The following case study illustrates issues with oil pans developing cracks shortly after being put into service on large, high-speed gas engines. Cracks developed on the bottom of several Williams’ engine oil pans after accumulating no more than 70 hours of run time. This prompted Williams, Caterpillar, and 6D Testing & Analysis to identify suspect oil pans and initiate an investigation. The process taken to identify the cause of the oil pan cracks including the details of the investigation, analyses, and test results are described in the following paper.
The removal and inspection of the oil pans, crack inspection, and examination of the history and previously completed vibration studies are covered in the investigation. The analysis details the finite element analysis performed by 6DOF used to create a test plan to measure high dynamic stresses, natural frequencies, and mode shapes. Finally, the impact test results, dynamic strain measurements, and operating deflection shape data taken on one of the oil pans are reviewed.
The goal of this case study is to highlight the engineering involved with investigating high cycle fatigue cracks due to resonance.
Introduction
Any failure represents a substantial amount of downtime, loss of production, and personnel resources. Specifically, unexpected failures with reciprocating compressors and engines require a methodical troubleshooting approach in order to determine the root cause. Therefore, it is imperative that the engineers involved evaluate all potential scenarios and substantiate recommended solutions with facts. A non-typical failure, such as the one described in this case study, can be particularly difficult to troubleshoot and will require more time and resources to come to a solution. However, the amount of down time and risk can be significantly reduced when done properly through thoughtful testing and analysis. This case study presents the facts and troubleshooting methods used to investigate cracks on several large high speed gas engines.
Background & History
The oil pans on the bottom of several Caterpillar (CAT) 3612 engines developed cracks. All of the cracks occurred on newly commissioned units and were on the bottom of the oil pan. Williams, CAT, and their testing contractor 6DOF took a methodical troubleshooting approach in evaluating the cause.
The first oil pan crack occurred after only 66 hours of operation. The crack occurred on the bottom of the pan. Therefore, it was necessary for CAT and Williams to immediately remove the oil pan from the engine to be shipped to CAT for non-destructive testing (NDT) to assess the cause of the unexpected failure.
The process of removing an oil pan from a large high-speed engine is not a typical procedure and requires special precaution and cribbing in order to maintain the integrity of the pan for later examination. While CAT performed NDT on the failed oil pan, Williams inspected other identical packages and performed a review of the design looking for other clues to the cause of the failure. (Read more here.)
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