Given how many cable systems are required to tolerate movement – whether small amounts during initial installation or repetitive daily motion – understanding the flex requirements of a specific application is crucial to choosing the right wire and cable. When looking at cable types, terms like “flex,” “flexibility,” and “flex life” show up often. What factors affect or influence flex? Do you know the best material options to make your solution more flexible or maintain a longer flex life?
Within the world of “flex,” flexibility and flex life may sound equivalent, but there are key differences. In fact, some high flex cables can have short flex lives, and other wires and cables with high flex life may not be particularly flexible.
Simply put, flexibility is a measure of how much movement a cable can tolerate at a given time. Flexibility comes in many types; torsional, rolling, bending, and variable all describe different kinds of flexibility. When a cable is bent, twisted, or pulled into positions other than its original state, its flexibility is tested. Flexible cable can bend significantly and stay bent for a large amount of time without being damaged.
Will your cable need to be bent or curved around infrastructure? If so, you may need high flex cable, or cable with strong flexibility.
Another way that people might refer to a flexible cable is by using the term ‘supple’. Often times when someone is looking for a very flexible cable, they want it to be very soft and supple because they want it to be easy to manipulate or bend back and forth. Think of a surgeon using an electrosurgical tool. In this type of application, the surgeon must be able to effortlessly manipulate the tool without the cable kinking or back twisting and ultimately pulling on the surgeon’s hand, resulting in fatigue.
Flex life deals less with how the cable moves, and more with how often it can move in these ways without taking serious damage. Many cables can bend to some degree for limited use (e.g., installation). High flex life cable can bend repeatedly and regularly without disrupting the cable’s intended use, such as power supply or data transfer.
Will your cable be subject to repeated movement such as is needed in robotics? If so, you may need cable with a high flex life.
Ultimately, the distinction comes down to use and application. If your cable needs to move well, it must be flexible. If your cable needs to move often, it must have a high flex life. In many applications, such as industrial machinery or factory automation, both flexibility and high flex life are required.
Whether we are talking about flexibility or flex life, there are different types of movement to consider. Different materials may be chosen depending on how your end application and the cable will move.
- Retractable. A coiled or spiral cable is intended to “snap back” into shape after being pulled or extended.
- Torsional. When a cable is twisted, pulled, or rolled around its axis, it experiences torsional flex.
- C track, Drag chain, or Rolling. Often seen in track systems, rolling a cable not only stresses the cable’s interior components, but also wears down the outer jacket over time.
- Variable. More “free form,” variable flex may make a cable move on multiple points in multiple directions – often seen in robotics.
- Bending. This “tic toc” motion often happens when part of the cable is stationary, and the other part is bent back and forth over a fixed object.
- Continuous. In many industrial applications, flex isn’t occasional – the cable may be part of equipment that requires constant, repeated movement for the lifespan of the system.
Repetitive flexing can cause serious damage to wire and cable systems. Internal conductors may break, outer jackets can erode or tear, and friction can cause overheating. Not only does this decrease the lifespan of the cable, it can cause system downtime or damage. Luckily, there are various ways to achieve flex characteristics to avoid downtime or damage. Material choices in a variety of components including stranding, conductor insulation, braid, strength members, jacketing material and so on all have materials that can help increase flexibility or extend flex life.
When choosing stranding for a cable requiring high flexibility, a high strand count is best. The typical rule of thumb is “the more strands, the more flexible and the longer the lifespan”. From a movement and motion standpoint, a rope lay construction is by far the most flexible configuration – it is also by far the most expensive, highlighting the trade-offs buyers and engineers must constantly consider when choosing materials. Concentric stranding is a better choice when aiming to achieve a high flex life.
With the rope lay configuration, there are a number of individual bundles of stranding twisted into groups that make up one larger, solid configuration.
In a concentric strand, there is a solid conductor running down the center that is surrounded by layers of rotating strands. Each layer is comprised of strands running the same direction, but the direction alternates for each layer.
Insulation and Jacketing
When selecting insulation and jacketing materials, it is a good idea to choose materials with a high dielectric strength. A higher dielectric strength means you can extrude a thinner wall, which means there’s less material in the way, making it more flexible. It is also best to choose a ‘soft’ material with a ‘shore a’ durometer of 80 or lower. The softer the cable, the easier it will be to bend and flex. A thinner wall and softer material will also increase the cable’s suppleness.
A strength member is the component of a cable with the function of adding support to preserve the cable integrity. Strength members add both function and protection to connectivity solutions. There are several options including steel, aramid fiber, or a stiff fiberglass rod that runs the length of the center of the cable. The strength member helps the cable withstand different types of movement including:
Aramid fiber, such as Kevlar® is a strong material ideal for extreme pulling strength while maintaining flexibility and flex life. Aramid fiber can be stranded in a variety of configurations to achieve differing levels of strength. For example, a bundle of four strands provides 450-600 Newtons of pull strength. While it is not rigid, it does offer high compressive strength without adding significant weight. If aramid fiber is used in a cable solution, special attention must be given to the cable’s termination, as incorrect termination can lead to weak points in the fiber. We have even seen cases where a customer comes to us with a competitor’s cable assembly that is failing in the field because the strength member was never terminated, so it was not being put to use.
Separators serve as barriers between the cable’s core components and the outer jacket material so that they do not bond together. Flex facilitating tapes such as PTFE or spun nylon will help increase both flexibility and flex life. These tapes can be wrapped in-line with jacketing and will help the cable flex without compromising the structure of the cable. It will also protect the cable from repeated flexing in a demanding, high flex life application.
Shielding provides protection by increasing both structural integrity and signal integrity. Selecting the right shielding materials and configurations can add strength, flexibility, and noise suppression. The primary types of shields include a foil shield, a box weave or basket weave braid shield, and a spiral shield. Foil provides a layer of shielding without adding a lot of weight or cost, but when high flex life is required, it may not be the best material. A spiral shield provides more flexibility, but may have a shorter life span in high flex environments. A box weave braid, while stiffer, will stay together throughout the flex life of the application due to its interlocking nature.
A retractile or coil can also be beneficial when flexibility or high flex life is required. If the cable will be used on a robotic arm or a security camera where torsional and rotational movement puts demands on the flex life of the cable, a retractile may be your best option as the coil will relieve much of the stress.
There are many options when it comes to terminating your cable to a connector especially when flex is a critical factor. An over-molded strain relief is going to give your assembly a much longer flex life than a boot. The over-mold will offer added protection from flexing near the connector or pulling that might occur from flex demands further down the cable. Additionally, a service loop inside the connector will offer a buffer, so that any pull on the cable will not immediately pull the conductors out of the pins.
Hopefully this article has you thinking about the requirements of your cable and different material options that may improve its performance. As you have probably gathered, flex, flexibility, and suppleness can be interpreted differently, so it is important to understand the needs of the end application to ensure a properly designed cable and assembly solution. Additionally, if you have a current cable that is underperforming, it is important to understand what is failing and where, so that a cable designer can select the right materials to rectify the situation. Whether you have a new project requiring a cable with maximum flexibility and/or a long flex life, or require a redesign, Northwire Application Engineers and Design Experts are available to partner with you to design a solution that fits your application’s specific needs.