Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation
|When it comes to threads on any fastener, caution should be given when considering the use of "single point" cut threads. Instead, the thread of choice is one that is rolled.
To the left are two stud bolts. The bottom one was the original design which was provided by the OEM. The threads were made by turning the stud in a lathe and using a single pointed cutting tool bit. There is a correct way and a wrong way of cutting a thread, all of which is beyond the scope of this article. Suffice it to say that if tiny tears are made in the material while cutting the thread, then that is most definitely the wrong way. The OEM stud bolt was redesigned and is shown above it in the picture. The threads were rolled into the material, and in addition, the threads were undercut (necked down portion between the center body of the bolt and the first thread).
|In the left picture the cut threads can be seen more closely. The machinist made four passes when cutting the threads. Two of those passes can be seen at the left hand side. The material tore as the tool cut the tread, the marks which run perpendicular to the direction of the thread are tear marks which travel into the body of the stud bolt. At the root of every thread can be seen very faint tear marks. The lower right picture is a view from the other side and showing the exact same gouge marks. Metal has been torn in the rough looking areas. Every tear mark leaves behind a tiny fracture which can, and will, grow over time.|
|Looking again at the upper right corner of the picture at the right, it can be seen that the very first fully developed (i.e. deepest) thread is the third ridge from the right. This is the thread that will more than likely load up first, regardless of whether the nut actually ever fits up that far. That's because this thread acts like a notch, and designers are always fussing about notch sensitivity, it is a sharp edge which concentrates the stresses at this location.|
|To overcome these problems the stud bolt was redesigned. The threads were rolled into the material. Rolling actually deforms and cold works the material, thus making it even stronger. There are no tears from which numerous cracks can begin. Notice in the picture at the right the general appearance of the rolled threads. The light reflection off of the thread face doesn't reveal any kind of rips and tear marks as in the cut thread picture directly above it. Another good design practice is to undercut the threads. In the picture at the right notice how the body tapers down from the right and going toward the left. Before the taper reaches the first thread a rounded radius is cut even further into the bolt. This design is actually stronger because the notch effect is removed from the loading scheme. In the upper right picture the stress on the first fully loaded thread is directed radially outwards toward the heavier section of the stud bolt, thus the sharp corner created by the thread root comes into play. In the lower right picture of the rolled thread, the stress from the first fully loaded thread cannot go radially outward into the body of the stud bolt because material has been removed by the undercut; thus the stress is directed radially inwards, and the "rounded" notch effect created by the root of the rolled thread is further minimized.|
|The two pictures above are of the same failed bolt, which had cut threads. This bolt lasted almost four years before failing. Although its cause of failure was from other events, this failure was revisited to see if the cracks began at tear marks from a thread cutting tool. The picture at the left shows the fracture surface from the top. The thread face can be seen intact 360 degrees around the outside. Notice the circumferential lines on the thread face; these are identical to those shown in the far top right picture at the beginning of this article. In the picture at the right notice that the crack starts after the second thread face, the third face which of course is missing, was the first fully developed thread face. Notice that there are tear marks on these two remaining thread faces. These tears are where the cracks start as they travel radially inward. Since their location is not on the same plane because of the thread angle, the cracks don't join evenly and there is an abrupt change in elevation, giving the appearance of cliffs. These "cliffs" are called ratchet marks. Their orientation always indicates the direction of travel of the crack.
In the picture at the left a different failed bolt was analyzed. A thread face is evident along the left hand side. The tooling tear marks are evident, and what's even more revealing is that the ratchet marks line up directly with each and every tear mark. This is no coincidence.
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