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Contemplating a switch of grease products, due to poor performance or process change, brings to light the critical issue of compatibility. Before implementing a new product, plant and maintenance engineers must weigh all consequences of grease intermixing and the impact on equipment reliability, production levels and the bottom line.
Innovative data and testing tools aid the decision process through the analysis of two or more co-mingled greases. A critical part of this assessment is working with a lubricant supplier with the specific industry know-how, and the proper testing procedures in place to ensure maximum grease compatibility and performance, promoting business success and profitability.
Understanding compatibility and contamination is the first step to optimal grease performance, plant operations and return on investment. Simply, compatibility results when the performance properties of the mixed greases are equal or better than the lower performing grease. In some cases, performance properties of the mixed greases may be less than the lower performing grease, yet still meet customer specifications and requirements.
Incompatibility can be identified from the performance or measured physical properties of two or more mixed greases. An incompatibility issue could be reflected in reduced lubricating performance due to the modified composition of the fluids and additives from intermixing. Changes in physical properties would be reflected by a softening or hardening of grease consistency, or decreased shear stability upon mechanical working and even reduction in thermal stability identified by a reduction in dropping point.
Maintenance and engineering personnel can minimize the risk associated with grease changes by doing each of the following:
The final determination of compatibility comes with proper testing of greases in the application for key performance properties.
More often than not, intermixed grease analysis shows minimal changes in grease performance, but this cannot be taken as the rule without proper assessment. Because 100 percent grease replacement is unlikely, some level of contamination is certain to occur, requiring the pre-implementation measurement of leftover grease with a new product before finalizing an upgrade or machinery process change. Taking this precautionary measure assures reliability and performance of equipment. In addition, because equipment users usually do not have the proper resources to perform testing themselves, it is the responsibility of the grease supplier to measure grease compatibility with dropping point, penetration and high-temperature performance test machines.
Optimally, supplier testing will show low levels of contamination where performance is not affected, making the transition to a new grease product quick and easy. However, a true incompatibility situation may result when low-performing grease affects the performance of the higher performing grease, meaning another grease with more desirable intermixing attributes should be considered.
Current technology pinpoints assessment of dropping point, consistency after shearing conditions, and change after storage at extremely high temperatures as measurements providing the most information with the least resource expenditure. These tests can be conducted concurrently or sequentially with varying ratios (for example, 50:50, 75:25 or 10:90) of mixed grease to characterize product performance and physical attributes such as friction, wear, evaporation rate, bleed, shear-stability, oil separation, soft metal corrosion and load-carrying ability.
An overview of a recently performed aviation grease study by Anderol demonstrates the necessity of proper grease testing within the actual manufacturing environment. In this example, a recent three-test trial of intermixed lithium soap-thickened greases reveals details of compatibility when observing grease consistency, antiwear and extreme pressure along with water contamination on extreme pressure grease performance.
For study purposes, two greases (sample No. 1a and sample No. 1) were designed with a thickener system and additive package for conformance to BMS3-33A specification, and one grease (sample No. 2) with lithium 12-hydroxysterarte, meeting the requirements of MIL-PRF-23827C. The low-temperature (-73ºC) properties and cone penetration ranges are shown in Table 1.
All three grease samples had similar low-temperature (-73ºC) properties and a similar cone penetration consistency range.
For this experiment, a low-level, 10 percent by weight, of contamination was implemented to avoid harmful degradation of the individual greases and to simulate a situation common to typical grease changeouts. With these attributes in place, cone penetration (per ASTM D217), four-ball wear (per ASTM D2266), and four-ball extreme pressure/load carrying capability (per ASTM D2596) tests were performed concurrently. It was found that all three greases showed good mechanical shear stability, antiwear performance and extreme pressure characteristics consistent with the specifications to which each individual grease was designed. Simply put, all the grease samples were found to be compatible with one another.
Even though the study found no grease incompatibility issues, changes in grease performance were measured and documented. In particular, the results of the load-carrying capability or four-ball extreme pressure testing illustrated the aspect of performance dilution upon contamination. Table 2 presents a selection of data points showing how low-level contamination by water and a lesser performing grease resulted in a reduction in performance of a higher performing product.
A graphic representation showing the dramatic difference in load-carrying capability between two grease samples (sample No. 1 and sample No. 2) designed to different specification requirements is illustrated in Figure 1.
Figure 1. ASTM D2596, Load Wear Indices of Individual Greases
The effect of performance dilution upon a grease with a high load wear index when contaminated by water and/or a lesser performing product is represented in Figure 2.
Number Sample Composition
Figure 2. ASTM D2596, Load Wear Indices of Contamination Samples
The lithium complex grease, sample No. 1, upon low-level contamination by the lithium 12-OH stearate grease, sample No. 2, or water exhibits approximately a 10 percent reduction in the measured load wear index. The performance dilution continues when the lithium complex grease is contaminated by both the lithium 12-OH stearate grease and water. The dilution in performance is again nearly equivalent to the level of contamination, that is a 20 percent reduction in the load wear index upon 20 percent contamination.
Figure 2 shows how the performance of a grease can be reduced by contamination from water or a lesser performing grease.
If incompatibility occurs, failure would be found in any of the tests of a given specification requiring no other testing; however, a borderline compatible reading requires additional application testing for conformance to the extreme temperatures and excessive pressure conditions of a given grease application. Generally, greases having similar thickener types will be compatible, but even greases of the same thickener systems can be deemed incompatible because of differing additive treatments and must be judged on an individual basis.
Regardless of the manufacturing industry - be it food processing, mining, power generation or aviation - plant managers and engineers need to work hand-in-hand with a supplier employing highly trained professionals with expertise in industrial lubricants and applications to ensure compatibility.
Ideally, a supplier with an intensive research and development program is willing to perform tests before a lubrication change is implemented to protect machinery from failure or wear. In addition, where possible the supplier should be present throughout a grease changeout to make sure the new grease runs correctly.
The supplier should also spend time with maintenance personnel to make sure they know what to look for going forward. Consideration must also be given the supplier’s continuing oil-monitoring services for process maintenance after the changeout has been completed.
Companies placing emphasis on research and development are more readily able to meet a customer’s business needs through monitoring and analytical testing in the effort to support industry specifications every step of the way. With these elements in place, companies will enjoy improved operating efficiency, increased machine life and an overall cost effectiveness.