It is often asked but seldom answered properly. In this section, we hope to lay-out some grease basic principles to guide the maintenance practitioner. To start, it has to be made clear that both over-lubrication and under-lubrication can lead to premature bearing failures.
There are many modes by which over-lubrication can occur, such as too frequent charging of grease, excessive grease quantities dispensed per interval, or perhaps a regime wherein there is no relief port for expired greases to exit. In general, having too much grease increases the operating temperature of the components thereby degrading the grease prematurely. The pressure built-up by having too much grease can also cause the bearing seals to burst, and thus allowing harmful, wear-causing contaminants to enter the bearing.
Infrequent re-greasing is the main cause, of course. Having too little grease generates excessive temperatures, too, (degrading the lubricant), due to the heat generated by excessive friction and wear. Voids in the bearing can also house wear-causing contamination that would have been otherwise “sealed” by the grease.
THE RIGHT QUANTITY
First, it is essential to determine the free bearing space, in cubic centimeters (cc), for each particular bearing. In the absence of reference materials, below is a table that provides a fairly accurate estimation on free-bearing space.
1.) Identify bearing inner diameter or bore of bearing in question, then,
2.) Determine bearing type / use bearing code as reference, then select which ‘curves’ from 1-9 to utilize as a reference,
3.) Refer to the x-axis on the left to calculate the grease volume in cc
The next step is to ascertain the fill quantity, as a percentage of the total free bearing space in cc. To do so, one has to calculate the speed factor of the bearing ((OD+ID)/2) x rpm)), then use the table below.
As a general rule of thumb, higher speed factor require less grease quantity, while lower speed factors indicate the need for a ‘fuller’ charge of lubricant.