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 Chris Marks

Bearing Solutions for Planetary Gearboxes

7/19/2010 10:00:00 AM | Bearing Applications | Comments | Chris Marks |

The wind energy industry’s trend toward leaner and more integrated machines presents particular challenges, especially in regards to reliability. For Timken, this is an opportunity to put 110 years of engineering history to work on developing wind-related technological innovations.

One Timken Wind Energy Knowledge Center blogger you should be well familiar with, Chief Technologist Jerry Fox, exemplifies this drive for engineering excellence and innovation. He was recently selected for inclusion in WindPower Engineering magazine’s inaugural listing of the “2010 Innovators in Wind Power.” The magazine cited, among Jerry’s many accomplishments the 14 patents he holds along with 14 other patent applications currently in process, as well as his next contributions, which will include developing solutions for hybrid systems and a patented new series of low-weight modular designs to ease installation and service.

Jerry and many other talented engineers on our team are also leading technological advances with gearboxes – primarily gear and bearing reliability. And much of the focus on how this factors into the design of speed-increasing gearboxes is related to the planetary stage.

This stage can be as simple as a single-stage or as complex as a differential-stage design. In either case, the gearbox must be sized according to the gear and bearing sizes, which are dependent on the actual gearbox power rating. For the planetary stage, the planet bearing is usually the constraint in the overall gearbox envelope due to the size required to meet the current wind industry bearing life requirements.

The latest standards for IEC and ABMA recommend two candidates for bearing solutions at the planet position: cylindrical roller bearings (CRBs) or tapered roller bearings (TRBs).

Common planet bearing designs today are the two- or four-row CRB arrangement, depending on the gearbox load requirements. For either arrangement, it's critical that the bearing radial internal clearance (RIC) is properly selected and considered during bearing life analysis. A large variation in RIC among the bearing rows may result in significant load reaction among the rows, meaning that one row may be loaded more, resulting in less L10 and leading to potential issues with reliability.

More often than not, gearboxes are designed using helical gears due to their increase in torque density and decrease in noise generation over their cousin, the spur gear. Due to the fundamental nature of the helical gear, an overturning moment is always constant for a given input torque due to the opposing axial gear forces at the sun and ring mesh points. This moment tips the gear in the radial plane and attempts to misalign the gear contact, the planet-gear and the planet-pin.

Since CRBs must maintain operating clearance, any RIC must be removed from each row before load can be supported by it. Variation in clearance among the rows impacts the bearing reaction and contact stress. Bearing manufacturers compensate for this by producing matched clearance bearing assemblies that improve the load distribution across all bearing rows. While it's most beneficial to minimize operating clearance in order to maximize the number of rolling elements carrying the load, a CRB design can actually take advantage of the compliant gear body, thereby decreasing the degree of clearance matching to reasonable requirements.

Fundamentally, gear forces acting at each planet gear mesh point tend to ovalize the gear under load, especially since the CRB operates in radial clearance. Properly considering these ovalization effects in a bearing life analysis can benefit the bearing load share properties and increase bearing predicted life. Due to unique combination loading, speed and clearance required for this arrangement, skidding and smearing damage is more often a cause for concern than classic subsurface bearing fatigue.

Unlike CRBs, TRBs can be set in preload or negative operating clearance. This benefit, coupled with a larger effective spread between bearing centers, can be used to oppose the helical gear load effect and improve the planet tilting stiffness. Also, during lightly loaded transient conditions, preload can offer a full 360° load zone and maintain traction forces that protect the bearing against skidding and smearing damage. Timken offers design solutions such as our new wear-resistant wind bearings to enhance bearing life in these types of application conditions. In addition, replacing four CRB rows with two opposing TRB rows also offers potential power density advantages.

Regardless of bearing type, one of the most difficult aspects of bearing selection is defining the operating environment. The resultant planet and bearing loads can greatly be influenced by gear load factors KHβ and Kγ. These gear load factors describe the load distribution across the face of a gear and load share amongst a gear set respectively. While these parameters can be estimated or calculated, often the complexity of the tolerances impacting these loads aren't completely understood. Planet carrier bearing tolerances; housing and carrier positional locations; housing compliance and many other underlying factors can greatly impact actual bearing performance. For this reason, a detailed bearing system analysis must be considered for each planetary application.

I hope this overview is helpful and invite you share your thoughts or ask questions regarding planetary bearings and proper bearing selection for wind turbine applications.



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