dropping point

Dropping point is the temperature at which a grease sample begins to flow as a liquid under standardized test conditions, measured in degrees Celsius or Fahrenheit. It marks the upper thermal limit of a grease's useful semi-solid state. When a grease reaches its dropping point, it loses the mechanical structure that allows it to cling to bearing surfaces and instead drains away, leaving lubricated components unprotected.

The dropping point test is conducted using a cup-and-ball apparatus specified in ASTM D566 or similar standards. A small sample of grease sits in a cup suspended above a thermometer. As temperature rises at a controlled rate, the grease softens until the first drop falls from the cup onto the thermometer bulb. That temperature is recorded as the dropping point. The method is simple and reproducible, which is why it remains the industry standard despite newer analytical techniques.

Dropping point varies widely depending on base oil viscosity, thickener type, and additives. Lithium complex greases typically drop between 160 and 190 degrees Celsius. Polyurea greases often exceed 220 degrees Celsius. Calcium soap greases, now less common in industrial use, drop much lower, around 70 to 100 degrees Celsius. A grease's dropping point does not directly indicate its safe operating temperature: equipment should not run near the dropping point, as the grease will already be thinning at temperatures well below it.

Practical limits and selection

Dropping point matters most when selecting greases for high-temperature applications such as oven bearings, furnace equipment, or industrial motors under sustained load. A grease with a dropping point of 160 degrees Celsius may perform acceptably in a bearing operating at 120 degrees Celsius, but risk bleeding out at 140 degrees. Conversely, a grease with an extremely high dropping point often contains synthetic base oils and premium thickeners, making it far more expensive than necessary for ambient-temperature applications.

Dropping point is one of several key properties used to specify grease, alongside penetration (which measures stiffness), oxidation resistance, and water washout. A grease can have a high dropping point but still fail in service if its oxidation stability is poor or if it separates under mechanical shear. Manufacturers and maintenance engineers must read the complete technical data sheet rather than rely on dropping point alone.