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Concerning the big megawatt wind turbines, if there is such a big trouble with gearbox bearings failing much earlier than expected and costly maintanence or replacement of huge gearboxes, what is keeping people from placing the whole power unit(gearbox, generator, transformer, etc.) down on the ground, connecting the gearbox and turbine rotor with a chain-sprocket drive?

With a belt or chain ,a certain margin of flexibility would compansate for misalignment of axis and absorb shock loads. Apart form delivery time, replacement of damaged parts would take much less time and effort.

Continuously variable transmission systems are already being adapted to wind turbines so such a drive should not be an unrealistic option. Turbines with hydraulic drives are already being built with down tower power units.

I know this would have been done if it had more advantages but what is the big disadvantage here that overcomes such a saving of time and cost?

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  • $\begingroup$ "if there is such a big trouble with gearbox bearings failing much earlier than expected" - who's telling you that there is? Whose expectations were these? $\endgroup$ – 410 gone Jul 24 '19 at 14:49
  • $\begingroup$ This isn't an answer, but a common solution to gearbox failures on large, offshore turbines (where the replacement costs are the highest) is to do away with the gearbox entirely, and to operate large, many-pole permanent magnet generators directly at rotor speed. $\endgroup$ – Flyto Aug 23 '19 at 21:01
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The math doesn't make any engineering sense.

A 5MW turbine typically rotates at about 10 RPM, near enough 1 radian per second.

Assuming you had reasonably big sprockets on the chain drive, say 2 meters diameter (!!) the linear speed of the chain would be 1 meter /second, and the tension in the chain would be about 5MN or about 500 tons force.

Something tells me that is not going to be more reliable than a gearbox - and of course if you want to reduce the chain tension by increasing the speed, then you still have a gearbox at the top of the tower which has to transmit the full 5MW of power to the chain.

In fact the chain makes the loads on the bearings worse, not better. As well as the thrust force from the turbine, the top bearing now has to support a force of 500 tons downwards to resist the tension in the chain. And of course the tower also needs to be strengthened, to support a turbine that effectively "weighs" 500 tons when it is working.

Maybe replacing the existing bearings if they fail is a better idea :)

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The extra parts required (remote drive units, drive mechanism etc) is more costly than the cost of the servicing issues encountered when the units are up top.

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The introducing an extra drive link - chain drive or belt drive - the efficiency of the system would decrease. Also another item would be introduced into the system that would need to be maintained, which would increase operating costs, in addition to increasing capital cost.

The more items within a system the increase in potential for something to malfunction.

There would also be additional noise produced which could upset neighbors.

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Gear box failures were not unexpected by engineers , only environmentalists. Over 15 years ago , at an American Society for Metals dinner meeting ( Houston chapter), I heard about the limited life of the gears and how difficult it was to get and install new ones. The speed increaser gears are very large so there were/are very few sources in the world capable of heat-treating them. The great majority of prime movers are high speed / modest torque : gas turbines,steam turbines, and internal combustion engines. Wind turbines are the exception ; they are low speed high torque. This combination puts great demands on the gears of a speed increaser - in particular the high torque. The speed increaser gears are at the limits of the steel in terms of bending load and contact fatigue load , so limited life was /is expected. Like driving a car 100 mph, the car will do it but not for nearly as far as driving at 65 mph. The rotational speed of the wind turbine must be increased to produce the proper electrical frequency with the limited diameter of the generator The suggested alternative requiring tons of chain and associated bearings, etc, would be a bigger problem. Also , how would the wind turbine rotate into the wind if it had a chain run to a stationary generator on the ground?

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  • $\begingroup$ Why have a chain drive? a hydraulic drive might be more sensible (motor / pump combination also needs reservoir treatment unit)... $\endgroup$ – Solar Mike Jul 24 '19 at 19:58
  • $\begingroup$ A hydraulic drive would still have issues with the pressures and flows involved -- although I think about how big a hydraulic motor or linear actuator is vs. the same-sized electric actuator, and maybe it'd make sense. $\endgroup$ – TimWescott Jul 24 '19 at 20:19
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    $\begingroup$ "Gear box failures were not unexpected by engineers , only environmentalists." This is not true. In larger capacity wind turbines, failure of gearbox bearings occurs well below the designed lifetime of the bearings. Despite adhering to good manufacturing practice and design, many bearings undergo premature failure well within the 20 years of their expected lifetime. There is an extensive amount of engineering literature regarding this, as a brief bit of research would reveal. $\endgroup$ – Involute Aug 12 '19 at 17:35
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    $\begingroup$ For instance, this paper covers these premature bearing failures: de Azevedo, H.D.M., Araújo, A.M. and Bouchonneau, N., 2016. A review of wind turbine bearing condition monitoring: State of the art and challenges. Renewable and Sustainable Energy Reviews, 56, pp.368-379. $\endgroup$ – Involute Aug 12 '19 at 17:37
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    $\begingroup$ Typical bearing failures occur via rolling contact fatigue, and these larger wind turbine bearings are also expected to fail by RCF after 20 years. The failure modes of these premature failures are different. For instance, the following paper investigates how wind turbines planet bearings instead fail via white structure flaking: Evans, M.H., 2012. White structure flaking (WSF) in wind turbine gearbox bearings: effects of ‘butterflies’ and white etching cracks (WECs). Materials Science and Technology, 28(1), pp.3-22. $\endgroup$ – Involute Aug 12 '19 at 17:44

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