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 m 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.