Here, hardened-steel raceways are mechanically inserted on light aluminum rail to withstand high contact loads and resist wear; this Integral V Technology (of PBC Linear) lowers replacement cost.

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.”

Some belt-driven linear actuators feature a profiled rail guidance system that allows large moment loads. With a carriage that employs THK caged ball bearings, the MXB-P (from Tolomatic) is capable of velocities of up to 150 in. per sec — suitable for handling, marking, and cutting applications. “It integrates a linear belt with ball-bearing load support and guidance,” says Dietrich. This allows machine builders to install a preassembled motion system without external guide rails or load support.

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.