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.
