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"We just purchased several new grease guns. Do they need to be calibrated before being used? If so, how is this done?"
Most people think of grease guns as just rugged carriers that deliver grease to a bearing through a Zerk fitting. Greasing typically is done using a specific number of pumps with the grease gun or until grease oozes out of the seal. However, these practices can lead to a variety of problems. For instance, overgreasing can cause heat to build up in a bearing, resulting in both grease and bearing failures. Undergreasing (starvation) can lead to excess friction with potential damage to the rolling elements.
A better approach is to think of a grease gun as a finely calibrated lubrication device that delivers a precise amount of lubricant to the bearing. A grease volume and frequency calculator can help you determine how much lubricant is needed and how often to grease your bearings. Equipment manufacturer manuals sometimes include greasing instructions for bearings as well. While these methods will help you decide how often to regrease your bearings and how much grease to use, rarely is any consideration given to how much grease is being delivered by the grease gun.
One way to calibrate a grease gun is by employing a small scale with a high resolution (tenths for grams or hundredths for ounces). With your grease gun, pump 10 slow, even strokes of grease onto the scale. Next, take the reading from the scale and divide it by 10 (the number of strokes). This will reveal the average amount of grease per stroke.
Keep in mind that even grease guns of the same make and model can vary in the amount of grease they deliver with each stroke. Therefore, every grease gun should be labeled with the amount of grease expelled per stroke and recalibrated on a regular basis. Also, be sure the same grease is used when calibrating.
A grease metering device is another option that does not require calibration. It is installed between the grease gun's hose and the hydraulic coupler used to attach to the Zerk fitting. When the operator pumps grease through the hose, the metering device provides a digital readout on a small screen, measuring the volume or weight in grams or ounces. While the digital reading is helpful, these devices can be bulky and may not be the best option for bearings in tight locations.