Machinedesign 1762 Straight Move Lowdown 0 0

Straight move lowdown

June 1, 2009
The shortest distance between two points is a straight line — so it's no wonder that nearly every motion system includes sections in which parts travel in straight paths.

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.

Similarly, the lateral freedom allows one or both rails to be mounted so that one rail pulls laterally away from the other. Once mounted, the rails do not allow play, but do move along the course that the nonparallel surfaces determine, instead of ‘fighting’ each other and binding. The end result is misalignment compensation as well as linear precision.

The whole kit and caboodle

Purchasing a finished actuator may be cost effective, or if a designer builds one machine to use in house, it is possible for him or her to save money by designing, purchasing, and then assembling the various components. “Many times builders assume some savings if some of the components are already on plant shelves. However, many times people do not count their own time at the drawing board and during assembly,” says Cook. “It begs the question: Are they really saving money and adding value to their machine?”

As in other designs, there is a trend to integrate electric actuators, motors, and controls into all-in-one (and easy) packages.

Increasingly electric actuation also addresses more complex motion-control challenges. “Connectivity and communication technologies such as Ethernet may allow increasing value,” predicts Dietrich. He forecasts that more conversion of traditional pneumatic applications to electric will make for greater integration of electric motion control. Simple, easy-to-use solutions will find increased utilization as machinery designers migrate to electromechanical linear motion.

“Our linear actuators also eliminate intermediate power transmission parts — like the gearbox that usually goes with a belt drive,” says Michalske. “Eliminating components and parts while doing the same work extends life and accuracy.”

There will always be applications that require gearboxes — for example, servomotor-driven systems. “However, whenever possible, we look for lower-cost solutions that can eliminate extra components.” Another example: Switching from a servo to a stepper motor may eliminate the gearbox.

Complete linear subsystems and actuators can also make sizing for application requirements easier — the biggest challenge in linear motion. “If an engineer overspecifies linear-motion elements, for example, he or she is likely building unneeded cost (and performance) into the machine. Underspecifying brings different issues: The final machine might be less expensive but not last as long or survive continuous operation,” says Gingerich.

To help engineers, linear motion suppliers are also developing new products to satisfy specific cost and performance targets. “One of our ball rail systems for light automation and handling tasks fills the gap between a linear bushing application and one requiring a high-precision rail-and-block type guide,” Gingerich notes.

Although developed primarily for special-purpose machines and assembly and handling applications, the precision and cost of their ball rail system have led to atypical applications such as sliding doors and walls, tradeshow displays, and furniture elements. “We've also found that lightweight aluminum-based components have been especially critical in some applications, saving as much as 65% of the weight compared to standard steel rail-and-block systems,” notes Gingerich.

Another option is belt-driven linear actuators that feature profiled rail guidance for large moment loads. How does this compare to other rails? “The profiled rail gives effective, compact load guiding; as part of a prepackaged, preengineered system it adds value. Other guiding systems may have similar capability, but may take up more room,” says Dietrich of Tolomatic.

Profiled shafting products resist moments without the need for additional shaft and bushings. “However,” says Cook of Rollon, “the C-shaped steel profiles on our compact rails can be mounted to nonparallel surfaces without adding additional preload.”

Clean living

Ball bearing-type guides are the gold standard of linear motion. Their weaknesses are that they require lubrication and can form particulate mater as they wear. This often forms environmental contamination that can cause problems in assembly and packaging industries. Additionally, the bearings are subject to application environments that can cause premature failure.

This can spell a recipe for disaster: “The two leading causes for bearing failure are contamination and lack of lubrication,” says Jonathan Schroeder of PBC Linear. “For example, washdown in a food processing plant infiltrates ball-type bearing seals and causes corrosion that ultimately leads to catastrophic bearing failure. The biggest failures, however, are from lack of maintenance; linear ball-type bearings need regular service.” In addition, many applications require quiet operation and the click-click-click of balls entering and leaving the load zones can be a problem.

In an effort to match the accuracy of bearing-type guides without the drawbacks, Schroeder's company uses a simultaneous integral milling operation to machine critical surfaces of an aluminum extrusion to less than 0.001 in. or 0.025 mm over a 6-m length, in the same range as some steel recirculating ball-bearing guides, also known as linear guides or profile rails. Aluminum, by itself, is too soft to be a good bearing raceway so it must be hardened in some fashion: For cam roller and recirculating ball bearings, machined aluminum frame is fitted with a steel race for longer bearing life. The mechanical crimping system used to secure the races also eliminates labor-intensive fasteners.

“There were a few applications that led us to develop our integrated V-track linear guides,” says Michalske. One application used two parallel rails mounted to an aluminum plate. The customer had a tight parallelism tolerance and their assembly line staff would spend hours trying to align the two rails. In the other application, a customer was using two linear guides mounted to a 100×200-mm aluminum T-slot structural frame. This customer did not machine the frame prior to assembly and would sometimes spend hours trying to align the linear guides so that they would move easily without binding. “Both systems now employ custom-designed integrated rail-carriage systems that decrease installation and alignment time to almost nothing,” says Schroeder. In short, the machining of simultaneous integral milling allows for dimensional accuracy and parallelism at a lower price point.

Looking to the future

Two areas to watch are super-efficient and small actuators. “Eventually, there will be need for efficient actuators to track the sun, for solar energy; miniature linear-motion systems will also be important, as we push the horizons of nanotechnology,” says Schroeder.

His company's stance is that efficiency and green engineering are ways of life for any company that expects to be around a few years from now.

Another area of growth is custom solutions. “For the price of an aluminum extrusion die and a few consulting engineering hours, we can supply a custom solution,” Schroeder says.

Their guide's aluminum extrusion dies range from a few hundred to a few thousand dollars, depending upon size and die complexity. Customers can do a simple ROI calculation to determine if the initial investment in a new extrusion die will yield return enough to justify the engineering change.

Generally speaking, customers spending more than $5,000 a year on installation, assembly, and alignment find that the switch saves money. “With 250 working days in a year, $5,000 works out to approximately $20 per day; if an average assembly worker makes $15 to 20 per hour, it takes little time to recover that initial investment,” Schroeder adds.

In fact, in the current economic environment, the most obvious trend has been the relentless pursuit of cost reductions. “In many cases, this may simply mean the replacement of components that were originally overspecified with lower cost components that can still get the job done,” says Gingerich of Bosch Rexroth. In others, it might mean that automated systems are replacing manual operations, and linear motion components are needed as part of the final automated approach.

“We believe that through innovation we can continue to achieve success,” concludes Cook of Rollon. “Design around what is. Don't just add onto existing products. Distill. Improve.”

For more information

Bosch Rexroth Corp.

Linear motion
Assembly technologies
(704) 583-4338

PBC Linear

(888) 389-6266

Tolomatic Inc.

(800) 328-2174


(877) 976-5566

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