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Release time:

2017-12-27

Trapezoidal and serrated threads are commonly used on lead screws that transmit motion and power. Trapezoidal threads are stressed on both sides of the teeth when working, while serrated threads are stressed on one side when working.

1. thread type

According to the tooth type can be divided into triangle, trapezoid, rectangle, sawtooth and arc thread;

According to the thread rotation can be divided into left and right hand;

According to the number of spiral lines can be divided into single-line and multi-line;

According to the thread parent body shape is divided into cylindrical and conical.

Elements of 2. threads

Thread consists of five elements: profile, nominal diameter, number of threads, pitch (or lead), and direction of rotation.

1. Tooth type

In the cross-sectional area through the thread axis, the profile shape of the thread is called the profile. There are triangular, trapezoidal, zigzag, circular and rectangular teeth.

2. Diameter

Threads have large diameters (d, D), medium diameters (d2, D2), and small diameters (d1, D1). The nominal diameter is used when representing threads, and the nominal diameter is the diameter representing the thread size.

Nominal straight of ordinary thread

3. Number of lines

A thread formed along one helical line is called a single thread, and a thread formed by two or more helical lines equally distributed along the axial direction is called a multi-thread.

4. Pitch and lead

Pitch (p) is the axial distance between two adjacent teeth corresponding to two points on the radial line.

The lead (ph) is the axial distance between two adjacent teeth on the same spiral line corresponding to two points on the middle diameter line.

For single thread, lead = pitch; for multi-thread, lead = pitch × number of threads.

5. Rotation

The thread that is screwed in when rotating clockwise is called right-handed thread;

A thread that is screwed in when turned counterclockwise is called a left-handed thread.

Marking of 3. threads

(1) Ordinary thread

Common threads are the most widely used, and the threads on threaded fasteners (bolts, studs, screws, nuts and other parts) are generally common threads.

Common thread is divided into coarse common thread and fine common thread. Fine common thread for precision parts and thin-walled parts.

In the marking of threads, the pitch of fine common threads must be noted, while the pitch of coarse common threads is generally not marked.

The marking of a common thread consists of five parts:

1) Feature code

M (common thread)

2) Size code

Dimension code: nominal diameter × pitch (the lead and pitch of multi-thread shall be noted, and the pitch of single-thread coarse tooth common thread shall not be marked).

"M10" means a single line coarse plain thread with a nominal diameter of 10 mm and a pitch of 1.5mm.

"M10 × 1" means a single thread with a nominal diameter of 10mm and a pitch of 1mm.

2) Code of tolerance zone of common thread

It consists of tolerance class (number) and basic deviation (external thread is represented by lowercase letters and internal thread is represented by capital letters), such as 5g6g, 6g, 6H, 7H.

When the thread middle diameter tolerance zone and the top diameter tolerance zone have different codes, they shall be noted separately, for example: M10-5g 6g

When the code of the tolerance zone of the middle diameter is the same as that of the top diameter, only one code shall be noted, for example: M10 × 1-7H

It is not marked when the tolerance zone code is 6g or 6H (nominal diameter ≥ 1.6mm).

3) The screwing length of the common thread

There are three kinds of long, medium and short, which are represented by codes L, N and S respectively.

M10-5g6g-S a thread with a short screwing length

M10-7H-L of thread with long screwing length

When the thread is of medium screwing length, the code N is not marked.

When special needs, the value of the screwing length can be indicated.

M20× 2-5g 6g-40

4) Rotary direction code

LH is marked when left-handed, but not when right-handed.

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

M10-7H-L is right-hand thread

(2) Pipe thread

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

marking of pipe thread:

It consists of thread feature code, dimension code and rotation direction. The size code is not the size of the thread major diameter, but the size of the pipe diameter (inch). The direction of rotation not written in the mark is right-handed.

Example of marking for 55 ° unsealed pipe threads:

G: Thread feature code of unsealed pipe thread;

G3/4: Single-line right-handed cylindrical internal thread with size code 3/4;

G3/4A or G3/4B: single-line right-handed cylindrical external thread with size code 3/4, marked A and B are tolerance grades of thread pitch diameter;

LH in G3/4LH and G3/4A-LH represents left-hand thread, and the thread pair composed of the two is only marked with the mark code of the external thread.

Example of marking for 55 ° sealing pipe thread:

Rp3/4LH: single-line left-handed cylindrical internal thread with size code 3/4;

Rc3/4: Single-line right-handed conical internal thread with size code 3/4;

Rp/R13/4 LH and Rc/R23/4: internal thread and external thread are screwed together to form a thread pair.

Rp: Thread feature code of sealing cylindrical internal thread;

Rc: Thread feature code of sealing conical internal thread;

R1: Feature code of conical external thread matched with cylindrical internal thread;

R2: feature code of the conical external thread matched with the conical internal thread;

(3) Trapezoid thread and sawtooth thread

Trapezoidal and serrated threads are commonly used on lead screws that transmit motion and power. Trapezoidal threads are stressed on both sides of the teeth when working, while serrated threads are stressed on one side when working.

Trapezoidal threads and serrated threads are marked in the same way as ordinary threads.

Examples of markings for trapezoidal threads:

Tr40 × 7LH-7e, trapezoidal thread (thread feature code is Tr), nominal diameter φ40, single line, pitch 7, left-handed, medium diameter tolerance zone code 7e; medium screwing length. Note: Only the code of the middle diameter tolerance zone is marked, and there are only two kinds of screwing lengths (codes N and L). When the screwing length is medium, N is omitted.

When the thread is a multi-line thread, the mark is:

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

The thread pair of trapezoidal thread is expressed as: Tr40 × 7-7H/7c, the tolerance zone of internal thread is in front, the tolerance zone of external thread is in back, and the two are separated.