Understanding Bearing Failure

Undesired bearing failures do occur. However there is a lot of information that can be taken from a bearing failure that can be used in the corrective action process.

Although bearing life calculations are the key design tool used in selecting bearings, only 1% actually fail due to pure fatigue. The majority of bearing failures are from a lubrication-related issue, with other contributing factors being contamination and improper mounting. What can be taken from this is that 95% of bearing failures can be either prevented or have their service life extended. One very important tool for doing this is root cause failure analysis.

When it comes to bearings, root cause failure analysis is generally viewed as a two-stage process: what is the root cause specific to the bearing, and what may have caused that specific bearing’s root cause.

Understanding that this may sound confusing, it is best explained via an example: Imagine a failed bearing being looked at in order to determine the root cause failure in the hopes of potentially avoiding a similar failure in the future. After examining the bearing in a systematic approach by cleaning, disassembling and investigating each component, it is noticed that the failure modes visible on the bearing indicates a breakdown in lubrication. Therefore the reason behind the bearing failure is inadequate lubrication.

But, logically, the next question would be: What caused the lubrication breakdown? Many times this second question cannot be answered by looking at the bearing alone. It is by answering this second question that real corrective actions can be taken.

Again, going back to our example, the next steps would be to look at the application as a system, the reason being that many times a bearing failure is a result of a system failure. The first questions to ask would be: What is the application and what are the operating conditions?

In our example, assume the bearing was taken from a dryer roll that failed prematurely. Were there any other bearing failures from the dryer system? No. Does one central lubrication system service the entire dryer? Yes. This already tells us some very valuable information in that only one bearing failed, but there is a common lubrication system for all the bearings. This could mean that there are more bearings potentially ready to fail, or that there could be something unique to this particular bearing location.

The next step would be to investigate more closely the surrounding environment: housing, steam joint, piping, etc. Again, in our example, it might be found that the piping was damaged, reducing the flow to the housing, thus starving the bearing of lubrication.

What is important to understand is that by just looking at a bearing alone, only a root cause for the bearing failure is possible. From this, possible factors leading up to that root cause can be suggested, but these can be further narrowed down by understanding the application and other influencing factors such as: process, maintenance schedules, looking at surrounding equipment, etc.

The most effective root cause analysis is that which is carried out by a team. For example, this team could involve a bearing specialist, a process engineer and a maintenance person. By understanding what normally should happen in the application and by looking for evidence of changes to this norm, root cause can be determined.

A common question from bearing users is “How much life is left in the bearing?” This can be associated with a bearing failure, or can be related to a bearing taken out of service without any signs of damage.

Up until now, the answer was mostly based on a calculated life minus the amount of hours already run on the bearing. Now technology exists to evaluate the residual life in the bearing. Using X-ray diffraction, it is possible to analyze the residual stress and the micro-structural decay in the failed bearing. This analysis gives indications of what loads the bearings have been subjected to. The sub-surface material response will be measured and compared to the predicted pressures and related to bearing fatigue life.

Minimum load can cause failures

It is easy to understand that a heavily loaded bearing should experience a shorter bearing life than that of a moderately loaded bearing, especially after understanding the life scenarios as described previously. However there is a point where the bearing will be too lightly loaded for it to perform properly.

Many times bearing application engineers have received inquiries from customers, asking why their bearing had failed so quickly. Upon being asked how much load the bearing was experiencing, the customer usually replies that the bearing had hardly any load at all, and, based on the life equation, it should have lasted forever. They are then even more surprised to hear that this light load is exactly the reason that the bearing failed.

To ensure proper bearing operation, there must be a minimum amount of load on the bearing. To understand this further, it is necessary to understand the concept of the bearing load zone.

Using the example of a bearing with inner ring rotation and unidirectional inner ring load, the load will be supported on a portion of the outer ring. This portion or section is known as the load zone, and is analogous to that of a piece of pie. The size of the load zone depends upon the magnitude of load applied. The larger the applied load, the bigger the load zone or piece of pie; the smaller the applied load, the smaller the load zone.

The total applied load is supported by the rolling elements in the load zone. Therefore any rolling element within this load zone is in a state of pure rolling along the raceway. As the rolling element leaves the load zone, the rolling element is unloaded and may not necessarily roll along the raceway; it may float along, suspended in the lubricant between the two raceways.

As the rolling element re-enters the load zone, it returns to pure rolling. In a bearing that has enough load (enough meaning it meets the minimum load requirement), the contact pressure between the rolling element and the raceway gradually increases as you enter the load zone, to a maximum at the middle of the load zone, then gradually decreases as it leaves the load zone. This is important since the rolling element would not see a sudden change of angular velocity.

If the bearing is operating in an under-loaded condition, the load zone is very narrow. What this means is that the contact pressure between the rolling element and the raceways suddenly changes from virtually nothing to maximum immediately as it enters the load zone, and is quickly released upon leaving the load zone. This results in very sudden changes to the rolling element’s angular velocity. This sudden change in angular velocity causes the rolling element to skid as it quickly accelerates to pure rolling in the load zone.

This is analogous to that of an airplane landing on the runway: on approach, the landing gear is down and the wheels are not moving, which is similar to the rolling elements. As the plane touches down on the runway, which is equivalent to the bearing load zone, the wheels must very quickly accelerate to match the speed of the plane. As it touches down, it smears the runway with rubber. This is exactly what happens in the bearing; the rolling element breaks though the lubrication film and smears the raceway.

There are various factors that affect the effect of minimum load situations in rolling bearings. The biggest factor is the load. A large-enough load zone must be present to prevent sudden changes in contact pressure. The faster the bearing speed, the more drastic the angular acceleration of the rolling element. Other factors include viscosity and bearing clearance.

There are some measures that can be taken to prevent bearing skidding, or to reduce the effects of the bearing skidding. As stated earlier, the single biggest factor is the load. Therefore, selecting the prop
er bearing size is crucial to ensure that the best load distribution is achieved.

There are, however, cases where the bearing size is predetermined by other factors: existing housing size or shaft size where the bearing will be lightly loaded. Some methods to minimize the effects of the minimum load include:

*Adjust the internal clearance: By reducing the internal clearance the load zone should increase, allowing for a smoother transition of the rolling elements in and out of the loaded area. Care should be taken to not reduce the clearance too much.

*Reduce the rotational speed of the bearing: The faster the rotational speed, the higher the minimum load requirement. This is primarily due to the greater rotation speed differential between rolling elements in the unloaded zone as compared to those in the loaded zone. This increased speed differential increases the amount of angular acceleration upon entering the loaded zone, thus increasing the likelihood of roller skidding.

*Use oil lubrication in place of grease: The thickener of the grease can have a slowing effect on the rolling elements outside of the load zone, potentially leading to increased angular acceleration as the rolling element comes into the load zone.

*Special coatings: These coatings don’t prevent the skidding but rather reduce or eliminate the damage associated from the skidding.

What is next?

As most industries will focus on increasing output though increased speed and reliability, rolling bearings will be called upon to support such improvement programs. However, it is doubtful that the bearing industry will see the addition of very many new bearing designs, if any at all, but rather the refinement of existing bearing designs.

This refinement will most likely focus on areas such as materials and steel cleanliness, improved manufacturing methods and quality control, and special products such as coatings or ceramic technologies.

Parallel to this, bearing manufacturers will continue to better understand applications and continually develop engineering support in the areas of life theory and application engineering to more accurately understand the use of bearings.

John Melanson is manager of engineering, SKF Canada Ltd., Toronto, Ont. For more information, use the reply number below or visit www.skf.ca.