Influence of thread parameters and parameter errors on interchangeability of fasteners

Basic tooth type of common thread of fastener

The geometric parameters of fastener threads depend on the basic tooth profile in the axial section of fastener threads. The basic tooth shape is a common theoretical tooth shape of internal and external threads formed by cutting H/8 from the top and H/4 from the bottom of the original triangle (equilateral triangle). The tooth shape is the basic size with thread.

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(1) Large diameter D or D

Large diameter refers to the diameter of the imaginary cylinder which coincides with the bottom of the inner thread or the top of the outer thread. The large diameter of internal thread is denoted by D; The large diameter of external thread is denoted by D.

(2) Path D1 or D1

Small diameter refers to the diameter of an imaginary cylinder that coincides with the top or bottom of an external thread. The diameter of internal thread is denoted by D1. The path of external thread is denoted by D1.

(3) Middle diameter D2 or D2

Middiameter refers to the diameter of an imaginary cylinder or cone whose busbar passes through the grooves and bumps of the thread profile where the width is equal. This imaginary cylinder is called middiameter cylinder. The bus line of the middle diameter cylinder is called the middle diameter line, and its axis is the thread axis. The basic dimensions of the middle diameter of the combined internal and external threads are equal, that is, D2= D2.

(4) Single middle diameter

Is the diameter of an imaginary cylinder, the width of which the busbar passes through the groove is equal to half of the basic pitch, regardless of the width of the tooth, and is used to represent the actual size of the middle diameter of the thread.

(5) the pitch P

Pitch refers to the axial distance between two adjacent teeth corresponding to two points on the central diameter bus.

(6) Tooth profile half Angle

Half Angle of tooth profile refers to the Angle between the vertical line of thread axis and the upper tooth profile. The half Angle of normal thread is 30°.

(7) Screw length

The screw length refers to the axial length of the contact part of the helical surface when the internal and external threads are screwed.

2. Influence of thread parameters of fasteners on interchangeability

In order to realize the interchangeability of fastener thread connection, it is necessary to ensure good coupling and certain connection strength. There are five main geometric parameters affecting the interchangeability of fastener threads: large diameter, small diameter, middle diameter, pitch and half Angle of tooth profile. These parameters in the process of processing can not avoid to produce a certain processing error, not only affect the fastener screw screw, contact height, fit tightness, but also affect the reliability of the connection, thus affecting the fastener thread interchangeability.

1. Influence of pitch error on interchangeability of fastener threads

Pitch error includes single pitch error and pitch accumulation error. The single pitch error is independent of the spin length, while the cumulative pitch error is related to the spin length or working distance. The cumulative pitch error refers to the maximum difference between the actual distance and the nominal value of any two teeth with the same name within a given length range. The influence of pitch error on thread interchangeability of fasteners is shown in FIG. 2.

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2. Influence of half Angle error of tooth profile

The error of tooth profile is the difference between the actual tooth profile and the theoretical tooth profile. The half-angle deviation of the tooth shape affects the screw quality and the uniformity of thread contact. The half-angle error of tooth profile may be caused by the inaccurate tooth profile Angle itself or the incorrect relative position with the axis, or it may be the result of the combined errors of both.

3. Influence of single median diameter error

In the manufacturing process, there will inevitably be some error, that is, the difference between single central diameter and its nominal central diameter. Single middle diameter error will directly affect the screw coupling and bonding strength. If only the influence of the middle diameter is considered, the screwing of the inner and outer threads can be guaranteed as long as the middle diameter of the outer thread is smaller than the middle diameter of the inner thread. When the middle diameter of the external thread is too small, the fit is too loose, it is difficult to make good contact between the tooth sides, affecting the reliability of the connection and reducing the strength of the connection. When the middle diameter of outer thread is larger than that of inner thread, it will affect the screwing. Therefore, the maximum central diameter of external thread and the minimum central diameter of internal thread should be limited in order to ensure the screw quality. In order to ensure the connection reliability of the thread, the minimum central diameter of the external thread and the maximum central diameter of the internal thread must be limited.

Because the pitch error and tooth type half Angle error are compensated by the middle diameter, the internal thread is equivalent to the middle diameter of the thread smaller, the external thread is equivalent to the middle diameter of the thread becomes larger, the middle diameter after this change is called the role of the middle diameter, that is, the actual role of the thread match in the middle diameter. When there is pitch error and tooth profile half Angle error of external thread, it can only be screwed with an internal thread with a larger middle diameter, which is equivalent to the effect of increasing the middle diameter of external thread. The value is equal to the sum of the compensation values of the actual middle diameter and pitch error of external thread and the half Angle error of tooth profile.

4. The judgment principle of the qualification of middle diameter of action and thread

Judge the thread diameter qualified, according to the thread limit size judgment principle (Taylor principle). That is, the role of internal thread diameter should not be less than the minimum limit size of diameter; The single central diameter shall not be greater than the maximum limit size of the middle diameter, the action of the external thread should not be greater than the maximum limit size of the middle diameter, the single central diameter shall not be less than the minimum limit size of the middle diameter.

5, fastener thread, small impact

In order to ensure the screw, the actual size of the inner thread is larger than the actual size of the outer thread, without affecting the mating and interchangeability. If the small diameter of the inner thread is too large or the large diameter of the outer thread is too small, it will affect the strength of the fastener thread connection, so its tolerance must be specified.


Post time: Oct-12-2021