Damage Analysis

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Damage analysis is used to prevent repeated failures. Often the damage is simply described rather than determining the cause of the damage.

Determination of the primary failure mode is the goal of the analysis activity. Understanding the failure mode is the first step in determining the responsibility for the damage and the activity needed to prevent its reoccurrence. Corrective actions may be needed at the turbocharger factory or on the engine.

Because turbochargers are rotary machines that operate at high speeds and temperatures, some of their failure modes are not readily apparent. Forces and frequencies developed in high speed machines often reach levels that are quite unexpected. The heavy damage often found is not in proportion with the forces that the unit sees when operating properly. A broken shaft or a destroyed bearing is usually not the defective part but rather the part that received the damage. Resultant severe and rapid damage progression hides clues and confuses the analysis process. Once the damage has been done checking the parts from the unit in question can give some useful information. Examination of the areas adjacent to the journal bearing travel may allow the determination of the condition of the finish and dimension of the shaft journals and bearing bores before damage.

Bearings and seal systems that are appropriate for turbocharger applications share little with those used on crankshafts and other lower speed devices. In operation when all is well, the bearings are very lightly loaded. Together with the lubrication film the bearings delicately position the rotating group with the wheels held close to the end housings. Most primary causes of failures that are difficult to determine are those things that prevent the stable positioning of the rotors at speed.

Examples of primary failure modes are, foreign object damage, insufficient lubrication, imbalance, incorrect dimensions and operation at excessive temperatures. Examples of incorrect dimensions are parts that were not properly manufactured, incorrect parts for the assembly, and parts that have been damaged by foreign material in the lubrication or other causes. Each of these modes will be covered in more detail but the variation seen in the damage allows for the development of only general guidelines for each mode. The longer a turbocharger has run, the less likely it is to be defective. High operating speeds tend to expose defects quickly. Determination of the primary cause of failure is made by evaluating the type and location of damage, and then, based on natural laws and a logical progression of events, establishing the conditions or forces that caused the damage evidence.

The parts examination is the closest thing to preventive service required for turbochargers. This procedure is absolutely essential for forestalling repeated damage of a new or remanufactured unit. In many cases, the parts examination will expose problems in the host engine - remember that these problems must be corrected before another turbocharger is installed. Whenever possible, advise the engine operator of the cause of the failure. This advice may help to ensure a "healthy" operating environment for the replacement unit.

The majority of turbocharger failures are found to be due to poor operating procedures; lack of, or improper maintenance; or incorrect repair practices.

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