I. Mechanism of Hydraulic Oil Aging and Deterioration
Hydraulic oil begins to deteriorate as soon as it is produced. Once added to a hydraulic system, the rate of deterioration accelerates significantly, gradually reducing its performance until it becomes unusable. Hydraulic oils can be classified according to their manufacturing methods into mineral oil-based hydraulic oils, water-based hydraulic oils, and synthetic hydraulic oils. Mineral oil-based hydraulic oils account for over 90% of the market; therefore, this article only discusses mineral oil-based hydraulic oils. The base oil of this type of hydraulic oil is mainly composed of carbon and hydrogen compounds—hydrocarbons—accounting for 96%–99.5%, with the remainder being sulfur, nitrogen, and ash.
After refining, crude oil is mixed with additives such as antioxidants, anti-wear agents, rust inhibitors, defoamers, extreme pressure enhancers, and pour point depressants to prepare the hydraulic oil required for production. During use, hydraulic oil undergoes physical and chemical changes due to external factors such as air, moisture, impurities, heat, light, and mechanical agitation, as well as internal factors such as the chemical composition of the hydraulic oil and the properties of additives, resulting in changes to the overall performance of the hydraulic oil. The causes, influencing factors, and results of hydraulic oil aging and deterioration are shown in Table 5-10.
Deterioration of hydraulic oil leads to equipment wear, and the amount of wear (metal particle content in the oil) increases with usage time. As shown in Figure 5-3, the wear curve of the hydraulic equipment shows that the wear increases slowly before point E, but rises sharply after point E. This indicates that the hydraulic oil has aged and deteriorated, and must be changed. Therefore, the time T (months) and metal particle content 9 (g) corresponding to point E are the indicators for when the oil should be changed. The rust prevention and corrosion resistance of hydraulic oil decrease with usage time. When it decreases to a certain level, the oil must be changed. Otherwise, hydraulic components will corrode or even be damaged.
Why does hydraulic oil age and deteriorate? How to determine the optimal oil change period?
II. How to determine the optimal oil change period?
Choosing the optimal oil change period for hydraulic equipment is a very important issue. There are three methods for determining the oil change interval: ① Regular oil changes; ② Determining whether to change the oil based on experience and observation of oil samples; ③ Specifying oil change criteria and changing the oil according to the quality of the oil sample analysis results.
Currently, regular oil changes are commonly practiced in Chinese factories. However, practice has shown that regular oil changes ignore the quality changes of hydraulic oil during use and fail to consider the actual working conditions of the hydraulic equipment. Therefore, frequent oil changes either waste oil or cause equipment failure due to the continued use of aged and deteriorated oil, which is unscientific. The second method involves periodically sampling oil from the tank for visual inspection or simple analysis. This method has some advantages, but it lacks scientific data.
The third method is the most reasonable and scientific. It ensures that hydraulic equipment will not malfunction, be damaged, or have its service life shortened due to hydraulic oil issues, and it also ensures the full and rational use of hydraulic oil, preventing waste. However, this method requires the factory to establish a laboratory. In the absence of a laboratory, the other two methods must be used. The following discusses how to select these three methods for oil change intervals.
III. The following factors should be considered when determining equipment cleaning and oil change intervals:
1. Ambient Temperature and Contaminants: High dust levels, humidity, and temperature in the environment will promote oxidation or degrade the oil, increasing its viscosity, residue, and acidic substances, which will corrode the equipment. In such cases, the determined equipment cleaning and oil change interval will be shorter.
2. Material of the Workpieces Processed by the Equipment: The cleaning and oil change interval will differ depending on the material of the workpieces processed by the same equipment. For example, the oil change interval is shorter when frequently processing cast iron workpieces than when processing ordinary metals because the cast iron powder easily contaminates the oil, accelerating its aging and deterioration.
Why does hydraulic oil age and deteriorate? How to determine the optimal oil change interval?
3. Equipment Sealing: Poor sealing allows contaminants in the air, processed metal fragments, and emulsions to easily enter the system, accelerating equipment wear and resulting in a shorter oil change interval.
4. Equipment Operating Time: For assembly lines or three-shift systems, longer operating times result in shorter cleaning and oil change intervals. Single-shift systems or systems with low utilization rates require longer cleaning and oil change cycles.
5. Oil Tank Filling Level: The cleaning and oil change interval for ground equipment varies depending on the amount of oil in the tank. Lower fill levels result in higher hydraulic oil circulation speeds, shorter residence time in the tank, poorer heat dissipation, and higher oil temperatures.
6. New Equipment or Equipment After Major Overhaul: Newly machined surfaces and mating parts have a certain degree of roughness, resulting in a smaller actual contact area and higher pressure per unit contact area. This leads to faster wear and greater wear during the initial operating period. Therefore, cleaning and oil changes should be performed within the first three months of operation, after which regular cleaning and oil changes should be performed as per usual guidelines.
7. Oil Quality: High-quality oil has a longer service life and longer cleaning and oil change intervals. The hydraulic oil replacement cycle is generally 9–12 months.
