Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/468007
40 | January - February 2015 | www.machinerylubrication.com INDUSTRY FOCUS Lubricating oils are not completely free of air. Whether during operation or storage in barrels, oils are constantly in an exchange process with their air-containing environ- ment. Even if the oil is free of air bubbles, it will have a proportion of dissolved air. This depends primarily on the gas solubility, but pressure and temperature also have an effect. Some mineral oils can have air content approaching 9 to 11 percent volume at atmo- spheric pressure and room temperature. As long as the air remains dissolved in the oil, this generally is not a problem. However, free air bubbles, which usually are caused by constantly immersing machine parts or through oil returning to the reservoir, can lead to serious disruptions in equipment opera- tion, including impaired cooling effect, increased oxidation tendency, shortened oil life, reduced carrying capacity of the lubricant fi lm, oil spills, decreased oil pump capacity, lack of lubrication, cavitation and microdieseling. Foam and Air Release Oil returning to a reservoir has enough time to separate air in the form of air bubbles. The main infl uences on the speed at which these air bubbles separate from the oil and rise include the size of the bubbles, the oil's viscosity and the oil temperature. The amount of dispersing additives, the oil's density and any impuri- ties also play a role. As air bubbles arrive at the surface, surface foam is formed. Therefore, foam consists of a series of air bubbles, which are each surrounded by a skin of oil. As a function of the oil's surface tension, this skin of oil can burst more or less rapidly. The time it takes for the ascended bubbles to burst and achieve complete separation from the oil is mostly dependent on the oil's viscosity and temperature, but the content of polar aging products, impuri- ties and certain additives also have a bearing. The oil property that describes how fast these ascended bubbles burst is called the foaming behavior. As seen in Table 2, air release cannot be improved by additives. However, the foaming behavior of lubricating oils can be improved by anti-foam additives, which reduce the surface tension of the oil, i.e., by the well-proportioned addition of silicone-containing compounds or oil-soluble polyglycols. Too many anti-foam additives can lead to a signifi cant deterioration of the air-release capability. Operational Causes The possible causes of foam formation in gears can be divided into two groups: transmission and lubricating oil. If lubricating oil mixes with other lubricants or contaminants such as dust or water, foaming can result along with oil aging, which leads to the forma- tion of polar oil-aging products, an increase in viscosity or fi ltering out of anti-foam additives by bypass fi lters. Transmissions with short residence times of the oil in the reser- voir are particularly sensitive to changes. Increased air entry due to BY RÜDIGER KRETHE, OILDOC IGEAR LUBRICATION How to Prevent Foaming and Air in Industrial Gear Oils DISSOLVED AIR UNDISSOLVED AIR Dissolved into molecule structure (mineral-based oils = approximately 9%) Not dissolved into molecule structure No "free" air; no bubbles present "Free" air; air bubbles present (entrained bubbles in the body of the oil or on the oil's surface, i.e., foam) CAUSES Permanent reaction with air from environment by diffusion; air content primarily depends on pressure and temperature Generated primarily by entrainment of air bubbles, e.g., by immersing machine parts, return of the oil into the reservoir, suction of air bubbles, etc. CONSEQUENCES No problems Problems Variations of pressure and temperature can "transform" dissolved air into free air bubbles Greatly increased compressibility, affected lubrication and cooling capabilities, increased oil oxidation, cavitation, microdieseling, etc. TABLE 1. Comparison of dissolved and free air Dispersions