Basic knowledge of fasteners

1m =100 cm=1000 mm

Chapter 1 Measurement

There are two main units of length measurement in the world today, one is the metric system, the unit of measurement is meter (m), centimeter (cm), millimeter (mm), etc., which is used more in Europe, China and Japan and other Southeast Asian regions, and the other is the British system, the unit of measurement is mainly inch (inch), which is equivalent to the city inch of China's old system, which is used more in the United States, Britain and other European and American countries.

1. Metric measurement: (base 10)

1m =100 cm=1000 mm

2. Imperial measurement: (base 8)

3, less than 1/4 of the product with the designation to indicate its size,

For example: 4#, 5#, 6#, 7#, 8#, 10#, 12#

Chapter Two: Spiral

First, the thread is a uniform helix convex shape on the cross-section of the solid outer surface or the inner surface. According to its structural characteristics and uses can be divided into three categories:

(1) Ordinary thread: the tooth shape is triangular, used to connect or tighten parts. Ordinary threads are divided into coarse and fine threads according to the pitch, and the connection strength of fine threads is higher.

(2) Transmission thread: tooth shape has trapezoid, rectangle, saw shape and triangle, etc.

(3) Sealing thread: used for sealing connection, mainly pipe thread, taper thread and taper pipe thread.

Second, screw fit grade:

Thread fit is the size of the slack or tightness between the screw threads, and the grade of the fit is the specified combination of deviations and tolerances acting on the internal and external threads.

(1) For unified inch threads, there are three screw grades for external threads: 1A, 2A and 3A grades, and three grades for internal threads:

1B, 2B and 3B, all of which are gap-fit. The higher the grade number, the tighter the fit. In inch threads, the deviation is specified only for class 1A and 2A, the deviation for class 3A is zero, and the grade deviation for class 1A and class 2A is equal.

The greater the number of grades, the smaller the tolerance, as shown in the figure:

1, 1A and 1B, very loose tolerance grades, are suitable for tolerance fit of internal and external threads.

2, 2A and 2B are the most common thread tolerance classes specified in the British series of mechanical fasteners.

3, 3A and 3B, screw together to form the tightest fit and are suitable for fasteners with tight tolerances for safety-critical designs.

4. For external threads, 1A and 2A grades have a mating tolerance, 3A grades do not. Class 1A tolerances are 50% greater than class 2A tolerances, 75% greater than class 3A tolerances, and Class 2B tolerances are 30% greater than 2A tolerances for internal threads. Class 1B is 50% larger than Class 2B and 75% larger than class 3B.

(二) Metric thread, external thread has three thread grades: 4h, 6h and 6g, internal thread has three thread grades: 5H, 6H, 7H. (The precision grade of daily thread is divided into I, II, III, and usually II) In metric threads, the basic deviation of H and h is zero. The basic deviation of G is positive, and the basic deviation of e, f, and g is negative. As shown in the picture:

1, H is the commonly used tolerance zone position of the internal thread, generally not used as a surface coating, or with an extremely thin phosphating layer. The basic deviation of G position is used for special occasions, such as thicker coatings, which are generally rarely used.

2, g is commonly used to plate 6-9um thin coating, such as the product drawing requirements are 6h bolts, the thread before plating using 6g tolerance zone.

3, the thread is best combined into H/g, H/h or G/h, for bolts, nuts and other refined fastener threads, the standard recommends 6H/6g.

3.the main geometric parameters of mechanical threads:

(1), large diameter/outer diameter of the tooth (D, d) : is the imaginary cylindrical diameter of the top of the external thread or the bottom of the internal thread. The main diameter of the thread basically represents the nominal diameter of the thread size.

(2), middle diameter (D2, d2) : D2=d2=D (d) -2x3H/8, where H is the original triangle height:

H= (√ 3/2) P=0.866025P(60O tooth hill Angle); H=0.960491P(55 O tooth hill Angle)

(3), small path/tooth base diameter (D1, d1) : refers to the diameter of the imaginary cylinder where the top of the external thread or the top of the internal thread coincide.

(4) Pitch (P) : refers to the axial distance of two points corresponding to adjacent teeth on the median diameter line or the distance between adjacent dental hills or two adjacent dental valleys. In the imperial system, tooth spacing is indicated by the number of teeth per inch (25.4 mm) (table below).