Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/136253
utilized for modern lubricants. This trend of using more refined base oils and synthetic alternatives is based on the fact that they generally have better characteristics such as higher aging stability. However, while the higher-quality base oils have many advantages, there are concerns over some of their changed properties, which can lead to problems, especially when unfavorable combinations occur. One such consequence is varnish, which can be due to the base oil's altered dissolving performance with regard to aging and reaction products. Another consideration is component and lubricant damage, which can be caused by electrostatic discharges. The lubricant's conductivity is an important factor in the charge buildup, and conductivity is dependent on the type of base oil used (see Table 2). Along with the base oil, additives have a significant effect on an oil's conductivity. The higher the proportion of metal-organic additives, the higher the lubricant's conductivity. A prime example would be metal-organic additives such as those frequently used in zinc dithiophosphate (ZnDTP). As a proven multi-purpose additive in engine and hydraulic oils, ZnDTP improves wear and corrosion protection while simultaneously functioning as an antioxidant. However, zinc is considered to have dangerous health implications, so ZnDTP should be largely avoided. This means that the oil's conductivity decreases and the risk of static charging increases. A lubricant's conductivity not only is influenced by the base oil and the additive package but also depends on temperature. The higher the temperature, the higher the oil's conductivity. Unfortunately, there is no linear correlation between the two parameters, as each oil type has its own conductivity/temperature relationship. Furthermore, at a constant temperature, conductivity still changes during operation due to additive reactions, wear metals, reactions with metal surfaces, water and the formation of aging and oxidation products. charges generally can occur if there is friction in the flow between the oil and the surfaces surrounding it. The strength of the static charge depends on many different and partly interconnected factors. The energy density, which builds up in the system and leads to subsequent discharges, is contingent on the oil's conductivity and volume flow. The more oil that flows through a circulation pipe and the lower the oil's conductivity, the greater the potential for an electrostatic charge. Electrostatic Charges Although monitoring conductivity so far has been unable to achieve much success in the area of sensor technology, it is gaining significance with regard to electrostatic charges and discharges in lubricant and hydraulic systems. In oil-circulating systems, electrostatic www.machinerylubrication.com | May - June 2013 39