Inspecting CFM56 Turbine Engine Bearings

Inspecting CFM56 Turbine Engine Bearings

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.