Flexible cables or high flex?

Robotic and automation applications require specialized cables that can handle repetitive flexing and movement. Design engineers must consider using custom cables designed to withstand flexing to prevent product failure and down time down the road. Robotic wire applications don't always require extremely flexible wire and cable, withstanding hi-flex environment is usually the main concern.

Flexible cables or high flex? These two terms are often misunderstood. Flexible can mean the limpness of a wire or cable and is used by some to describe high flex. The term high flex wire and cable is usually used to describe repetitive flexing. Can you have both? Yes you may. Ultra flexible wire designs use high strand count conductors for higher flexibility as does high flex wire. High flex wire normally will use alloys for the conductor material while maintaining the high strand count. High strand count conductors are made up of many fine strands of wire, the higher the number of strands the more flexible and resistant to breaking the wire will be. Depending on the wire gauge, strand counts can be in the thousands. Typical off the shelf wire is available in solid, 7 or 19 strands. These stranding configurations are better suited for static type of applications. When a wire has fewer strands, the strands will fatigue and begin to break under repetitive bending, vibration and flexing situations. As the strands break, conductivity will be compromised and may result in damage to the wire insulation. Conductor stranding is available in a variety of configurations.

For best results for high flex robotic cables need to be designed with high strand count conductors preferably using an alloy for added strength. Insulation can vary depending on the application. When designing the insulation and jacket, the environment must be considered. A few considerations are:

• Chemical resistance
• UV resistance
• Abrasion resistance

For super flexible cables, silicone insulation and/or jacket is a great choice. Silicone is resistant to chemicals and one of the most flexible of material choices. The downside to silicone is abrasion resistance. Other flexible wire insulation and jacket materials choices include TPE, polyurethane and Calmont's Superflex which is a special flexible PVC to name a few. Cable construction and shielding options also play a role in flexible wire and cable. Consulting a custom cable manufacturer and discussing the application, environment and use of the robotic cables prior to product design is recommended. Making the right cable design choices from the start will prevent down time and product failure in the future.

Medical cables and UL certification.

There are times where medical cable designs requiring super flexible wire and the available UL styles don't match the design requirements. This happens often. UL sets the standards for different categories of wire and cable. The standards include wall thickness, material, voltage rating, flammability, type of conductor and other factors. What these standards do not take into account are the requirements for individual medical devices for ultra flexible wire or cable, high flex wire applications and sterilization techniques. Custom wire and cable manufacturers can manufacture the same construction to meet the UL specification while making improvements on performance for example using higher strand conductors, alloys for added flexibility and strength or insulation and jacket material improvements.

Recently, we received design request for a flexible flat medical wire for a two conductor cable with multiple lumen, the customer required a UL style for the wires but not the lumen. We often receive requests for medical cable designs requiring UL certified conductors but not all the components in the cable.

The reality is that UL certification for individual components becomes null and void when used in medical devices. If the device needs to be UL certified then the entire device needs certification not just the components individually. Custom cables can be manufactured to perform better than a UL cable. Some examples of improvements are:

• More flexible construction and materials
• Withstand sterilization and chemicals
• Hi-Flex and vibration resistant
• Conductor sizes outside the UL style

There are many UL styles for wire available and finding a close match to your needs is possible however, finding a close match is not the same as having a cable to meet or exceed your requirements. Wire can be designed to meet voltage, flame and other requirements and doesn't have to be UL certified to do so. When designing your wire and cable it is important to look at your individual product requirements and design the wire to meet those needs.

If your medical device is required to be UL certified, then design the wire and cable to meet your product's needs instead of designing your device around available UL styles. In the end the entire device will need to be certified.

Flexible Flat Cables: Having it your way

Need a super flexible flat cable? Need a 36AWG ribbon cable with 21 conductors all in pink? No problem.

Off the shelf flat cables are often available in limited sizes and flexibility which creates problems for designers who need more robust or flexible flat cables. Off the shelf flat cables are normally made of PVC and the conductor stranding is 7 or 19 strand. For applications requiring high flex cables, resistance to chemicals or flexibility off the shelf cables may fail to meet your expectations. Off the shelf cables do not give options for:

• Flexibility and hi-flex applications
• Odd number of conductors
• Uncommon sizes
• Unusual configurations
• Custom colors
  
  

Flexible flat cables are ideal for hand held devices, automation and robotic applications. Engineers and product designers do have options by working with custom wire and cable manufactures to design exactly what they need. Custom wire and cable manufacturers can offer a variety of sizes, configurations and flexible flat cable options. There are two basic types of flat cables:

Ribbon cables - Ribbon cables are multiple conductors connected in a parallel configuration where the conductor spacing is controlled. Silicone ribbon cables are a popular choice for applications where an ultra flexible ribbon cable is needed. Using silicone along with high strand count conductors and/or alloys for added strength and resistance to fatigue for hi flex applications will add to a ribbon cable's flexibility other flexible material are TPE, Polyurethane, and even PVC (Calmont's Superflex PVC).

Top 3 Choices for Ultra Flexible Wire or Cable

Flexible wire in terms of how limp it is starts with the wire design options. The choices you make can greatly impact how flexible the wire or cable will be. The right choices can make the difference between flexible wire and a super flexible wire. Designing a flexible wire or flexible cable begins with the design. We start at the core, conductor stranding and construction.

1. Conductor stranding: Conductors are made up of fine strands of wire assembled together into several configurations. The more strands in the construction, the more flexible and resistant to repetitive flexing the conductor will be. Conductor stranding options range from a solid strand (one strand) to over a thousand strands depending on the wire gauge. The construction of conductor stranding is important as well. Stranded conductors can be manufactured in a variety of configurations, the most common being concentric (true concentric, equilay concentric, unidirectional concentric, and unilay concentric), bunched and rope.

Conductor stranding with rope construction makes for a more flexible wire. This construction has the advantage of increasing flexibility by using a larger number of finer strands while maintaining a tighter diameter tolerance than a simple bunched construction. Ropes are more evident in the larger AWG sizes, such as 8 AWG and larger, but there are also many applications that require the flexibility of rope constructions in the smaller gauges. Constructions vary and can contain hundreds or thousands of strands. To achieve the most flexible wire design, be sure to use the higher stranded conductors. For flexible wire with repetitive flexing applications, consider using an alloy for fatigue resistance.

2. Insulation: Flexible wire and cable requires a flexible insulation material. There are many choices for insulating materials. The most flexible wire insulation is silicone, together with the right conductor stranding can be as limp as a wet noodle. While silicone is the most flexible wire insulator, it does have poor abrasion resistance. Choosing the right material will depend on your application and use, consult your wire and cable manufacturer to determine which material will work best for your application. Flexible insulation and jacket materials include TPE/TPR, polyurethane and even PVC, Calmont's Superflex® uses a special PVC formula for increased flexibility. When choosing an insulation or jacket material, consider your requirements. Is your application requiring sterilization? Is abrasion resistance required? Temperature, chemical contact and bio-compatibility are other requirements to consider. Calmont's insulation and jacket material chart is a helpful tool.

3. Cable construction: Cable construction is another component for super flexible cables. Cable lay length and shielding can increase the cable's flexibility. The cable lay is how the conductors are cabled together, the wires can run parallel or be twisted around each other. The length between each twist can be adjusted for maximum cable flexibility. Shield options include spiral, braid and Mylar or foil wrap. The application is important when considering shield types. For flexible cables which undergo repetitive flexing, spiral and foil type shield are not good options as the shield coverage will be compromised over time.

If you want a more flexible cable design, contact Calmont Wire & Cable to learn more about material choices.

Conductor Stranding: Poor designs lead to breakage.

Conductor stranding is an important component for applications with hi flex cycles or vibration. The higher the conductor stranding the better the resistance to stress caused by flexing. Custom cables with high strand count conductors are the best choice for these types of applications. Higher conductor stranding also makes for an ultra flexible wire when using the right insulation. Discussing the flex requirements with your wire and cable manufacturer is critical to choosing the right cable construction.

Are there circumstances when conductor stranding doesn't help with wire breakage and loss of conductivity? Yes! Recently, I was asked what type of wire I would recommend for an application that required the wire to be soldered onto a PCB. This person was repairing PCBs experiencing conductor breakage at the solder point due to vibration and he thought the wire was to blame. This is a common problem when the wire is soldered onto PC boards especially now with the use of lead free solders.  Applications such as aerospace that are subject to vibration and movement of the wire can cause stress at the solder point.  The problem has nothing to do with conductor stranding or flexibility of a wire, while it is true that high strand conductors and the use of alloy materials adds strength to the conductors. The reason for this has nothing to do with the conductor stranding or how flexible a wire is, but the termination. Termination with connectors is recommended to solve this problem. Cable assembly and contract manufacturers understand this problem and are using edge cards and other types of connectors for termination with newer PCB applications. The problem is more prevalent among legacy boards that are being repaired where retrofitting connectors is usually not an option. Often times a strain relief cannot be installed due to the nature of the application. The solution to repair this type of problem with the old boards may be as simple as using an epoxy to secure the wire to the board to prevent stress at the termination point.

Having the right strain relief is important especially with handheld devices. The lack of or using the wrong type of strain relief can actually cause a stress point and product failure. Strain reliefs can vary in sizes and materials, consult your cable assembly house or contract manufacturer for guidance. I had a customer experiencing conductor breakage on his handheld tool and sent us a sample for analysis. Though his design used high strand count conductor strands, we found the conductor stranding to be damaged at the strain relief, it was discovered the strain relief design had been changed to a spring which caused added strain to the conductors. He changed his design and the returns on his product ceased.

The moral to the story is that though high stranded conductors do resist the stresses of flexing, the cause of the problem may not be in the cable design. Take the time to consult your cable manufacturer and the contract manufacturer for the best material options for your device.