Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/388231
24 September - October 2014 | www.machinerylubrication.com with different thickeners mixing together, the grease containing water or other foreign fluids, or the grease shearing into small parti- cles under extreme loads. Linear Sweep voltammetry Linear sweep voltammetry, which is also known as the RULER test, establishes the amount of amino and phenol oxidation inhibi- tors in a grease sample. Because lubricating greases are affected by factors such as time and temperature, they usually contain antioxi- dants along with extreme-pressure and anti-wear additives. These additives can break down. Therefore, the relubrication intervals and amounts must be adapted to the decreasing additive levels. Infrared spectroscopy can identify the development of oxidation in mineral-oil-based greases but not for synthetic base oils. The RULER test is used for these types of greases. By comparing the curves of fresh and used greases, the remaining lifespan of a grease can be determined as well as the best time for the next relubrication. Soxhlet Extraction Since base oil viscosity is a key factor in calculating bearing lifes- pans, most grease manufacturers provide this essential information about the base oil. However, no regulations currently exist regarding this issue. Generally, high viscosity is considered better. In order to demonstrate high viscosity, all of a fluid's compo- nents including the oil, additives and viscosity index (VI) improvers must be mixed. The viscosity is stated based on this mixture. This viscosity value has little in common with the way viscosity is calcu- lated for oil used in roller bearings because the grease no longer releases some parts of the thickener onto the bearing track. A Soxhlet extractor can separate the liquid grease component from the thickener. Oil that is extracted in this process only contains liquid components. Polymer or adhesive supplements, VI improvers and even solid lubricants remain in the thickener. After the oil-based components have been extracted from the soap, information can be provided about the grease's oil and thick- ener levels. Separating the components into solids and oil makes it possible to carry out a detailed analysis of the base oil with respect to its viscosity, composition and proportion of additives neutralization number Even greases can turn "sour." Oxidation of the base oil, break- down of anti-wear additives or entry of salted fluids will result in the development of acids in the grease. These acids can destroy alkaline thickeners, causing the grease to have a soup-like consistency and the base oil and soap residue to separate. The grease then will run off the bearing surface, and bearing failure may occur. Various reactions can lead to the fat becoming acidic. When this happens, relubrication should take place at more regular intervals. The neutralization number can help determine when a grease is not suitable for use and if the base oil or thickener should be improved. Water Resistance Although it is easier to seal lubrication points with grease than with oil, grease should be resistant to water. Hydrophobic grease will provide a seal against splash water. If a grease can emulsify water, there is a risk of corrosion and hydrogen embrittlement. A simple test for water resistance involves placing a thin layer of grease on a strip of glass. If the layer of grease turns a cloudy, milky color or dissolves from the glass strip entirely, the grease will absorb moisture when stored in water. If water runs off the layer of grease, the grease can be considered water-resistant. Copper Corrosion For this test, a copper strip is covered with grease on all sides and placed in a specimen container with the same grease. The sealed sample container is stored for a certain amount of time in a warming bath. After the test period, the copper strip is removed, cleaned with solvent and carefully dried. The level of corrosion is established by using a color scale to compare the copper strip's discoloration. This technique is utilized to investigate a grease's corrosive char- acteristics in the presence of copper, as the sulfur level alone does not provide enough information on the expected corrosion of metallic machine parts. The test can reveal whether the additives that are supposed to reduce the effect of sulfur compounds still work and how the grease will behave in terms of non-ferrous metal corrosion when used in bearing cages. Of course, all of these methods will depend on the ability to obtain a representative grease sample. Without this essential element, quali- fied grease analysis and a reliable diagnosis will be impossible. GREASES