Posts in:Bearing Failure

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.

More Causes of Bearing Failure

Posted May 27, 2015 by admin

The response to our recent post about bearing failure pointed out something we forgot to mention.  That’s certainly not an exhaustive list of failure causes.  In the interest of being thorough, we thought we would continue the list with a few more common causes of bearing failure and damage.

Vibration Brinell - Also known as false brinell, this is a situation where the balls are moving rapidly around the raceway even when idle.  Without the bearing continuing to spin, lubrication will settle to the underside of the bearing and parts of the metal will wear down as the balls continue to spin.

Poor Lubrication - On the subject of lubrication, this chemical additive to the rotation process is crucial for maintaining the integrity of the bearing structure.  With incorrect or improper lubrication, the balls can increase temperature and wear at the raceways.

Stress Fatigue - When a bearing takes on an excessive load from one side, the stress reversals on the interior ball bearings can create flexing of the metal.  This flexing wears on the metal, puts in tension and compresses it.  This metal will eventually begin to flake away as a result.

High Temperatures - Most premature bearing failure in the field is caused by excessive temperatures and conditions.  High temperatures, be they atmospheric or due to excess friction, can cause the lubricant inside the bearing to boil and bleed reducing its effectiveness.  Any oxidation that occurs in these conditions will deposit bits of carbon in the bearing that can cause blockage and failure.  Even the metal itself can take damage from excessive heat.  High temperatures are a big enemy of bearings overall.

A few other culprits have claimed their fair share of bearings including rust caused by dampness in storage and a failure to properly clean and maintain the bearing.  The important part is the diagnostic process.  Trust only the best and most careful engineers to quality test and maintain your bearings.