REAL-WORLD CASE STUDIES
Friction Forces Synthetic Oil
Higher Friction Forces
Mineral Oil
Fig. 1. Fluid friction involves a series of molecular plates sliding over each other like the spreading of a deck of cards. The resistance to this sliding results in energy losses and heat generation.
Fluid friction—which is dependent on the molecular structure of the base stock and its viscosity—can be characterized as a series of molecular plates sliding over each other like the spreading of a deck of cards. The resistance to the sliding results in energy losses and heat generation. Figure 1 illustrates this point. Compared to that of a mineral-based product, the molecular structure of a synthetic (like a PAO) is more consistent, thus making sliding between the metal and lubricant film and sliding between the layers of molecular structures easier. Mineral oils have a variety of molecular components based on size. As shown in Fig, 1, this makes shearing or sliding of the lubricant film more difficult, resulting in higher energy consumption. During the EHD lubrication regime—where a solid-like film is produced—the force required to shear the film is called traction coefficient. PAOs have a much lower traction coefficient than mineral oils resulting in energy savings in shearing of the film in rolling-element bearings and the pitch point during meshing of gears. While base stocks contribute to the lubricity and shearing of the lubricant film, resulting in energy savings, another important factor is additives, such as friction modifiers, that go into the finished product. In some cases, additives can have more of an effect than the base stocks. A well-formulated lubricant will incorporate a synthetic base stock with the proper additives to reduce friction, resulting in the maximization of energy savings. Some small, specialty-lubricant companies promote energy-savings potential from the use of PAO base stocks with proprierty additives such as liquid moly. Documenting results Testing methods can be among the most difficult factors to deal with when attempting to justify a switch to synthetics based on energy savings. Some companies turn to sophisticated power meters and try to control as many variables as possible; others do a quick study that only measures amperage drop without any adjustments. Although there seem to be countless energy-saving “success stories” out there, it’s difficult to assess the accuracy of their results without acutally evaluating the data. The fact is, the more efficient a piece of equipment or system component is, the more difficult it is to see significant energy improvements. Take, for example, nonworm gears that have efficiencies from 90-95%, and worm 10 | LUBRICATION MANAGEMENT & technology
gears that operate with efficiencies in the 65-80% range: It’s usually easier to see energy improvements in worm gears than other industrial gear types. Here are several accounts of real-world evaluations that attempted to make the case for using synthetics based on the energy savings they could generate. I. The case of a coal pulverizer plant worm gear. . . A major lubricant company conducted a study on the efficiency improvements in worm gears through the use of a PAO. ■ Initial laboratory tests were conducted on a worm gear
with a 20:1 reduction ratio operating at 1750 rpm. ■ ISO 460-compounded gear oil was used. Based on the
viscosities at the operating temperature, an ISO 320 PAO was used to more closely match the viscosity of the ISO 460 oil. This resulted in a 3.5% efficiency improvement in the laboratory test. Initially when an ISO 460 PAO was used, the efficiency improvement was only 0.9%. This illustrated another advantage of synthetics because with the high viscosity index one lower ISO grade could be used, giving the same protection with less fluid friction. ■ Under carefully controlled conditions, the actual field
trial tested a utility’s coal pulverizer with reduction ratio of 22:1, running at various load levels. Because the operating temperatures were low, it was decided to compare an ISO 320 EP mineral oil with an ISO 320 non-EP PAO (so the viscosities would be fairly similar at the operating temperature). ■ At different loadings, energy savings of 9.8%, 8.7% and 8.5%
were realized—far exceeding the laboratory test results. II. The case of a crushed-rock-mining PUG mill… Energy savings were evaluated on a PUG mill using an ISO 150 PAO and ISO 150 PAO/mineral oil. The major difference between these two lubricants was their additives. ■ Fifteen-minute runs were conducted with each product,
keeping the variables on production as close as possible. MAY/JUNE 2013