28 February 2001
Flexible Cables and Design Considerations
The use of robotics and automated manufacturing systems has created a growing demand for high-quality, flexible cables. Learning the basics of cabling will provide you with the ability to specify the correct cable for every application. By choosing the correct cable, you maximize cable life and minimize the downtime and costs associated with cable failures.
There are three standard categories of motion that determine the cable design:
- Static. In a static application, there's a lack of motion. Solid wire and low-grade compounds are commonly used in such cable designs.
If the application creates a harsh environment (e.g., oils or chemicals), make sure the cable is designed to resist those compounds.
- Continuous Rolling or Bending Flex. Another type of movement to consider is a continuous rolling or bending flex motion.
Continuous flex cables are typically found in track applications. The cable is designed with stranded wire, with specially formulated thermoplastics used for insulation, depending on the application, and a configuration that is designed specifically for such rigorous movement. Consider asking your cable supplier for specific test data, which should simulate your application as much as possible.
- Torsional or Multiaxis. This is typically found on a robot arm or any application having two directions of motion. Stranded wire is used in the special cable configuration. Again, depending on the application's requirements, special thermoplastics may be used in the insulation materials.
Required Bend Radius
A cable's overall construction, including stranding, compounds, inner conductors, fillers, and jackets, influences its bend radius. The unique construction of continuous flex cables makes them ideal for use in high-speed, automated equipment that flexes in a linear motion (Figure 1), including industrial robots, pick-and-place machines, and automatic handling systems. Conversely, torsional cables are designed for twisting and bending applications (Figure 2).
A cable's environment affects its longevity. Harsh chemicals eat away at certain jacketing materials, and temperature extremes impair a cable's flexibility.
Anticipated Flex Life
It's important to project the anticipated flex life of a cable by deciding the number of complete cycles you'll need the cable to travel. In return, your cable manufacturer should provide detailed test data on a cable's performance, rather than projected figures.
Your cable supplier should offer cables and accessories with a variety of worldwide approvals, including UL, CSA, and CE. In addition, "harmonized" power supply cables, hookup wires, and multipair cables satisfy European standards such as VDE and DIN, which is necessary when exporting equipment to Europe.
A Cable's Inner Workings
In order for a cable to continuously flex or twist, it must be constructed of certain materials and assembled in a manner that allows the conductors freedom of movement. In essence, the cable itself becomes a mechanical system with a number of moving parts (Figure 3).
One of the most critical considerations in a cable's flexibility is the compound used in the outer jacket, which must have memory (resilience). This memory ensures that the jacket will return to its original unstressed condition upon completion of a flex cycle, in order to relieve stress. A compound with a poor memory will start the next flex cycle in a stressed condition and continue to be stretched further, until it fails.
The number of chemical solvents and oils used in manufacturing is growing steadily, with each one causing harm to control cables. In fact, some of these chemicals are so potent, they destroy cables in two ways: Either the chemical is drawn into the jacketing material, causing it to balloon, soften, and ultimately disintegrate, or it draws a similar chemical from the jacketing material, drying the jacket out and causing it to become brittle.
Not all jacketing materials are affected by all chemicals and oils, so different jacketing compounds should be used for each operating environment and the hazards present. For instance, polymeric plasticized PVC is good for oil resistance, while polyurethane is suited for abrasive conditions. Finally, thermoplastic elastomers are extremely chemical resistant for use in the harshest environments.
Single or multipaired conductors should be finely stranded and made of bare copper to allow the individual strands to move freely. The conductors are then encased in flexible insulating and jacketing compounds to increase flex life and provide resistance to minerals and cutting fluids.
When electrical interference in an application distorts signal transmissions, or in cases where emissions need to be suppressed, flexible cables can be protected by a tinned copper braid (a spiral shield is used in torsional applications). This eliminates distortion from electromagnetic interference and allows cables to transmit data flawlessly.
Many times, a cable fails because it has been improperly installed. Basic installation rules include making sure there are no twists or kinks in the cables and unwinding cable from the outside layer.
Take care when installing cables in the flexible tracks that organize, guide, and support the cable during the flexing cycle. When installing cables in these tracks, leave a space of at least 10% of the cable diameter between cables, and make sure the weight is evenly distributed, with heavier cables on the outside. If the cables are separated from one another and tensioned correctly, they won't bend on the track's inner or outer surfaces. Correctly installing a cable in a track greatly enhances its flex life.
The demands placed on flexible control cables increase dramatically with the continued increase in speed and sophistication of automated equipment. Continuous flexing, bending, and exposure to destructive solvents and oils can all take their toll on flexible control cables. Fortunately, by learning some fundamental cabling considerations, you can specify the proper cable for each application and dramatically increase the life of your cable products. This leads to the benefits of less downtime from cable failures, lower operating cost, and greater reliability.
Remember that the application is key. The motion that the application calls for will determine the type of cable to use.
There are three basics to remember:
- Know the motion.
- Three standard categories of motion determine the cable design: static, continuous flex, and torsional.
- Consider the environment.
Figures and Graphics
- Figure 1. Cable rolling/flexing back and forth in a linear motion.
- Figure 2. Cable twisting clockwise/counterclockwise with angles covering a 90°360° range.
- Figure 3. This back and forth "tick-tock" flex occurs when one end of the cable is held stationary.
- PDF of "Flexible Cables and Design Considerations"
John Gavilanes is a design engineer for Olflex Wire & Cable, Inc. He received his BSME from the College of New Jersey, minoring in computer science and mathematics. John has 13 years' experience in designing and developing robotics, torsional, and continuous flex applications and is a member of the Open DeviceNet Vendor Association. Contact him at 30 Plymouth Street, Fairfield, NJ 07004; tel: (888) 456-3539 or (973) 575-1101; fax: (973) 575-1267.