Dropping point
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Dropping Point is a qualitative property of a lubricating grease that gives a general indication of the temperature at which a grease passes from a semi-solid to liquid state under specific test conditions. It is a measure of the heat resistance of a grease.
Dropping point is used in combination with other test-based properties to determine the suitability of greases for specific applications. Dropping point is applicable to greases that contain soap thickeners. Greases with other thickeners, such as many synthetic greases, will not exhibit a change of state. Instead, they separate oil and the dropping point property does not apply.
The dropping point test is defined in ASTM standards D-566 and D-2265. The test apparatus consists of a grease cup with a small hole in the bottom, test tube, two thermometers, a container, stirring device and an electric heater. The inside surfaces of the grease cup are coated with the grease to be tested. A thermometer is inserted into the cup and held in place so that the thermometer does not touch the grease. This assembly is placed inside a test tube. The test tube is lowered into the container which is filled with oil. Another thermometer is inserted into the oil.
To execute a test, the oil is heated, while being stirred, at a rate of 8 to 12°F per minute until the temperature is approximately 30°F below the expected dropping point. The heat is reduced until the test tube temperature is 4°F or less than the oil temperature. The dropping point is the temperature recorded on the test tube thermometer when a drop of grease falls through the hole in the grease cup. If the drop trails a thread, the dropping temperature is the temperature at which the thread breaks.