General Gearbox problem in Wind Industry

Something from me to be shared based on my search from the experts:

According to Andy Milburn, gear and bearing consultant, Milburn Engineering, Inc.,
Seattle, gearboxes in wind turbines are prone to failures stemming from their
application in wind energy systems. The following are the most common problems:

Highly variable load and speed: Wind is an intermittent energy source. It alternates
between gusting and still. Therefore, the load that these gearboxes are trying to
transmit is a lot more variable than it is in a plant operation. "There are some
applications where the load in plants is pretty variable," says Milburn. "But I think wind
turbines are probably the worst of the lot."

Low gearbox safety factors: The drive system in wind turbines is designed to be
compact. "They try to make things as small as they can, so that means the safety
factors that they’re using are low compared to a typical industrial application where
they don’t have to be too concerned about weight," says Milburn.

Flexible foundation: Typically, a plant gearbox and motor are mounted on a large
concrete foundation or a steel structure that’s bolted to a concrete foundation. "Up in
the nacelle, we don’t have that luxury," says Milburn. "The nacelle is flexing and the rotor itself is causing lots of loads in the whole structure. This causes misalignment
between the generator and the gearbox."

Periods of no rotation: The gearboxes operate only around 30% of the time, so they
are often idle. "Most rotating machinery doesn’t like to sit at rest," says Milburn. "They
run into problems with lubrication then."

Operating as a speed increaser: Wind system gearboxes operate as a speed
increaser instead of a speed reducer. "Most gearbox applications use a high-speed
motor that drives something that’s turning slower," says Milburn. "In this case, you’re
taking the blade that’s rotating slowly, and you’re increasing the speed up to the
generator speed. So that has some affect on things in terms of lubrication."

Extreme operating environment: The turbines have to operate in extremely cold or
extremely hot settings. Although the gearbox is in a nacelle and protected from rain it
can still be subjected to extreme temperatures.

High operating temperature: Manufacturers are resistant to adding large radiators to
wind turbines. "They allow these gearboxes to run pretty hot, and that means the oil
viscosity gets low," says Milburn. "When they’re rotating slowly, you don’t get a thick oil
film between bearings and gears, so you get metal-to-metal contact — and that’s a
problem."

Design details: Design flaws may cause failures, even if they occur slowly over time.