2/18/2010 11:00:00 AM | Bearing Applications
| Jerry Fox
I recently read a report about an offshore wind farm project located off the eastern coast of England. The report details the site’s financial performance during a one-year start-up period and how poorly performing gearbox bearings – specifically at the intermediate and high-speed shaft positions – resulted in unplanned work, negatively impacting operating and maintenance (O&M) costs.
According to the report, corrective actions were made but the example serves as a reminder of how important gearbox bearing designs can be to the success of an investment project.
Over the years, I have participated in numerous discussions relating to wind turbine gearbox designs. Some defend the merits of using conventional bearing styles such as spherical roller bearings and ball bearings while others, including consultants and operators, report recurring problems in the field.
So why were spherical and ball bearing styles selected for gearboxes in the first place? It appears to be a combination of several factors. These bearing types on paper have acceptable L10 life and are usually attractively priced; the designs are also pre-adjusted and typically easy to mount. But unfortunately, common bearing failure modes such as smearing and micropitting are often overlooked. In my opinion, while the L10 life based on classical fatigue looks acceptable, the actual field performance is dismal.
To address this issue, Timken has applied bi-directional tapered roller bearing technology to the locating positions of low, intermediate and high-speed shafts in wind turbine gearboxes. Since 2006, the Timken® locator bearing has been used successfully in 1.5 MW gearboxes without report of damage.
The locator bearing is a single-row tapered roller bearing that can support radial loading from any direction and thrust loading in both directions along the axis of the shaft. It is designed to support combined radial and thrust loading across the roller body while the turbine is operating in the "power generation mode." The internal geometry of the bearing is designed to ensure that during this mode, the bearing rollers are in contact to create a 360 degree load zone, thereby operating with a true rolling motion. This helps to maximize the load carrying capacity of the bearing, center the shaft for improved sealing and gear contact and eliminate opportunities for roller skidding or raceway smearing.
During the "motoring mode” when the gear thrust reverses, the bearing partially unseats. The radial reaction is supported across the roller body on a smaller load zone while the smaller thrust load is supported between roller corners and flanges on both the inner and outer raceways.
Besides its load carrying features, the locator bearing eliminates an entire bearing row, helping to reduce overall gearbox length, weight and cost. It is a bearing style that merits further consideration as a modern alternative to creating a lean and reliable gearbox design. For more information, download the technical paper.
One final point: creating a reliable bearing system for parallel shafting is very possible but requires that equal attention is given to both the locating and non-locating positions. Stay tuned for more information on this topic and what can be done to help eliminate competing bearing failure modes affecting these positions.
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