Posts in:November, 2015

Why is proper lubrication important?

Posted November 9, 2015 by wowdev

After experiencing numerous axle hub failures on new production, a customer called upon VNC to help determine the root cause and solution.

The very next day, VNC traveled to the customer’s site in order to examine the failed axle hubs and review all of the known facts.

While keeping an open mind to all possible root causes, all early indications pointed towards a problem involving inadequate lubrication, however, upon reviewing the site’s assembly and lubrication processes no irregularities were observed.

With inadequate lubrication seemingly ruled out, several failed bearings, as well as the mating components, were disassembled and taken back to VNC’s inspection lab for a more in depth examination.

Forensic Examination:

Although most of the bearings were badly destroyed and difficult to inspect, the forensic examination continued to point towards inadequate lubrication as the root cause of failure.

During the examination the bearing fragments from the raceways and rollers consistently demonstrated severe discoloration and scoring. At such an early stage in their field life, such signs would not occur under proper lubrication conditions.

In addition to the failed bearings, sampling was completed on the current bearing inventories and all bearings were found to meet specifications.

The Hypothesis:

With all components meeting design specifications, no signs of external contamination, and inadequate lubrication seemingly ruled out, the team turned towards the one observation that could not be explained.

Despite the numerous failed axle hubs on new production, every single failure was occurring on the curb side of the axle. If a product defect were causing failures in the field, this affinity to one side of the axle would not occur. Instead the failures should exhibit a random pattern among the assembly locations.

With renewed focus on the curb side of the assembly line, the assembly and lubrication processes at the customer site were reviewed once again in greater detail.

The Solution:

Several days later after an in depth investigation of the lubrication portion of the assembly line, the customer was finally able to identify the root cause for the recent axle hub failures.

With the renewed focus on inadequate lubrication, it was discovered that the lubrication machinery used on the curb side of the assembly line was improperly modified in order to decrease the time to assemble.

This modification was found to directly cause less lubrication to be applied to the curb side axle hubs. With less lubrication, the bearing components were not receiving ample lubrication in order to protect the bearing components and perform properly.

Once the machinery was returned to normal operating conditions, the curb side axle hubs were once again receiving a proper amount of lubrication during assembly. With the root cause corrected, the axle hub failures were eliminated.

Contact VNC Bearing today to find out how we can help you.

Why is proper radial clearance important?

Posted November 9, 2015 by wowdev

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.

Forensic Examination

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.

The Hypothesis:

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.

The Solution:

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.

Contact VNC Bearing today to find out how we can help you.