Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation



Machine Design - Case No. 140: Cutting Square Corner into a Radius Creates a Stress Concentration

View of Fractured Gear Attachment At the SplitlineSharp edges should always be avoided when designing and fabricating Close Upcomponents.

The picture at the left shows a bolt hole from a fractured section of a 12 foot diameter split ring gear. The bolt hole was located where the two sections of the split ring gear come together, and are bolted up solid. The gear is unusual in design in that a portion of it had to be hobbed out in order to make room for the bolts. The corners of this hobbed out region were well radiused, thus eliminating any stress risers; however, in order to make room for the washer which goes underneath the bolt nut, the shop actually cut into the radius and this created a very sharp corner. The circular machining cut marks can be seen in the picture at the left. The crack initiated at the sharp corner of this circular cut (foreground both pictures) and proceeded downward; note the chevron marks located directly below the bolt hole and beginning at the outer circular machine cut. The picture (above right) shows the location of an additional fracture piece, the top of which locates the beginning of the radius. Essentially, the radius was completely cut out.

Fractured Bolt at Gear SplitlineThe bolt which went through this hole is shown at the left. It failed in an unusual manner. Instead of failing in tension, the bolt failed in bending. This is due to the fact that the gear foot shown above was flexing more and more as it's own crack progressed, and changed the way the bolt carried the load. The bolt cracks initiated at many points along the thread root at the right (i.e. multiple chevron marks) and progressed toward the left; the loading changed as the crack continued, and the final rupture is in shear, and created the lip in the picture (far left).


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