High-speed Applications
During the frequent plant visits of our technicians, they often are 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.
A clear example are overhanging fans. This fans are frecuently belt-driven at a 1-to-1 ratio from a large electric motor with an average speed of the motor of 1,750 revolutions per minute (rpm).
If there are no reduction or increase in pulley size, it is safe to assume the speed of the bearings is quite similar. Frecuently these bearings are greased with a product that is 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 find 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 first learn how to determine the speed factor of a bearing.
Calculating the Speed Factor
The speed factor is a term that helps define the relationship of the speed at which a bearing rotates with the size of the bearing. There are two main ways to calculate this factor. The first is known as the DN value, which uses the bearing inner diameter 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 diameter, 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 lubricant. Among these properties would be the viscosity of the oil and the National Lubricating 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 determines how thick or thin the lubricating film 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 100 to 220 centistokes. While these type of greases may work fine for moderate speeds and loads, when the bearing speed increases, the viscosity must be reduced accordingly.