Friction is present in all motion systems — even in bearings and linear motors that have layers or air to separate moving parts. However, friction becomes a cause for consideration in systems with contacting solid surfaces. The difference between the historically idealized perpetual motion machine and real-world machines is friction. Says Frederick E. Bryson, a consultant in Bryson City, N.C., “Until nature repeals the Second Law of Thermodynamics, any system that moves generates friction, with resulting energy losses.” Electrical technology is the most efficient in converting energy into work; in other words, it operates with the lowest losses, much of which is due to friction. Large motors, for instance, can operate at efficiencies of up to 98%.
Mechanical technology — for example, hydraulic systems, pumps, conveyors, and actuators — is substantially less efficient. “At the least efficient end of the scale, internal combustion engines tend to convert energy into work with efficiency between 25 and 40%. At the upper end of the scale of mechanical systems are centrifugal pumps, at about 80% efficiency,” Bryson explains. However, mechanical systems are often necessary for reasons of convenience or torque multiplication. Sometimes they are the only way to get a specific job done.
Because friction is energy loss, it increases the cost of manufacturing by whatever is spent to overcome it. “Manufacturers of rotating equipment reduce this entropic loss by using rolling element or hydrodynamic bearings and low-friction coatings to reduce sliding friction,” he adds. Other solutions include using lubricants that have low viscous losses, or designing cooling fans with low drag.
But sometimes friction is beneficial to systems. Antifriction bearing manufacturers, specifically of bearings and screws used in linear motion applications, have held that there is no wear of bearing surfaces until the fatigue life is approached — because wear has negative connotations. “Indeed, designers often think that wear in bearings is an undesirable attribute,” says Wes Howe, chief engineer AST Bearings, Montville, N.J. “However, one look at initial wear might change their minds. Initial wear in industrial components is as often as beneficial as the initial wear in automotive engines, to use a familiar example.” This break-in period makes for a better-seated system and smoother motion. During the initial stages of operation the rolling elements reduce the more aggressive asperities in the honed raceways, which makes the bearings run more smoothly.
As Dennis Barnes at the Precision Alliance, Fort Mill, S.C. explains further, “On precision-ground gothic arch surfaces, there is a burnishing that takes place on both the rolling elements and raceway surfaces. Thirty percent of this wear occurs in the first one million cycles of recirculation, and it can be as much as 5 mm on miniature systems, and more on larger parts.” Relative wear on the individual components is dependent on the length of travel and other factors, but typically, wear is evenly distributed between the longer raceway (either the rail or screw), the recirculating component raceway (either the bearing block or nut), and the rolling elements themselves. The balls or rollers usually show a bit less wear than the raceway surfaces. “Allowing the bearings to orient themselves reduces friction and increases bearing life,” agrees Howe.
Friction useful elsewhere
Friction can also be useful when used to help dampen the motion or inertia of a system. For example, backdriving of linear motion elements such as ball screws is sometimes undesirable. “In these situations, friction is useful because it prevents backdriving,” says George Jaffe of Steinmeyer Inc., Burlington, Mass. “In the ideal motion system, friction is totally eliminated and the servo control only has to deal with pure load. The next best case is to maintain constant friction during the motion cycle.” In the case of ball screws, when friction is caused by preloading the ball nut, it is optimized by accurately controlling the diameter of the shaft's ball track.
Friction is also useful for vibration dampening, thus improving settling times for rapid and short start/stop movements. “There are some applications that simply cannot handle overrunning or backlash. In these cases, friction brakes or clutches must be utilized,” Howe says. According to David Bortz, founder of Tribco Inc., Cleveland, “One solution is to use space-age materials to solve friction-lining problems in brakes, clutches, and other components.” He points out that pure Kevlar-fiber textile material used for brake and clutch applications is longer lasting than other alternatives. The non-molded and non-asbestos material lasts three to five times longer than traditional graphitic, sintered bronze, and paper friction materials — for reduced downtime and lower maintenance costs. How does the material make friction brakes and clutches last longer? “The Kevlar material does not wear down opposing mating surfaces,” says Bortz. It is specified by designers as the friction material for parts with outside diameters from 0.875 in. (for cameras used in outer space) to 92 in., for metal stamping plates. The textile-reinforced polymer composite friction material is also a more heat-resistant material.
Friction is beneficial in other situations where motion needs to be engaged, stopped, damped, or oriented. “Mechanical clutches and brakes that rely on friction are the simplest means of engaging or stopping motion, but friction dampers reduce the energy of vibration as well. Too, dancer bars that employ friction dampers take up slack in moving webs of film and paper,” says Bryson.
Coatings to the rescue
Kevlar is one material that extends the life of products designed for optimized friction. Similarly, Teflon reduces the wear of engaged components that need the smallest possible friction coefficient. “We employ a low-friction plating containing a matrix of Teflon captured in a hard nickel base, 2 to 5 µm thick. It provides both low friction and excellent corrosion protection,” says Barnes. “And because the Teflon is captured inside the nickel plating, it does not peel or scratch off.” The nickel also helps support high contact pressures so that the Teflon isn't worn through as on typical bearing surfaces sprayed with Teflon. This technique reduces both wear and friction in sliding surface bearings, and can dramatically increase life in severe-duty applications.
Several new coating processes can add both lubricity and hardness to key motion components. So how should designers use them? Matching the needs of the application to the properties of these new coatings is key to optimizing sliding friction and wear. And Barnes explains that when duty cycle, speed, or accuracy dictate the use of anti-friction components, controlled friction can be added to the system with special lubricants if required.
If a designer has the choice between sliding friction and viscous friction, he or she should choose viscous friction because it tends to result in lower losses. “The most obvious examples are air bearings (where the lubricant is literally air) and prime movers that float on air such as trains that employ linear motors,” says Bryson. But in conventional journal bearing applications, designers are finding that a thin layer of low-friction coating reduces friction (and subsequent wear) during conditions of heavy load or start-stop operation. And Bryson says that oleophobic coatings can reduce viscous shear losses in engine crankcases and gearboxes.
Dry running tests show that graphite sleeve bearings overcome friction effectively — without the help of lubrication, and without damaging pumps and other components. Rolling element bearings do have a lower coefficient of friction. Robert C. Stowell of Graphite Metallizing Corp., Yonkers, N.Y,. explains, “There's no doubt that rolling element bearings do a better job of fighting the influences of friction. That said, in some applications they just don't work.” One example is in an application where lubrication contamination or washout is a concern. So the real strength of sleeve bearings is the ability to overcome friction as well as the extreme environments where it can occur.
“There aren't any direct, documented comparisons of rolling-element bearing performance to graphite sleeve bearings, but again, that's because the two are appropriate for different situations. The main advantages of graphite grades are the ability to carry current, and the ability to fight friction without any grease or oil to fail,” says Stowell. The graphite provides consistent lubrication to overcome the friction where traditional lubrication can also fail due to temperature.
“Even the smallest miniature ball bearings exhibit as much as 500 mg-mm of starting torque when lubricated with one drop of Mil-L-6085 oil,” Howe explains. But friction and wear have been reduced significantly with the advent of vacuum-degassed bearing steel as well as newer stainless steels. “The latter contains smaller, more evenly dispersed primary carbides than those in older archaic 440C stainless steel.” Too, since the mid-1960s the grinding and honing of raceways has been conducted using centerless grinding principles. This has made for bearings that operate with torque significantly reduced over the bearings with components previously manufactured using chucking.
Ironically, one of the greatest contributors to friction is the overfilling of rolling-element bearings with the wrong lubricant. “We worked with one manufacturer of optical encoders that experienced an 11% drop in noise, a 9% drop in operating temperature, and a 9% increase in machine life due to a change in lubricant — as well as reduced lube fill,” comments Howe. If lubricant is too thick, it adds friction directly and slows systems down. The lower viscosity of thin oil, on the other hand, can allow metal parts to come into direct contact. “Provide the proper lubricant for the application, in the correct amount,” stresses Jaffe.
But lubrication is sometimes used to induce friction on purpose. As mentioned, friction can be useful when used to help dampen the motion or inertia of a system, especially in high-performance linear positioning systems. “When friction can be controlled and calibrated, it provides a uniform reluctance of motion which, when used in small amounts, helps to prevent overshoot in high performance servo systems,” explains Barnes. This type of friction can also offset the effects of a poorly tuned servo loop, providing better precision in the final application. In fact, newer oils with specific viscoelastic properties add predictable, uniform friction to positioning systems. By using different amounts of these lubricants, the amount of friction can be calibrated to match the needs of the specific drive system.
The issue of preloading
Positioning systems use linear guideways to control straight motion and a drive mechanism to produce motion. “Unless using hydrostatic or air bearing components, friction will be prominent in these systems,” says Jaffe. “Linear drive and guideways components that use recirculating rolling elements are normally preloaded to eliminate play and achieve a higher degree of accuracy.” Preloading does increase friction though, which causes wear. The degree of wear, and eventual loss of accurate movement, depends on speed and duty cycle.
On the other hand, the practice of interference fitting of smaller bearings should be avoided at all costs. Usually the contiguous components (specifically, the shafts and housings) have geometrical accuracies measured in tenths of thousandths of an inch at best. In contrast, geometrical accuracy of bearings is measured in millionths of an inch. “So if interference fits are used to mount bearings, the bearings will conform to the inaccuracies of the shaft or housing — and cause excessive torque and heat,” comments Howe. What is the proper way to mount these bearings? “Smaller bearings in particular should be mounted with a slip fit and secured with an adhesive,” recommends Howe. “If the shafts are rotated as the adhesive sets, the rolling elements and inner and outer rings settle into the paths of least resistance, which in turn decreases torque and heat — and extends bearing life.”
Some high-efficiency ballscrews substantially reduce friction, and thus heat and wear, while maintaining system stiffness. But before components are selected for a design, Jaffe recommends that designers first determine whether or not the desired system accuracy requires play-free (that is, preloaded) components. “If not, friction can be greatly minimized, if not almost eliminated, without resorting to more expensive solutions such as air bearings,” he says. Jaffe also points out that designers can keep the completely nonfunctional friction of misalignment to a minimum by ensuring that elements are properly mounted.
Heat is the word
Several things can make mechanical components fail. One is overheating — from contamination, misalignment, excessive loads, and excessive speed. Of course, a high ambient temperature or a chemical or mechanical breakdown of lubricant can also make systems overheat in a hurry. As a general guideline: “Use 1570°F as a benchmark. For every 200°F increase in temperature you can cut the expected life of the lubricant in half,” says Howe.
Lubricants do two things: They provide a film that separates wear surfaces, and they carry away heat to cool down mechanical components. Howe elaborates. As temperature increases, the film strength of lubricant diminishes, and mating surfaces come into contact with more aggressive surface asperities. It is then that these asperities micro-weld to the surface of the mating component. “In ball bearings, these weldments eventually hit the raceways and break off, making an abrasive slurry of the lubricant.” At this point bearings become very noisy.
There is one caveat. Explains Bryson: “Wear in components in which sliding friction is present is greater than that of components experiencing viscous friction. But viscous friction can be improved, too.” In a few cases, it has been reduced by the application of a coating to the surfaces of fluid moving equipment. “The coating's relatively rough surface has the same effect on fluid flow as dimples on golf balls, extending laminar flow near the surface and reducing energy wasting turbulent flow.”
Bryson was involved in coating pistons of diesel engines with a polyimide/amide/PTFE low-friction material. “In this application, the coating increased the lubricity at the piston-cylinder interface, and prevented piston slap.” Efficiency of the engines was increased by almost 15%, as demonstrated in a 200,000-mile test. Dynamometer trials showed a corresponding increase in power of 16%.
Heat also causes rolling elements to experience thermal growth — expansion of the metal — if the bearing is not stopped. What's more, “this thermal growth is very seldom uniform. Rolling elements can grow enough to bind between the raceways. The free rolling elements will then drive the element's separator into the stalled element, causing the separator to explode,” warns Howe. With a few more revolutions the bearing fails totally.
Cool-running components also benefit from the accuracy of larger systems. “We work with a major U.S. machine tool builder with a reputation for axis accuracy on water-cooling ballscrews,” says Jaffe. They recently introduced a new line of milling machines without any cooling, using ballscrews that run about 67% cooler than conventional designs. “The reduced friction also makes the machine run significantly quieter.”