Many robotic cells and automation systems use two different types of motion: motion along rails or gantries, and motion around multiple-axis robot joints. The cable construction determines robotic cable vs drag chain cable. Drag chain cables are ideal for repeated linear bending within carriers, and torsion-rated robotic cables for twisting and 3D movement are typical around robot arms. Selecting the incorrect type can result in early conductor breakage, damage to the jacket or even unexpected downtime affecting the production schedules.
This guide takes engineers and integrators through the significant differences, allowing them to select the appropriate cable for each part of the robot cell for increased reliability and lower long-term cost.
Motion Types in Robotics and Automation Cells
The first step in making the right selection is to have a good understanding of the dominant motion profile for each cable strike.
Linear Bending in Cable Carriers and Gantries
Linear motion is along fixed paths like seventhAxis tracks, overhead gantries, conveyor shuttles, or along rails (on the floor). In this case the cable moves back and forth, mostly bending in one direction in the cable carrier. Torsional stress is minimal or none, the primary concerns are bend radius, travel distance, speed, acceleration and environmental factors such as oil, weld spatter, etc.
Torsional and 3D Motion in Robot Joints
The robot joints add torsional or twisting motion around the longitudinal axis of the cable generally with bending in several directions. Typical 6 Axis robots have wrists, elbows, and shoulders that rotate ±360° or more per cycle. This generates complex 3D stresses, which typical linear rated cables cannot handle for extended periods of time. Cables used for dress packs near the arm must be able to withstand the twists imposed without kinking, corkscrewing, or migration of the conductor inside the cable.
What Is a Drag Chain Cable in Robotic Cells?
Drag chain cables, otherwise referred to as continuous-flex or high-flex cables, are designed to move in a continuous linear path within energy chains or carriers.
Design Focused on Linear Continuous Flex
With extra-fine stranded conductors, optimized lay lengths and tough yet flexible jackets (often PUR), these cables can withstand millions of bending cycles in one direction. They focus on abrasion resistance, tight bend radius and long travel lengths, but not high levels of torsion. Shielding designs are based on performance under flexing conditions of rolling, but not twisting.
Typical Locations Where Drag Chain Cables Are Used with Robots
Common applications of drag chain cables include supplying power, control, Ethernet and sensor signals between the robot base and the control cabinet, between the floor or overhead tracks and the robot, and in peripheral linear motion systems. These runs are performed prior to the robot base becoming fully three dimensional, and linear-rated cables are the practical and cost-effective solution.
What Is a Robotic (Torsion-Rated) Cable?
Torsion-rated robotic cables are specially designed to meet the dual bending and twisting requirements of articulated robot arms.
Construction for 3D Bending and Torsion
Special stranding methods, sliding layers between cores or low friction wrapping between cores together with strong, but flexible jackets enable the cable to take up the torsional forces without binding and diameter laps. Many designs feature fillers or tapes to allow individual elements to move independently when twisted. Such cables may include explicit torsion numbers as: ±180°/m, ±360°/m or even more, which are tested in millions of cycles.
Test Methods and Ratings for Torsion Applications
Cable manufacturers use specialized torsion testing apparatus called a “torsion rig” to test robotic cables and apply the same loads of twisting repeatedly at design angles and speeds, and, in some cases, at multiple angles and speeds to the cable simultaneously. When selecting a torsion cycle life and angle per meter rather than linear flex numbers. A good drag chain cable is not necessarily suitable for joints in robots.
Failure Modes When Using the Wrong Cable Type
One of the most popular and costly robot system design errors is mismatching the cable to the motion profile.
Drag Chain Cables Misused on Robot Joints
A linear optimised cable will corkscrew, crack the jacket, break the shield and after some time, develops conductor fatigue when it is driven through rotating joints. A wrist cable may last for several months rather than years, leading to erratic problems, safety shut-downs, or even axis failure during manufacturing.
Over-Engineering or Misplacing Robotic Cables in Simple Linear Runs
Torsion-rated cables are used in drag chains but they are generally more expensive and slightly more bulky than required for normal linear applications. Implementing robot cables throughout adds costs to the project but doesn’t provide increased value on simple linear axes. When the motion is correctly matched with the cable, there is no need to overspend while maintaining the performance.
Mapping a 6-Axis Robot and Cell Layout to Cable Types
A clear mental picture of the robot cell helps the engineer to assign the correct cable to each segment.
Base to Robot: Linear Supply Paths
Use drag chain cables from the control cabinet to the robot base (usually on floor tracks, overhead rails or stationary structures). These runs tend to be relatively long runs with bending within one plane and generally include power, communication and I/O lines before the motion gets more complex.
Along the Robot Arm: Joints and Dress Pack
After cables depart from the base and pass through several rotating joints, torsion-rated robotic cables are a must-have. As seen in the dress pack, the segments from axis 3-6 are subjected to the highest torsional load and need cables which are suited to the rotation angle, speed, and likely life of the robot OEM.
Key Specification Parameters for Each Cable Type
Pay attention to parameters which are directly related to the motion profile.
For Drag Chain Cables (Linear Motion)
Troubleshoot continuous-flex cycle rating, minimum bending radius, maximum travel length, speed and acceleration limits, jacket material (PUR for oil and abrasion resistance) and temperature range. Always check these against actual linear axis or carrier motion profile.
For Robotic Cables (Torsion Motion)
Consider: Torsion angle per meter, Torsion tested cycle count, Bending radius under torsion, Dress pack inside / outside, and other resistances (weld spatter, chemicals). These values have to be consistent with the particular rotations of the joints and with the general duty cycle of the robot.
Simple Decision Framework: Which Cable for Which Motion?
Apply this practical checklist to the assessment of any axis or cable run:
- Is the cable being operated inside of a linear cable carrier that bends mostly in one plane? → Select a high-rated drag chain cable.
- If the cable may become twisted around its length, like at the robot wrist or elbow, install a torsion rated robotic cable.
- Does the motion include a combination of short linear motions and significant torsion? → Check with the supplier to see if they can provide a combined rated cable which is more likely to be a robotic grade cable.
If in any doubt, give the supplier the details of the motion: axis angles, speed, number of cycles, travel lengths, bend radius, and environmental conditions. Partners who ask these questions are typically more reliable and effective with longer-lasting solutions.
We produce high flexibility drag chain cables and torsion rated robotic cables with strict quality control at Hulk Electric, they are certified by UL, CE, TÜV and other certifications. Our engineering team can assist you to map your individual robot cell and suggest the best cable for the particular area which will minimize risk and keep your automation smoothly running.