Every finished “ready to use” lubricant is manufactured to a proprietary formula. Each is expected to combat friction according to various design, function and operating conditions under which bearing surfaces must be kept separated. Lubricating oils—either as liquids or as thickened forms known as grease—are blended products consisting of a base-oil stock and an additive package. The base-oil percentage of a finished oil can range from 75% to 99%, depending on how much of an additive package is used to enhance, suppress or contribute new properties to the oil. Base-oil stocks come from three primary sources that classify an oil as an animal/vegetable, mineral or synthetic lubricant.
Animal/vegetable base oils
The Industrial Revolution ran for a long time on olive oil and rendered animal fat. Eventually, the problems associated with animal/vegetable oil’s inability to arrest rapid acid formation under ever-increasing speeds and loads were solved by the discovery of crude mineral oil. One of the few exceptions was the use of sperm whale oil—prized for its exceptional lubricating properties and used up until the early 1970s as a base oil for automotive ATF (automatic transmission fluid). The animal/vegetable oil classification is now generally reserved for cooking purposes.
Mineral base oils
Often defined as petroleum-based oil, mineral-base-oil stock is overwhelmingly the most popular base-oil stock in use today. Mineral oil comes out of the earth in a crude form that must be refined to remove impurities (i.e., aromatic hydrocarbons, sulfur compounds, acids and wax) and improve the base oil’s desirable properties (i.e., its Viscosity Index [VI], pour point and stability). (Refining separates crude-oil molecules by size and weight to produce a variety of petroleum-based products. Interestingly, lubricating-base-stock oil constitutes only 1-2% of a barrel of crude oil’s yield. At approximately 25%, gasoline accounts for the highest percentage of refined product yield from a barrel of crude.)
Where (in the world) the crude oil originates establishes the base-oil properties and, in turn, the type of service application for which the finished lubricant is suited (as determined by the levels of paraffin and napthene present in the crude stock).
Paraffinic crude oils, which are generally found in the Mid-Continental U.S., the Middle East and the British North Sea, are favored for the manufacture of crankcase oils, gear oils, bearing oils, turbine oils and most hydraulic fluids. Paraffinic crudes will contain to 60% paraffin and up to 10% wax, giving this type of oil an excellent VI rating—in the range of 95 to 105.
Napthenic crude oils, which are generally found in coastal U.S. and South American regions, are favored for the manufacture of compressor oils, refrigerant oils and locomotive oils. Napthenic crudes contain up to 75% napthene and show only traces of wax. While such percentages give these oils an improved (lower) pour point than their paraffinic siblings, they also lead to a lower flash point and a less-desirable VI rating—in the range of 30 to 70.
Synthetic base oils
Synthetic base-oil stocks are, as their name implies, man-made. Designed with an improved, more uniform molecular structure than mineral-oil stocks, they display more predictable fluid properties and can work better under the types of severe conditions that are unsuitable for mineral-based lubricants. (Element #5 of this series will explore synthetic base stocks and their characteristics in more detail.)
The quality of any base-stock oil is measured by its resulting properties that define how well the oil will perform in service, and what additives will be required to enhance its performance. There are five major properties identified in a base-oil specification.
#1. Viscosity. . .
Defined by the oil’s molecule size, viscosity is recognized as a lubricant’s measure of resistance to flow. With larger molecule sizes, an oil’s resistance to flow increases, thus slowing down the flow rate. Higher-viscosity (i.e., “thicker”) lubricants will have slow flow rates, whereas lower-viscosity (i.e., “thinner”) lubricants will demonstrate faster flow rates.
Viscosity will change based on the ambient temperature and load. When the temperature increases, the lubricant becomes thinner and the viscosity decreases. Inversely, as the temperature decreases, the lubricant thickens and viscosity increases, making it more difficult to pour or pump around.
When a lubricant comes under extreme load, its viscosity will increase. This is a phenomenon experienced in the elastohydrodynamic lubrication (EHL) film state found in rolling-element bearings. It occurs when the ball or roller moves into the direct loading contact area known as the Hertzian contact area, causing the component to elastically deform, trap and pressurize the lubricant momentarily. This raises the viscosity, which causes the lubricant to change from a fluid state to a solid and back again, as the roller or ball moves through the direct-load area.
Base oils are rated with a viscosity number according to a recognized Viscosity Index numbering system. The most commonly used imperial system for industrial-lubricating oils is the SUS (Saybolt Universal Seconds) rating index, which charts the viscosity rating at two different temperatures of 100 F and 210 F. Its equivalent metric rating is the ISO VG rating index that follows the Kinematic viscosity rating measured in centistokes @ 40 C and 100 C. For example, an ISO VG 220 gear oil is the equivalent viscosity to an SUS 1000 @ 100 F gear. Different oil types have their own rating systems, as depicted in Fig. 1.
#2. Viscosity index. . .
The Viscosity Index (VI) is a measure of oil’s viscosity change due to temperature. Oils with higher VI ratings are more desirable, as they are more stable under changing temperature conditions, and reflect a narrower change in viscosity over a standard temperature range. As noted above, paraffinic oils have much higher VI ratings than napthenic oils, which makes them more stable and desirable where a wide operating-temperatures range is experienced. Oils can be grouped and classified by their VI property as shown here in Fig. 2.
#3. Specific gravity. . .
Specific gravity rates an oil’s density relative to water.
#4. Flash point. . .
The flash point rating is used to determine a lubricant’s volatility. Flash point is the lowest temperature a lubricant can be heated before its vapor, when mixed with air, will ignite but not sustain combustion. Paraffinic oils have higher flash points than napthenic oils.
#5. Pour point. . .
The pour point defines the lowest temperature at which the oil will still pour, or flow. Because of their level of wax content, paraffinic oils are said to have a wax pour point, which is not as low as the viscosity pour point that napthenic oils are described as having. Lower-viscosity oils will have lower pour points.
An oil base stock is a canvas for the additive package that makes up the final lubricant blend designed for an intended application purpose. LMT