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Difference Between Root Area and Tensile Stress Area of a Threaded Fastener



The effective cross-sectional area of the threaded rod that resists rod fracture is the tensile stress area. It has been observed during the testing of threaded rod that an unthreaded rod, having a diameter equal to the mean of the pitch diameter and the minor diameter has the same tensile strength as the threaded rod. This cross-sectional area called the tensile stress area is used for the purposes of calculating the tensile strength of the rod.



For metric series, the tensile stress area (for steel) = (PI/4)*(D-0.938194P)2

For inch series, the tensile stress area (for steel) = (PI/4)*[D-(0.9743/n)]2

where

D = nominal diameter, P = thread pitch, n = threads per inch, PI = 3.1416

Don't be confused with Root area - a more conservative stress area that is still widely used such as in ASME B31.1 code. Root area is based on the root or minor diameter of the threads, and therefore its stress area is smaller than the tensile stress area. Root area is not based on experimental data. It is designed to introduce a factor of safety in thread strength calculations. The designer pruposely assumes a "root" stress area smaller than the "real" tensile stress area to be sure that the rod isn't overstressed in service.

For metric series, the root area = (PI/4)*(D-1.3P)2

For inch series, the root area = (PI/4)*[D-(1.3/n)]2

ASTM A193

Originally approved in 1936, this specification is heavily utilized in petroleum and chemical construction applications. The ASTM standard covers alloy steel and stainless steel bolting materials for high temperature service. This specification includes fasteners intended for use in pressure vessels, valves, flanges, and fittings. Although, this material is often available in national coarse (UNC) thread pitches, if being used in traditional applications, threads are specified 8 threads per inch (tpi) for diameters above one inch.

Below is a basic summary of a few of the common grades. ASTM A193 covers a number of other standard specifications not covered in this description including B5, B6, and B16.




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