From... Guide to Design Criteria for Bolted and Riveted Joints by Kulak et al...
Experience with the one full turn method indicated that it was impractical to use finger or hand tightness as a reliable point for starting the one turn. Because of out-offlatness, thread imperfections, and dirt accumulation, it was difficult and time consuming to determine the hand-tight position. Bethlehem Steel Corporation
developed a modified “turn-of-nut” method, using the AAR studies and additional tests of their own.4.16, 4.17 This method called for running the nut up to a snug position using an impact wrench rather than the fingertight ondition. From the snug position the nut was given an additional ½ or ¾ turn, depending on the length of the bolt. The snug condition was defined as the point at which the wrench
started to impact. This occurred when the turning of the nut was resisted by friction between the face of the nut and the surface of the steel. Snug-tightening the bolts induces small clamping forces in the bolts. In general, at the snug-tight condition the bolt clamping forces can vary considerably because elongations are still within the elastic range. This is illustrated in Fig. 4.18 where the range of bolt clamping force and bolt elongation at the snug tight condition is shown for 7/8 in. dia. A325 bolts installed in an A440 steel test joint. The average clamping force at the snugtight condition was equal to about 26 kip. The bolts in this test joint were snug tightened by means of an impact wrench. This modified turn-of-nut method was
eventually incorporated into the 1960 specification of the council.
For bolts equal to or greater than about ¾ in. dia., snug position provided by an impact wrench is approximately equal to the tightness attained by the full effort of a man using an ordinary spud wrench. For longer or larger diameter bolts, the force produced by this snug load will be less than that for the “standard” case, and for shorter or smaller diameter bolts it will be more. These differences are accommodated in the specification by prescribing the same definition of snug tight...
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