Common sense sharing of threaded fasteners

Thread is a shape with uniform spiral protrusions on the cross-section of the solid outer or inner surface, and the fit of threaded fasteners refers to the degree of tightness between the threads that are screwed together, and the grade refers to the internal thread and the The combination of deviations and tolerances on external threads.

For inch threads, there are three levels of external threads: 1A, 2A, and 3A. There are three levels of internal threads: 1B, 2B, and 3B, all of which belong to clearance fits. The higher the level, the tighter the fit. For inch threads, the deviation of 3A grade is 0, and the deviation of 1A and 2A is the same. (Guide: Material selection requirements for high-strength bolts)

And 1A and 1B are very loose tolerance levels, suitable for the tolerance matching between external and internal threads; 2A and 2B are the most common tolerance levels for inch mechanical fasteners; 3A and 3B, Can be screwed into the tightest fit, generally used for fasteners with tight tolerances, and a key safety design.

There is a fit tolerance between 1A and 2A of external thread, and 3A does not. Generally, the tolerance of 1A is 50% larger than that of 2A and 75% larger than 3A. ; The tolerance of internal thread 2B is 30% larger than 2A, 1B is 50% larger than 2B, and 75% is larger than 3B.

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