Choosing Cable for Constantly Flexing Applications

The selection of high performance cable for automated equipment is often an afterthought in system design. Finding the best cable for automotive, food/beverage, biotechnology and semiconductor industries can be confusing. Automated robotic, materials handling and assembly systems used for product manufacturing and processing applications require special designs for cable that is constantly moving.

Metal under constant motion can break due to the heat generated from the friction of movement. Try this: bend a wire coat hanger back and forth at the same point until it breaks. Now feel the wire where it broke. Notice how hot it is? This heat is the ultimate reason for cable failure in flexing applications. Cables designed for high fl ex applications must beat the heat, so to speak, by reducing friction. This is done by integrating design features into the cable that promote reduced friction and choosing the appropriate cable jacket material.

Cables that seem pliable and those specifically designed for a flexing lifecycle are not the same thing. Just because a cable is limp in your hands does not mean it will have a long lifecycle in an application. In reality, a stiff outer cable jacket allows the conductors to move more freely resulting in less friction and a longer lifecycle.

The components of highly flexible cable are: copper conductors, insulation, filling material and an outer jacket.

Copper conductors can be made from many fine strands or a lesser amount of thick strands of copper. The alloy of the copper also has an affect on the flexible cables lifecycle. It may seem that a larger number of very tiny strands would lead to a longer lifecycle, but this is not necessarily true. Cable designers need to keep the cost of copper in mind, as many thin strands are more expensive than fewer, thicker strands.

Insulation material is extruded around the strands of copper. It should be firm and have a low coefficient of friction between the outer jacket and filler materials.

Fillers are used to keep the core of the cable rounded, enhancing the cables flexibility. The number of twists imparted to the core during construction also affects the shape of the core. Cables can be made using a greater amount of filler and a lesser amount of insulation, or with no filler at all.

Manufactures that design cable specifically for extreme flexibility may bypass the filling material and opt for a different material that can physically reduce the friction between the cable’s core and outer jacket. This material may be tapes or textiles that are wrapped around the conductors to prevent heat buildup caused by friction, thus elongating the cables flexing lifecycle.

The cables outer jacket needs to be able to withstand constant flexing while providing protection against mechanical and chemical damage. Most manufacturers use custom jacket materials that withstand various environmental rigors to different degrees. For instance, most cables are rated for use in cold environments, but some manufacturers’ proprietary cables can tolerate extreme temperatures of up to - 40 degrees Celsius. Depending on the application, resistance to other environmental factors, such as chemicals, abrasion and moisture, the jacket may be subjected to are also important considerations when choosing a cable jacket material.

If the inner components of a cable have been designed for increased flexibility, jacket material can be determined respective to these environmental conditions. Cable jacket material can be made from multiple different materials, some better suited for certain environments than others. Types of cable jackets include:

PVC (Polyvinyl Chloride) cables generally provide excellent resistance to oils, ozone, acids, alkalis, alcohols, greases and waxes. In most cases PVC is cost effective jacket material and provides resistance to weather and atmosphere conditions. This jacket material is especially suitable for applications in the food and beverage industry where frequent washdowns occur.

PUR (Polyurethane) jackets provide resistance to oils and lubricants including cutting oils, transmission fluids and gasoline, as well as exceptional abrasion resistance comparable to thermoset rubber cables. PUR also has outstanding memory properties, making it an ideal jacket material for retractile cords that are used in constantly moving applications. This jacket material is usually used in manufacturing applications that use metal parts.

Using thermoset CPE (Chlorinated Polyethylene) jacket material over EPDM (Ethylene-propylenediene monomer) rubber insulation provides resistance to flame and temperature extremes, as well as resistance to tears, cuts and abrasions. This cable is ideal for welding applications.

Be sure when choosing cable you are aware of all conditions the cable will be subjected to. It is important to consider environmental conditions, the physical location the cable will be placed and installations, as well as what the application requires, i.e. voltage, current, bend radius, connections, junction boxes.

Where cabling is subjected to linear, angular or rotational motion between two points, always allow adequate cable length to absorb the energy imparted by the motion. Use of coiled cords, mechanical support mechanisms, or large, well supported cable loops will also maximize cable life.

As long as a manufacturer has indicated a cable is designed for flexing or constant motion, and the other factors outlined have been determined, the cable should be perfectly suited for the particular application and perform for millions of flexing cycles.