Because these traversing systems are so widespread, every aspect of linear electromechanical design is continually being improved — from guidance and load support to actuation and control.

With its origin linked to nothing less than the development of the wheel, linear motion design dates back at least to the pyramids, when Egyptians solved the problem of moving heavy loads by placing tree trunks under blocks of stone and using water as lubricant. “This same basic principle is used today in linear motion guides, although rolling elements recirculate within a guide instead of being placed by hand,” says Kevin Gingerich, Bosch Rexroth Corp., Linear Motion and Assembly Technologies, Charlotte, N.C. Linear motion was further improved later with bronze bushings; fast-forward to the 18th century, and efficiency and (reasonably) predictable life was then enhanced with ball bearings. “More recently, life was also increased through use of formulated polymer coatings,” adds Glen Michalske of PBC Linear - Pacific Bearing Co., Roscoe, Ill. “This was all done to improve on a great idea: To get from point A to point B while carrying or moving something without expending a lot of effort.” In the U.S., linear motion really began with round shafting and ball bushings, according to Andrew Cook, general manager, Rollon Corp., Sparta, N.J. This design dominated for decades until profiled shafting with its advantages came into prominence.

Today's applications are substantially more demanding in terms of precision and cost, and linear motion suppliers have continued to develop more sophisticated guides, ballscrews, actuators — even complete Cartesian robotic systems — to meet these needs.

Significant surfaces

Today, chemistry and more advanced riding surfaces continue to improve linear motion systems. Ceramic-coated surfaces are in wider use; the development of embedded steel tracks on aluminum frames has enhanced systems where it counts, at critical points.

“Nearly all surface treatment enhancements for linear guides have the purpose of increasing surface smoothness and straightness to reduce friction,” points out Gingerich of Bosch Rexroth, “although a few, such as chrome plating, may have more to do with the ultimate operating environment.”

Surface improvement techniques include such things as plastic or metal hole plugs to seal the mounting holes in a rail, or even metal cover strips to create a more uniform sliding surface over a rail's entire length.

“For example, as one of our ball rail systems use an aluminum rail, hardened steel inserts were added for the structural rigidity necessary for precision,” says Gingerich. The use of aluminum for rail material, besides keeping cost lower, also allows the system to be more accommodating of uneven mounting surfaces, because it is less rigid than standard steel ball rail guides.

“For plain bearings, we use a ceramic coating process, which creates a hardened surface that is nonstick to prevent most materials (weld splatter, paint, sugar, and so on) from adhering to the shaft and causing premature bearing failure,” Michalske of PBC Linear explains. Combined with a bearing liner, the design reduces friction and maintenance, for linear motion that withstands environmental abuses.

One caveat: Surface treatments and design are just part of the larger system. Aaron Dietrich, manager of electric products, Tolomatic Inc., Hamel, Minn.: “Our engineering design philosophy is to supply components that meet design parameters and provide dependability; it doesn't directly relate to just guiding or surfaces.”

Another approach is to bypass surface imperfections, macroflaws or otherwise. “Some newer designs take advantage of less costly materials but so far, nothing has proven better for increasing load capacity and decreasing cost than cold-drawn carbon steel,” says Cook.

He explains that engineers often believe that builders can make the parallel surfaces so easily drawn in CAD files. “Indeed, there are ways to do this,” says Cook. “Manufacturers can machine surfaces and spend time ensuring parallelism. However, this adds cost. Can this cost be passed to the customer?” This depends on the customer and the application. When it cannot, the cost reduces bottom-line profit.

“With both welded steel frames and structures, or those made from aluminum extrusions, it is difficult to ensure that linear bearing mounting surfaces are parallel. Anyone who has mounted round or profiled shafting products knows that two rails mounted together must be mounted perfectly parallel,” says Cook. In fact, that is printed (in so many words) in catalogs of some shafting manufacturers — and mounting to nonmachined surfaces makes for poor movement or even bearing failure.

One Rollon system corrects for these nonparallel mounting surfaces with a pair of rails. One rail allows slider rotational freedom before mounting; the other allows lateral freedom. Rotational freedom allows one or both rails to be mounted in a turned position, or twisted relative to each other. One rail can also be mounted higher or lower than the other.