Reason for Low Tight Axle Strength of 10.9S Sets of Torshear Type High Strength Bolt for Steel Structures

The reason of low tight axle strergth of the M20 mm×80 mm 10.9S sets of torshear type high strength bolt used for the steel structure was analyzed by means of test for chemical composition,hardness,wedge load,assured load and tlght axle strength. The results showed that the quality of the sets of bolt was eligible,but under the effect of dry circumstance,the phosphide film on the blot and nut was airslaked,the lnbricative capability was reduced and the friction in axes direction was increased that led to the low tight axle strength.