Machinery Lubrication

Machinery Lubrication Jan-Feb 2018

Machinery Lubrication magazine published by Noria Corporation

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www . | January - February 2018 | 27 not. ese would include the two lithium-complex greases shown in the table above: It's more than just the soap that determines compatibility. Even when the thickeners are the same, differ- ences in the base oil types, viscosities and additives can render a mixture inadequate or even damaging to a critical machine. For the two lithium- complex greases in the table above, many of the considerations matched up, including the base oil type (PAO), NLGI grade and even the manufacturer. However, a mixture of these products would certainly not produce an optimal outcome for a given machine, as each is designed for a very different application (speed, temperature and lubricant film type). In addition to these issues, the compatibility charts themselves are often in direct contradiction. For example, some charts indicate that "polyurea" grease is compatible with calcium complex, while others list them as being incompatible. Barium complex and clay greases are iden- tified as being compatible in several charts, but this is contradicted by other charts that describe the mixture as incompatible. After some review, it becomes clear that basing your grease mixing decisions on these charts is fraught with danger. Trust but Verify Some turn to lubricant suppliers for guida nce. A lthough most suppliers are quite familiar with the performance capabilities of their products, few have conducted testing on the thousands of competing lubricants to establish which are compatible and which present prob- lems. A "trust but verify" policy can be a sound strategy to ensure success. If you are consulting with a lubricant supplier on a product changeover and are concerned about the impact of grease mixing, take the supplier's advice under consideration but ask for a copy of the testing that was performed to determine if the product will be compatible in the mixture of concern. Laboratories that test in-service grease samples report that one of the most frequent causes of failure is grease mixing. In electric motor repair, it is quite common to receive reports of "overlubrication" leading to grease being found in the motor windings instead of the bearing housing. However, this sometimes may be a result of grease mixing, which produces a thinning of the grease mixture and allows it to be drawn past the housing shields and into the windings where it is distrib - uted by movement and air flow. e risks and incidents of grease mixing are widespread, so how do you know if two greases are incom- patible? In 2011, ASTM issued D6185, the "Standard Practice for Evaluating Compatibility of Binary Mixtures of Lubricating Greases." It involves preparing ratios of the greases in question and performing three tests. e mixtures are 50-50, 10-90 and 90-10, but could also be 50-50, 75-25 and 25-75. e tests are cone penetration (D217), dropping point (D566 or D2265) and storage stability at an elevated temperature, which is also determined by cone penetration. It is interesting to note that these methods are cited in ASTM standards as not being useful in predicting the performance of in-service greases. erefore, after a few minutes of mixing, a test that does not predict how a grease behaves in a machine is used to evaluate the suitability of a mixture to perform in bearings or gears for months or years with constant mixing. Addressing these concerns, an ASTM committee has created a working group to evaluate more advanced approaches used by some laboratories to test grease compati- bility and decide if improvements in the D6185 standard are warranted. Some of these enhanced tests include extended grease mixing and working prior to testing, as well as using techniques such as a stress rheom- eter to predict performance issues like hardening, separation of the oil and thickener, and tendencies of the mixture to tunnel or channel in the machine housing. Cer ta inly, avoid ing grea se mixtures should be the strategy employed whenever possible. Of course, sometimes it is not cost- effective to do so, such as when products in use are discontinued or when lubricant consolidation has been implemented as a cost-savings initiative. Successful organizations will carefully consider all the perfor- mance parameters and differences between the greases in question. Simply checking a box on a compati- bility chart is not sufficient to protect your critical machinery. ML PRODUCT MANUFACTURER A 'BEARING' GREASE MANUFACTURER A 'SPECIAL' SEVERE CONDITION GREASE Thickener Lithium Complex Lithium Complex NLGI Grade 2 2 Base Oil Type SHC Polyalphaolefin (PAO) SHC Polyalphaolefin (PAO) Additives Anti-Wear (AW) 11% graphite/1% moly (EP) Base Oil Viscosity 100 cSt at 40°C 1,000 cSt at 40°C Intended Use Higher speed motors Extremely slow speeds, higher temperatures, boundary lubrication Even when you know the right grease to use, the opportunities for the wrong grease to be mixed are many." "

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