Diagnose bearing failures beyond root cause analysis

Take a ‘CSI’ approach to root cause analysis to look beyond the obvious to determine why bearings are failing. Six types of failures and 12 tips for determining the root cause are featured.


Figure 1: Example of fatigue spall. Courtesy: Motion IndustriesA common phrase that is used throughout industry when a machine is experiencing unscheduled downtime is, "The bearing failed." Even though the rotating component fails to turn, in most cases, it is more accurate to say that we failed the bearing. I've heard it said, by different reliability specialists, "Bearings don't fail, they are murdered." In my 40-plus years of selling, sizing, installing, observing and analyzing failed bearings, I have found this often to be the case. People find new and creative ways to kill their bearings every day.

With that in mind, for the sake of discussion, let's ask the question, "Why do bearings wear out, lock up and go bang in the night?" In order to answer that question, we must understand that even though improvements in material science and bearing design geometry make for a potential theoretical design life of infinity, rotating components prematurely fail from a variety of causes.

The subject of bearing failure involves metallurgy, tribology and the operating environment, as well as the consideration of the applied loads. In some cases, a single event may have caused the failure. Other times, numerous complex incidents and conditions may have come together in a nexus of poor human decisions, harsh environmental settings, severe operational conditions and poor maintenance practices. All of these contribute to premature failure. Ultimately, this results in catastrophic failure of the rotating or reciprocating component and additional ancillary damage to associated mechanisms. Due to the high cost of lost production and the time constraints placed on maintenance departments, reality dictates that the problem-solving process be simplified and expedient.

The unverified quote attributed to Albert Einstein, "Everything should be kept as simple as possible, but no simpler" is a useful mindset when determining the root cause of a bearing failure. In order to correctly determine the reason why a bearing failed, a systematic way of problem solving needs to be employed by a reliability team. Root cause failure analysis (RCFA) is a problem-solving method used to determine the cause of an incident and separate it from the effects, as well as filter out the nonauditory noise, which can prevent the team from solving the problem and implementing the necessary corrective actions.

Avoiding assumptions and preconceptions during the process is sometimes difficult but absolutely necessary to define the nature of the problem. The team must dig deep, ask questions and approach the challenge with a CSI mentality, and of course, determine, implement and follow up on the corrective action and countermeasures.

Look beyond the obvious

At first glance, a bearing failure may appear to be caused by not enough grease. But this answer is a too simplistic and a convenient excuse. The actual root causes of the bearing failure may be that it lacked the correct lubricant viscosity, proper delivery method, or intervals for the application.

The obvious effects may be heat, smoke, auditory noise and unscheduled downtime. The analytical noise in the problem-solving process is extraneous and will hinder the determination of the root cause and implementation of corrective measures. Nonauditory noise is actually an ancillary effect. Examples are: complaining, bad tribal knowledge and distractive thinking. If the problem-solving team stays focused and continues to dig deep, the root cause may actually be a lack of lubrication training and a laissez-faire culture towards tribology.

Figure 2: Water and oxygen, along with the sulphur compounds found in the lubricant, combine to acid etch the surface of the steel. Courtesy: Motion IndustriesFor the sake of simplification, let's group bearing failures into six major categories. They are: fatigue, handling/installation, operational, environmental, lubrication and defect. Even though there are a myriad of often times confusing terms used to describe bearing failures and the visual effects observable on the bearing, these six types of failures will be used to categorize the typical failures occurring in industry. Categorization is not absolute, but used for the sake of problem solving.

1. Fatigue

Stress over time, which is fatigue failure, is the right reason why bearings should eventually stop rotating. This cause of failure is different than wear. Fatigue failure is related to the stress the bearing is subjected to over time. Fatigue failures in bearings typically originate sub-surface and eventually will propagate to the surface, and in time, will appear in what is referred to as a spall (see Figure 1).

Fatigue failure is, in part, interrelated to the purity and the quality of the steel. With advances in steelmaking over the last few decades, micro-inclusion impurities within the bearing steel are minimal. Calculating the fatigue life of a bearing takes into consideration, load, speed and cycles or time. Bearing engineers use the L10 life of a bearing, along with dimensional restrictions and the bearing's design, for selecting a bearing. L10 life is a life calculation where 90% of identical bearings are handled, installed, lubricated and run under the same operating and environmental conditions, without metal fatigue. This is the ideal reason for the cause of failure.

2. Handling/Installation

Improper handling and installation practices often kill the bearing before it has been installed or rotated. Storage in a wet or highly contaminated environment is detrimental to the bearing's life. Bearings should be stored, flat, dry, clean and in good order. The temperature should not vary widely from summer to winter. Humid environments are not conducive to extended bearing life. Access should be controlled to prevent unnecessary opening and movement.

If bearings are stored, or a machine is idle for an extended time, the lubricant within the bearing may leach out and puddle at roller intervals-causing an acid etching of the surfaces.

If the fit between the shaft and inner ring, or the housing and outer ring, is not within the recommended specifications for the application, fretting-corrosion on the surfaces will be evident. This is a different type of corrosion than that caused by invasive water and oxygen. Fretting-corrosion occurs when there is movement between a bearing ring and shaft or housing because the fit is too loose. Microscopic steel particles break off due to movement and oxidize.

This will result in the appearance of areas of corrosion on the surfaces of the rings. In extreme cases of inadequate fit, the outer or inner ring may turn or creep, resulting in galling of the surfaces. The remedy for this is to measure all mating components prior to installation and make sure they are within the fit recommendations. If not, it's time for a new shaft or housing.

Figure 3: True brinelling from force fitting. Courtesy: Motion IndustriesAmbient vibration during storage should be minimized, and bearings should never be stored upright on a shelf. False brinelling of the metal surfaces is caused when the roller and ring are in contact with each other and subjected to vibration over time, resulting in the wearing away of the metal.

If the bearing is dropped while it is being handled, a form of mechanical damage referred to as a true brinelling will result. This material displacement or dent, where the rollers and races contact each other, may also be caused by force fitting the bearing (see Figure 3), or through the use of an improper mounting tool, such as a hammer.

Striking a steel bearing directly with a hammer is dangerous and could result in the bearing exploding. The human condition that all too often leans towards carelessness and urgency is a big part of this failure equation.

I recall a time when I was passing through a customer's facility and noticed a man on a forklift truck delivering a new motor for installation. When the driver turned the corner, at too high of a speed, the motor rolled off the pallet and hit the floor, shaft first. That bearing was murdered before it had a chance to operate.

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