First, the mechanical performance level
China has provisions for bolts, screws and studs of different mechanical performance grades.
Self-tapping screws and self-tapping screws
Ordinary self-tapping screws are not classified according to mechanical properties. As long as the appropriate specifications are selected, they can meet their general requirements. The mechanical performance grade of self-tapping locking screws is divided into two levels, A and B. Class A is a high strength class; Class B is a low strength class.
Second, the thread grade
Thread is a shape with uniform spiral protrusion on the outer or inner surface of the solid, and the fit of the threaded fastener refers to the tightness between the threads that are screwed together, the grade refers to the internal thread and A combination of deviation and tolerance on the external thread.
For inch threads, the external thread has the following three grades: 1A, 2A, 3A. The internal thread has the following three grades: 1B, 2B, and 3B, all of which belong to the clearance fit. The higher the grade, the tighter the fit. For inch threads, the 3A deviation is 0, and the 1A and 2A deviations are the same.
Classes 1A and 1B are very loose tolerance grades and are suitable for tolerance fit between external and internal threads; Class 2A and 2B are the most common tolerance classes for inch mechanical fasteners; Class 3A and 3B are capable of spinning The tightest fit is typically used for tight tolerance fasteners and a critical safety design.
There is a tolerance between the 1A and 2A grades of the external thread, and the 3A grade is not. In general, the tolerance of the 1A grade is 50% larger than the 2A grade and 75% larger than the 3A grade; The 2B level tolerance is 30% larger than the 2A level, the 1B level is 50% larger than the 2B level, and 75% larger than the 3B level.
Determine specifications
1 Diameter specifications Try to select the first series of values, as far as possible not to use the values in the brackets in the product standard size table and the values in the specification table that should not be used.
2 Length specifications generally do not use specifications other than those specified in the product standard. When the bolt or nut is normally connected, the length of the thread end is preferably (0.2-0.3) d.
3 The size and length of the matching specifications should be preferred within the specifications of the product specifications.
