When maintaining machinery, you must decide what lubricants are suitable for your equipment and application. One important consideration when selecting a grease is the dropping point. This is the temperature at which grease begins to drip or flow. It is determined by how well the lubricant resists thinning as heat increases. But, what factors influence this characteristic for greases and oils? Should you always select a lubricant with a high dropping point? This should clear up those questions for you.
What is Dropping Point?
The dropping point of grease is among its key characteristics. To put it simply, it is the temperature at which grease turns into a liquid. To put it another way, it is the temperature at which a heated grease sample will start to flow through a hole. However, the dropping point is always far beyond the grease’s high-temperature performance limit. So, it is not of the highest consequence for performance. To ensure that your lubricant stays in place, you must use grease at an operating temperature below its dropping point.
The Dropping Point Test
Most grease technical data sheets list their dropping point results. To conduct the test, grease is placed on the interior walls of a tiny cup that has a hole in the bottom. Next, a thermometer is inserted, making sure not to touch the grease. Afterward, the device is heated until only one drop of oil separates and drips out of the cup’s bottom. The dropping point of the grease is the temperature at which this happens.
Dropping Point vs. Operating Temperature
It’s important to realize that this test measures the highest temperature at which grease will maintain its shape, not the highest temperature at which it can function. From the results of a grease’s dropping point test, you can determine the maximum operating temperature using three straightforward rules:
- For results less than 300°F, subtract 75°F.
- For results between 300°F and 400°F, subtract 100°F.
- For results over 400°F, subtract 150°F.
If you plan on longer relubrication intervals, then you must subtract even more from the dropping point temperature to get the ideal operating temperature. Additionally, keep in mind that higher temperatures require more frequent regreasing.
What Impacts this Characteristic?
Grease thickener is the component with the largest impact on the dropping point. The fibers of the thickener impact not only this but also channeling, bleeding, and general consistency. Additionally, the thickener’s resistance to oxidation affects grease performance at higher temperatures.
When to Use a High Dropping Point Grease
For bearings that operate at high temperatures, a high dropping point is crucial. But, keep in mind that this doesn’t mean the base oil can survive high temperatures. So, it is best to always maintain a substantial temperature difference between the operating temperature of the bearing and the grease’s dropping point. The operating temperature of the bearing is another important factor to take into account when choosing a high-speed grease. Checking the grease’s dropping point will help you make sure it will function properly at high temperatures.
Below you’ll find a table of the different types of grease and temperature comparisons.
|Type||Dropping Point||Max Operating Temperature|
|Anhydrous Calcium||140°C||90 to 110°C|
The Bottom Line on Optimal Grease Temperatures
The dropping point is a measure of how well grease withstands heat. When the temperature of the grease rises, you will lose the proper consistency as the grease liquefies. Only a few greases can, after cooling from the dropping point, return to their original structure. That’s why this one tip is essential: To prevent excessive bleeding and potential bearing failure, the dropping point of the grease should be significantly higher than the operating temperature.