Grease test specifications
Oxidation Stability
Oxidation stability reflects the anti-aging performance of grease. For some industrial greases with a long service life, this property is particularly important and thus serves as a key indicator for such lubricants.
The common method for determining oxidation stability is to oxidize a quantitative amount of grease in an environment containing air (or oxygen) and metal catalysts at a certain temperature for a period of time, followed by testing the changes in its acid value, viscosity and the formation of precipitates.
All greases exhibit different auto-oxidation tendencies due to differences in their chemical composition and operating conditions. During service, greases will gradually oxidize and generate substances such as aldehydes, ketones, acids, resins and asphaltenes. Oxidation stability refers to the ability of grease to resist the generation of the above-mentioned harmful substances.
Rubber Seal Compatibility
Rubber seals are widely used in hydraulic systems, and greases are required to come into contact with sealing materials. If the compatibility between grease and rubber is poor, it may cause the rubber to undergo swelling, shrinkage, hardening and even cracking, thus losing its sealing function. Therefore, lubricants must have good rubber compatibility.
In hydraulic oil standards, this performance is often evaluated by the rubber seal compatibility index, which is measured by the changes of rubber test pieces of specified dimensions after immersion in the oil for a specified period of time.
Oiliness and Extreme Pressure (EP) Property
Oiliness refers to the ability of polar substances in grease to form a physicochemical adsorption film on the metal surface, thereby withstanding high loads and reducing friction and wear.
Extreme pressure (EP) property refers to the performance that, under high-temperature and high-load conditions, polar substances decompose and undergo tribochemical reactions with the metal surface to form a low-melting-point and plastic extreme pressure film, which in turn plays a lubricating role in shock resistance, high-temperature resistance and high-load resistance.
Corrosion and Rust Prevention
Grease may cause corrosion of steel and other non-ferrous metals during service due to oxidation or the effect of additives.
The corrosion test is usually performed by immersing copper strips in the oil at 100 °C for 3 hours and observing the color changes of the strips. The rust prevention test generally involves mixing 30 ml of distilled water or artificial seawater with 300 ml of test grease, inserting steel rods into the mixture, agitating it at 54 °C for 24 hours, and then inspecting the steel rods for rust.
High-quality grease should have the ability to resist metal corrosion and rust; both of these properties are usually mandatory test items in industrial grease standards.