Part 2 of our 9-part series on bearing lubricating procedures covers ensuring proper fill quantity. Over-lubrication can be as detrimental as under-lubrication. We review the procedures to avoid lubrication starvation and other fill-related problems. Next time we’ll cover the related subject of bearing free space.
The proper fill quantity is important to ensure that all contact surfaces are provided with a suitable lubricating film over the designed operating life. Over-lubrication can be as detrimental as under-lubrication. With over-lubrication, there is an increase in the internal friction of the component as excess lubricant is moved through the free space. This results in increased heat generation and therefore a shorter application operating life. With under-lubrication, a boundary lubrication condition will occur, as all contact surfaces are not supplied with the proper quantity of lubricant. This condition may lead to wear and/or lubrication starvation resulting in shorter operating life.
The correct lubricant quantity is determined by the design, operating speeds, reservoir volume, and the extent of sealing or shielding found in the application. The objective of the lubricant fill quantity is to provide the contact surfaces with a consistent lubricating film, thick enough to prevent metal-to-metal contact and support full fluid film lubrication.
Fill quantity becomes of particular interest when the application requires the use of grease lubricated bearings. The bearing modified speed factor (ka•nDm – see table below for correction factors) for the application becomes an important factor in determining the proper grease quantity.
During run-in, when the lubricating film is setting up, excess lubricant will be expelled from the contact surface area. It is important that this excess lubricant is not restricted from leaving the contact surface area so as not to increase the internal friction of the system. When the application is sealed or shielded (no exit path for the excess lubricant) it becomes especially important to choose the proper fill quantity.
By providing a cavity outside of the contact area greater than the static free space of the bearing, there is enough of an area available for the excess lubricant. This cavity will also provide a lubricant reservoir that may continually feed the lubrication point, through capillary action, during operation. This cavity must be large enough to be able to contain the total volume of excess lubricant, but small enough to ensure the proper capillary action. The availability of a proper grease reservoir can aid in sealing and extend the overall lubricant life.
Most bearing and grease manufacturers recommend the following rules of thumb for bearing fill quantity. The area marked in yellow indicates the grease fill as a function of the speed factor.
Note: Special attention must be given to relubrication quantities so that an over-lubrication condition is avoided. Typical relubrication amounts are 1/3 to 1/2 of the original fill quantity.
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