Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/655822
38 | March - April 2016 | www.machinerylubrication.com IN THE TRENCHES being pulled through the pump is lower than what is needed to adequately lubricate the motor. If the oil gels or the viscosity becomes too high, it can lead to another condition known as air binding. In this condition, an air void is created within the oil in the sump. The oil is too thick to fill the void, and thus the pump just pulls in air. This adversely affects the health of the equipment, as it can lead to boundary conditions, excessive wear and ultimately premature failure. Other methods can also be used to test an oil's cold-tempera- ture characteristics. ASTM D3829 is the standard test method for predicting the borderline pumping temperature of engine oil. With this test, the goal is to identify the temperature at which an engine oil can no longer be pumped. The test results can indicate whether a candidate oil remains fluid enough at certain temperatures or if a different oil should be selected. Gelation in Gear Oils Gearsets are another area where the gelation or cold-tempera- ture characteristics of an oil become important. Gear oils generally have a high initial viscosity, which leads to a much higher viscosity at low temperatures. Studies on gear oils lubricating wind turbine gearboxes have shown that these oils can become quite cold at several hundred feet above the ground and in cold climates. The cold temperatures coupled with moisture contamination resulted in the formation of gels in some of the in-service gear oils. This condition can be just as detrimental to the health of the gearbox as the motor oil condition discussed previously. Gelation Factors Several factors should be taken into account when determining how well a lubricant will work at colder temperatures and the prob- ability of gelling at these temperatures. These include the base oil, wax content, pour point and the base oil's refining process. All of these will have a marked impact on gelation and the lubricant's cold-temperature characteristics. If your equipment is operating in extreme cold temperatures, you should consider the base oil used in the lubricant. Mineral base oils have a wide operating temperature range but are often discarded in favor of comparable lubricants with a synthetic base oil. Synthetic oils generally have a higher viscosity index, which means they will remain more fluid in cold conditions and thicker in hotter temperatures. For machines that require mineral oils, take note of the API base oil category or how refined the base oil is. Crude oils from the ground naturally have a bit of wax in them, which can negatively affect the oil's tendency to gel in cold temperatures. The majority of this wax can be removed through refining. During the dewaxing process, the wax content is reduced or the wax structure is converted to a different structure with better properties. The cold-temperature characteristics are also improved. Typically, the more refined a base oil, the higher the viscosity index and the better the low-temperature properties. API base oil Groups II and III have lower volatility and lower pour points. When in doubt about which API group a particular base oil falls into, contact the oil manufacturer or consult the technical data sheets. An oil's pour point is another property that should be analyzed before selecting a lubricant to be used in cold environments. The pour point is the temperature at which the oil will no longer flow due to gravity. As an oil is cooled, waxes remaining in the oil begin to crystalize and congeal together, making the fluid more solid until it stops flowing. Even oils that are virtually free of wax will have a pour point associated with them. If choosing a lubricant for a machine that will operate in extremely cold environments and there are several oils with the same properties except for the pour point, pick the one with the lowest pour point to avoid issues stemming from reduced flow at cold temperatures. Synthetic base oils are synthesized from different compounds and mostly have no waxes in them. They also have a lower pour point than mineral oils and are often selected for cold environments due to their higher viscosity index and lower pour point. However, synthetic base oils are still at risk of gelation if they become contaminated with certain contaminants such as water and glycol. Routine oil analysis should be performed to look for these common culprits. Preventing Gelation When it is impossible to find an oil that stays thin enough in cold environments, a common solution to avoid the pitfalls of restricted flow or excess viscosity is to install a lube oil heater. These types of