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What Are The Symbols Of The Thread

Dec 28, 2019

(1) General thread


Threads are most commonly used, and the threads on threaded fasteners (bolts, studs, screws, nuts, etc.) are generally general threads.


General threads are divided into coarse threads and fine threads. Fine threads are generally used on precision parts and thin-walled parts.


In the symbol of the thread, the pitch of the general thread of the fine tooth must be noted, and the pitch of the general thread of the coarse tooth is generally not marked.


1) Feature code


M (indicates general thread)


2) Scale code


Scale code: Nominal diameter × pitch (leads and pitches of multi-threaded threads should be noted, and the pitch of general thread pitch for single-line coarse teeth is not indicated).


"M10" indicates a single-thread coarse thread with a nominal diameter of 10 mm and a pitch of 1.5 mm.


“M10 × 1” indicates a single thread fine thread with a nominal diameter of 10mm and a pitch of 1mm.


2) Code of official service belt for general thread


It consists of the service grade (number) and basic deviation (indicated by lowercase letters for external threads and uppercase letters for internal threads), such as 5g6g, 6g, 6H, 7H.


When the code of the threaded middle diameter public service belt and the top diameter public service belt are not the same, it should be separately injected, such as: M10-5g 6g


The middle diameter is the same as the top diameter public service belt code. Only one code is noted, such as: M10 × 1-7H


The official service belt code is 6g or 6H (nominal diameter ≥ 1.6mm).


3) Turning length of general thread


There are three types: long, medium and short, which are indicated by codes L, N, and S, respectively.


M10-5g6g-S is short thread


M10-7H-L is a thread with a long turn-on length


When the thread is of medium twist length, the code N is not indicated.


When special needs are required, the value of the twist length can be indicated.


M20 × 2-5g6g-40


4) Turning code


LH is indicated when left-handed and not indicated when right-handed.


M10-7H-L-LH is left-handed thread


M10-7H-L is right-handed thread


(2) Pipe thread


Pipe threads are generally used in the connection of pipelines (water pipes, oil pipes, gas pipes, etc.). The symbol of the pipe thread is marked with a guide. The guide line points to the large diameter of the thread.


Symbol of pipe thread:


It consists of thread feature code, scale code and rotation direction. The size code is not the size of the large diameter of the thread, but the size of the diameter (inch) of the pipe. All symbols in the symbol that do not indicate a rotation are right-handed.


(3) Trapezoidal thread and zigzag thread


Trapezoidal and zigzag threads are commonly used on screws that transfer motion and power. The trapezoidal thread works on both sides of the tooth, while the zigzag thread works on one side.


The symbols for trapezoidal and zigzag threads are similar to general threads.


Example of symbols for trapezoidal threads:


Tr40 × 7LH-7e, trapezoidal thread (thread characteristic code is Tr), nominal diameter φ40, single wire, pitch 7, left-handed, middle-diameter official service belt code 7e; medium twist length. Note: Only the code of the middle diameter public service belt is marked, and there are only two types of rotation lengths (codes N and L). When the middle rotation length is used, N is omitted.


When the thread is multi-threaded, the symbol is:


Tr40 × 14 (P7)-7e, in which "14" is the lead, "7" is the pitch, and the double-thread thread.


The trapezoidal screw thread pair indicates: Tr40 × 7-7H / 7c, the internal service belt of the internal thread is in front, and the external service belt of the external thread is at the back, separated by “/”.