How to diagnose bearing issues
Bearings are key mechanical components of rotating machines and motors are obviously no exception. Bearing failures make a considerable portion of overall motor failures. In this post we look at factors influencing bearing lifetime, explore the failure modes and provide some hints how to assess and diagnose bearing health condition and eventually prevent certain failures.
Bearings underwent massive development over the last decades. Deeper knowledge thanks to the progress in material science or advances in the design and geometry of bearings were the main contributors. The potential design lifetime can be almost infinite. Despite that fact bearing do fail. And there are multiple reasons for that. Finding a root cause can be compared to crime scene investigation known from popular TV series.
Sometimes the failure may be caused by just one event. In other cases it is a consequence of combination of several factors. Human factor is often one of them. When performing a root cause analysis one shall not just focus on things that can be seen. You need to go deeper.
6 types of bearing failures
In this section we will classify bearing failure root cause into 6 main categories. This classification is not absolute. However, it is used as part of problem-solving approach.
Category 1: Material fatigue
Category 2: Installation
Category 3: Operating conditions
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1. Material fatigue
Material stress over the time is an fatigue type of failure and often the true root cause. It is a different mechanism than aging. Fatigue typically starts under the surface and spreads towards the surface until it finally appears in form of spalling.
Fatigue is partly linked with the purity and quality of steel. Thanks to progress in steel making over the last decades the micro-inclusive impurities inside the bearing steel have marginal concentration.
When calculating fatigue lifetime of a bearing it is necessary to consider loading, rotational speed, load cycles etc. Bearing designers must consider dimensional and construction restrictions and apply basic bearing lifetime L10. It is a calculation methodology…
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2. Handling and installation
Incorrect handling and installation often have destructive impact on the bearing even before it starts to provide its function. Storage in environment with high humidity or pollution is harmful for bearing lifetime. Bearings shall be stored flat in dry and clean environment. Temperature in the warehouse should not largely fluctuate throughout the year. In case of long term storage or long period of idle condition of the machine the lubricant may leak from the bearing and create acid causing local surface damages.
Arrangement between shaft and inner ring shall respect recommended tolerances. Otherwise there is high risk of friction/vibration corrosion. Such vibration corrosion is likely to happen when the arrangement of bearing ring and shaft is too lose. In worst case the bearing ring may slip causing severe mechanical damages.
3. Operating conditions
This category includes all negative effects of operating conditions except for environmental conditions. Excessive axial loading, eccentricity, extreme vibrations or bearing currents are some of the severe factors leading to a bearing failure.
These types of failures can be observed when investigating the surface of the orbits (particularly in the area of loading of the bearing rings). Good axial alignment is a key factor for long lifetime of rotating parts.
High vibration due to non-circular shape of rotating parts causes “jumps” or “slips” of the balls or cylinders due to time variable loading. As a consequence, the orbits suffer from scratches and so called false hardening.
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