Support for the long haul

Like ancient explorers whose journeys expandedthe boundaries of the known world, developers at W.L. Gore & Associates (Elkton, MD) have nearly quadrupled the travel range of unsupported hybrid planar cable, from approximately 16 inches to at least 60 inches. For light manufacturing applications requiring distances in between, the new Gore^(TM) Trackless Cable Assemblies are said to eliminate the space, time, and cost required to install and maintain cable tracks or other support systems. And, the company says, they also eliminate the vibration and noise associated with cable tracks.

For Gore's efforts, Design News readers voted the Trackless Cable Assemblies Best Product of the Year.

"Customers told us, 'We love your product but hate to have to stick it in this track,'" recalls Kusha Sheikholeslami, an applications engineer in Gore's High Flex Life Planar Cable Assemblies Group. Gore is a major supplier of cables for high-speed pick-and-place applications in semiconductor manufacturing, medical diagnostics, food processing, and other industries. The company was keenly aware of the need to support longer travel distances reliably-without cables buckling, bouncing, or jumping-and to streamline cable management systems.

"In the semiconductor industry, for example, in order to increase throughput customers are working with larger wafers, and they wish to handle multiple wafers at a time. In both cases they need longer cable travel distance," Sheikholeslami explains.

Gore's goal was a 60-inch travel length and a bend radius of 2 to 3 inches, figuring that a bend radius any smaller than that would place too much stress on the conductors. "The challenge was how to make a cable that's extremely limp and flexible in one direction but strong and rigid in the other direction. Whatever we did had to be small enough to fit inside the cable and be practically invisible from the outside, and it had to withstand more than 20 million flex cycles while carrying the weight of several cables stacked," says Sheikholeslami.

"We tried stiffeners, but they made the cable larger and heavier than we wanted it to be-and it still sagged," he explains. "We thought about stacking the cable on a stainless steel belt, but that made it too thick. We considered using a stiffer jacket material, but that would have given us too large a bend radius. We even tried shrinking the size of the track."

Ultimately, Gore settled on the idea of putting supporting elements inside its flex cable to give it structural support and limited bending freedom. The supporting elements had to be small enough not to change the uniform profile of Gore's current cables, yet rigid enough to carry the weight of stacked cables.

"We studied the design of the human backbone, looking at how vertebrae interacted with intervertebral discs and interlocking facets," says Sheikholeslami. "We went through a series of design concepts for these miniature backbones and flex tested each one to see how many flex cycles it would survive." The flex testing took approximately four months per 10 million cycles, and Gore's goal was 20 million cycles. Materials were tested for dimensional stability in manufacturing, fatigue resistance, elongation, deformation, and cold flow.

"We developed a characterization method to correlate the deflection of a support element to the flex life of a cable," Sheikholeslami notes. "Once the model was established, each data point collected allowed us to make adjustments and corrections along the way to the design and materials selected without waiting for flex life data. This allowed us to shorten what could have been an even more time-consuming development effort."

The endoskeleton design uses liquid crystal polymers for their dimensional stability and resistance to deforming, and stainless steel for a hinge-free design that eliminates wear. Aramid fibers were considered because of their resistance to elongation, but rejected because of their poor flex-life performance and complexity in manufacturing stage. The support member is encapsulated in the same cable jacket previously used on Gore's planar flex cables. The jacket consists of an expanded PTFE (polytetrafluoroethylene) outer layer and a polyurethane inner layer.

According to Sheikholeslami, the expanded PTFE eliminates the problems of particulation and abrasion that can occur with urethane or with other extruded-type jacket materials such as PVC. As a result, Gore's trackless assemblies are suitable for use in clean rooms, vacuum applications, and harsh temperature and chemical environments. "The expanded PTFE allows cables to slide over one another more easily, which relieves the stress that can build up when cables roll back and forth on top of one another in flexing applications," Sheikholeslami explains.

Prototypes of Gore's patented Trackless Cable Assemblies have endured as many as 28 million rolling flex cycles, traveling back and forth up to 60 inches, with a 2.5-inch bend radius, at speeds in excess of 100 inches/second and accelerations up to 4Gs without sagging or buckling.