1. What are fasteners?
Fasteners are a general term for a type of mechanical parts used to fasten two or more parts (or components) into a whole. They are also called standard parts in the market.
2. It usually includes the following 12 types of parts:
Bolts, studs, screws, nuts, self-tapping screws, wood screws, washers, retaining rings, pins, rivets, assemblies and connecting pairs, and welding nails.
Identification and inspection of threads
1. Uses and characteristics of threads:
Threads are widely used, from airplanes and cars to water pipes and gas used in our daily lives. Most threads play the role of fastening connections, followed by the transmission of force and motion. There are also some special-purpose threads, which are of many types but limited in number.
The reason why threads can be used for a long time is that they have the characteristics of simple structure, reliable performance, convenient disassembly, and easy manufacturing, making them an indispensable structural element in various electromechanical products today.
According to the use of threads, various threaded parts should have the following two most basic functions: one is good screwing; the other is sufficient strength.
2. Classification of threads
a. According to their structural characteristics and uses, they can be divided into four categories:
- Ordinary threads (fastening threads): The tooth shape is triangular and is used to connect or fasten parts. Ordinary threads are divided into coarse and fine threads according to the pitch. The connection strength of fine threads is higher.
- Transmission thread: The tooth profiles include trapezoid, rectangle, saw and triangle.
- Sealing thread: used for sealing connection, mainly pipe thread, tapered thread and tapered pipe thread.
- Special-purpose thread, referred to as special thread.
b. Threads can be divided into metric thread (metric thread), British thread, American thread, etc. according to region (country). We usually refer to British thread and American thread as British thread. Its tooth angle is 60°, 55°, etc., and the diameter and pitch and other related thread parameters are in British size (inch). In our country, the tooth angle is unified to 60°, and the diameter and pitch series in millimeters (mm) are used to name this type of thread: ordinary thread.
3. Ordinary thread tooth profile
4. Basic terminology of thread
- Thread: On the surface of a cylinder or cone, a continuous protrusion with a specified tooth profile formed along the spiral line.
- External thread: A thread formed on the outer surface of a cylinder or cone.
- Internal thread: an internal thread formed on the inner surface of a cylinder or cone.
- Major diameter: the diameter of an imaginary cylinder or cone tangent to the external thread crest or internal thread root.
- Minor diameter: the diameter of an imaginary cylinder or cone tangent to the external thread root or internal thread crest.
- Center diameter: the diameter of an imaginary cylinder or cone whose generatrix passes through the place where the groove and the protrusion on the tooth profile have equal widths. This imaginary cylinder or cone is called a center diameter cylinder or center diameter cone.
- Right-hand thread: a thread that is screwed in when rotating clockwise.
- Left-hand thread: a thread that is screwed in when rotating counterclockwise.
- Tooth profile angle: the angle between two adjacent tooth sides on the thread tooth profile.
- Pitch: the axial distance between two points corresponding to two adjacent teeth on the center diameter line.
5. Thread measurement
For general standard threads, thread ring gauges or plug gauges are used for measurement.
Due to the large number of thread parameters, it is impossible to measure each parameter of the thread one by one. Usually, we use thread gauges (thread ring gauges, thread plug gauges) to make comprehensive judgments on the threads. This inspection method belongs to the acceptance method of simulated assembly, which is not only convenient and reliable, but also comparable to the accuracy requirements of ordinary threads. Therefore, it has become the most common acceptance method used in actual production.
6. Thread measurement (middle diameter)
In threaded connections, only the middle diameter size determines the nature of thread fit, so how to correctly judge whether the middle diameter is qualified is very critical. Based on the goal that the middle diameter size should ensure that the most basic performance of the thread can be achieved, the standard stipulates the judgment principle of the middle diameter qualification, namely: "The effective middle diameter of the actual thread cannot exceed the middle diameter of the largest solid tooth profile. And the single middle diameter of any part of the actual thread cannot exceed the middle diameter of the smallest solid tooth profile."
There are currently two more convenient methods for measuring a single middle diameter. One is to measure the middle diameter with a thread middle diameter micrometer, and the other is to measure it using the three-needle method (our company uses the three-needle method).
7. Thread fit grade:
Thread fit is the size of looseness or tightness between screwed threads. The fit grade is the specified combination of deviation and tolerance acting on internal and external threads.
(1) For unified British threads, there are three thread grades for external threads: 1A, 2A and 3A, and three grades for internal threads: 1B, 2B and 3B, all of which are clearance fits. The higher the grade number, the tighter the fit. In the imperial thread, the deviation is only specified for 1A and 2A grades, the deviation of 3A grade is zero, and the grade deviation of 1A and 2A grades is equal.
The larger the grade number, the smaller the tolerance, as shown in the figure:
1) Grades 1A and 1B are very loose tolerance grades, which are suitable for tolerance fit of internal and external threads.
2) Grades 2A and 2B are the most common thread tolerance grades specified for imperial series mechanical fasteners.
3) Grades 3A and 3B form the tightest fit when screwed together, which are suitable for fasteners with tight tolerances and are used for critical designs for safety.
4) For external threads, grades 1A and 2A have a fit deviation, while grade 3A does not. The tolerance of grade 1A is 50% larger than that of grade 2A and 75% larger than that of grade 3A. For internal threads, the tolerance of grade 2B is 30% larger than that of grade 2A. Grade 1B is 50% larger than that of grade 2B and 75% larger than that of grade 3B.
(2) Metric threads, external threads have common thread grades: 4h, 6e, 6g and 6h, internal threads have common thread grades: 6G, 6H, 7H. (Japanese standard thread accuracy grades are divided into three levels: I, II, and III, usually II) In metric threads, the basic deviation of H and h is zero. The basic deviation of G is a positive value, and the basic deviations of e, f and g are negative values. As shown in the figure:
1) H is the common tolerance zone position for internal threads, which is generally not used for surface coating, or with a very thin phosphating layer. The basic deviation of the G position is used for special occasions, such as thicker coatings, and is generally rarely used.
2) g is often used to plate a thin coating of 6-9um. For example, if the product drawing requires a 6h bolt, the thread before plating uses a 6g tolerance zone.
3) The best combination of thread fit is H/g, H/h or G/h. For refined fastener threads such as bolts and nuts, the standard recommends the use of 6H/6g fit.
