This 5/16-18 x 5/8 Grade 5 unslotted hex washer screw has a thick flange and serrations on the bearing surface side. Designed for use in applications where loosening forces are present (vibration), the serrations grip to the bearing surface so that significantly more torque is needed to loosen than to tighten the screw. The flange provides more strength than a standard machine screw and eliminates the need for a lock washer or locknut. These are heat treated, quenched and tempered at a minimum temperature of 650 degrees (F) with a Proof Load of 85,000 PSI and Tensile Strength of 120,000 PSI.