Machinery Lubrication magazine published by Noria Corporation
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determines how thick or thin the lubricating fi lm will be based upon the load, speed and surfaces in contact. This must be matched to the needs of the bearing. Most general-purpose greases have a base oil viscosity of around 220 centistokes. While this type of grease may work fi ne for moderate speeds and loads, when the bearing speed increases, the viscosity must be reduced accordingly. There are many ways to calculate viscosity. By utilizing the speed factor mentioned earlier, you can use standardized charts to iden- tify an appropriate viscosity for the bearing at the operating temperature. In the previous example of the fan bearing, the NDm value of the bearing was 293,125, which led to a base oil viscosity of approximately 7 centistokes. The bearing was operating at around 150 degrees F. With a standard viscosity index of 95, this equates to an ISO 22-32 base oil viscosity. If you were to use a standard multi-purpose grease, this bearing would receive about 10 times the viscosity needed. Although some excess viscosity isn't necessarily a bad thing, this level would be a bit extreme. Excessive viscosity can lead to excess heat generation and increased energy consumption. Both of these are detrimental to the health of the bearing and the lubricant. The hotter the bearing runs, the lower the viscosity of the grease becomes. This can cause increased grease run-out and require more frequent applications of fresh lubricant. The energy consumption can also add up over time, resulting in money lost due to nothing more than the increased drag from excess viscosity. With grease, it is common to be able to lubricate bearings easily until they reach speed factors greater than 500,000. This is when specially formulated high-speed greases are employed. Some greases on the market are touted to work up to speed factors of 2 million. However, it is worth noting that not all greases are created equal, and not all can perform well at varying speed levels. Channeling Characteristics One property of a lubricating grease that can determine how it will lubricate at high speeds is called channeling. This term is used to defi ne how well grease can fl ow and fi ll a void left in its surface. Method 3456.2 of Federal Test Method Standard 791C offers one way to test the channeling characteristics of a lubricant. In this test, grease is applied to a container, and the surface is leveled off. After the temperature has been stabilized, a steel strip, known as the channeling tool, is pulled through the grease, leaving behind a void or channel in the grease. After 10 seconds, the grease is checked to see if it has fl owed back into the channel or covered the bottom of the vessel. If the grease has fi lled the void, it is known as non-channeling. If the grease did not fi ll the void, it is labeled as a channeling grease. Channeling greases are more easily pushed out of the way of the element as it rotates, thus leading to less churning and less temperature gain. Greases that are non-channeling fl ow back into the path and can result in the generation of excess heat. Thickener Type Aside from the base oil viscosity, another grease property that impacts its channeling characteristics is the thickener type. The thickener in a grease is commonly referred to as the sponge that holds the oil. The structure of the fi bers in the thickener can affect certain grease properties, such as channeling, bleed, dropping point and overall consistency. Some grease thickeners have long fi bers, while others have short fi bers. Short-fi bered thickeners will have a smoother texture. More complex thickeners, as well as those ML IMPACT OF BEARING CONDITIONS ON BASE OIL VISCOSITY SELECTION ISO VG (cSt@40°C) Application Examples Load Speed Oil Sepa- ration* Pump- ability* 22 High-speed Spindles Low High High High 100 Large, High- speed Electric Motors 150 Wheel Bearings 220 Paper Mills, Multi-purpose, Industrial 460 Paper Mills, Steel Mills 1000 Mining Equipment Crushers, Bearings, etc. 1500 Very Low Speed, Heavy/Shock Loads High Low Low Low *Oil separation and pumpability are also influenced by grease consistency and thickener type. **Scales indicate directionality. 6 Factors for Selecting a High-speed Grease 1. Base Oil Viscosity – Ensure the viscosity adequately provides the lubri- cating fi lm but is not too thick to cause excessive heat and drag. 2. Channeling Characteristics – The grease should be able to channel so excess heat isn't generated from grease churning. 3. Dropping Point – The dropping point of the grease should exceed the operating temperature by a wide margin to avoid excessive bleed and possible bearing failure. 4. Thickener Type – Choose a thickener that can provide the proper dropping point, channeling and bleed characteristics. Also, if you use multiple greases, check the thickener types for compatibility in case of accidental mixing. 5. NLGI Grade – The consistency of the grease will have an impact on the bleed characteristics and channeling properties of the fi nished lubricating grease. 6. Additive Load – Most applications require additives to help the oil lubricate. For greases, a wide variety of chemical and solid additives can be blended to aid in fi lm strength and reduce friction and wear.