I. Types of Threads
Threads are divided into two major categories according to their purposes: connecting threads and transmission threads.
1. Connecting Threads
Connecting threads are divided into two types: ordinary threads and pipe threads, mainly used for component connection. There are four commonly used standard threads, namely: coarse-pitch ordinary threads, fine-pitch ordinary threads, pipe threads, and tapered pipe threads.
① The thread form of ordinary threads is an equilateral triangle (thread angle is 60°). The difference between fine-pitch and coarse-pitch threads is that under the same major diameter, the pitch of fine-pitch threads is smaller than that of coarse-pitch threads.
② The thread form of pipe threads and tapered pipe threads is an isosceles triangle (thread angle is 55°). Pipe threads are mainly used for the connection of water pipes, oil pipes, gas pipes and other pipelines. Pipe threads are divided into cylindrical pipe threads and tapered pipe threads, both of which are in inches, and the pitch is expressed by the number of threads within a 25.4mm thread length.
Pipe threads are further divided into:
● Non-sealed pipe threads (G): Pipe thread taps are used for internal thread processing, and dies are used for external thread processing;
● Sealed pipe threads (R): High precision is required, and there are two fitting methods: cylindrical internal threads and tapered external threads form a "cylinder/taper" fit; tapered internal threads and tapered external threads form a "taper/taper" fit.
(1) The size of the pipe thread is an approximate value of the pipe inner diameter, not the pipe outer diameter. For example, 1/2 inch corresponds to DN15.
(2) The thickness of the pipe thread form is expressed by the number of threads per inch, and the converted pitch is a decimal. For example, a G1 inch pipe thread has 11 threads along the axis, and its pitch is 25.4 ÷ 11 ≈ 2.309mm. Pipe threads are mostly used for the connection of pipe fittings and thin-walled parts, with small pitch and thread form size.
● Metric threads are expressed by pitch, while American and British threads are expressed by the number of threads per inch.
● Metric threads have a 60° equilateral thread form, British threads have a 55° isosceles thread form, and American threads have a 60° isosceles thread form.
Note: Insiders usually use "fen" to refer to thread size. 1 inch is equal to 8 fen, 1/4 inch is 2 fen, and so on (e.g., 1/2 inch is 4 fen, 3/4 inch is 6 fen).
2. Transmission Threads
Transmission threads are used to transmit power or motion, and there are four commonly used standard threads:
1) Trapezoidal threads: The thread form is an isosceles trapezoid with a thread angle of 30°, which is the most commonly used transmission thread. Compared with rectangular threads, its transmission efficiency is slightly lower, but it has good processability, high root strength and good centering performance. The lead screw of machine tools uses trapezoidal threads to transmit power bidirectionally, and the thread code is Tr.
2) Sawtooth threads: A type of transmission thread that bears unidirectional force. The thread form is an isosceles trapezoid, one side forms an angle of 30° with the vertical line, and the other side forms an angle of 3°, forming a thread angle of 33°, with the thread code B. It is only used to bear unidirectional power. Due to its higher transmission efficiency and strength than trapezoidal threads, it is often used in unidirectional force-bearing mechanisms such as screw presses and hydraulic presses.
3) Rectangular threads: Mainly used for force transmission. Its characteristic is that the transmission efficiency is higher than other threads, but the processing difficulty is large and the root strength is low, so its application is limited.
4) Module threads: Also known as worm gear threads, with a thread angle of 40°, which has the characteristics of large transmission ratio, compact structure, stable transmission and good self-locking performance, mainly used in reduction devices.
II. Mechanical Properties of Bolts
1. Grades: The strength grades of metric bolts mainly include 10 performance grades: 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, 12.9.
Distinction and meaning of high-strength bolts: Bolts of grade 8.8 and above are collectively referred to as high-strength bolts, and the remaining grades are called ordinary-strength bolts.
2. Meaning of bolt performance grade marking: The bolt performance grade marking consists of two parts of numbers, representing the nominal tensile strength value and yield ratio of the bolt respectively. For example, the meaning of a bolt with performance grade 4.8 (Note: Grade 4.8 is an ordinary-strength bolt, not a high-strength bolt) is:
(1) The nominal tensile strength of the bolt material is 400MPa grade;
(2) The yield ratio of the bolt material is 0.8;
(3) The nominal yield strength of the bolt material is 400×0.8=320MPa grade.
3. The mechanical performance grade of bolts mainly has the following four indicators:
a. Strength indicators (tensile strength, yield point, yield strength, guaranteed stress);
b. Hardness indicators (Vickers hardness, Brinell hardness, Rockwell hardness, surface hardness);
c. Plasticity and toughness indicators (elongation, wedge load strength, impact absorption energy, head firmness);
d. Decarburization layer indicators (minimum height of non-decarburized layer of thread, maximum depth of full decarburized layer).
4. Noun Explanation
1) Tensile strength (σb) (N/mm²): The maximum tensile force that a product can bear per unit area, referring to the maximum stress that a metal material can bear before breaking.
2) Guaranteed load (SP) (N/mm²): According to the grade and specification of the product, a certain load is applied to it for a certain period of time, and the product can bear it without any measurable permanent deformation.
3) Yield point (σs) (N/mm²): The point where the strain increases but the stress does not increase when the material is stretched. In the tensile curve of general low-strength products, an obvious yield point can be displayed, which is the boundary between elastic deformation and plastic deformation of the material; in the tensile curve of high-strength products, there is no obvious yield point. When the yield point cannot be measured, it is allowed to use the method of measuring yield strength instead.
4) Definition of yield strength: It is the yield limit when a metal material undergoes yield phenomenon, that is, the stress that resists micro-plastic deformation. For metal materials without obvious yield phenomenon, it is specified that the stress value that produces 0.2% residual deformation is its yield limit, which is called conditional yield limit or yield strength. External force exceeding this limit will cause permanent failure of the part, which cannot be recovered. For example, the yield limit of low-carbon steel is 207MPa. When the external force exceeds this limit, the part will produce permanent deformation; when it is less than this limit, the part can return to its original shape.
Remarks:
a. Material deformation is divided into elastic deformation (can return to original shape after external force is removed) and plastic deformation (cannot return to original shape after external force is removed, and the shape changes, such as elongation or shortening).
b. When the stress exceeds the elastic limit, it enters the yield stage, and the deformation increases rapidly. At this time, in addition to elastic deformation, part of plastic deformation will also occur. When the stress reaches the yield point, the plastic strain increases sharply, and slight fluctuations occur in stress and strain. This phenomenon is called yield. The maximum and minimum stresses in this stage are called upper yield point and lower yield point respectively.
Since the value of the lower yield point is relatively stable, it is used as an indicator of material resistance, called yield point or yield strength (ReL or Rp0.2).
5) Hardness: The ability of a metal material to resist the indentation of a harder object is called hardness. It is a comprehensive physical quantity of material performance, indicating the ability of a metal material to resist elastic deformation, plastic deformation or fracture within a small volume (common indicators: Vickers hardness HV30, Brinell hardness HB, Rockwell hardness HRB and HRC, surface hardness HV0.3).
6) Wedge load strength: Apply a wedge load test to hexagon head, square head (four-corner), hexagon flange face or socket head cap bolts, that is, test the tensile strength of the product after adding a wedge block under the head, aiming to detect the tensile strength of the product and its head firmness.
7) Elongation (δ): The elongation of a product is the ratio of the elongation after fracture to the original length before fracture.
① Yield point: The stress at which the sample can continue to elongate (deform) without increasing the force (keeping constant) during the test.
② Upper yield point: The maximum stress before the force first decreases when the sample yields.
③ Lower yield point: The minimum stress in the yield stage when the initial transient effect is not considered.
Some steels (such as high-carbon steel) have no obvious yield phenomenon. Usually, the stress at which micro-plastic deformation (0.2%) occurs is taken as the yield strength of the steel, which is called conditional yield strength.
8) Head firmness: Install the product into a support with an inclined hole, and strike the product head. For full-thread bolts or screws, as long as no head-off occurs, even if cracks appear on the first thread, it shall be regarded as meeting the requirements of this test; for half-thread products, no cracks shall be generated at the head, the supporting surface and the transition fillet between the supporting surface and the screw rod. According to GB/T 3098.1, this test shall be carried out for bolts and screws with specification ≤ M16 and too short length to conduct the wedge load test.
