Servo cables are designed to meet the precise mechanical requirements of the application. A cable that performs well in a stationary application between a drive cabinet and a fixed motor may fail quickly in a drag chain where it is subjected to constant flexing or in a robot arm application where it is under regular tension. It is the basis for long-term reliability in industrial automation to specify servo cables for various motion profiles: static, continuous flexing, or torsional.
This guide covers the difference between each type of move and the cable and comes with an explanation of the underlying cable construction technology, as well as the practical considerations engineers must have when choosing servo motor cables for CNC machines, robotics and automation systems.
Why Motion Profile Comes Before Cable Part Number
A “servo cable” is typically used to describe electrical performance; namely, power delivery, feedback signals, brake control, and EMC performance. The mechanical motion profile identifies the physical quality of the cable that can be used for the application.
When choosing an industrial servo cable, the first step is always to classify the path as static, flexing, torsional or a combination. This influences much more than just nominal voltage or cross-section for the decisions concerning stranding, lay length, shielding, fillers and jacket materials.
Static, Flexing and Torsional Motion Are Different Mechanical Loads
- Static: The cable is essentially fixed after installation and only vibrates slightly, moves thermally or is accessed for service every so often.
- Repeated bending in a single plane such as rolling or reverse bending within a cable carrier on linear axes, also known as continuous flexing.
- Torsion: Twisting around the cable’s length (along the longitudinal axis), is typical when rotating robot joints and spindles.
Bending, torsion, vibration and abrasion are often all combined in many applications today, particularly in robot dress packs.
Why “Flexible” Is Not a Complete Specification
It’s not until the millions of cycles are performed that many standard cables will feel flexible during manual installation. Fatigue of conductors, tearing of shields, and cracking of jackets due to repeated stress if not stranding and geometry properly designed. Motion type, bend radius, travel distance, cycle rate, torsion angle and environment should be clearly specified in each specification.
Static Servo Cable Applications: When a Standard Fixed Installation Is Enough
All servo axes don’t need to be high flex. A special fixed installation servo cable may be superior in performance and much less expensive in true static applications.
Typical Static Servo Cable Installations
Common examples include:
- Internal connections to the drive motor in a mechanical frame.Internal wiring to the drive motor in a stationary machine frame.
- Connect to stationary motor/gearbox from a control cabinet.
- When the spindle or pump motor is not moving, they will be in fixed position.
- Axes that have infrequent vibration or maintenance motion.
What a Static Servo Cable Still Needs to Handle
There is no such thing as low performance with static. These cables must be rated for current/voltage, have a good EMI shielding, withstand oil/coolant, have a proper temperature range and meet the UL, CE, RoHS, or other standards.
Risks of Using Static Cable in Moving Equipment
The use of a static-rated cable in dynamic service generally results in conductor fatigue at bend locations, degradation of the cable’s shield, cracking of the jacket and eventual failure of the signal or power. Correct motion profiling is necessary as internal damage may happen before external damage.
Continuous-Flex Servo Cables for Drag Chains and Linear Motion
Unlike flexible cables, continuous-flex servo cables are specially designed to bend repeatedly in cable carriers and linear axes.
Where Continuous Flexing Happens
These cables are found on CNC machine axes, linear gantries, automated storage systems, packaging lines, material handling equipment and pick-and-place units.
Construction Features That Support Flex Life
Some important features are the fine-stranded annealed copper conductors, shorter lay lengths optimized, low-friction fillers/insulations, EMI stable shielding systems that deliver consistent performance, and rugged, flexible jackets (typically PUR for abrasion-prone applications).
Flex-life is highly sensitive to the manufacturer’s test environment, which include bend radius, travel, speed, acceleration, temperature and carrier design. Always check data with real application parameters.
Bend Radius, Travel Length and Cable Carrier Layout
It is essential to do it right the first time. Keep minimum dynamic bend radius of the cable, do not overfill carriers, do not place power and feedback cables in the same carrier and give good strain relief. Even high quality constructions are defeated when cables are forced below rated radius or are rubbed. ns.
Torsion-Rated Servo Cables for Robots and Rotating Axes
Torsion-rated servo cables are cables that are designed to withstand twisting forces on the cable itself, which is a very different type of stress from linear bending.
Where Torsional Motion Occurs
The typical applications are the 6-axis industrial robot, SCARA robot, robot dress pack, rotary tables, rotating tools and multi-axis automation cells.
How Torsion-Rated Cable Construction Differs
They are designed with special internal cable lay to ensure controlled movement, optimized cable lay for torque distribution, reinforced but flexible shield to ensure protection and flexibility and strong jacket that will not crack when twisted. It is important to understand that a standard continuous-flex cable will typically cause corkscrewing, core migration and early failure of the shield.
Combined Bending and Torsion in Robotic Dress Packs
Often bending and torsion are applied to the robot simultaneously. The optimal solutions are those that incorporate flex and torsion qualifications, careful dress-pack routing and strain relief. Discuss exact axis movements and angle of design with supplier at the start of design.
Servo Cable Construction Features That Control Motion Life
Knowing the inside workings gives engineers the edge in all scenarios of motion.
Conductor Stranding and Copper Fatigue Resistance
The finer stranding spreads the mechanical load over a greater number of individual wires to provide a much greater flex and torsion life for dynamic use.
Lay Length, Fillers and Core Stability
The reduced internal friction and no conductor migration during repeated motion is made possible by controlled lay lengths and movement friendly fillers.
Shielding and Jacket Selection
EMC performance is critical in servo systems. The jackets are selected based on the actual conditions: PUR for high abrasion, special compounds for extreme temperatures or special chemicals. Never use generic ratings as references, always use actual datasheets.
Common Mis-Specification Errors and Their Failure Patterns
The most common reason for early failure in a servo cable is an insufficient specification of the motion.
Using Static Cable in a Drag Chain
This causes conductor fatigue, jacket splitting and the damage of the shield at the primary bend point.
Using Flex Cable for Uncontrolled Torsion
Causes the cork to corkscrew, spiral and displace its core.
Ignoring Bend Radius, Cycle Rate and Routing Details
The leads of even good cables can deteriorate quickly when they are installed too close to the equipment, when there is too many leads in the carrier, when no strain relief is used or when sharp edges or contaminants come into contact with the cables.
Practical Specification Checklist for Static, Flexing and Torsional Servo Cables
Give your supplier all the details for a correct recommendation:
- Servo drive and motor type, along with necessary power, brake, encoder or hybrid functions
- Static, Continuous-Flex, Torsional, or Combined motion profile
- Cable path and routing method (cabinet, tray, drag chain, robot dress pack)
- Travel distance, speed, acceleration, cycle rate, number of expected cycles
- The minimum bend radius and the carrier dimensions.
- Torsion angle per metre (when applicable)
- Current, voltage, and shielding requirements for electrical needs.
- Human factors: fatigue, stress, human element, washdown, etc.Operating conditions: temperature, oil/coolant, chemicals, abrasion, washdown etc.
- All necessary certifications (UL, CE, RoHS, REACH, halogen-free, etc.)
The clear motion profile enables recommendations and/or custom motion cable design from Hulk engineers to address your equipment’s actual duty cycle and minimize unexpected downtime and total ownership cost.