Forgotten practice can save US industry $500 billion a year
Kevin C Craig, PhD - January 23, 2013
US engineering advancement has been catalyzed over the decades by external threats: the Space Race threat from the Soviet Union in the 1960s; the economic threat from Japan’s low-cost, high-quality manufacturing in the 1970s; the demographic threat from post-World War II engineering retirements in the 1980s; the global threat as US competitiveness declined in the 1990s; and now the environmental threat driving the need for global energy conservation and sustainability. In addition, the impending retirement of the Baby Boom generation challenges the engineering profession, as these workers will take with them a vast amount of knowledge and skill that must be replaced.
One critical area, essential in manufacturing and transportation, is tribology, officially defined in 1966 as the science and technology of interacting surfaces in relative motion. An undergraduate engineering student receives less than one hour of instruction in tribology during a four-year program, so this matter demands urgent attention.
Tribologists, through the development of air-bearing technology between 1960 and 1980, enabled the extremely fast, reliable, precise, and accurate operation of a computer hard-drive read/write head (approximately 0.05×0.04×0.01 in.) riding over the disk surface on a cushion of air at a height of 15 nm and an average speed of 53 mph. The challenge for tribologists today is to understand the potential modes for wear and surface damage in an endless variety of mechanical systems.
Whether the goal is to reduce parasitic friction or enhance friction in a design, the designer must employ a proper tribological approach—the right combination of geometry, materials, and lubrication—to ensure safety, performance, and energy-efficient operation. Estimates are that the correct application of tribology throughout US industry could save $500 billion annually.
Friction, inevitably accompanied by wear, accounts for most of the energy consumed in our society. The object of lubrication is to reduce friction, wear, and heating of machine parts, which move relative to each other. A lubricant is any substance that, when inserted between moving surfaces, accomplishes those purposes.
Several types of lubrication exist: Hydrodynamic refers to full-fluid-film lubrication, hydrostatic refers to lubricant introduced under pressure to create a full film, elastohydrodynamic refers to lubricant films between elastically deformable surfaces, boundary refers to a fluid film several molecular dimensions thick, and solid refers to solid lubricants used at high temperatures. Unlubricated surfaces have a friction coefficient of about 1.0 with heavy wear; for boundary and thin-film lubrication, the value is about 0.01 with slight wear; and for thick-film lubrication, the value is about 0.001 with no wear.
Figure 1 This plot of the coefficient of friction versus Hersey number for a journal bearing under test is a good measure of the health of the bearing.
The most common fluid-film bearing is the journal bearing, in which a sleeve of bearing material is wrapped partially or completely around a rotating shaft to support a radial load. Figure 1 shows a plot of the coefficient of friction versus Hersey number (μN/P, where μ is the absolute viscosity in centipoise, N is the shaft speed in rpm, and P is the average pressure in psi) for a journal bearing under test conditions, and is a good measure of the state of health of the bearing.
There are other areas of science and technology that, like tribology, are threatened. Once these skills and knowledge are lost, getting them back will be nearly impossible.
Kevin C Craig, PhD, is the Robert C Greenheck chairman in engineering design and a professor of mechanical engineering at the College of Engineering at Marquette University. For more mechatronics news, visit mechatronicszone.com.