The Influence Of Fastener Thread Parameters And Parameter Errors On Interchangeability

1, Basic tooth profile of ordinary threads for fasteners

The geometric parameters of fastener threads depend on the basic tooth shape within the axial profile of the fastener thread. The basic tooth shape is the theoretical tooth shape shared by the inner and outer threads formed by cutting off H/8 at the top and H/4 at the bottom of the original triangle (equilateral triangle). This tooth profile has the basic dimensions of a thread.

Figure 1 Basic tooth profile of ordinary thread for fasteners

(1) Major diameter D or d

Major diameter refers to the hypothetical cylindrical diameter that coincides with the inner thread base or outer thread top. The major diameter of the internal thread is represented by D; The major diameter of the external thread is represented by d.

(2) Trail D1 or d1

Minor diameter refers to the hypothetical cylindrical diameter that coincides with the top or bottom of the internal or external thread. The minor diameter of the internal thread is represented by D1; The minor diameter of the external thread is represented by d1.

(3) Mid diameter D2 or d2

Mid diameter refers to the diameter of an imaginary cylinder or cone, whose generatrix passes through the groove and protrusion width on the thread profile, and this imaginary cylinder is called a mid diameter cylinder. The generatrix of a diameter cylinder is called the diameter line, and its axis is the thread axis. The basic dimensions of the combined inner and outer thread diameters are equal, that is, D2=d2.

(4) Single pitch diameter

For an imaginary cylinder diameter, the generatrix of the cylinder passes through a point where the width of the tooth groove is equal to half of the basic pitch, regardless of the size of the tooth width, to represent the actual size of the thread pitch.

(5) Pitch P

The pitch refers to the axial distance between two adjacent teeth on the corresponding two points on the radial axis.

(6) Half angle of tooth shape

The tooth profile half angle refers to the angle between the tooth side on the thread profile and the vertical line of the thread axis. The half angle of the tooth profile of a regular thread is 30 °.

(7) Thread turning length

The thread turning length refers to the axial length of the contact part of the spiral surface during the internal and external thread turning.

2, The influence of fastener thread parameters on interchangeability

To achieve interchangeability in threaded connections of fasteners, it is necessary to ensure good screw fit and a certain degree of connection strength. There are five main geometric parameters that affect the interchangeability of fastener threads: major diameter, minor diameter, pitch, and tooth profile half angle. These parameters, which are not applicable during the machining process, inevitably result in certain machining errors. They not only affect the screw fit, contact height, and fit tightness of the fastener thread, but also affect the reliability of the connection, thereby affecting the interchangeability of the fastener thread.

1. The influence of pitch error on the interchangeability of fastener threads

Pitch error includes both individual pitch error and cumulative pitch error. The individual pitch error is independent of the screw length, while the cumulative pitch error is related to the screw length or working distance. The cumulative error of pitch refers to the maximum difference between the actual distance between any two teeth with the same name and the nominal value within a given length range. The influence of pitch error on the interchangeability of fastener threads is shown in Figure 2.

Figure 2 Influence of pitch error on the interchangeability of fastener threads

2. The influence of tooth profile half angle error

Tooth shape half angle error refers to the difference between the actual tooth shape half angle and the theoretical tooth shape half angle. The deviation of the half angle of the tooth shape affects the screw fit and the uniformity of the thread contact. The half angle error of the tooth shape may be caused by the inaccuracy of the tooth shape angle itself or its incorrect relative position with the axis, or it may be the result of the combined error of the two.

3. The impact of single pitch diameter error

During the manufacturing process, there is inevitably an error in the pitch diameter of a thread, which is the difference between a single pitch diameter and its nominal pitch diameter. A single pitch diameter error will directly affect the screw fit and bonding strength of the thread. If only the influence of pitch diameter is considered, then as long as the pitch diameter of the external thread is smaller than that of the internal thread, the rotational fit of the internal and external threads can be guaranteed. When the pitch diameter of the external thread is too small, the fit is too loose, making it difficult to achieve good indirect contact on the tooth side, affecting the reliability of the connection and reducing the connection strength. When the pitch diameter of the external thread is greater than that of the internal thread, it will affect the fit. Therefore, in order to ensure the screw fit of the thread, the maximum pitch diameter of the external thread and the minimum pitch diameter of the internal thread should be limited; To ensure the reliability of thread connections, it is also necessary to limit the minimum pitch diameter of external threads and the maximum pitch diameter of internal threads.

Due to the compensation of pitch error and tooth profile half angle error using pitch diameter, for internal threads, it is equivalent to a decrease in thread pitch diameter, and for external threads, it is equivalent to an increase in thread pitch diameter. The pitch diameter after this change is called the effective pitch diameter, which is the actual effective pitch diameter in the thread fit. When there is pitch error and tooth profile half angle error in the external thread, it can only be screwed onto an internal thread with a larger pitch diameter, which is equivalent to increasing the pitch diameter of the external thread. Its value is equal to the sum of the actual pitch diameter and pitch error of the external thread, as well as the pitch compensation value of the tooth profile half angle error.

4. Principles for determining the qualification of the effective pitch diameter and thread pitch diameter

Determine the qualification of the thread pitch diameter based on the principle of determining the limit size of the thread (Taylor principle). The effective pitch diameter of the internal thread should not be less than the minimum limit size of the pitch diameter; The single pitch diameter should not exceed the maximum limit size of the pitch diameter, the effect of the external thread on the pitch diameter should not exceed the maximum limit size of the pitch diameter, and the single pitch diameter should not be less than the minimum limit size of the pitch diameter.

5. The influence of large and small diameters on fastener threads

The manufacturing of fastener threads ensures that the actual size of the internal thread's major and minor diameters is greater than the actual size of the external thread's major and minor diameters, without affecting fit and interchangeability. If the minor diameter of the internal thread is too large or the major diameter of the external thread is too small, it will affect the strength of the threaded connection of the fastener, so its tolerance must be specified.