Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/875060
42 | September - October 2017 | www.machinerylubrication.com environments. This has led to the creation of advanced lubricant formulations, such as pour point depressant additives and multi-viscosity oils, as well as advanced engine designs with the integration of block and pan heaters. Lubricant Failure To comprehend the potential machine failure modes associated with a cold start, it is essential to review the ways in which a lubricant can fail from these conditions, both physically and chemically. Not only does the lubricant become more viscous at cold temperatures because of the tempera- ture-viscosit y relationship, but it can eventually congeal when it passes below a certain temperature point called the pour point. Under test methods defined by ASTM D97, the pour point describes the cold temperature point in which the oil begins to become immobile and will no longer flow. For example, gear oil at a cold enough temperature can be placed on the floor and stood upon like a block of ice. However, unlike water, which maintains a constant viscosity until it solidif ies at an exact temperature, oil will gradually become thicker and congeal at varying rates. This makes it challenging to set lubricant selec- tion standards for the equipment operation. Therefore, even though the oil has not yet reached the pour point, the viscous nature of the cold oil can cause substantial flow restriction in machine components like hydraulic or circulating oil pumps. The consequences of a pump flow restriction can be catastrophic, with effects such as surface wear from vaporous cavitation and bearing wear from starvation. Some additives are affected by the low temperature conditions of machine startup. For instance, cer tain ex treme-pressure additives are only activated at elevated temperatures as a result of surface fric- tion from high speeds or high loads. If the machine is operating at slower speeds during startup and in a cold environment, the additive can become significantly less effective. Other additives like rust inhibitors can have poor solubility and stratify to the bottom of sumps and storage containers during prolonged stagnant periods in cold temperatures. If the oil continues to operate at cold temperatures, the foam tendency can increase, especially with low viscosity oils. All these degradative effects on lubricant additives will result in limited protection of the machine components from corrosion, premature wear and foam. Filter Resistance and Failure All machines behave dif ferently to high viscosit y in cold-star t conditions, depending on the design of the machine and the component sensitivities. For example, in any system where fluid is circulated, such as a forced circulating lube oil system or a wet sump circulating system, the oil should flow through a filter. The filter elements will naturally cause flow restriction. Higher flow restriction results in a higher pressure drop, leading to the filter bypass valve opening (if equipped), the filter element rupturing or both. Because the viscosity increases during a cold star t, the pressure drop also increases due to increased resistance through the filter element. Unfortunately, the potential for failure is not isolated to the filter in these condi- tions. Not only can the filter rupture, but it can also trigger a chain reaction with other secondary failures on the machine. When the oil starts to channel in between cracks as the filter ruptures, the buildup of contaminants on the filter has a chance to push through all at once. The mass ingres- sion of particles will then directly promote wear on the machine. Generally, the leading cause of failure from cold-start conditions is from increased flow resistance. Even if the filter doesn't rupture, the viscous oil will result in damage. The table on page 43 illustrates how viscosity can change as a result of a temperature change. A 220 ISO VG oil with a viscosity index of approximately 100 will increase to more than 5,000 centistokes when the temperature drops to 0 degrees C (32 degrees F). Lubricant Starvation Most mechanical methods that help supply oil to lubricated components will become hindered when temperatures drop. LESSONS IN LUBRICATION Beware of Vaporous Cavitation Cold Engine Starts (Viscosity Too High) Vaporous cavitation is particularly concerning, as it is symptomatic of high pressure differentials in the pump, such as during high viscosity cold starts. As oil is drawn in on the suction side of a hydraulic pump, dissolved air is desorbed from the oil coalescing with vaporous bubbles, which then continue to expand. These larger vaporous bubbles respond destructively when compression occurs during the pump's transition from the suction side to the high-pressure side. This sudden and drastic increase in pressure forces these bubbles to collapse, leading to damage of the nearby oil and machine surfaces from the adiabatic effect. CONDITIONS • Extreme cold • Gradual cooling • Wrong wintertime viscosity • Cold temperature plus high soot load EFFECTS • Air binding or flow limited • Loss of oil pressure • Dry start • Engine wear SOLUTIONS • Quality oil • Correct wintertime viscosity • Engine/oil pre-heater