ILMA Compoundings – September 2019 - 25

"In some cases, where the temperatures don't differ
radically, one viscosity may be enough for the application.
In other cases, a multi-viscosity oil may be formulated using
viscosity-index improvers, which will allow these oils to be
used over a wider range of temperatures," he said.
Typically, a range of greases are formulated with different
viscosity oils to be able to cover the array of viscosities
needed for various applications, Kaperick said. "The grease
thickener's primary function is to hold the oil in place
and allow the oil to do its job of providing a protective
lubricating film," he explained, adding that some of that is
accomplished by adjusting the viscosity of the oil and some
is carried out with performance additives.

The Risk of Corrosion

Corrosion is caused by environmental moisture, such as
humidity, rain, condensation and salt spray. Corrosion
screening for both oils and greases can be done using the
dynamic EMCOR bearing test, Kaperick said. "The procedure is well-documented through ASTM (D6138) and is
included in several specifications for premium gear oils and
greases used in wind turbine applications," he added.
Kaperick explained that the test employs a double-row ball
bearing lubricated with the test oil or grease. "The bearing is
rotated for three eight-hour periods through a water solution
during the one-week test and then evaluated for the lubricant's ability to prevent rust from forming on it," he said.
"Often, a saltwater solution is used to mimic conditions experienced by offshore wind turbines and gauge the oil's ability to
protect bearings against this corrosive element."
Another form of corrosion in wind turbines is caused
primarily by vibrational wear called fretting and can take
place in grease bearings in a wind turbine, Kaperick said.
"The screening for the ability to protect against fretting
corrosion is carried out with the Riffel test. This test is
only used to screen greases for wind turbine applications
and applies very severe conditions with a 70-kilonewton
load vibrating for a million cycles in the presence of a 1%
saltwater solution," he explained.

The Life of Lubricants

The cost of a lubricant alone for a complete drain, flush and
fill for a midsized turbine can exceed $5,000, Kaperick said.
Then, there are the overhead charges for the crew, transport
and potential crane callout. "It is no wonder that extended
oil drain intervals are of interest to wind farms," he said.
The useful life of the lubricants can vary quite widely
depending on several factors, including the quality of the
initial formulation, the use of mineral or synthetic base
stocks, and the conditions it is subjected to, such as extreme
temperatures, water and salt spray.

"Due to the cost of changing the oil or
relubricating greased bearings, the
use of premium synthetic oil formulations, which can be used for longer
periods of time, is often justified."
Joe Kaperick, senior advisor, Afton Chemical
Changing lubricants within wind turbines can be
labor-intensive and may involve getting to remote locations, such as mountains or offshore sites, and climbing the
tower to access the lubrication points. "Due to the cost of
changing the oil or relubricating greased bearings, the use of
premium synthetic oil formulations, which can be used for
longer periods of time, is often justified," Kaperick said.
Shuangwen Sheng, senior engineer at National Renewable
Energy Laboratory, agrees, noting a move toward synthetic
formulations, such as polyalphaolefin (PAO)/ester, ester and
polyalkylene glycol (PAG)-based. "The synthetic oils offer
longer life and so reduce the need for oil changes," he said.
Because PAOs provide excellent viscosity index and
low pour point, Sheng said they are a fluid of choice for
applications characterized by wide ranges of operating
temperatures. With a PAO/ester blend, there have been
hydrolysis issues - breakdown in the presence of water -
making the selection of hydrolytically stable products a
critical issue, Sheng added. PAGs offer improved resistance
to micropitting, but they have compatibility problems with
coatings and seal material, he said.
Turbine manufactures or gearbox providers typically
recommend that lubricants be replaced every five years,

ILMA Compoundings – September 2019

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