一个螺纹孔需要多少螺纹啮合？

是否可以确定一个螺纹孔所需的最小螺纹啮合，以防止发生螺纹滑扣？

每个螺纹接头都必须遵循一个简单的工程设计规则：any fastener failure must occur in the shank of the male fastener – bolt or stud. It implies that the shear strength of both internal and external threads overcomes the tensile strength of the bolt. Therefore the choice of the fasteners and the design of the assembly must avoid any possibility of thread stripping while making sure the weakest element in the joint is the bolt or the stud.

The shear resistance of any thread can be calculated by following E.M. Alexander’s ­method¹, depending on four main para­meters:

• The Young modulus E of the material
• The shear strength tm of the material
• The shear area of the thread, related to both the clearance between internal and external threads, and the effective thread engagement which is equal to the depth of tapped holes, minus the height of ­entry chamfers.

Nevertheless, tolerance can mod­ify the previous indications: the wider the thread clearance, the smaller the thread stripping load. Calculation tools according to Alexander’s method can ­deliver accurate results. 

¹ Analysis and design of threaded assemblies, E.M. Alexander, 1977