These bearings use sliding motion and a thin film of oil to provide theoretically infinite life.
by Steven Pasternak
November 15, 2018
Circulating oil flows directly onto the thrust plates.Image 6. Circulating oil flows directly onto the thrust plates.

In addition to water cooling, this application also uses circulating oil lubrication. Circulating oil systems not only filter the oil, removing harmful contaminants that can damage the bearing or shaft, but also deliver the filtered oil directly into the thrust cavity, as shown in Image 6. This further facilitates oil film formation at the thrust faces and in the radial direction. The bearings are also lubricated by bronze trapezoidal oil rings, which provide valuable redundancy in the event that the circulating oil system shuts down unexpectedly. Circulating oil systems often serve more than one bearing, but for additional redundancy, this end user chose to use one small system per bearing, as shown in Image 7.

image 7Image 7. Each bearing is supported by both water cooling and a small circulating oil system.

To support any thrust load generated, the inboard bearing on each fan was supplied with a thrust collar and babbitted steel thrust plates with bi-directional tapered lands. Tapers in both directions allow the bearings to provide the same thrust capacity, regardless of the direction of shaft rotation. Tapered land bearings have high load capacity due to the slight taper that facilitates oil film formation, resulting in much higher thrust capacity and lower operating temperature than comparable flat-land thrust bearings.

Maintenance & Condition Monitoring

When a fan is designed, its performance in ideal conditions is not all that is considered. This equipment is designed to last for decades and is often in critical applications, making downtime very expensive. In this case, it has been estimated that an unplanned shutdown could cost as much as 1 million Euros per day. Therefore, the designers must consider ease of maintenance when selecting bearings. In this application, all bearing components are fully split, which significantly reduces the amount of downtime required for repair or replacement. The radial and thrust bearing surfaces are made of a soft babbitt material that is designed to protect the fan shaft from damage in the event of a failure. The liners, thrust plates, thrust collars, oil rings and seals are also replaceable individually, allowing for very quick replacement in an unplanned downtime situation, although it is recommended to replace liners and housings together when possible.

Condition monitoring is critical for hydrodynamic bearings, especially in outdoor applications. Detecting irregular vibration patterns or temperature spikes early on allows potential problems to be addressed before expensive secondary damage occurs. These fans were all equipped with accelerometers on the bearing housings to measure vibration, although the bearings are also machined to accept proximity probes. They were also equipped with resistance temperature detectors (RTDs) that measure bearing operating temperature.

Conclusion

When originally commissioning the fans, the end user requested fans that could run without failure for at least six years. To meet this need, the designers selected bearings with high load capacity, exceptional dynamic performance, water cooling and circulating oil lubrication that allows them to perform reliably in all conditions. To further ensure that any potential problems are addressed before becoming detrimental to the system, the designers equipped each bearing with RTDs and accelerometers. By choosing the right bearings for the job, the fan designer has confidence that their customer will be satisfied over the life of the fan. To date, there have been no reports of any of the six fans experiencing a failure.

References
1. He, M., Cloud, C.H., and Byrne, J., 2005, “Fundamentals of Fluid Film Journal Bearing Operation,” Proceedings of the Thirty-Fourth Turbomachinery Symposium, Texas A&M University, pp. 155-176.

2. Hamrock, B., Schmid, S., and Jacobson, B., 2004, Fundamentals of Fluid Film Lubrication. McGraw-Hill, New York, NY.

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