10/24/2012 8:00:00 AM | Bearing Ratings
| Jerry Fox
For the last 26 years, application engineers at The Timken Company have been successfully applying the Timken 1986 bearing rating C90 in bearing life calculations for all the industries that we serve with the exception of the wind industry. For wind turbine bearing applications, our application engineers apply the ISO bearing rating C1 in bearing life calculations because the certification bodies for new wind turbines have stipulated that practice in order to promote more transparency among all the bearing suppliers. To quantify the impact of that practice, if all geometry is equal, a bearing rating calculated for 1M revolutions C1 using the ISO rating equation will be approximately 10% lower than a bearing rating C1 calculated using the Timken 1986 rating equation. That difference corresponds to a reduction on average of 30% in the calculated bearing life. Perhaps such a conservative approach was felt necessary, given the variability in performance levels offered by so many different bearing manufacturers competing to supply.
For the past several years, Timken has been working with two wind turbine certification bodies, Det Norska Veritas (DNV) and Germanischer Lloyd (GL), in an effort to gain their approval for using the Timken 1986 bearing rating when making a bearing life calculation according to the method prescribed in ISO TS16281. Last year we made a mid-course adjustment, presenting both certification bodies with our most recent life test data taken from our International Bearing Performance Audit (IBPA). The IBPA program regularly samples and life tests bearings from all our manufacturing facilities to ensure each plant maintains or exceeds the Timken brand promise. Due to continuous improvements in manufacturing technology that is shared among all of our plants and the consistency of bearing internal geometry resulting from those improvements, the life test data accumulated while testing thousands of bearings over the last 10 years clearly supports yet another rating increase beyond the 1986 rating. This latest increase we refer to as the Timken 2011 bearing rating.
DNV and GL have just recently endorsed a modified 2011 bearing rating increase for Timken tapered roller and cylindrical roller bearings used in wind applications. This endorsement now opens the door for wind turbine designers to begin taking advantage of a higher Timken bearing rating.
Starting now, for many of the bearings that we apply to wind applications, the increase in bearing rating C1 can be as much as 25% higher than the ISO bearing rating. Applying such a difference, bearing fatigue life calculated for a Timken bearing can increase by as much as 2.1 times.
You might be asking yourself how this development can benefit the design of your wind turbine, perhaps for your main shaft system or planetary gearing system. The answer is simple – being able to calculate a higher bearing life means that a higher value of calculated reliability can be achieved as well. Wind turbine power train designers find this extremely important as they attempt to demonstrate with calculated results to the certification bodies that their bearing selections can meet higher levels of reliability, particularly the case for offshore applications.
Not so long ago, the typical fatigue life design requirement at most wind turbine manufacturers had been 20 years L10, setting the expectation that 90% of a population of bearings would survive for 175,000 hours of operation for the given duty cycle. Today the major wind turbine producers commonly place higher bearing life requirements in their specifications. These bearing life requirements still vary among the manufacturers, but one might be 25 years L5 (95% survival) which equates to 342,000 hours L10 (based on a 3 point Weibull curve with a slope of 1.5). This increase in required reliability (or in other terms higher required calculated bearing life) can force the designer to scale the bearing up at the same time he is trying to reduce his purchased bearing cost or to cut weight. Therefore a bearing that offers increased rating C1 and higher power density helps the designer avoid this dilemma. Taking advantage of the new Timken 2011 wind bearing rating C1 is an excellent step towards lean design and becomes a very worthwhile consideration in almost any wind turbine project.
In fact, the bearing size in a wind turbine power train is based upon conformance to a maximum permissible Hertzian contact stress criteria in the rolling contact as prescribed by the standards organizations. For example, the maximum permissible contact stress at equivalent load applied for a low speed main shaft bearing would be 1650 MPa. For wind turbine gearbox bearings, the maximum permissible contact stress for the planetary idler bearing also at equivalent load is 1450 MPa and for the high speed shaft bearing, 1200 MPa. Once these maximum permissible stress criteria are satisfied, the analysis turns to calculated life which must satisfy the designer’s reliability requirement most often stated in terms of an L10 criteria in the bearing specification document. By selecting a Timken bearing with a 2011 wind bearing rating, the life to be calculated using the ISO TS16281 method increases on average by about 2.1 times. The design benefits immediately while scaling up to a larger size bearing is often avoided.
This endorsement by the certification bodies means that going forward, a competitor bearing having a rated capacity C1 derived using the ISO equation that delivers a predicted 20 years L10 can now be replaced with an equivalent geometry Timken bearing that can meet a customer requirement of 25 year L5 calculated life. This advancement at Timken represents yet another opportunity for you to create leaner, more competitive designs. We invite you to consider this advancement in bearing technology for your own applications and are anxious to assist you in the process.