Abstract
When the bolt is tightened, the rotational force working on the nut is transferred to the flank within the engagement length and transformed to the longitudinal bolt force. It has been recognized, however, it has not been defined as a calculation formula so far. Somebody says it is equivalent to four threads, the other says fifty percent of the nominal bolt size. The modern bolt tightening method measures the bolt elongation during bolt tightening by means of ultrasonic equipment and controls the longitudinal bolt force. The elongation of bolt is calculated based on the bolt length including the engagement length of bolt and nut. Therefore, accurate estimation of the engagement length leads to achieve the correct bolt longitudinal force. Focusing on the unified coarse thread exceeding one inch in nominal size in eight threads series, how to estimate the correct engagement length of bolt and nut during tightening based on the tensile experiment data of 1 1/4-8un will be described.
