tailieunhanh - Engineering Tribology Episode 1 Part 5

Tham khảo tài liệu 'engineering tribology episode 1 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Lubricants and their Composition 75 ASTM D-566 ASTM D-2265 . The schematic diagram of a drop point test apparatus is shown in Figure . Although frequently quoted drop point has only limited significance as a grease performance characteristic. Many other factors such as speed load evaporation losses etc. determine the useful operating temperature range of the grease. Drop point is commonly used as a quality control parameter in grease manufacturing. Figure Schematic diagram of a drop point test apparatus. Oxidation Stability The oxidation stability of a grease ASTM D-942 is the ability of the lubricant to resist oxidation. It is also used to evaluate grease stability during its storage. The base oil in grease will oxidize in the same way as a lubricating oil of a similar type. The thickener will also oxidize but is usually less prone to oxidation than the base oil. Oxidation stability of greases is measured in a test apparatus in which five grease dishes 4 grams each are placed in an atmosphere of oxygen at a pressure of 758 kPa . The test is conducted at a temperature of 99 c and the pressure drop is monitored. The pressure drop indicates how much oxygen is being used to oxidize the grease. The schematic diagram of the grease oxidation stability apparatus is shown in Figure . oxidized grease usually darkens and acidic products accumulate in the same manner as in a lubricating oil. Acidic compounds can cause softening of the grease oil bleeding and leakage resulting in secondary effects such as carbonization and hardening. in general the effects of oxidation in greases are more harmful than in oils. Thermal Stability Greases cannot be heated above a certain temperature without starting to decompose. The temperature-life limits for typical greases are shown in Figure 27 . The temperature limits for greases are determined by a number of grease characteristics such as oxidation stability drop point and stiffening at low temperature. TEAM LRN 76 .