Recurring Oil Pan Cracks on Large High Speed Gas Engines
Cracks were developing in oil pans of several Caterpillar engines used to drive reciprocating pumps at a gas pumping facility. Removing an oil pan for inspection and replacement from such an inaccessible, large installation required special precautions and extensive cribbing.
Because the engine design had been in service for many years, it was a mystery why cracks were suddenly developing after less than 70 service hours.
Identifying the Problem
6D Testing & Analysis was called in to help identify the root cause of the problem. Initial vibration testing of several pumping units indicated that the cracking did not appear to be caused by a system-wide issue. Subsequent strain gage testing was initiated in parallel with a finite element simulation activity.
A finite element model was developed to predict the natural frequencies and mode shapes of the oil pan focusing on the various “panels” created by the internal baffle structure. Together with knowledge of the cracking patterns determined from inspections, simulation results were used to define strain gage locations for the instrumented oil pan.
An oil pan was instrumented with strain gages and accelerometers. Prior to installing this oil pan, an impact modal test was conducted on the standalone oil pan. This test was directed at experimentally measuring oil pan mode shapes and resonant frequencies. Theses measured mode shapes were subsequently compared to those predicted in the finite element simulation. Finally, the instrumented oil pan was installed and operating tests conducted over a range of available speeds and pumping loads.
Testing results indicated that the strain gages in the failure region showed a strong resonant peak at the 2nd order at about 31 Hz (915 rpm). This caused approximately a 10 to 1 strain magnification factor. Operating Deflection Shapes (ODS) were also developed from the operating test data. The ODS pattern confirmed that the oil pan cracking was not due to a system wide resonance, but rather could be attributed to a local resonance of the one of the oil pan bottom panels.
Initial comparisons of the oil pan panel resonance frequencies as measured in operation did not compare well with the simulation predictions nor the oil pan modal test results. However, one of the mode shapes for the largest oil pan bottom panel matched the crack pattern.
A subsequent test conducted over a range of oil levels in the pan, showed that the oil pan resonant frequencies were significantly affected by the oil level. When the simulation model was adjusted to include the mass of the oil, the resonant frequencies matched the operating test results. This provided increased confidence that the physical mechanism causing the cracking was completely understood.
Implementing the Solution
The oil pan design was modified to increase the natural frequency of the subject panel. A similar impact modal test was performed on the modified oil pan to determine the bottom panel natural frequencies and mode shapes.
The FEA provided confidence that the modified oil pan, when installed in the unit and filled with oil, would provide a sufficient increase in frequency and stiffness to eliminate the cracking problem. A validation test confirmed that the natural frequencies and stiffness were increased sufficiently to prevent future fatigue failures.
The early use of a finite element simulation supporting the testing program was critical to initially identifying the root cause, understanding the physical mechanisms, and later providing confidence that the solution was correct.
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