"In our plant, we are looking at building a stronger greasing program. Do you have any advice on how to select greasing frequency and quantity for each application (bearing size, speed, temperature, etc.)? At the moment, we are probing our bearing with an ultrasonic gun, but this often leaves the bearing undergreased."
To answer this common question, you must first gather all the necessary information to understand the conditions in which these grease applications are working.
To calculate grease quantity, you need the bearing’s physical dimensions (primarily the outside diameter and width). Once you have these two measurements (in inches), you multiply them together and then multiply by 0.114. The resulting answer is the grease quantity in ounces. This formula can be used with metric measurements as well; just use 0.005 instead of 0.114. The result will be grease quantity in grams. This formula works for all bearings and is widely accepted as the best way to calculate grease quantity.
Once the required grease volume has been calculated, you will need to determine how often to apply that amount of grease. This timeframe is known as the regreasing frequency. This calculation is more complex. The machine’s operating conditions must be collected along with some additional bearing information.
Temperature is known to affect both grease and oil, so naturally one of the first correcting factors that must be collected is operating temperature. The hotter the environment, the more frequently the grease must be replenished. The ambient contamination (how dusty the environment is) and moisture (how humid the environment is) work in a similar manner. The dirtier and wetter the environment, the more often the bearing must be regreased.
Simple things like the bearing’s physical position and vibration will also affect the grease’s run-out and frequency of reapplication. If the bearing is mounted on a vertical shaft, the grease has a tendency to run out of the bearing quicker, thus needing to be replenished more often. When the bearing is subjected to vibration, it causes the oil in the grease to separate from the thickener, allowing it to drain away from where it is needed much faster. Therefore, these two correcting factors — position and vibration — should be taken into consideration.
The final correcting factor is that of the bearing element shape. This is important because each different element stresses the grease differently. For instance, a ball rolling across the race churns the grease differently than a spherical element, which churns the grease much more heavily.
Below is a chart of these correcting factors and how they operate together. Once they have been calculated properly, the final pieces of the puzzle are the bearing ID and speed of rotation. Through some simple math, you are left with a run-time (in hours) frequency for regreasing. This new timeframe and real-time feedback from the ultrasonic probe will allow you to fine-tune the frequency to the optimum level.