Testing & Analysis

Case Study: Oil Pan Cracking

Recurring Oil Pan Cracks on Large High Speed Gas Engines

Oil pans on the bottom of several Caterpillar (CAT) engines were developing cracks after only 66 hours of operation. Williams, CAT, and 6D Testing & Analysis (6D) took a methodical troubleshooting approach in evaluating the cause.

Testing

Inspection of the oil pan found a crack along the bottom plate of the oil pan, adjacent to the weld.  6D was contracted to analyze the vibration signature of the failed component and propose a plan to make the oil pan more stress resistant.

To help determine if this was a system or unit issue, comparison vibration measurements were made on two different CAT engines. Testing revealed that cracks were not attributed to a system-wide issue.  A more detailed look was required at the local region.  To do this, the finite element model (FEM) was reviewed to determine the mode shapes that could cause high dynamic stresses in the orders that were measured during the first round of testing.

A finite element model of the oil pan revealed that the stress patterns did not match the crack pattern; again indicating that the issue was not system-wide.  However, it was not yet clear why a particular panel was experiencing a problem.  It was critical to determine the forcing function exciting the mode shapes.

Pinpointing the Problem

Another round of testing was conducted to answer the following questions:

  • What is the strain in the crack area?
  • What is the frequency of this strain?
  • What panel or system mode shape is associated with this strain frequency?
  • What engine or compressor order is providing the forcing function?

Testing indicated that the strain gauges 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 developed from the vibration survey data. The ODS pattern confirmed that the oil pan cracking was not due to a system wide resonance, but due to a local resonance of the subject oil pan bottom panel.

Implementing the Solution

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.

The original oil pan design was modified to increase the natural frequency of the subject panel.  A validation test showed that the natural frequencies and stiffness were increased sufficiently to prevent future fatigue failures.

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