Every new generation of lubricant specifications comes with a new set of challenges. Many of these challenges are in the form of added industry tests. New industry tests, and customer specific tests, can be a bottleneck in the development of new fluids because of long queues and unavailability. Wind turbines currently have one of the more demanding test landscapes, due to the number of necessary tests and changing specifications.

While the concept of wind power goes back thousands of years, recent advances in turbines have pushed the need for specialty fluids. This has created a plethora of industry and manufacturer specifications that change rather quickly. To add to the complexity, even with all the antiwear and fatigue tests required by OEMs, white etching cracking (WEC) is still a significant problem in wind turbine gearboxes. While this is not the most common problem, the downtime that is incurred due to this issue has made it a high priority for the industry to understand and solve.

In lieu of a screener test for WEC, an understanding of how a lubricant may reduce the occurrence of etching would be beneficial for the next generation of fluids. Theories regarding the cause of WEC agree that stress at the contact plays a role. Understanding how the fluid can impact this stress through friction and tribofilm formation is incredibly important.

The other main challenge for wind turbines is the extreme environments in which they are installed. To cope with this, the low temperature properties of a fluid must be understood.

Bench Test Advantages

There are multiple advantages to incorporating bench tests into fluid development. The main attraction of bench tests is usually that they are cheap (relative to engine tests) and fast. It is easy to employ a set of bench tests that utilize just a small amount of fluid. However, one important step must not be overlooked: the initial work to ensure the bench tests produce meaningful data.

All too often, bench tests are used in a standard test mode that has little to do with the end application. Results may fill a table but have little correlation to the application of interest. For a bench test to be meaningful, researchers must find the proper test with the proper conditions. Test fit can be measured by showing differentiation of existing commercial fluids with known performance characteristics.

Once bench test data is produced, it tends to be less noisy than more complex testing, allowing more reliable information to be extracted and analyzed. With a designed experiment and a good statistical software package, there can be a deep understanding of the impact of individual additives and additive interactions on performance.

Bench testing also lets formulators think outside the box. Many bench tests can be run with singular additives or binary systems without worry of harm to the system, like in a rig or engine test. This opens up the door to testing fluid combinations that might otherwise be too expensive or have a short shelf life. Giving formulators the ability to think outside the box can advance understanding of additive chemistries and the impact of additive interactions.

This approach also adds value after the product is commercialized. The understanding, as well as the tests that were developed, can be used to troubleshoot problems in the field. Furthermore, as specifications change in the future, the base of fundamental knowledge can be used to start developing the next cutting-edge product.

Rheologys Role

One discipline that is often underutilized, when it comes to the value of its bench-top testing, is rheology. While many rheometers have the flexibility to look at fluids under many conditions, oscillatory rheology (OR) excels at examining low temperature properties. OR can be used to gain a better understanding of low temperature properties of a fluid as compared to other industry techniques such as scanning Brookfield, fixed Brookfield and mini-rotary viscometer (MRV).

The scanning Brookfield technique records viscosity as a function of temperature. To make the measurements, there is always a significant shear rate on the sample, which constantly interrupts the formation of molecular networks in the fluid. Unfortunately, it is these networks that are of interest since they degrade performance. Techniques like MRV and fixed Brookfield viscosity let these networks form by not making any viscosity measurements until a desired temperature is reached. The end result is a viscosity at only one temperature and no information during the cooling cycle.

A low temperature sweep on an OR can produce viscosity measurements across a temperature range. By using small oscillations, the motion does not interrupt network formations, and the back and forth motion actually gives more information than just viscosity. Lubricants are often referred to as visco-elastic fluids, and OR can actually measure the viscous and elastic attributes of a fluid. In fact, this technique is sensitive enough to differentiate between low temperature networks formed by the paraffins in the base oil and interactions between the polymers used to modify viscosity.

Tribologys Role

Advances in tribological measurement equipment have been a dream come true for product developers. For example, a mini-traction machine (MTM) makes it possible to find the coefficient of friction across all friction regimes. Tribofilms can be observed by adding a spacer layer imaging option (MTM-SLIM), and the elemental composition of the tribofilm can be measured with a scanning electron microscope outfitted with an energy dispersive x-ray detector (SEM-EDX). The ability to quickly and reproducibly look at friction and tribofilm formation makes it easier to balance friction and antiwear properties. This type of analysis can also improve the balancing of other components like detergents and solubility enhancers that may compete for surface space.

Since tribofilms can increase friction, many friction modifiers work by controlling tribofilm formation. Formulators must be careful not to reduce friction at the cost of forming no tribofilm at all. By monitoring friction and tribofilm formation simultaneously, it is possible to optimize the components of a blend to achieve a thin, smooth tribofilm with superb frictional characteristics.

Put Testing to Work

To gain better understanding of how wind turbine fluids could affect WEC, a series of meaningful bench tests were identified. By collecting wind turbine fluids with known field performance, it was possible to set up these tests to differentiate performance and create a calibration curve. The tests included a combination of low temperature OR, friction measurements (MTM) and tribofilm analysis (MTM-SLIM, SEM-EDX). This slate of tests could then be used to understand the impact of the tested additives on significant fluid properties and create fluids that surpassed current commercial blends.

This approach showed that the fluid should:

Control friction as load increases.

Control friction as slide-to-roll ratio changes.

Form a thin, smooth tribofilm that is rich in phosphorus.

Maintain tribofilm composition as load increases.

Avoid low temperature network formation, maintaining pumpability even in extreme temperatures.

Lower elasto-hydrodynamic friction to reduce surface and subsurface stress. Base oils have the most impact on EHD friction. PAOs provide the lowest friction, testing indicates, and renewable API Group III+ oils can also provide benefits.

A fluid may be good at any one of these, but to be a wind turbine fluid, it must be good at all of these.

To keep up with lubricant specification changes, formulators have to adapt to meet these new demands or the industry may leave them behind. This adaptation must also extend to test methods. A reasonable effort in fundamental understanding can go a long way in new product development. Through a combination of industry performance tests and well-designed bench testing, formulators can truly gain insight into the effectiveness of a lubricant in any application. z

Jeffrey Guevremont, Ph.D., is a principal scientist and manager of the applications group at American Refining Group. His work focuses on development of innovative and sustainable products. Previously, he worked at Afton Chemical where he gained a deep understanding of lubricant additives, with much of his research concentrated on tribofilm formation, additive-surface interactions and metal fatigue. Contact him at jguevremont@amref.com.