A METHOD AND APPARATUS FOR DETERMINING WHEN A THREADED FASTENER HAS BEEN TIGHTENED TO A PREDETERMINED TIGHTNESS

摘要

Apparatus (25) for determining when a fastener has been tightened to a predetermined tightness level by an impact wrench (1 ) comprises determining the number of hammer impacts to advance an anvil (9) of the impact wrench (1 ) through a predetermined angle and comparing the number of hammer impacts with a predetermined number of hammer impacts. Initially, the cumulative angular displacement of the anvil (9) from the first detected hammer impact is determined from the cumulative angular position of the hammer (5) for the corresponding hammer impact from the first detected hammer impact. The cumulative angular displacement of the hammer (5) is determined when the rotational speed of the hammer (5) is zero, at a minimum or approaching zero just prior to the direction of rotation of the hammer (5) reversing at the corresponding hammer impact. On determining the cumulative angular displacement of the hammer (5) at each hammer impact, the cumulative angular displacement of the anvil (9) at the corresponding hammer impact is determined by computing the difference between the cumulative angular displacements of the hammer (5) at the corresponding hammer impact and at the previous hammer impact and subtracting an angle through which the hammer (5) freely rotates between each hammer impact from the computed difference. The result is added to the cumulative angular displacement of the anvil (9) computed for the previous hammer impact.