Overview of lubricant bubbles

Overview of lubricant bubbles

People who have worked in the field of equipment lubrication for many years understand the necessity of keeping lubricating oil clean and doing good pollution control. Air will also cause "pollution" to the lubricating oil. The lubricating oil should not only contain moisture, impurities, and other oil liquids, but also avoid air-whether it is the foam on the surface of the oil or the bubbles contained in the oil. The reason why air is mixed into the lubricating oil is more complicated, and sometimes it may not be the problem of the oil itself. Therefore, adding anti-foaming agents and changing the oil cannot solve the fundamental problem.

　　Bubble and foam are harmful to lubricants and machinery. Bubbles will accelerate the rate of oxidative deterioration of lubricants, accelerate consumption of additives, affect heat dissipation, fail to form a complete lubricant film, and cause wear and tear on equipment. Air bubbles in the high-pressure system can also cause local high temperatures, causing the oil to quickly deteriorate. In addition, air is easy to compress, and there is gas in the lubricating oil, and the thickness of the oil film will become thinner or even rupture. The direct friction between mechanical parts causes wear. The bubbles burst under pressure, which will cause cavitation damage to the metal surface. In addition, air bubbles can cause unstable operation of the equipment.

　　 Regarding the problem of bubbles in lubricating oil, oil testing experts and lubricating oil technicians have conducted a series of studies. Some interesting phenomena are worthy of our understanding:

1 　　Pure oil itself does not produce foam. If other things are added, it will cause foam, such as foreign impurities and additives. The lubricating oil is prepared with base oil and additives. The base oil is the oil before adding any additives. At this time, the oil is pure and does not produce foam. However, in order to enhance the performance of the oil, it is necessary to add some additives, such as anti-wear agents, Pressure agents, detergents and dispersants, etc. The more the amount of polar additives, the more likely it is to cause foam. For example, antiwear agents and extreme pressure agents are all polar additives.

2 　　 The size of the 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 stable bubbles, which are not easy to burst and disappear.

　　3 　　 The entry of water into the lubricating oil will reduce the interfacial tension of the oil and make the lubricating oil more prone to foam. For most oils, as long as there is a water content of 1000 ppm, the oil will continue to generate foam.

　　4 　　 The higher the viscosity of the lubricating oil and the smaller the bubbles in the oil, the slower the bubbles will be released, which will result in more bubbles in the oil. If the bubble volume*2, then the time for the bubble to release will be shortened to 1/4. This is well understood, the larger the bubble volume, the easier it is to float on the surface of the oil.

　　5 　　The volume of bubbles is inversely proportional to the viscosity of lubricating oil. Low-viscosity oil is prone to produce large bubbles, and high-viscosity oil is prone to produce small bubbles. The larger the bubble, the easier it is to rupture, while the small bubble is relatively "stubborn" and not easy to rupture, so it is more stable after it is generated and it takes a longer time to dissipate.

　　6 　　 Low-viscosity lubricating oil (ISO VG 10~32 oil) is the easiest to generate bubbles during cold start at low temperature. 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. .

　　7 When the air mixed in the lubricating oil reaches more than 30%, it is generally regarded as 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 air bubbles on lubrication is obvious.

8 　　 When the bubbles pass through the high-pressure area as the oil flows, they will quickly heat up under compression, and the temperature can reach several hundred degrees Celsius, causing the oil to become locally hot and deteriorate.

9 　　Deep and narrow fuel tanks are more prone to foam problems, while spacious and low fuel tanks are relatively less prone to foam problems. This is well understood. The deep and narrow fuel tank is not conducive to the release of bubbles. In addition, the fuel tank is narrow, and the interval between the oil return area and the suction area is short. The oil from the return pipe is sucked away before the bubbles are released, causing the bubbles to remain in the system. Inside.

　　10 The obstruction of the oil flow caused by the mechanical structure can also cause bubble problems, such as the turning of the pipe, the restriction of the oil flow due to the poor oil path, etc., which will release the air dissolved in the oil to form bubbles, which will also cause the mechanical Cavitation and local high temperature of the oil.