Bubbles in lubricating oil: 10 interesting facts you don't know
author: Leila
2024-09-20
People who have worked in the field of equipment lubrication for many years understand the necessity of keeping lubricating oil clean and doing a good job of pollution control. Air can also cause "pollution" to lubricating oil. Lubricating oil should not only be free of water, impurities, and other oil liquids, but also avoid containing air - whether it is foam on the surface of the oil or bubbles contained in the oil. The reasons for the mixing of air in lubricating oil are more complicated, and sometimes it is not necessarily a problem with the oil itself, so adding anti-foaming agents and changing oil cannot solve the fundamental problem.
Bubbles and foam are harmful to both lubricating oil and machines. Bubbles will accelerate the oxidation and deterioration of lubricating oil, accelerate the consumption of additives, affect heat dissipation, fail to form a complete lubricating oil film, and cause wear to equipment. Bubbles can also cause local high temperatures in high-pressure systems, causing the oil to deteriorate rapidly. In addition, air is easy to compress, and if there is gas in the lubricating oil, the thickness of the oil film will become thinner or even break, and the mechanical parts will directly rub against each other, causing wear. Bubbles burst under pressure, and cavitation damage will also occur on the metal surface. In addition, bubbles can also cause unstable operation of equipment.
Regarding the problem of bubbles in lubricating oil, oil testing experts and lubricating oil technicians have conducted a series of studies, and some interesting phenomena are worth knowing:
Pure oil itself will not produce foam. If other things are added, it will cause foam, such as foreign impurities and additives. Lubricating oil is prepared with base oil and additives. Base oil is the oil before any additives are added. At this time, the oil is pure and will not produce foam. However, in order to enhance the performance of the oil, some additives need to be added, such as anti-wear agents, extreme pressure agents, detergents and dispersants, etc. The more polar additives are used, the easier it is to cause foam. For example, anti-wear agents and extreme pressure agents are both polar additives.
The size of bubbles in the oil is related to the interfacial tension of the oil. The smaller the tension, the smaller the bubbles formed. The smaller the bubbles, the easier it is for the oil to form more and more stable foam, which is not easy to break and disappear.
The entry of water into the lubricating oil will reduce the interfacial tension of the oil, making the lubricating oil more likely to produce foam. For most oils, as long as there is a water content of 1000ppm, the oil will continue to generate foam.
The higher the viscosity of the lubricating oil and the smaller the bubbles in the oil, the slower the bubbles are released, which will result in more bubbles in the oil. If the bubble volume is *2, the time it takes to release the bubbles will be shortened to 1/4. This is easy to understand. The larger the bubble volume, the easier it is to float to the surface of the oil.
The volume of the bubble is inversely proportional to the viscosity of the lubricating oil. Low-viscosity oil is prone to generating large bubbles, and high-viscosity oil is prone to generating small bubbles. The larger the bubble, the easier it is to break, while small bubbles are relatively "tenacious" and not easy to break. Therefore, they are relatively stable after they are generated and take a long time to disappear.
During low-temperature cold start, low-viscosity lubricating oil (ISO VG 10~32 oil) is most likely to generate bubbles. If it is under the operating temperature of the equipment (30℃~60℃), the situation is just the opposite. High-viscosity oil (ISO VG 460~1000 oil) is most likely to generate bubbles.
When the amount of air mixed in the lubricating oil reaches more than 30%, it is generally considered to have a foam problem. For many lubricating oils, when the volume of air mixed reaches 10%, the viscosity of the oil will increase by about 15%, so the negative effect of bubbles on lubrication is obvious.
When bubbles pass through the high-pressure area during the flow of oil, they are compressed and heat up rapidly, and the temperature can reach several hundred degrees Celsius, causing local high temperature of the oil, thus deteriorating.
Deep and narrow oil tanks are more likely to have foam problems, while spacious and low oil tanks are relatively less likely to have foam problems. This is easy to understand. Deep and narrow oil tanks are not conducive to the release of bubbles. In addition, the oil tank is narrow, and the interval between the oil return area and the oil suction area is short. The oil coming out of the oil return pipe is sucked away before it can release the bubbles, causing the bubbles to remain in the system.
The mechanical structure causes the oil flow to be blocked, which can also cause bubble problems. For example, at the turning point of the pipeline, the oil path is not smooth, resulting in restricted oil flow, etc., which will release the air dissolved in the oil to form bubbles. This situation will also cause mechanical cavitation and local high temperature of the oil.
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