United States

United States

The blame does not lie with Gearboxes and bearings

My compliments to you for the searching and comprehensive analysis of current gearbox problems (Windpower Monthly, November 2005). For me, you have highlighted the fact that there is still a lack of quantitative science in the application of gearbox technology to wind turbines -- and your invitation for comment on the "facts and issues" was irresistible.

My main observation is that the designers and manufacturers of gearboxes are getting a "bum rap." They have been suffering from the fact that many if not most wind turbine developers cannot tell them what loads their gearboxes will actually encounter. Then to make things worse, they have had to deal with designs where the gearbox suffers from internal deflections due to rotor bending loads and even thrusts.

Wind turbine designers who understood gearbox technology would not impose such penalties on the gearbox supplier. And the gearbox supplier would not accept those penalties if he were not anxious for the business and convinced that they were required for the functionality of the turbine.

Then there's the matter of varying success in control of the actual torque loads that pass through the gearbox. It is generally true that we have had a long history of only semi successful control of torque overloads. Many used to rely on stall control, which never did the job. Then there were a large number of systems where a hyperactive pitch control would go unstable and create large torque oscillations. Typically, those problems ended in compromises where, to remain stable, the control could not sufficiently attenuate loads due to turbulence.

Sorry if I come across as a wise guy, but I have had both technical and financial responsibility for the design and development of gearboxes going back to helicopters in the 1950s, and on numerous wind turbines during the last 30 years. I have had close contact with the design and development of seven different wind turbine models, five of which initially had very bad torque control performance. Some of them were greatly improved by suitable introduction of damping, and some of them ended up with excellent protection against torque overloads.

I do not buy the idea that the new epidemic of failures is being caused by the scale up of wind turbine sizes. Working on contract to the National Science Foundation back in 1975 we played with a comprehensive simulations of turbines with rotor diameters of up to 500 feet (152 metres), ridiculous as that was. It is from that perspective that I believe present-day scale up should not be the cause of technical problems -- economic yes, but not technical.

Although I am known as a non-believer in the classic wind turbine model with a three-bladed rotor, I greatly admire the success with which they have been and are being devolved. They are making great strides with what an old friend calls their "build'em and bust'em" development method. It can be done with less sophisticated technology than is required for a good two-bladed system. They will, however, always end up with vibratory loads in the blades, in the nacelle and the tower that cannot be eliminated. To solve their present gearbox difficulties, they mainly need to evolve more successful torque controls, whether constant speed or variable speed.

Anyway, my sympathies go out to the gearbox and bearing guys!

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