Lubrication Fundamentals: Lubricating Oil Basics

Editor's Notes

• #4: Physically separate opposing friction surfaces with the oil film reduces friction reduces wear reduces the amount of energy needed Additives enhance the ability of the oil to reduce friction and wear
• #5: Oil absorbs heat generated by the friction surface Transfer heat to reservoir where it is dispersed Oil can pass through a cooler to assist in heat exchange (left image) A newly commissioned fire water pump at a Middle East refinery repeatedly suffered high non drive end (NDE) bearing temperatures during periodic test runs and eventually caused a bearing failure. It was revealed that bearing failure was caused by a lack of lubrication since the failed bearing showed signs of lube oil starvation. However, the constant level oiler sight glass showed the oil level as full. (right Image) This bearing displays the results of high running temperatures commonly associated with over-greasing. When the dropping point is reached, the bearing begins to bleed, the grease runs dry and eventually failure occurs.
• #6: Oil holds contaminants in suspension Carries them away from friction surface settle out in reservoir filtered out Additives disperse solids, avoiding sludge formation Oil is periodically changed removing contaminants from system
• #7: Physical boundary (oil film) protects surfaces from corrosion Additives enhance rust and oxidation inhibiting properties. Some oils use alkaline additives to neutralize acid as it is formed Lubricant Viscosity Oil viscosity is just as important as oil quantity to ensure adequate lubrication. Required viscosity depends on operating temperature. Inadequate lubricant viscosity appears as a highly glazed or glossy surface. As damage progresses, the surface appears frosty and eventually spalls. This type of spalling is fine-grained compared to the more coarsely grained pattern produced by fatigue failure. In the frosty stage, the fine slivers of metal pulled from the raceway create a “nap” that can sometimes be felt. The frosted area will feel smooth in one direction but have a distinct roughness in the other. As metal is “pulled” from the surface, pits appear and frosting advances to pulling.
• #8: Oil forms a viscous seal to keep water and dirt out Grease serves as a physical barrier
• #9: Fluid under pressure actuates moving parts Oil cleanliness is essential Oil must have anti-foaming characteristics Some systems require fire resistant fluids (Image) Too much grease volume (overgreasing) in a bearing cavity will cause the rotating bearing elements to begin churning the grease, pushing it out of the way, resulting in energy loss and rising temperatures. This leads to rapid oxidation (chemical degradation) of the grease as well as an accelerated rate of oil bleed, which is a separation of the oil from the thickener. The heat that has been generated over time along with the oil bleed eventually will cook the grease thickener into a hard, crusty build-up that can impair proper lubrication and even block new grease from reaching the core of the bearing. This can result in accelerated wear of the rolling elements and then component failure.
• #15: Chemical compounds added to impart specific properties Added to improve performance characteristics Formulated to provide specific set of performance parameters Can have detrimental side effects
• #17: Oil additives are chemical compounds that improve the lubricant performance of base oil Anti-Wear Additives Zinc Phosphorus EP Additives Phosphorus Molybdenum Detergents & Dispersants Magnesium Sodium Calcium Alkaline Additives Calcium Anti Foam Additives Silicon Viscosity Index Improvers Pour Point Depressants Rust & Oxidation Inhibitors Barium Tackifiers
• #20: ISO Viscosity Grade Refers to Kinematic Viscosity at 40 degrees C ISO VG 68 = 68 cSt @ 40C
• #21: SAE - Oil Weight Automotive Grade (10w-30, 30, 5w-40) Gear Grade (75w-90, 80w-90) Oil’s Kinematic Viscosity at 100 degrees C is compared to SAE scale Image description Steel balls of equal weight dropped into test tubes filled with motor oil fall at different rates. Their rate of fall depends on the viscosity of the oil. The ball traveling through the light SAE 20 oil has traveled farthest, while the ball in the heavy SAE 50 has traveled least