Vibrating equipment can be machines, such as vibrating screens, conveyor pans or a combination of both, that are designed to transport, sort or dry materials. Various types are used in many processing industries. They typically vibrate at 500 cycles per minute (cpm) and accumulate vibratory cycles quickly. These conveyors contain many welds and, while welds may not be a concern with constantly applied stresses, they can be with cyclic stresses.
These conveyors are excellent weld fatigue testing machines, even though they are not intended to be used as such. When a weld is not designed for fatigue, it will likely fail. Welds contain unavoidable defects and, with elevated cyclic stresses, cracks can grow quickly from these defects.
The author is not a metallurgist, but he has analyzed many such failures as a consultant. Over-stroking, broken springs, incorrect operating procedures on well-designed equipment or poor attention to details on new designs are usually the causes of such failures, shown in FIGS. 1A, 1B and 1C. Some sources (1) say that 90% of failures in engineering components are due to high-cycle fatigue.
FIG. 1A is from a poorly designed, 20-ft-long conveyor/dryer that was recently installed. It had cracked at so many welds throughout the conveyor that a decision on whether to rebuild or scrap the unit was required.
FIG. 1B failed after a diverter gate became loose, causing high-impact stresses on the welds and resulting in cracks. The meandering nature of the crack growth indicates a low cyclic...