer. G position basic deviation for special occasions, such as thicker coatings. g is commonly used to plate general thin coating, such as the product drawings require 6h bolts, and the thread before plating uses 6g tolerance zone. It is recommended to combine the threads into H/ g, H/h or G/h. For refined fastener threads such as bolts and nuts, it is recommended to use 6H/6g.