Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/544388
38 | July - August 2015 | www.machinerylubrication.com IN THE TRENCHES WE S CA SH | NORI A CORPOR AT ION ost industrial facilities have bearings that rotate faster than normal processing equipment. When it comes to lubri- cating these pieces of equipment, not all lubricants behave the same way. For grease-lubricated components, the effects of the grease on the bearings can lead to increased heat, drag and ultimately prema- ture failure. By properly selecting a grease that can handle these higher speeds, you can help minimize any potential failures caused by mismatching the lubricant to the application. High-speed Applications During my frequent plant visits, I often am asked about the temperature at which bearings should operate. Inevitably, the bearings that seem to be running the hottest are the ones that rotate the fastest. For example, on a recent trip, I inspected an overhanging fan. This fan was belt-driven at a 1-to-1 ratio from a large electric motor. The speed of the motor was set at 1,750 revolutions per minute (rpm). Since there was no reduction or increase in pulley size, it is safe to assume the speed of the bear- ings was quite similar. These bearings were greased with a product that was much too thick for them, leading to the generation of excess heat and shortening the bearing life. By matching the grease properties more closely to the bearing needs, you can help prolong the life of the bearing. While this example paints a picture of a type of machine in most plants (fans), it is common to fi nd high-speed applications in other components as well. For instance, some pumps that are directly coupled to a motor and have grease-lubricated bearings may spin in excess of 2,000 rpm. The same holds true for certain mixers, agitators and blowers. These components may suffer if a multi-purpose grease is simply applied without much regard to the needs of the bearing. To understand what the bearing requires in terms of lubrication, you must fi rst learn how to determine the speed factor of a bearing. Calculating the Speed Factor The speed factor is a term that helps defi ne the relationship of the speed at which a bearing rotates with the size of the bearing. There are two main ways to calcu- late this factor. The fi rst is known as the DN value, which uses the bearing inner diam- eter multiplied by the speed at which it rotates. The second method is known as the NDm value. This uses the bearing's median size, also known as the pitch diam- eter, and the rotation speed to calculate the speed factor. The speed factor can help you determine a variety of lubricant properties, which you can then utilize to select the proper lubri- cant. Among these properties would be the viscosity of the oil and the National Lubri- cating Grease Institute (NLGI) grade of the grease for the application. Viscosity The most important physical property of a lubricant is the viscosity. Viscosity is what CHOOSING a HIGH-SPEED GREASE M GREASE TYPE BASE OIL VISCOSITY (40°C) SPEED FACTOR (NDM) Slow-speed, high-pressure, industrial grease 1,000-1,500 cSt 50,000 Medium-speed, high-pressure, industrial bearing grease 400-500 200,000 EP, NLGI #2, multi-purpose grease 100-220 100,000- 200,000 High-speed, high-tempera- ture, long-life grease <70 600,000 High-speed, long-life grease 15-32 >1,000,000 G r e a s e s OPERATING TEMPER- ATURE DN (speed factor) NLGI NO.* -30 to 100°F (-34.4 to 37.7°C) 0-75,000 1 75,000-150,000 2 150,000-300,000 2 0 to 150°F (-17.7 to 65.5°C) 0-75,000 2 75,000-150,000 2 150,000-300,000 3 100 to 275°F (37.7 to 135°C) 0-75,000 2 75,000-150,000 3 150,000-300,000 3 * Depends on other factors as well, including bearing type, thickener type, base oil viscosity and base oil type.