Test Method for Oxidation Stability of Lubricating Grease
Lubricating grease, commonly known as “butter”, is a semi solid or paste like lubricant blended from base oil, thickener and additives. It is a stable solid/semi solid product formed by dispersing thickener into liquid lubricant.
Base oil: Accounts for more than 70 % of total weight and provides core lubricating performance. Mineral oil, synthetic oil, vegetable oil and other types can be adopted.
Thickener: Forms the skeleton structure and determines the consistency of the product. It falls into two categories: soap based (lithium, sodium, calcium and other metal soaps) and non soap based (graphite, polyurea, bentonite, etc.).
Additives: Added as required to improve specific properties such as oxidation resistance, rust prevention and anti wear performance.
It remains solid at ambient temperature and adheres to vertical metal surfaces without flowing off. Under compression, it releases oil film to achieve lubrication.
Compared with ordinary liquid lubricating oils, it features a wider operating temperature range and longer service life, and is suitable for friction points with open or poorly sealed conditions.
Test Purpose
The core purpose of testing the oxidation stability of lubricating grease is to evaluate its resistance to oxidative deterioration during storage and service, providing critical reference for product R&D, quality control and working condition matching.
Core performance evaluation
Determine anti oxidation capacity: Quantify the inherent anti oxidation level of grease via indicators such as oxidation induction period and changes in acid value / penetration after oxidation. Longer time and smaller indicator variation represent better stability.
Predict long term service life: Simulate practical working conditions including high temperature, oxygen and metal catalysis to forecast performance degradation cycle of grease in equipment and prevent premature failure.
Production and R&D
Support formula screening and optimization: Compare anti oxidation performance of combinations of different base oils, thickeners and antioxidants during new product development to select the optimal formula.
Control finished product quality: As a mandatory factory test item, it ensures the oxidation stability of each batch complies with relevant national and industrial standards and guarantees product consistency.
Industrial application
Avoid equipment operation risks: Identify potential problems such as increased acid value, sludge formation and abnormal consistency caused by oxidation in advance, so as to prevent corrosion of lubricated components, oil circuit blockage and abnormal equipment wear.
Meet special working condition requirements: Verify whether grease can operate stably for long periods under harsh conditions such as high temperature, long life cycle and food contact scenarios. For example, food grade grease prevents food contamination caused by migrated oxidation products.
Test Sample and Apparatus
Test sample: Lubricating grease
Test apparatus: SH0325B Fully Automatic Lubricating Grease Oxidation Stability Tester, compliant with ASTM D942.
Test Procedure
Sample preparation: Take five clean and dry glass sample dishes. Accurately weigh 4.00 g lubricating grease into each dish and spread the sample into a uniform thin layer. Place the dishes on the lower layer of the holder and leave the top shelf empty to avoid condensate dripping onto samples. Place a small piece of glass wool at the bottom of the pressure vessel.
Vessel assembly and gas purging: Put the sample holder into the pressure vessel, fit the sealing gasket and tighten bolts evenly. Purge air inside the vessel by repeating oxygen injection and pressure release for four cycles. Charge oxygen according to ambient temperature so that the pressure stabilizes at 750 kPa ±14 kPa after heating in the oil bath.
Constant temperature oxidation: Immerse the pressure vessel in an oil bath maintained at 100 ℃ ±0.5 ℃. Stabilize for 2 h, check for leakage and confirm pressure stays at 750 kPa. Start timing. Record vessel pressure every 24 h for a total test duration of 100 h. If pressure drops abnormally and continuously, leakage is confirmed; invalidate this test and repeat.
Test completion: Stop heating upon finishing the specified test duration. Cool the vessel to room temperature, vent residual oxygen and take out samples.
Result calculation: Pressure drop ΔP = Initial pressure − Final pressure, unit: kPa. Report the average value of parallel samples as the final result.
Test Result
The pressure drop is 68 kPa after 100 h test. The relatively low pressure drop indicates good oxidation stability of this lubricating grease.
