Blade fracture analysis of a motor cooling fan in a high-speed reciprocating compressor package

This paper presents the root cause analysis of the blade fracture of the motor cooling fan in a reciprocating compressor package. The dynamic analyses results showed that the first-order torsional natural frequency of the fan was within ±5% of the fifth-order excitation frequency. In addition, the maximum dynamic stress of the fan operating in conditions of torsional resonance was 40.88 MPa, and exceeded the allowable stress of 36.12 MPa. Furthermore, the maximum dynamic stress occurred at the same locations where the cracks initially emerged. It was concluded that the excessive dynamic stress caused by the torsional resonance was the root cause of the fatigue fracture of the fan blade. Design improvements were implemented to adjust the natural frequencies of the fan, and included reducing the height of the blades, increasing the number of blades, and changing the weld locations. After the implementation of these modifications, the first-order torsional natural frequency of the new fan avoided the resonance regions, and the maximum dynamic stress of the new fan was 0.69 MPa, which was considerably lower than the allowable stress. No fracture problems occurred on the new fan.