Careful analysis points to causes of bearing failure

The accurate diagnosis of a bearing failure is imperative to prevent repeated failures and their related expenses. While performing failure analysis is commonly left to outside vendors, the in-house collecting of information can make a great difference in correctly diagnosing a bearing failure.

On-site personnel have the first-hand advantage of inspecting the bearing before, during and after disassembly. At every stage, information can be recorded for future consideration.

One of the most overlooked items when removing a bearing from service is taking a lubrication sample for analysis. Conclusions may never be drawn without the knowledge of the lubricant’s condition and quantity, as found in the equipment upon disassembly.

Similarly, omitting information from someone providing bearing analysis can cause confusion and be misleading. For this reason, it is important that not only the bearing be returned for analysis, but information about the equipment and the equipment’s history accompany the bearing.

Gradual deterioration is based on operating conditions and is normally the first sign of bearing damage. This period of deterioration can occur early on, or a few months down the road, prior to a machine’s breakdown.

Here is an example of a bearing failure (see Fig. 1). One set of a cup and cone tapered roller bearing was installed on a truck front wheel axle. After 5,810 km, the outer bearing exploded. The cage was completely fractured into several pieces.

An oblique groove about 12 mm in width was found on the inner ring of the bearing cone (see Figs. 2 and 3). The bottom width of the groove is black. The inside diameter of the inner ring is mostly smooth and bright.

There are some seizing marks on the inner ring bore as result of the inner ring creeping on the shaft. Cold welding and scoring can be observed on the raceway of the bearing cup. The colour of the cup is lighter than the cone.

In all, 14 rollers were found. All of them were out of shape, whether by wear or by pressure. Some of them were overheated and melted together.

Analysis

The oblique groove on the inner of the bearing cone is the result of flame cutting so that the inner ring could be removed from the shaft.

The fit between the inner ring of the bearing cone bore and the shaft was loose. The result of this loose fit is revealed by the bright marks, and additionally, sliding wear and scoring on the surface of the inner ring. Cold welding and circumferential scoring of the inner ring face are both additional evidence of the loose fit between the bore of the cone and the shaft.

Conclusion

The failure was caused by a loose fit between the inner diameter of the inner ring and the shaft. The axial support of the inner ring was insufficient, which allowed inner ring creep on the shaft and created sliding wear with the shoulder of the shaft or bearing spacer.

These conditions yield friction and resulting high temperature, which leads to lubrication failure. As the lubricant deteriorates the wear will accelerate and the heat will increase. The condition will continue until the heat-treated surface becomes soft and agglutination (the adhesion and clumping of particles) occurs.

The cage will fracture due to the rollers not rotating smoothly. The spacing of rollers will be compromised and they will gather together. Eventually, the bearing will not rotate any more, leading to scoring, cold welding and system failure.

Recommendations

The automotive wheel bearing is a fragile component that is easily worn and these parts are often replaced. After a certain number of bearing replacements, the diameter of the wheel case cover hole and the axle diameter will change and deform, causing much difficulty in the installation of the standard bearing.

In cases of extreme deformation, the installation of new bearings will be impossible. One maintenance solution is to use a bearing design with an an enlarged outside diameter or a smaller bore diameter to accommodate the deformations taking place in the bearings.

For example, KML Bearing can provide different types of enlarged size bearings, including enlarged outside diameter double row angular contact ball bearings and enlarged outside diameter in general, between 0.030 mm to 0.070 mm. Each kind of bearing can be provided in one of three groups of enlarged outside diameter sizes: Group 1, enlarged size of 0.030 mm; Group 2, 0.050 mm; and Group 3, 0.070 mm.

Enlarged outside diameter tapered roller bearings are available between 0.040 mm and 0.080 mm. Each kind of bearing is offered in two groups of enlarged outside diameter: Group 1, 0.040 mm; and Group 2, 0.080 mm.

The bearing bore diameter reduction must be determined for each specific application.

Peter Kwok is a project engineer with KML Bearing Canada Inc., Mississauga, ON. He can be reached at 905-629-4255 or by e-mail at peterk@kml-bearing.com.