Kingsbury Eliminates Subsyncronous Vibration For Directed-Lube Bearings
Over the past five years, the use of directed lube tilting pad journal bearings has grown due to ever increasing speeds of turbomachinery. Directed lube bearings provide cool lubricating oil directly to the leading edge of the tilting pads, thus reducing power losses and oil film temperatures.1 (DeCamillo, Brockwell 2001) This technology has been used with thrust bearings for two decades; however, the use with journals is somewhat new.
New technology brings new problems. In late 1999, a customer approached Kingsbury with evidence of a new type of subsynchronous vibration (SSV) on a directed-lube journal bearing. The vibration was considered "new" because it occurred only in this directed-lube class of bearings - never in a flooded bearing. This new vibration was originally detected on a competitor's bearing but then also appeared on Kingsbury's LEG® journal bearing. Thus far, it has occurred on high-speed, lightly loaded shafts typically found on centrifugal compressors and turbines.
The new SSV found on directed lube tilting pad journal bearings does not appear to be unstable and threatening as the more well-known types listed above. Unlike the classic SSV, this new SSV has a very low amplitude - on the order of 0.1 to 0.2 mils pp - and a low, random frequency that is less than 20 Hz. The classic SSV locks in a fixed frequency, whereas this new vibration's frequency has been termed "hash" because of its random and, somewhat, broadband nature. (see Figure 1)
Since 1985 the patented Leading Edge Groove (LEG) bearing (see Figure 2) represented a significant technological improvement for Kingsbury in their market. Previously, the typical means of lubricating a bearing was to flood the entire area around the shoes and shaft. This method, while simple, exposes the rotating shaft to a larger volume of oil in and around the shaft, which adds to churning losses and heating of the oil. Instead of a flooded system, Kingsbury's LEG design supplies oil directly to the leading edge of each shoe and allows the bearing cavity to be evacuated. Its design provides customers with better machine efficiency, smaller lubricating systems, lower capital costs and increased load capacity. Needless to say, finding a solution to the subsynchronous vibration became a high priority.
Faced with this challenge, engineers from Kingsbury's Research & Development department set out to eliminate the subsynchronous vibration. They simulated the SSV anomaly by modeling their test equipment to replicate the conditions used by the OEM. Their in-house test rig was equipped with a similar LEG bearing and configuration then run under conditions to match the test performed by the OEM.
The SSV phenomenon proved to be especially complex. Most tests at Kingsbury's testing facility take about four weeks to complete. In this case, the R&D team developed and tested theories about this SSV for three months. They knew from early on that the vibration could be dampened to an acceptable level by simply flooding the bearing with oil (see Figure 3), but this defeated the entire purpose of the LEG design. Focusing on flow rates, they operated the bearings in many different configurations, including studying the effects of different seal ring designs, leading edge groove designs, pivot offsets, bearing clearances and preloads. They increased the supply of oil to the upper pads. Then they increased the flow to the lower pads. Nothing worked.
The question among Kingsbury's R&D team became, "How can we capture oil from the side leakage and re-introduce it back into the bearing?" The engineers considered baffling schemes, shields and a number of other ideas. Eventually, they settled on putting grooves directly into the babbitted surface of the pad (see Figures 4 and 5). Located on both sides of the pad, the grooves captured and redirected side leakage back into appropriate areas of the shoe. By reintroducing oil to the pad, the grooves recycled side leakage and lowered the amount new oil needed to maintain the "critical oil level".
The goal of Kingsbury's R&D program is to develop new products, improve existing products, resolve customer problems, mold data into a useful form, and investigate patentable ideas. Over the past few years, Kingsbury's R&D department has investigated topics other than the normal bearing related issues such as manufacturing costs, tinning compounds, adverse operating conditions, warranty claims, seals and baffles, shaft current damage, turning gear failure, oil contamination, investment castings, synthetic lubricants and additives.
As Scan DeCamillo, Manager of Research and Development said, "It is our experience that R&D provides an important advantage over our competition and contributes to our growth and profitability. Our goal is to best utilize our R&D resources to maintain leadership and technological advantage in the fluid-film industry".
1 DeCamillo, S., and Brockwell, K., "A Study of Parameters that Affect Pivoted Shoe Journal Bearing Performance in High-Speed Turbomachinery," Proceedings of the 30th Turbomachinery Symposium, Texas A & M University, pp. 9-22 (2001).
About the author...
Joe Wilkes is the Director of Engineering at Kingsbury, Inc. In addition to overseeing the Engineering department, he is responsible for all Research & Development activity at Kingsbury.