Precision Lubrication for Grease-lubricated Bearings

There are four primary components to precision grease lubrication for bearings: lubricant selection, application method, the volume of lubricant to be delivered and the frequency with which it is applied.

Many different methods are available for specifying these values, and opinions vary concerning which approach is best. For this reason, the best approach may be to use more than one method to develop a strategy that defines default values, which are then fine-tuned based on feedback from visual inspections, operating temperature, ultrasonic instrumentation and other factors.

Due to the variability of operating conditions and machine design, it may be impossible to be precise without introducing the condition-based component to the formula. Condition-based lubrication should be part of any world-class program, and this subject is gaining interest in the lubrication industry.

By acquiring knowledge of the fundamental parameters used to define grease lubrication values and applying the latest technological tools, today’s lubrication engineer can improve the reliability of machinery, increase the efficiency of PM activities and continue to shift the lubrication program to the preferred and fiscally sound proactive strategy.

Figure 1. Regreasing Example

Grease Selection

When studying the grease lubrication strategy, a good starting point is selecting the right grease. Simply choosing the best-quality grease is not as important as picking the correct grease for a given application.

Too often, grease selection is oversimplified and the key properties overlooked. Grease selection is more complicated than lubricating oil selection. Due to the lack of specificity in most grease recommendations, it is important to learn how to properly select greases for certain applications.

Proper grease specification involves all of the components of lubricating oil specification including base oil viscosity, additive requirements and base oil type, as well as other considerations including thickener type and concentration, consistency, dropping point and operating temperature range.

While most users acknowledge grease quality, many don’t stop to make sure the right lubricant is being used. For a comprehensive discussion on grease selection, see the referenced article concerning this issue.¹

Application Method

There are several different methods for applying grease. Grease can be applied through centralized application systems, single-point auto application systems, hand packing and, the more common, manual grease gun.

A compelling argument can be made for the superiority of continuous application systems, but unfortunately, they are impractical for most applications. Therefore, the manual grease gun is the most common application method and is the primary focus of this article.

Manual grease application, when performed correctly, is an effective method and provides certain advantages over automatic systems.
One requirement of manual application is for the technician to be in close proximity to the lubricated component.

This allows for inspections to be made in conjunction with relubrication. In addition to sensory observations such as sight, smell and sound - instrument inspections, such as ultrasonic monitoring and temperature readings, may be used to provide component condition information and to fine-tune the lubrication activity.

The addition of ultrasonic monitoring to an already developed greasing strategy will help take the lubrication program to a world-class level.

Relubrication Volume

One of the more important and frequently botched components of the greasing strategy is relubrication volume. When asked how individuals determine the amount of grease to apply to a particular component, a common response is “however much it takes.”

This is obviously not a precise method. There are many acceptable methods for estimating the appropriate relubrication volume. A common method is recommended by SKF, where grease replenishment volume is defined by the following equation:

Gp = 0.005 * D * B

where
Gp = grease replenishment amount (gm)
D = bearing outside diameter (mm)
B = total bearing width (mm)

This method generally provides positive results, but does not always take every factor into consideration. For example, it does not account for differences in bearing housings and application points.

Instead, it assumes the grease is added at the optimum location. Also, it is not always possible to know the amount and condition of old grease in the housing at the time of reapplication. For these reasons, it may be advantageous to modify the calculated values with a condition-based approach. The most advanced condition-based technique is ultrasonic instrumentation, which optimizes the relubrication volume.

By establishing a baseline value and determining a statistically appropriate limit, the volume of grease added can be optimized. It is the opinion of the author that this hybrid approach is the best method for arriving at the optimum relubrication volume rather than relying on calculated values or instrument feedback.

Application Frequency

The component of the greasing strategy that holds the most variability is the frequency of relubrication. Many factors must be considered to be even reasonably precise in determining the best application frequency. Such factors as operating temperature, seal type and condition, particle contamination, moisture, vibration and grease type all play a role in determining reapplication frequency.

Although there are several methods for calculating frequency, some of which take many of these factors into consideration, they can still generate different values. Figure 2 charts correction factors from SKF - a common method used to calculate frequency.

Figure 2. Frequency Calculation
(Courtesy of SKF)

Figure 3

Once again, to be precise, feedback is needed from the lubricated components to verify the proper frequency has been chosen. For frequency, as with volume, ultrasonic instrumentation might be the best tool available to help dial in the optimum relubrication frequency.

A conservative approach is to use a frequency generation method as a starting point, and continuously refine that value based on feedback from the ultrasonic equipment or other physical observations.

Summary

Optimizing a grease lubrication strategy can be tricky. The best approach is to combine the latest technology with proven traditional methods and of course, the valuable intuition that comes with experience and knowledge should never be underestimated.

Put simply, just remember to use the right lubricant in the right place, and use the right application method in the right amount at the right time. To bring all of these elements together and do the best job possible, one must be aware of all available techniques, theories and technologies.

Reference
“Step-by-Step Grease Selection”. Machinery Lubrication magazine, September 2005.