WINTER 2014

170 THE DISTRIBUTOR’S

170 THE DISTRIBUTOR’S LINK FASTENER FAILURES continued from page 168 In this case, the cap screws were not torqued evenly, in a criss-cross pattern nor in a sequence. Other failure clues will involve assembly abuse. By looking at all of the parts, we can piece together the details of what happened. The following is a head failure which occurred during installation of an A307A fastener. Again, it is unusual for these low carbon steel fasteners to fail but this was of the correct steel composition, unlike the A307B. The picture shows and air gap between the threads indicating the fastener experienced yield. If you do not have a thread gauge, another method would be to take a new fastener of the same diameter and thread pitch and lay the threads along the entire thread length of the two fasteners. If they do not match, the fastener in question has been stretched beyond its yield point. From examining the nut we can observe that one side of the nut is normal, while the obverse was the side against the joint. This side is severely swiped, more so than when just using a hand wrench. The only clear assembly method used here was a power wrench that caused the high speed spinning marks. The head fracture was completely ductile, as evidenced by the ‘cups and cones’ appearance of the fracture surface. The hardness was also within specification. By looking at the shank of the bolt, a small gouge can be observed. This was due to the fastener spinning, which was then caught by a burr from the hole which caused the deeper impression. When taking a closer look at the wrenching corners of the nut, it is clearly evident that a power wrench was used for assembly. There are indentation markings on the corners of the nut in the ‘on’ position. There was some back-lash of the wrench that caused the mark on the opposite corner (top) of the nut. The next step is to check the thread pitch to determine if the fastener had been tensioned into yield. This is done by placing a thread gauge along the threads nearest the head, not the end of the fastener. The threads closest to the head are the threads that stretch and will remain stretched if into yield, not the threads inside the nut, or grip zone. The back-lash is also caused by a high assembly speed; the faster the speed the greater the compression of the joint which rebounds in relation to the compression relaxation. In this case, the fastener was being tightened against a metal plate and wood, which increased the compressibility and rebound. Although not mandatory by the ASTM A307 standard, the manufacturer can stress relieve the fasteners, which would minimize any potential damage to the head during installation. The installer could be less aggressive with the speed wrench and watch for any accidental lubricants on the threads. Lubricants will greatly reduce any assembly friction and cause more tension than torsion.

THE DISTRIBUTOR’S LINK 171 INSPECTING SCREW AND BOLT STRAIGHTNESS continued from page 108 To determine the exact amount of camber or bow, the part is placed between the rails so that the sliding rail is at its closet position to the stationary rail. To find this spot, the bolt would be rotated 360º to determine the lowest indicator reading. The indicator is then “zeroed out”. The part is then rotated until the maximum indicator reading is achieved. The reading on the indicator is the exact amount of camber or bow in the measured part. Unlike the fixed jaw style of gage, you do not add the screw diameter to the allowable bow. Using this style of gage and our 1/2-13 x 4” fastener, you cannot exceed the .024” max allowable bow in screw or bolt to determine if it is within the acceptable straightness limits of the standard Inspect straightness to avoid complaints Bent screws and bolts can cause their users major assembly problems. When supplying screws and bolts that have lengths greater than eight times their body diameter, suppliers should inspect them for straightness to avoid potential customer complaints and/or rejects.