Robots on rails: using linear axes as a 7th axis
Why add a 7th axis to industrial robots?
Industrial robots are increasingly asked to do more - reach further, cover larger work areas and adapt to changing production demands. While six axis robots offer flexibility and precision, their working envelope is fixed.
Adding a linear axis as a 7th axis - effectively placing the robot on rails - allows the robot to travel along a defined path. This simple concept dramatically increases reach while maintaining positioning accuracy and repeatability.
For OEMs, machine builders and automation engineers, robots on rails offer a scalable way to extend robotic capability without moving to larger or more complex robot models.
What is a 7th axis robot system?
A 7th axis system integrates a linear motion axis beneath or alongside a robot, enabling additional horizontal or vertical movement beyond the robot’s native range.
This linear axis can:
Extend the robot’s working envelope
Allow one robot to service multiple stations
Enable repositioning without retooling or relocation
The robot controller typically synchronises motion between the robot and the linear axis, ensuring coordinated movement and consistent path accuracy.
Key benefits of robots on rails
Extended reach and coverage
A linear axis allows a single robot to move between multiple work zones, for example welding, picking, inspection or assembly cells, without duplication of equipment.
More efficient layouts
By moving the robot instead of duplicating robots, manufacturers can reduce floor space, simplify layouts and optimise material flow.
Scalability and flexibility
Systems can be extended over time by adjusting rail length, adding stations or reconfiguring processes, supporting modular and scalable automation strategies.
Optimised robot utilisation
A 7th axis enables higher robot utilisation, particularly in processes with multiple sequential tasks or distributed workstations.
Design considerations for linear axes in robotic applications
Specifying linear axes for 7th axis use requires careful consideration of both mechanical and control requirements.
Load and dynamic behaviour: The axis must support the combined mass of the robot, tooling and payload while maintaining stiffness and positioning accuracy under dynamic motion.
Accuracy and repeatability: Consistent positioning along the rail is critical, especially when the robot performs precision tasks at multiple locations.
Drive and guide selection: Belt driven, rack and pinion or spindle driven linear axes may be selected depending on speed, payload and travel length requirements.
Integration with robot control systems: Successful 7th axis systems rely on seamless communication between the robot controller and the linear axis drive system.
Typical applications for 7th axis robot systems
Robots on rails are widely used in:
Automotive body and final assembly
Welding, cutting and surface processing
Pick‑and‑place and material handling systems
Palletising and packaging lines
Large‑area inspection and testing
In these applications, the linear axis becomes a critical part of the automation architecture, not just an accessory.
Supporting flexible and future‑ready automation
As automation demands continue to evolve, 7th axis systems provide a practical way to extend robot capability without redesigning entire production cells.
By combining linear motion with robotic flexibility, manufacturers can create automation solutions that are easier to scale, reconfigure and future‑proof.
Summary
Using linear axes as a 7th axis allows robots to:
Cover larger working areas
Serve multiple stations efficiently
Deliver flexible, scalable automation
For many applications, robots on rails represent a smart balance between performance, efficiency and long‑term adaptability.
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