When presented with an opportunity to resolve a noise issue, VNC proceeded with a systematic approach in order to determine the root cause and solution.
It all began when elevated noise levels began to be reported in approximately 20% of the customer’s assemblies.
During the initial investigation, the shaft and housings were measured and found to be within tolerance. Bearing inventories were also measured and individually tested for noise and were found to be good.
The forensic examination began by carefully disassembling the bearings. Microscopic investigation of the bearing races yielded evidence of brinelling in about 20% of the inspected bearings.
Brinelling is a material surface failure caused by excessive contact stress. When the contact stress exceeds the material limits, plastic deformation occurs and small indentations can be seen.
In addition to the signs of brinelling, small axial scratches were observed on the raceways at ball spaced intervals. The combination of the axial scratches and brinelling pointed to the failure originating during the installation process.
At this point VNC returned to the customer in order to verify proper installation procedures were being used in the assembly areas. Once the team confirmed that all technicians were using acceptable installation procedures, the focus turned to the suitability of the assembly fits.
Although each of the components were meeting their print specification, the resulting assembly fit, including the statistical variation of each component, must be considered. Even if nominal dimensions yield a desirable assembly fit, the statistical variation of each component must be included in order to consider every possible assembly scenario.
This type of situation, where only a portion of the potential assembly scenarios create undesirable results could explain why only 20% of the customer’s assemblies were exhibiting brinelling.
After detailed calculations, it was determined that brinelling could occur when the shaft was at the high end of the tolerance range and the housing and the bearings were at the low end of the tolerance range. In this scenario the resulting assembly fit completely removes the radial clearance within the bearing and gives the bearing excessive radial preload. This excessive radial preload correlates to excessive contact stresses which in turn, cause brinelling.
After more calculations, VNC advised that the issue could be resolved up by changing the bearing’s internal radial clearance specification to a “C4” rating.
Working with the customer and their subcontractors, VNC produced 200 bearings with the new C4 radial clearance rating. These bearings were specifically numbered and special attention was paid to each bearing’s radial clearance and noise measurements.
Before assembly, the other assembly components were also measured, numbered, and tracked. By tracking each component in this manner, the resulting fit of each assembly could be accurately identified and recorded.
Over the next several weeks, the performance of the assemblies was monitored and the results were recorded. Testing proved that the “C4” radial clearance rating successfully eliminated the brinelling and noise issues seen in the previous assemblies. With the root cause and solution identified, the customer changed their drawings to require the “C4” radial clearance rating for all future production.
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