Bearing Lubrication Procedures:
Part 3—Determining Bearing Free Space
Part 3 of our 9-part series on bearing lubricating procedures presents three ways to determine bearing free space. Knowing the cavity volume that can hold lubricant is the secret to avoiding over or under lubrication. Next time we’ll examine run-in procedures that improve bearing and lubricant performance.
The proper fill quantity of a grease lubricated bearing is often specified as a percentage of the bearing’s free space. Therefore, it is important that the bearing free space is accurately determined. Three methods for determining a bearing’s free space are described below. These methods are listed according to both accuracy of results as well as simplicity, with the simplest and most accurate method presented first.
1) Published Engineering Data
Many bearing manufacturers have already determined the free space for a number of their catalog bearings. Because each manufacturer maintains design information on his bearings, a simple phone call to the engineering department of the bearing manufacturer will result in the most accurate value for a specific bearing’s free space. Unfortunately, due to the large number of catalog bearings available, coupled with the relatively minor importance of this information, some bearing manufacturers do not have ready access to every bearing’s free space.
2) Published Reference Charts
Many major bearing manufacturers have developed generalized bearing free space reference charts. These charts help the user to calculate a specific bearing’s free space based on the bearings design configuration and inner diameter. Compiling this information for all bearing configurations makes these charts efficient reference tools. However, one must keep in mind that the free space information presented on these charts is generalized. Modifications to the bearing, such as the addition of shields or seals, may change the actual free space. In addition, due to differences in internal design configurations, information from one manufacturer’s chart may not always be transferred to another manufacturer’s product.
3) Rule of Thumb Equation
Probably the most complex method of determining a bearing’s fill quantity is the “Rule of Thumb” equation. However, this method is just that, a rule of thumb, and has limited accuracy. Keep in mind that some applications, such as those with available lubricant cavities or those operating with low speed factors, may not require an extremely accurate measurement of the bearing’s free space. In these cases, it is sufficient to use the following equation:
Free Space (cubic centimeters) = (w(D2–d2)–74251Wb)/1273.24
w = bearing width (mm)
D = bearing OD (mm)
d = bearing ID (mm)
Wb = bearing weight (lbs.)
What bearing lubrication challenges do you face? Let us know in the comments below.