PDSs Reveal Meaningful Lube Specs
EP Editorial Staff | December 1, 2021
Product data sheets provide the test results that help you specify the correct lubricant for your assets.
By Mark Barnes, PhD, CMRP, Des-Case Corp.
Choosing the correct oil or grease for a given application is the most fundamental decision any lubrication process owner needs to make. Lubricant selection often starts with the OEM recommendation. However, depending on the age of the machine and level of detail provided, many OEM lubricant-selection guides offer little more than generic information (“use a good quality AW68 mineral hydraulic fluid” or “use a high-quality EP 220 synthetic gear oil such as ACME Gear SYN 220”).
When more detailed recommendations are provided by the equipment manufacturer, changes in lubricant formulations, coupled with changes from one lubricant vendor to another over the years, can lead to “specification creep.” This occurs when the lubricant no longer meets the requirements set forth when the equipment was first installed.
To select the correct lubricant, it’s always preferable to use the OEM recommendation as a starting point, i.e., a guide to the minimum performance required of any lubricant. From there, specifics about the application, such as possible shock loading, unusually high or low operating temperatures, or extended service intervals, should be used to refine lubricant selection.
To do so, requires an understanding of lubricant physical and chemical properties, based on laboratory tests. These properties are usually listed on the lubricant’s product datasheet (PDS). What follows are explanations for some of the more common tests used for industrial lubricants.
Kinematic viscosity at 40 C, ASTM D445: Selecting the right viscosity based on load and speed is the most critical aspect of lubricant selection. For most industrial fluids, lubricants are specified by the ISO viscosity grade (VG) (ISO 3448) which is based on kinematic viscosity at 40 C. To fall within a specific ISO grade, the new oil must fall within a ±10% range of the grade.
Kinematic viscosity at 100 C, ASTM D445: Kinematic viscosity at 100 C is more commonly listed for engines oils and other automotive lubricants. Some industrial fluids that are intended to run at higher temperatures will also have the viscosity listed at 100 C. All fluids are tested at 40 C and 100 C so that the oil’s viscosity index can be calculated.
Viscosity index, ASTM D2270: The viscosity index of an oil can be calculated from the formulae and tables listed in ASTM D2270. It is a unitless number that represents how an oil’s viscosity will change with temperature. An oil with a high viscosity index (VI) would show a smaller change in viscosity with temperature than an oil with a low VI. Viscosity index is most important when considering fluids that will operate across a wide range of temperatures, such as low-temperature start up or high operating temperatures.
Pour point, ASTM D5950: An oil’s fluidity at low temperatures can be estimated from the pour point, which is the lowest temperature at which the oil will still flow. Ideally, the lowest ambient operational temperature for cold start up should be 10 C higher than the pour point, unless supplemental heating is provided.
Flash point, ASTM D92: Flash point is less common but is important in high-temperature applications where a source of ignition is present. Flash point is defined as the lowest temperature at which the vapors above the oil will “flash” or ignite if a spark occurs.
Acid number, ASTM D664: The acid number of an oil is primarily used to measure the degree of degradation of an in-service oil, indicated by weak organic acids from oil oxidation. As such, the acid number of a new oil is reported more as a reference for future in-service oil analysis and is less important in predicting the in-service performance of a new oil. A high new-oil acid number does not necessarily mean the oil is corrosive and is not a measure of corrosivity. Many new oils have an acid number greater than zero due to the presence of anti-wear and extreme pressure additives.
Base number, ASTM D2896: Base number is the opposite of acid number. It measures the presence of alkaline additives; specifically over-base detergent additives found in engine oils. Unlike acid number, the base number is relevant to new-oil selection since it should be selected based on the known amount of sulfur in a fuel such as diesel. However, with the emergence of low and ultra-low sulfur fuel, the importance of the base number has waned, except in certain railroad and marine applications.
TOST, ASTM 943: The TOST, or turbine oil stability test, measures the anticipated oxidative life of long-service-life oils such as turbine oils and hydraulic fluids.
RPVOT, ASTM D2272: The rotating pressure vessel oxidation test (RPVOT) is used to gauge the oxidative stability of in-service turbine oils. Because of the extreme conditions used in the RPVOT test, it should not be used to compare the performance of two different turbine oils. Measuring a high RPVOT value when new, does not always guarantee a long service life.
Air release, ASTM D3427: Air release measures how quickly an oil can detrain air. It is an important test for high-flow/high cycle-rate oils, such as hydraulic fluids, where air entrainment can cause cavitation and micro-dieseling.
Foam tendency and stability, ASTM D892: The foam tendency and stability test measures how readily an oil will form foam when agitated and how quickly foaming will collapse.
Rust prevention, ASTM D665: Rust prevention is a go/no-go test that measures an oil’s ability to stop rust formation with either water or saltwater ingression. All new oils typically pass the rust test, making it of minimal value when selecting a new oil.
Copper strip corrosion, ASTM D130: This test measures an oil’s corrosivity to copper and copper-containing alloys such as brass and bronze. A measure of no or low copper corrosivity is important in applications such as worm drives or bearing lubrication where a brass cage is in use.
Demulsibility, ASTM D2711/D1401: In some industries, such as power generation, steel, and pulp and paper, water is a pervasive problem. Where the presence of water is anticipated, having an oil with a good oil-water separability is important.
Four-ball wear, ASTM D4172: The four-ball wear test is one of several tests that measure an oil’s wear-prevention characteristics and is used for circulating oils where mixed-film lubrication is expected. The test uses four steel balls, in a pyramidal arrangement, immersed in the test oil (Fig. 1). The wear scar is measured where the top rotating ball touches the supporting three balls after the defined test period.
Four-ball weld, ASTM D2783: Like the four-ball wear tests, loading is slowly increased until the balls weld due to adhesive wear. This test is important in heavily loaded applications such as gearboxes.
Timken OK load, ASTM D2782: In this wear test, a weighted lever arm applies friction to a rotating cup rubbing on a test block. The lever is sequentially loaded until the maximum weight prior to welding occurs.
FZG test, DIN 51354: Named for the German Center for Gears and Drives, this test measures scuffing and wear as gears are sequentially loaded while lubricated with the test oil. Results are reported as stages of wear, indicating oil load-carrying capability.
When using lubricant product datasheets to make lubricant selections, avoid the temptation to compare two lubricants and conclude that one product is necessarily “better” just because one or more of the test results appear to show a performance difference. As an example, if a lubricant is being used inside at a minimum ambient temperature of 20 C (68 F) and the machine never runs above 50 C (122 F), what’s the point of having an oil with a very low pour point or high viscosity index?
When selecting a lubricant, choose the optimum product based on application, ambient conditions, and other pertinent considerations. Take time to understand how new lubricants are tested and what the test parameters listed on a lubricant PDS mean. EP
Mark Barnes, CMRP, is Senior Vice President at Des-Case Corp., Goodlettsville, TN, (descase.com). He has 21 years of experience in lubrication management and oil analysis.