Machinery Lubrication magazine published by Noria Corporation
Issue link: http://www.e-digitaleditions.com/i/601893
4 | November - December 2015 | www.machinerylubrication.com AS I SEE IT a defective cooler can drive down viscosity. Sometimes the temperature issue is localized or specific to a transient operating condi- tion. For example, a machine may develop a hot spot for various reasons that can cause viscosity to plunge in that same vicinity. When extreme, these hot spots can also crack the oil's molecules, leading to perma- nent and severe loss of viscosity. Viscosity can drop by oil contamination and distress. Fuel dilution in engines and chemical contamination (solvents, refriger- ants, natural gas, etc.) can all result in a sudden drop in viscosity. Water contamina- tion can thin many oils that have a high solvency for water. In the case of natural and synthetic esters, water may disassemble the ester molecule to sharply reduce viscosity by a chemical reaction called hydrolysis. Some lubricants have additive systems that include viscosity index improvers (VII). VII molecules are extremely large, and when the oil is hot, they unfurl, making them extremely susceptible to rupture by mechanical shearing in the machine's fric- tional zones (cam/follower contacts, swashplate/slipper contacts, pumps and rolling-element bearings). These ruptures reduce the oil's viscosity over time. Engine oils and most hydraulic fluids are at risk for VII viscosity shear thinning. CONSEQUENCES OF INADEQUATE VISCOSITY Machines starved of viscosity suffer from a range of problems that translate to operational costs and impaired reliability. In certain situations, viscosity deprivation can lead to sudden catastrophic death of machine components such as bearings and gears. In other cases, the effects are milder and may only slightly shorten the life of the machine. The following consequences can occur when viscosity is lower than ideal: Mechanical Wear In many circumstances, viscosity is the most important lubricant property that prevents or mitigates wear. When viscosity falls below a critical threshold, mechanical wear is accelerated. This includes abrasive wear (two-body and three-body), adhesive wear (scuffing and galling), surface fatigue (micropitting, etc.), and delamination wear. Thermal Circle of Despair Low viscosity causes wear and friction, which generate heat. Heat lowers oil viscosity, leading to more friction and wear as well as more heat. This is the thermal circle of despair. The accelerated wear shortens machine life, and the heat shortens the lubricant life. Short Lubricant Life When frictional surfaces are deprived of viscosity, the lubricant's additives are affected in three ways. The first is from heat (as mentioned above). This heat accelerates the depletion of additives such as antioxidants. This results in base oil oxidation. The second is the rupture of VII additives, which leads to more loss of viscosity. The third is the mechanical friction from low viscosity that causes anti-wear and extreme-pressure (EP) additives to sacrificially deplete more rapidly. Leakage Low viscosity will increase the rate of leakage. This includes both out-leakage and internal leakage. Out-leakage causes a loss of lubricant, while internal leakage affects machine function (speed and control in the case of hydraulic systems) and energy consumption. Engine Oil Consumption and Environmental Effects Low crankcase oil viscosity increases the rate of oil consumption in diesel and gaso- line engines, which is an operational cost. The oil that is released to the exhaust path produces hydrocarbon emissions, which endangers human health and leaves a carbon footprint. RECOGNIZING VISCOSITY- STARVED MACHINES Don't assume the lubricant in your machine has the right viscosity simply because it is the one specified in the machine's service manual. Challenge conventional recommendations for viscosity. Some machines are operating at conditions far afield from that intended by the machine designer. Machine applica- tions vary sharply due to duty cycle, work environment, temperature, close-proximity contaminants and operating conditions. RISK-HEDGING STRATEGIES FOR MACHINE VISCOSITY STARVATION RISK FACTOR STRATEGY High temperature from low coolant, high ambient conditions, low oil levels, machine hot spots Monitor temperature frequently and correct the root cause. Use high VI oil. High loads and/or slow speeds Select viscosity to match machine loads and speeds. Periodic shock loads, frequent stops/ starts Use a lubricant with film-strength additives to protect against two-body abrasion, adhesion and galling. Examples are anti-wear (AW), EP and friction modifiers. Mechanical unbalance or misalignment Monitor for these conditions and remediate accordingly. Use additives to mitigate such as AW, EP and friction modifiers. Fuel dilution or chemical contamination Use oil analysis to detect and prescribe a remedy. Hydrolysis of esters from water contamination Proactively control water contamination. Use oil analysis to detect and prescribe a suitable remedy VII shear thinning Use oils resistant to excessive VII shear thinning. Dirt ingression risk Employ contaminant exclusion strategies. Use more effective contaminant removal methods (e.g., filters). Monitor particle levels regularly. Consider increasing oil viscosity.