A mark on the face or flats of a nut may be harmless, or it may be a forging crack or seam that weakens it. When a visible discontinuity on an ASTM A194 nut is in dispute, the cone proof load test settles it. The nut bears on a hardened 120° cone that presses into it around the bore, pushing the nut outward while its threads are loaded: ASTM F606 calls this simultaneous dilation and stripping. Because the cone concentrates the load, the test load is the plain proof load times (1 − 0.30 × the diameter in inches): for a 1"-8 2H heavy hex nut, 175 ksi × 0.606 in² × 0.70, or 74,200 lbf, against 106,000 lbf in the plain test. The nut must carry it for at least 10 seconds without stripping or rupture.
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model dataCone proof stress from ASTM A194 Table 5 (2H heavy hex); cone proof load from the ASTM F606 formula and ASTM A962 Table 1; mandrel, conical washer, loading speed, hold time and acceptance from ASTM F606 and ASTM A962. Tensile stress area from the ASME B1.1 formula; nut drawn to ASME B18.2.2 heavy hex dimensions. The test machine, fixture and grip are generic.
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