Threaded fastener fit level knowledge

A thread is a shape with uniform spiral protrusions on the cross-section of the solid outer or inner surface. The fit of threaded fasteners refers to the degree of tightness between the screwed threads, and the grade refers to the combination of deviations and tolerances on internal and 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: Analyze the reasons for the fracture of connecting rod bolts of diesel engines)

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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