Inspecting CFM56 Turbine Engine BearingsBy Dan Berg October 1999
Have you ever considered the benefits to be gained by inspection of aircraft bearings? One reason of course is to ensure that the aircraft remains safely in the air. However, an additional meaning of the inspection process is to be able to predict, with some degree of accuracy, when the bearing is likely to fail. The guidelines of the engine shop manual are designed to help us in that endeavor.
Interpretation of the shop manual is critical. The criteria set forth are minimum requirements for the airworthiness of the bearing. Brand new bearings, in most cases, have criteria that are more stringent in comparison. Therefore, the knowledge of the bearing inspector, interpretation of the shop manual, and his or her interpretation of specific defects are critical to the survival of the aircraft engine.
CFM56 engine bearings
Visual inspection of aircraft bearings can be very stressful even for the experienced inspector. Because of the rolling contact between the rolling elements and the rings, the main shaft bearings on the CFM56 engine are referred to as anti-friction types.
The No. 1 position bearing is classified as an angular contact bearing. Angular contact is a term used for radial rolling bearings with a nominal contact angle greater than zero degrees up to and including 45 degrees. Nominal angle for the No. 1 bearings is 30 degrees. This allows the bearing to take a combination of radial and thrust loads.
Position 2, 4, and 5 bearings are classified as cylindrical roller bearings. These bearings are radial rolling bearings using cylindrical rollers as the rolling elements. Unlike the number 1 position bearing, these bearings are designed for a purely radial load. These bearing configurations that are used are very specific to their application. The strict requirements in aerospace applications mean that high performance materials must be used. Each element is measured for resistance to fatigue, wear, extreme temperatures, and corrosion. Only the core metals with the best characteristics are considered when manufacturers select their raw material. In addition, the resistance of the metals is further increased by the use of heat treatment. This allows the manufacturer to assign different metallurgical properties to different parts of the same bearing.
Before any inspection criteria can be applied, we must first determine the functionality of the bearing surface involved. These surfaces are classified as being either "active surfaces" or "non-active surfaces". Active surfaces can be defined as moving surfaces that are in effective contact with the rolling element or that functionally support the cage. Non-active surfaces are defined as all other surfaces including clamping surfaces
. SNR Bearings is authorized by CFM International to replace up to and including three components in order to complete the repair and return each bearing to service. Bearing positions 2, 4, and 5 require the retention of the inner ring at all times. Conversely, the No. 1 bearing position requires the retention of the outer ring throughout this process. The No. 3 bearing is not within the scope of shop manual 72-09-01 for repair and can only be inspected and replaced in its entirety if found unserviceable. Any non-conformance with these "intimate" components prohibits serviceability and repair for the bearing.
Disposition of visual imperfections is usually determined by the use of a scribe with a designated nose radius. The scribe is held perpendicular to the surface being checked; grasped lightly between the thumb and forefinger with only the weight of the scribe resting on the surface to be checked; and guided across the widest part of the imperfection. In order to fail the scribe test, the imperfection must be definitely felt with the scribe, that is, the scribe must hesitate or "catch" as it is drawn across the imperfection. If the scribe does not catch, the imperfection should be treated as being non existent. In most cases this means, "If you can't feel it, you can't see it." Many times this philosophy is difficult to accept for new inspectors.
After a defect has been detected, it must be identified. This must be done to ensure that the proper inspection limitations are used in determining the conformity of the defect according to the shop manual. Frequently these defects cannot be identified with the naked eye, thus the use of magnification is recommended in making these determinations.
Rarely mentioned and often overlooked, the oil holes are the critical lifeline to the bearings' survival. Think of a bearing as a human brain. Then, picture the oil holes as the arteries for this brain. One clogged artery and the brain's performance can be greatly reduced. With two or more arteries clogged, look out! The same holds true for bearings. Without the proper flow of lubrication via the oil holes, the bearings life span is significantly reduced. By design, the oil holes are small and it takes very little obstruction to impede their performance. With the use of a small pin, these holes can be checked to ensure that they are free from obstruction.
For cylindrical roller bearings, some bearing inspection facilities take the process one step further. A systematic change of the cage is included with the inspection for all SNR origin bearings, for instance. This allows us to examine the inner ring raceway on bearing positions 2, 4 and 5 ensuring their serviceability.
After the serviceability for the component has been established, polishing of the raceway and all other surfaces is performed. This process helps eliminate any minor corrosion and indentations that may occur under normal operating conditions. To guarantee that there is no significant removal of material, the shop manual limits the use of emery paper to 600 grit and higher for this process.
Currently, one the biggest problems found is damage due to improper handling. Indentations in the raceway normally occur when foreign particles are pressed between the rolling elements and the rings during operation. However, indentations may also develop for other reasons. Excessive or heavy impact loads can cause localized indentations at the point of ball or roller contact. These dents may not be obvious to detect.
Unlike foreign particle indentation, this type of denting is characterized by a larger and much shallower type of imperfection or flat. It is important to note that this damage is often caused during removal or transportation well before the bearing reaches inspection. Detection of this type of indentation can sometimes be aided by imperfections found on non-active surfaces of the bearing. When an imperfection is found on these "non-active surfaces", we need to distinguish when the damage happened.
Raised metal or shiny-bottomed indentations are a classic telltale sign that damage occurred after removal of the bearing. An imperfection with a dull finish and rounded edges would indicate that it was there before installation or occurred during operation. The first place to check for active surface damage would be the raceway directly opposite the imperfection. Then, each rolling element and the remainder of each raceway need to be inspected. Another characteristic specific to this type of damage is inner ring/outer ring out-of-roundness. The out-of-round tolerance in most cases is very tight and it takes very little impact to alter them.
Though classified as a "non-active" surface, the seal teeth diameter of the No. 4 bearing is often prone to this type of handling damage. Acceptance levels for seizure marks, scratches, nicks or indentations on this feature are very stringent (any amount, 0.003-in./0.08mm deep, SM72-09-01 Rev.47 Dated Nov 15/98). There have been many cases of perfectly serviceable bearings being scrapped due to improper handling of this component.
Measuring and gauging
Measurements of dimensions and run outs can be performed with different degrees of accuracy with various types of test equipment. Current FAR's requires the "use of tools and equipment necessary to assure completion of the work in accordance with acceptable industry standards." In a perfect world, it would be best to use the identical test equipment utilized at the time the bearing was originally manufactured. This would further reduce any discrepancies in measurements that may be caused by gauging correlation. Since this is not practical, it is important that when measuring in decimals that the gauge being used must measure to the next decimal of accuracy.
Determination of dimensions is performed by comparing the actual part with an appropriate gauge block or master whose calibration is traceable to the National Institute of Standards and Technology. Before each measurement is made; the part to be measured, the equipment being used, and the master must to be brought to ambient temperature. Any heat transfer to the part being measured will greatly affect the accuracy of the measurement. The recommended room temperature is 68 degrees F with a variation of ±2 degrees F. In order to avoid any deflection of the components being measured, the force applied to the component should always be just enough to drive the indicator. Too much or too little force will give a false measurement.
Radial clearance is the arithmetical mean of the radial distances through which one of the bearing rings may be displaced relative to the other. This measurement normally specifies a specific load to be applied in each direction. There are various gauges and techniques used to obtain a correct measurement for this inspection. The correctness of the measurement will be determined by the amount of deflection exhibited between the rolling element and the rings. By using too much force an inspector can very easily compress the elements and get a false reading. Even worse, this method can produce a repeatable reading. One way to check for too much or too little force being applied is to compare readings with another inspector. Each inspector should be able to obtain the same repeatability when checking radial clearance.