Cables pose specially challenging demands when it comes to compact robots and rotary axes. Inconsistent routing around the corners and repeated tension and bending can reveal the weaknesses of ordinary cables quickly, causing conductor fatigue, corkscrewing of the cable jacket or unforeseen downtime. The torsion cables used for compact robots and Rotary Axes need to be specially engineered to accommodate tight bends, high cycle counts, compound motion, and low profile as well as slots.
This guide is based on actual harness installation and robotic design experience to illustrate how the construction of the cable, routing and the installation practices are matched to these factors. Reliable performance, just like in larger systems, can be guaranteed by space-conscious engineers and integrators who put space constraints on the same level as any other design criterion.
Motion and Space Constraints in Compact Robots and Rotary Axes
All design thinking questions begin with a clear picture of the actual motion profile and physical envelope. Traditional arms and joint spacing is reduced in compact robots, increasing the stress on cables.
Typical Motion Profiles in Compact Arms and Rotary Axes
The cables are typically routed inside the compact 6-axis arms with narrow hinges, which allow the wrist to move ±180° or more and the elbow to bend at a high angle. Pick-and-place robots come in equipped with high-speed rotary heads which rotate cables dozen times a minute. Continuous or oscillating torsion must be accommodated in a very small diameter joint between rotary indexing tables, which necessitates the use of cables.
By mapping these zones, this information can be used to determine where there are areas of high torsion angles and where bend radii are falling below 5× the outer diameter of the cable (typical limits for standard flex cables).
Space Limitations – Narrow Channels, Low-Profile Arms, Small Diameter Rotary Joints
The internal channels in compact arms often are just a few millimeters larger than the bundle of cables. The low profile designs don’t allow a lot of room for traditional dress packs. Small diameter rotary joints decrease service loop length and increase the amount of twist per metre of cable.
Such truths necessitate the design incorporating torsion-resistant structures and smart segmentation and management systems.
Torsion Cable Design Features That Matter in Tight Routing
Cable construction is important in limited areas. The internal geometry and protective layers are correct to ensure no uneven stress is applied, which causes failure.
Bundled Core Geometry and Symmetry for Compact Torsion Applications
Bundled cores (also called concentrically designed or symmetric) are cores whose conductors are placed in symmetric and balanced layers of power, signal, and data. This symmetry is able to spread compression and tension forces evenly when bending and twisting simultaneously. Asymmetric designs are prone to very fast corkscrewing in short cable runs common to compact robots, resulting in increased chance of conductor failure.
Inner Jackets, Torsion Protection Braids and Tribology Layers
Inner jackets are pressure extruded to occupy voids and to keep them round when repeatedly twisted. Torsion protection braids disperse shear forces from conductors to prevent possible damage to the conductors, and low-friction fleece or slide tapes enable internal wires to move without causing heat and abrasion inside tight channels.
The features help to stabilize the cable when it is forced through small rotary joints or narrow arm segments.
Designing Routing Paths – Bend Radius, Neutral Axis and Dress Pack Strategy for Tight Spaces
No matter how good the torsion cable is, if it’s not routed properly, it fails.
Respecting Bend Radius in Compact Robots and Rotary Axes
Minimum bend radius (typical 4-5x OD for torsion cables) is to be adhered to. When space limitations ensure that using the recommended size torsion cable radius is not feasible, there are three viable engineering choices: use a smaller OD version of the torsion cable, increase the path slightly with a better design of the joints, or accept the shorter service life with a regular scheduled replacement. The most frequent failure mode of compact arms is bending below rated values.
Neutral Axis Placement and “Less-Is-More” Cable Management
Keep CableBundle close to neutral axis of carrier or channel to reduce stretch and compress. The “less is more” principle is to split the robot arm into smaller independent sections of the harness with separate strain relief and service loops instead of using a single, continuous harness through the arm.
Rotary Energy Chains and Modules in Tight Installation Spaces
Special rotary energy chains and compact rotation modules direct cables around controlled paths on rotary axes. They ensure a proper torsion distribution and avoiding kinking even in very tight joint envelopes. These modules are now common in many mini pick-and-place systems that provide a high-range of rotation while maintaining a long cable’s life.
Reliability Pitfalls in Tight Routing – What Goes Wrong When Design and Installation Are Misaligned
Typical compact robot issues are:
- Torsion or locked-in torque in jackets caused by corkscrewing.
- Abnormalities in the channel such as localized hard spots and abrasion due to rubbing of cables against channel walls
- Fatigue of the conductors at repeated over bending or twisting points.
Such problems frequently go back to incorrect installation, such as the introduction of residual twist, or to incorrect routing, caused by not taking into account the positioning of the neutral axis.
Locked-In Torque and Poor Installation Practices
When cable is pulled straight from static reels, wound in the opposite direction of the stranding lay, or forced around sharp edges during assembly, it is referred to as “locked-in torque” and is not recoverable. This is the case with even minor residual twist in short compact robot harnesses.
Always rotate payout tables, install payout cables in the direction of table rotation, and check for any residual twist before closing access panels.
Practical Design Steps for Torsion Cables in Compact Robots and Rotary Axes
This is a motion first process:
- Specify motion profiles, torsion angles, cycle counts and exact routing dimensions for each arm segment and rotary axis.
- Use CAD to calculate bend radius and space required.
- Choose the right cables for torsion, and the right core geometry and protective layer combinations.
- Draw segmented dress packs that meet the requirements for neutral axis placement, enough clearance and service loops.
- Define rotary energy chains or modules for continuous rotation.
- Outline how to install without locked in torque.
Compact Harness Strategies – Segmenting, Junction Boxes and Modular Leads
Modularizing long runs with junction boxes eliminates crowding in narrow runs and ease maintenance. Smaller OD multi-core design and standard connectors also help to simplify routing without compromising reliability.
RFQ and Design Checklist – Specifying Torsion Cables for Tight Routing Spaces
Provide suppliers with complete information:
- Motion profiles like torsion angle, speed and lifetime cycles per axis
- For routing, the channel sizes and the minimum bend radius will be considered.
- Environmental factors (Temperature, weld spatter, oil)
- Use a safety margin for target service life.
Questions to Ask Suppliers About Torsion Cables for Tight Spaces
- In short runs with high torsion how does the core geometry prevent corkscrewing?
- Are there any data available on the torsion angle/bend radius performance of compact applications?
- What have they observed about this cable in rotary energy chains or low profile carriers?
- Are you able to provide reference installations in like-sized compact robots or rotary axes?