from Elektromechanik I (June 2009)
by Rudolf Reichert and Kristin Rinornter
When considering the specifications for a system, engineers sometimes fail to consider proper cable management and the significance of selecting the correct cables. Cable failure ultimately leads to equipment downtime, which can be enormously costly. Approximately 70 percent of device and equipment failures can be traced back to faulty connector technology. A significant part of these problems can be eliminated through adequate functional testing, such as measuring resistance, thickness, and voltage, as well as measuring high-frequency performance.
Mechanical properties, application parameters, and manufacturing materials strongly influence a cable's behavior and lifespan. As the cable moves, its core bends, slips, and twists, potentially damaging the conductors and insulators. Also, if the materials used for the cable’s jacket are too soft, the cable can fail due to abrasion. For instance, an incorrect bend radius of a cable chain can cause tension in the core of a round cable, which in turn damages the conductors and insulators, leaving insufficient shielding.
Higher accelerations, velocities, and vibrations generate forces up to several Gs, which can increase friction and tension. In addition, the demand for smaller devices with more functionality can lead to electromagnetic disturbance and undesired thermal effects.
Therefore it is becoming increasingly important to investigate the long-term mechanical behavior of different cables by creating real-life conditions during testing of bend strength, torsion, fatigue, and duration. The benefits of this type of testing are two-fold: the cable manufacturer can optimize customer-specific designs, and the customer can be confident in the cable’s reliability.
One cable manufacturer that emphasizes the quality of its products is W. L. Gore and Associates. Gore is a global leader for polytetrafluoroethelene (PTFE) and expanded PTFE (ePTFE) applications. The company's production sites are in the USA, Germany, Scotland/UK, Japan and China. In Germany, Gore has three production sites, one of which is located in Pleinfeld, Bavaria. At this site cables and cable assemblies are produced for industrial applications, aviation and space travel, defense, medical cabling and cables for test and measurement. In addition the Pleinfeld site houses a special measurement laboratory – the Flex Laboratory where testing is done on the mechanical, thermal and electrical properties of the company's own cables, as well as those from other companies.
Gore operates this extensive cable-testing laboratory because of its commitment to quality. Bob Gore, the son of the company's founder, stated clearly, “Our products do what we say they do. It’s that simple.” This philosophy not only emphasizes the need for excellent customer service, but also incorporates the issues of product reliability and a good customer relationship. “When making a purchase, the customer is buying a product on the basis of trust,” declares Rudolf Reichert, director of the testing laboratory. Quality, accurate predictions, and thorough testing of the cable’s properties are the basis of good products.
A materials technology and metallographic laboratory, as well as a raster electron microscope deliver additional information via damage- and fracture-related images, as well as position and state of the conductor after failure.
The engineers primarily test the dynamic fatigue of large quantities of cables without connectors as well as complete cable systems. In addition, they investigate the influence of cable materials on fatigue caused by bending both independent of and dependent upon on the cable’s design (see Image 1). This knowledge enables Gore to optimize production processes and analyze the influence of production parameters on the flex-life of a cable. Another focus for the laboratory is to study the basic relationships between cable-design parameters and the mechanical-dynamic long-term load for power lines, signal lines, and high-frequency transmissions.
For both standard and special tests, the lab works closely with customers in planning and performing the tests, focusing on the final application. This approach gives the customer a reliable statement based on empirical data regarding the expected lifespan of the products.
The movement patterns are run through the classical trapezoidal course profile as well as the freely programmable curved disk to reflect the stress put on the cable in the customer’s application.
To improve response time and the meaningfulness of tests with several independent variables, various Designs of Experiments are used, which allow Gore to identify the lowest number of tests to perform before achieving statistical relevance. In addition, mathematical analysis tools such as MatLab and SAS are linked directly to the Oracle database and raw data are processed directly.
Through the flex lab, signal duration, impedance, frequency spectrum and degradation can be measured, while the position of each core can be determined. By combining this research with that of the high-frequency lab — where demands regarding characteristics such as digital signal transmission and concurrent flex fatigue often conflict with each other — Gore is able to determine the optimal combination of criteria from high-frequency technology and the lessons of dynamic fatigue. New opportunities have emerged, especially from the various structures of GORE™ Cables that are being investigated in application-oriented stress tests with 20 million and more cycles.
Over many years a great deal of competence has been developed for the optimal stress-test investigation, such that Gore itself has designed the stress-test equipment and engineered the entire set of technologies for measurement and control that are needed for meaningful investigations.
Using Gore’s testing equipment and methodology, it is possible to connect individually customized, application-oriented stress tests with meaningful analyses. For example, using so-called “cable killers,” the lifespan of cables up to 50 meters long can be tested, or the performance of cables that operate high-speed equipment for constructing circuit boards can be evaluated on a device that simulates up to 15 G. Using an application-specific multi-axis test, Gore can identify the optimal C-shaped curve to be used in X-ray equipment.
Testing cables for medical technologies is especially sensitive because a failure could potentially put a human life at risk. For this reason other self-designed measurement equipment has been developed based on the specific needs of the medical technologies. “For this reason you'll also find testing devices with peculiar names such as “Snow White's Casket”, Tombstone, Shower Room, etc.,” stated Reichert with a grin. An application-specific example of a medical technology simulation includes stressing a cable in a C-shaped curve in X-ray equipment. A combination of torsion, rotation, kink and alternate bending strength are reproduced in this testing equipment in Pleinfeld.
Rudolf Reichert works in research & development at W. L. Gore and Associates in Pleinfeld, Germany, where he directs the Flex Laboratory. Kristin Rinortner is editor of ELEKTRONIKPRAXIS.