After removing a failed bearing, we saw that the cage was a dark bluish color. Does this provide any indication as to why the bearing failed?
Once a bearing or any component fails, it is important to look at the evidence left behind to determine a root cause. There could be pits, spalls or in this case a bluish tint to the cage. Each failure mode is indicative of a specific problem the bearing has faced. For the cage to change colors, you more than likely are looking at a problem involving heat.
Heat can be generated in a bearing by a few different processes. Overgreasing or packing too much grease into the bearing cavity can create a heat issue. Grease does not dissipate heat as well as oil does, so it tends to stay hotter longer. Also, if a bearing is completely filled with grease, it can cause churning and produce heat in the form of friction. This is similar to running through water. It is easier to run when the water is only ankle deep as opposed to waist deep. Now imagine the bearing trying to displace all the grease that has been packed around it. This is how the friction and heat are generated.
Understanding the loading that the bearing is experiencing can also help you identify where excess heat could be coming from. If the load is severe enough that the elements are being forced into the race with excessive force, this can lead to boundary lubrication, with heat being produced by the friction between the metal surfaces (the rolling element and the race).
Different types of rolling element bearings can withstand different levels and types of loading. Learning the difference between axial and thrust loading will help you determine if you are using the correct type of bearing. Some loads may be too severe for a ball bearing, but a spherical roller bearing might adequately support the same load. Just looking at the design strength and the recommended loading of the bearing will tell you if you are using the correct style.
It is important to point out that a single bearing can fail due to multiple reasons, but if this failure becomes common, you should be proactive and attack the root cause. To mitigate overgreasing, calculate the volume of grease needed to properly relubricate the bearing and ensure this volume is not exceeded. To prevent heat caused by overloading, confirm that the proper style of bearing is being used and that it is sized accordingly to handle the load it is encountering.