Key Design Criteria for High Flex Cable Constructions By Mike Levesque, Dale S. Long, and Ron Crouch of C&M Corporation

Transcription

1 KeyDesign CriteriaFor HighflexCable Constructions

2 Key Design Criteria for High Flex Cable Constructions By Mike Levesque, Dale S. Long, and Ron Crouch of C&M Corporation With the continued growth of factory automation as a strategy for improving manufacturing efficiency and reducing cost, the need for properly designed high flex cable products will continue to increase. For the purpose of this discussion, we will define high flex as a cable that can withstand repeated flexes, usually defined in the millions of cycles, without losing functionality. Unfortunately, as some cable users have learned, there is a difference between a flexible cable and a high flex cable, and high flex cables designed for given applications are generally not interchangeable. When evaluating high flex options, cable designers begin by seeking an understanding of the application parameters. This includes not only the flex requirements of the equipment the cable will be part of, but also the expectations for performance set by the OEM. The flexing requirements most often seen in the area of high flex cable products fall into one of four types: Variable/Omni-directional: These high flex cables move in multiple directions that can change as the unit is reprogrammed for new processes. Examples include cables that might be found on a robotic arm. Torsional: These products see a twisting or wringing type motion during use. Often they are also subjected to tensile loads while in operation. A cable used as part of an automated nut runner application would be an example of a torsional flex product. Rolling: Track systems are an example of a rolling flex application where the cable is flexed into a C shape then straightened on a continuous basis. Bending: Often referred to as tick tock this flex type creates specific bend points at which the cable is flexed on a repeated basis. A pick and place unit would be a good example of this type of high flex application. As with many aspects of cable design, certain basic rules exist. An example of one of these rules is the call for a 10:1 ratio of outside diameter (OD) to bend ratio, where the OD of the cable is less than or equal to ten times the bend radius when the bend radius is greater than 2 inches. Where this rule is not applicable due to the requirements of the application, special design considerations must be applied. High flex life cables are often subject to these unique rules. High Flex Composite Cable

3 When the application calls for a combination of flex types, the engineer must balance the design requirements for each flex type with the performance requirements of the customer. Material Selection While understanding the application and performance requirements is at the foundation of any robust high flex cable design, the choice of materials cannot be discounted relative to the importance it plays in the success of the final product. When evaluating the material options for an application, the design engineer has several material groups to consider: Conductor: The considerations for conductor include gauge of the individual strands, the strand count, construction, and the metal choice. Relative to the latter, the main options are copper or alloy. Alloys will generally outperform copper, all things being equal, but often increase product cost significantly and are rarely necessary. Relative to stranding, the smaller the gauge of the individual strands the more strands that are required to meet the AWG size requirements. While smaller strands generally provide a longer flex life compared to larger gauge strands, there is a point of diminishing returns when it comes to the strand gauge/strand count equation. Insulation: Designers must consider not only the durometer of the available plastics, but the electrical requirements of the final cable as well. While lower durometer (softer) plastics are generally a good choice when the desired characteristic is flexibility, for flex cables higher durometer materials are often preferred. Many designers choose to evaluate a plastics based on their flex modulus (also referred to as flexural modulus or flexural strength) as opposed to durometer. A material s flex modulus is often used as an indicator of a material s stiffness when flexed as well as its tendency to bend. Another criterion for consideration is the material s coefficient of friction, as a key goal for any high flex design is a state of maximized lubricity, where components slide, as opposed to rub or stick, when they are moved against each other. Cabling: In high flex cables, the goal of eliminating backtwist is paramount as it represents a stored energy within the cable construction that will work directly against flex life. This is generally accomplished using planetary cablers, whose construction neutralizes all components without the use of additional equipment, or neutralizers that produce a similar effect when added to rigid bay cabling units. The designer may also opt for a contra-helical cable lay which will increase the cable s size but make it more flexible compared to a unilay type approach. The cabling operation also establishes the cable s lay length, which is a critical design parameter. Lay length is defined as the distance required for one conductor to complete one revolution about the axis around which it is cabled. While, for example, a rolling flex cable would utilize a tight cable lay, a torsional flex cable would require a much longer or looser lay length. Unilay Contra-helical

4 Tapes: Generally, when it comes to flex cable design, the use of tapes of any type should be minimized. When tapes are required, consideration should be given to their ability to promote lubricity between the cable s components. Shielding: As stated in the discussion of tapes, foil shields are not recommended due to their limited flex life. While much more robust, braid shields have limited life in high flex applications as well. The type of flex (torsional, rolling, etc.) as well as the expected flex life as specified by the OEM will determine the viability of a braid shield as part of the construction. Should a braid be deemed acceptable, criteria such as wire gauge and shield angle become important. For example, the shield angle on a Tick-Tock flex cable would be approximately 45 while the angle for a torsional flex cable would be as low as is possible. Another option in the shielding area is reverse spiral shields. Relative to this shielding strategy there seems to be some disagreement over whether or not it provides any substantive advantages over a braided shield in high flex applications. Performance aside, should the design engineer opt for a reverse spiral shield, he or she must balance the flex life requirements of the application with the potential inductance created by this shielding choice. Jacket: Much like the wire insulation, both durometer and flex modulus are generally considered. Somewhat surprisingly, it is not the softer, more flexible jacket that is the choice when the consideration is only flex life. However, in most cases, there are additional considerations when choosing a jacket material that go beyond simple flex life. Abrasion resistance and resiliency would be two such examples. Additionally the application of the jacket during the extrusion process is critical as it must not bind to the cable core and restrict it from moving freely. Often a separator tape, generally paper or PTFE will be used to ensure separation of the jacket from the inner conductors or shield. Additional Considerations While most engineers would enjoy a world where the only design consideration was flex life, the fact is there are multiple parameters that must be weighed as part of the final cable construction. Fortunately or unfortunately, additional performance, safety, and environmental concerns are often part of the final design considerations. These might include but are not limited to, operating temperature, flame resistance, the ability to withstand cutting oils, grease, cleaning fluids, etc., sunlight resistance, and the requirements of any applicable agency approvals. Each of these variables is usually addressed via additives to the virgin jacketing material that provide the plastic with the necessary characteristics. These additives, however, do not generally promote flex life. When high flex cables are required in an environmentally controlled application where the only performance concern outside of flex life is out gassing or particulate generation, the designer does have flex friendly material options that will meet these performance parameters.

5 Electrical performance is another variable that is of key concern to designers. While in most cases no additives are required to the plastic in order to support performance, the desired electrical characteristics themselves can force the engineer to a material choice that does not maximize flex life. Similarly, environmental and safety performance criteria, when applicable, most often work in opposition to a design based solely on flex life. An experienced design engineer can balance the considerations of performance against these additional requirements and formulate a construction that can satisfy the conditions of the application. Testing and Failure Modes While the ability to test fully each high flex construction to assure compliance to any flex life requirement would be ideal, the timing of such evaluations makes it impractical. For example, a cable with a 20 million cycle flex life requirement completing one flex per second on a test unit running 24 hours per day, 7 days per week, would take approximately 8 months for testing to be completed! It is for this reason that high flex designs are usually created based on available test data from previous evaluations of the given construction, test data from similar constructions, materials data, and field data that summarizes the flex performance of the product, or similar high flex products, during actual use. From the perspective of documentation, the performance requirements for high flex products are usually specified in terms of minimum flex life as opposed to maximum. Unlike static applications, the preparation of the finished cable before installation, as well as fixturing during installation, plays a critical role in the product s performance almost as much as the design itself. Unlike static cables, failure to properly install flex cables is a well recognized failure mode. Flex cables should be unreeled and relaxed before installation, a process that allows it to regain the characteristics it possessed before packaging. Ideally, the cable should be hung in a non-terminated form for 24 hours before installation but laying it out on a flat surface is also a viable option. Any additional handling or termination must maintain the cable s new relaxed state. When clamped as part of the installation process, the clamps must be applied tight enough to hold the cable in place but not so tight as to prevent the conductors from moving during the equipment s operation. Failure to follow proper installation guidelines can derail the performance of even the best flex cable constructions. As with any manufactured product, performance failures can occasionally occur. Assuming proper installation, a mismatch of the application for which the cable was intended as compared to the application in which it was actually used, or improper manufacturing techniques, can create multiple failure modes that will vary between constructions. Where the failure is related to poor design, poor manufacturing processes, or improper installation, some common failure modes include: Knuckling: A sharp bend in an individual wire or the cable core. This failure mode is common in rolling flex cables.

6 Conclusion Hard Knuckling: Multiple sharp bends in an individual wire or cable core usually located in close proximity to each other. Again, this is a common rolling flex failure mode. Wire Break: Either a complete break in an insulated conductor(s) or the breaking of the copper strands beneath the conductor s insulation that can result in either an intermittent condition or complete circuit failure. Found most often in Tick Tock flex applications. Shield Failure: The breaking of the wire strands used to construct the braid shield. When the shield is used as a conductor the failure will break the circuit and stop the equipment from functioning. This failure mode is common in torsional flex applications. In an environment that relies on automation, high flex cables will be an integral part of the equipment utilized. Where the goal is continuous, reliable equipment performance, any failure of the cabling can result in a significant loss of revenue and production time. Successful cable designs begin with an understanding of the application and the OEM s performance expectations. The success or failure of the cable design will be a direct result of the design engineer s ability to balance the requirements of the application with the available materials, environmental conditions, and available manufacturing options. C&M Corporation, headquartered in Wauregan, CT, is an integrated manufacturer of bulk cable, coil cords, and cable assemblies (both molded and mechanical). C&M is a leader in providing engineered interconnect solutions through reliable customer service, world class quality systems, and a wide breadth of manufacturing capabilities that include bulk cable production, internal mold tool fabrication, and innovative assembly constructions. Please contact C&M at for more information or visit C&M at