KEY CONCEPTS
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Grease analysis and monitoring require consistent, representative sampling and an understanding of the greases involved and the components they lubricate.
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Regular grease analysis can identify systemic issues and opportunities to make an operation more efficient.
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Grease monitoring can help prevent catastrophic failures, resulting in less downtime and significant cost savings.
Meet the Presenter
This article is based on an Industry Insights presentation titled
Keys to Implementing a Successful In-Service Grease Analysis Program by STLE member Rich Wurzbach on March 15, 2022. Industry Insights is a monthly virtual event series available for free to STLE members. These events provide a forum for members to collaborate with STLE industry peers and an expert on a specific topic who gives a brief presentation, then facilitates conversation by asking discussion questions and providing actionable takeaways. For more information, click
here.
STLE member Rich Wurzbach is president of MRG Laboratories, a company that provides lubricant analysis solutions, based in York, Pa. He has 25 years of experience in developing condition-based maintenance programs and the applications of diagnostic technologies for industrial equipment. His expertise lies in the applications of infrared thermography, lubricant analysis, airborne ultrasound and the integration of various diagnostic technologies for equipment assessment. You can reach him at
rwurzbach@mrgcorp.com.
Rich Wurzbach
Oil analysis is widely used and accepted as a means of condition-based monitoring and early warning of wear, contamination and other incipient problems. Grease analysis programs, although less common, can provide the same benefits. Failure of even one grease-lubricated gear or bearing in a critical machine can bring an operation to a halt. Regular grease analysis can give an indication of problems in the early stages, and corrective measures can be taken prior to failure. In addition, grease analysis can uncover the root causes of some types of problems—contamination, wrong formulations, mixing of different grease types or specific wear modes—and help in planning corrective actions.
Grease manufacturers routinely test their products to ensure their performance and quality. Several industry organizations devote ongoing efforts to developing standards for sampling and testing new and in-service greases
(see Standards Development for In-Service Grease Testing). End-users are less likely to implement routine grease analysis programs, but those who do have seen substantial benefits.
Standards development for in-service grease testing
U.S. organizations
Electric Power Research Institute
Effective Grease Practices
MOV gearbox sampling and analysis studies
ASTM D7718: Standard sampling greases from in-service equipment
ASTM D7918: Standard method for in-service grease analysis by die extrusion (integrated tester method for six tests)
American Wind Energy Association
Recommended practices 812-815 for grease sampling and analysis with the a sampling and analysis system.
European energy companies
DONG Energy (now Ørsted) and Vattenfall
Main bearing research project 2011-2013
DONG Energy and Statkraft
Blade bearing 2014-2016
From drum to grease gun to bearings and gears
In-service grease monitoring is not like oil monitoring, and standard oil test methods are not necessarily applicable to greases. Oil viscosity is a key indicator of its lubricating ability, but grease behavior is non-Newtonian—its viscosity varies with shearing or other applied stress, which can vary from gear to gear or bearing to bearing. In-service grease also is less visible than oil. It goes from drum to grease gun to bearing or gear without the operator actually seeing it, and because it stays close to the interface it lubricates rather than circulating into a reservoir—taking samples from inside the machine can be difficult to impossible.
An effective grease monitoring program starts at the beginning: New greases should be checked before they are put into service. Unlike oils, greases are not filtered before or during circulation, so initial cleanliness is especially important. Grease cleanliness (moisture content and particulate count), even from the same supplier, can vary widely, depending on conditions during manufacturing and shipping. For example, a worn gear in a transfer pump involved in the manufacturing process could release metal particles into the grease. Also, greases made using open kettle processes are sometimes less clean than those made using closed kettle or continuous processes.
Once a new grease has been checked, it should be followed at every step between the drum and the machine. Contamination, in the form of particulates, moisture or other types of greases and oils, can occur at any point, from large drums to smaller storage containers, grease guns and injection ports on a machine.
However, not all particles in greases come from contamination. Some solid extreme pressure additives that are part of the formulation can show up during a particulates analysis. On the other hand, some molybdenum additive particles can be too small to show up on a routine test. Problem particles can be distinguished from additive particles using microscopy.
Factors in an effective grease analysis program
Grease is not thick oil, and the common attitude that “grease is grease” misses the point. Effective grease testing requires knowing the types of grease in service, the interfaces where they are most likely to come under shear and the condition of the grease when it went into service. The timing, location and conditions of grease sampling are critical to obtaining meaningful analysis results that can be compared with previous analyses. However, the time and effort can pay off by providing data beyond simple wear statistics.
Not all components will be accessible for grease sampling. For instance, a sealed bearing can’t be examined until after it fails. For other components that can be sampled, it’s important to identify the “live zones,” where the grease moves during operation. Sampling should be done from consistent locations and under consistent conditions to ensure that test results can be compared over time. Accessibility and operating conditions will influence how frequently sampling can and should be done.
Consistent testing pays off by establishing what’s “normal” for a given machine or component, which aids in spotting when conditions are moving away from this baseline. In addition, testing can reveal systemic issues with regards to grease cleanliness, matching the grease formulation to the application, mixing types of greases or other contamination and worker training in selecting and handling greases.
The types of tests and the equipment used should be appropriate for the individual components being monitored, and appropriate equipment should be used to take representative samples. The most common analyses are particle counting and typing and moisture analysis.
Die extrusion is one means of particle counting. The die presses a grease sample into a 1 mm-thick ribbon on a transparent substrate. Image recognition software analyzes a magnified video scan to count particle sizes, shapes and classifications. This method gives good results, but it requires special equipment and software, and it can be time intensive.
David Turner, senior technical services representative at Citgo Petroleum Corp., describes a simpler procedure using a Hegman gauge, commonly used for pigment particle analysis in the paint industry. ASTM methods are being developed for using this device to determine fine particle size distributions and concentrations in grease. The gauge is small, portable and inexpensive, and testing requires only a small grease sample (50 grams or less). However, results are obtained visually so there is some subjectivity. The Hegman gauge consists of a flat steel plate with measurement markings along its length. Two parallel channels running along the plate are deeper at one end, becoming progressively shallower toward the other end. A flat-bladed tool is used to spread a sample of grease on the plate, starting at the end where the channels are deepest and working toward the shallower end. Particle size is estimated by where along the plate particles leave tracks in the surface of the grease. Larger particles leave tracks in the deeper end, and smaller ones leave tracks in the shallower end.
Variations in moisture content can be a result of various formulations and manufacturing processes or water uptake or loss during storage or service. Different types of greases have inherently different moisture levels. Some grease types must be water-free for optimum performance, and heat produced by the machine can dry a grease during use. However, “less moisture isn’t always better,” Turner says, noting that water is a byproduct of manufacturing soap thickeners, and water can even be a stabilizer in some thickeners. Some calcium sulfonate greases are more moisture-tolerant than other soap-thickened greases, and clay-thickened greases require a certain amount of water.
Making the case for grease analysis
Remy Bakker of Poseidon Systems, a manufacturer of oil sensors, says that wind turbine operators often rely on mechanical monitoring (e.g., vibration sensors) for grease-lubricated components, but this can cause them to miss early signs of wear.
An early repair can cost $10,000 rather than a $200,000 repair after part fails, he says, but knowing when to step in requires having the data available. This kind of early intervention is available for oil-lubricated parts now, since inline sensors can monitor oil continuously. Grease analysis currently relies on spot testing, and it requires a technician to take samples from machine parts that can be difficult to access. Bakker’s company offers a tool that allows a technician to collect grease samples from up-tower bearings and insert them into an onsite sensor on the ground to get instantaneous readings.
Sometimes, taking a proactive measure like a hot oil flush can solve or alleviate a problem if it is detected early enough. Wind turbine operators also can buy time by downrating their assets (running a turbine at less than full power) if they see early signs of a problem. This allows them to continue to operate the turbine until they can schedule a repair. Repairs can be scheduled when the weather is warm, and multiple maintenance jobs can be done during one site visit—a real benefit for wind farms located at remote or offshore sites. As wind turbines grow progressively taller and larger, Bakker says, the cost of downtime and repairs rapidly outstrips the cost of routine testing.
Jorge Alarcón concurs, noting that replacing a large bearing can cost $100,000 to $200,000. Alarcón, global technical manager for oil condition monitoring at Bureau Veritas—a testing, inspection and certification services company—notes that some operations perform grease analysis based on how much the analysis costs rather than the price of the component that the grease lubricates, which can be orders of magnitude higher. “Cheap analysis can result in expensive repairs,” he says. Grease consistency is important, he says, but you also need information on the specific modes of failure. There is not one most common failure mode for wind turbine parts, he says. One study, based on approximately one million grease samples, found that wind turbine failure modes varied with the grease manufacturer, the type of grease and the geographic location of the wind turbine (which affected the types and sizes of loads that the turbines experienced).
“People do not understand the role of grease analysis as a maintenance tool,” says Wojciech Majka, CEO of Ecol, an oil analysis and lubrication services company. Customers can be more focused on the small amounts of grease involved or the small size of a single mechanical component, he adds, causing them to overlook the potential cost of even a small part failure in terms of substandard performance or downtime. His company holds discussions with OEMs before they approach the end-users, recruiting the OEMs to advocate grease analysis programs to their customers.
Wurzbach notes that these types of accelerated or simplified analysis methods and tools could be a way to increase adoption of grease analysis programs. Another powerful strategy is to demonstrate the value of grease analysis by gathering and publicizing success stories—how much time and money do companies save as a result of their programs, and what disasters have they averted? He adds that grease analysis programs have uses beyond preventing catastrophic failures. Regular analysis can guide operations toward more efficient grease usage and maintenance practices, and significant cost savings accumulate over time. He mentions one military helicopter operation that went from “best guess” grease maintenance to implementing a regular grease analysis program, optimizing their sampling schedules based on results from their analyses. Eventually, reductions in downtime saved the program $100 million annually.
Nancy McGuire is a freelance writer based in Albuquerque, N.M. You can contact her at nmcguire@wordchemist.com.