Design considerations of reliable screw joints

Pitch P, angle of thread 60º, pitch diameter d2=D2, major diameter of external thread d,  minor diameter of internal thread d3, major diameter of internal thread D, minor diameter of internal thread D1, root radius R, height of fundamental triangle H.

The critical areas of stress of screwed fasteners are:

The effective cross section area, or tensile area.
The shear area of the external thread (screw, bolt)
The shear area of the internal thread (nut, tapped hole)

 Case: Effective length of thread engagement Problem: Calculate the required length of engagement of a tapped hole in a casting of spheroidal graphite cast iron. The thread strength must be able to carry the full load at which a M12-10.9 bolt fails.   Given: The tolerance class of the dimensions of a standard nut are assumed to be representative for the tapped hole. The ratio of shear strength to tensile strength of the SG cast iron is taken as τ/σ=0.9, where the tensile strength is σ=500 MPa.   Solution: The ratio of shear strength of the bolt and the tapped hole is 0.58 1000 / 0.9 500 = 1.3. The required length of thread engagement becomes 1.3 0.8d = 12.5 mm.   Accurately calculating the required thread engagement length with a sophisticated program available at www.boltscience.com , in which a large number of influence factors are considered, finally results in a required effective thread engagement length of 12.9 mm.A generally used practical calculation method for the tread stripping strength is given by the formula Ath=0.5πd0L0. where d0=(d2+d3)/2. Applying this formula results in the effective length of thread engagement of L0=11.5mm.Note that the shear strength of a metric fine thread exceeds that of a standard metric thread because of the slightly larger d0-value (approximately 6%).

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