SUBJECT: Troubleshooting premature bearing failure 5-3

As discussed in technical article 4-12 on this web site, bearings have no wearable surfaces, they are instead designed to fatigue after many hours of service.

In a properly operating bearing the race ways and rolling elements will become dull in appearance. This dullness is not an indication of wear and has no affect on the life of the bearing. These dull surfaces form visible paths, so their appearance and location is important in analyzing any type of bearing failure.

When we install a bearing into a piece of rotating equipment the general rule is to have the interference fit on the race that is rotating and, therefore, carrying the load. Almost all centrifugal pumps, motors, and a high percentage of other types of rotating equipment have the bearings installed with the inner race an interference fit and rotating with the shaft . The outer race remains stationary, or in a fixed position.

In the following paragraphs I will be discussing various load conditions and the resultant appearance of the raceways and rotating elements in this type of an installation:

The radial load is rotating with the shaft, This is caused by an unbalanced rotating assembly or a bent shaft.

The radial load is unidirectional. This is what we would expect to find with a properly operating piece of equipment. If the equipment is operating off of its best efficiency point, is misaligned, or if there is excessive pipe strain the pattern will be the same, only more pronounced.

The radial load is multidirectional . Cavitation, too tight an interference fit, preloading, or cooling a bearing outside diameter are all common causes of this problem.

The axial load is unidirectional. This is the normal condition of all end suction centrifugal pumps.

An oval compression of the outer ring. Caused by an out of round housing.

The inner ring was misaligned. Normally happens during the installation process.

Now that we know what some typical wear paths look like, we'll inspect the only two things that are visible to the trained trouble shooter.

Look for damage caused by solid particles. These particles will be rolled into the race ways and can:

Look for lack of lubrication that can eventually cause the bearing to seize:

Look for smearing of the metal . When two non lubricated surfaces slide against each other, under load, the material transfers from one surface to the other.

Look for evidence of static vibration. You will see indents in the raceway that could be either shiny or rusted in the bottom. The frequency of the vibration has no affect, but greater energy causes greater damage. Roller bearings are more susceptible to this type of damage because the balls, in a ball bearing, can roll in many directions. Rollers, how ever, can roll in only one direction. Movement in the other directions takes the form of "sliding".

Look for electric current damage. It will show up on both the races and the rolling element. The bottom of the depression will be dark in color.

Look for flaking or spalling of the metal race way. Since there is nothing in a bearing to wear out, flaking or spalling is a sign of normal fatigue. Overloading however, can cause premature fatigue. Look for the following causes of bearing overloading:

Overloading is often accompanied by a change in appearance of the lubricant. You will see varnish or coke as the lubricant is subjected to this high heat.

In addition to overloading there are additional sources of heat that can destroy the lubricant :

Look for cracks in the metal.

Look for signs of corrosion.

The major bearing companies do a good job of providing the literature and photographs that you need to do effective "comparison troubleshooting". Check with your bearing supplier for the availability of this information.

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