Robot-transfer unit (RTU) and other linear-transfer system considerations

As robotics applications proliferate, robot integrators have begun to apply more 7th-axis tracks — also called robot-transfer units or RTUs — as well as linear tool-tray transfer systems.

Benedict Talan • President | LazerArc of Motion Index Drives

Robots excel in repetitive assembly and process operations that include welding, trimming, routing, drilling, fastening, composite layup, coating, and painting. Traditional installations mount the robot arm to a fixed station which in effect limits its reach. In contrast, facilities that use RTUs or linear tool-tray transfer systems extend the reach (and usefulness) of robots.

Recall that RTUs are long tracks in facilities that carry robots through work cells and between work stations. Another advantage of 7th-axis robotic tracks is that plant engineers can install multiple robot arms on a given RTU to boost plant flexibility and productivity.

A similar concept, linear tool-tray transfer systems extend the work reach of robots much like that of 7th-axis systems. However, instead of carrying the SCARA or other robot along a line, linear tool-tray transfer systems move workpieces and work fixtures past a servicing robot that remains stationary. Such setups are increasingly practical as robotic arms have become less expensive and more flexible.

Key to both RTUs and tool-tray transfer systems is that the designs are heavy-duty. That’s why some manufacturers use high-precision helical rack-and-pinion sets to handle the high dynamic loads of SCARA positioning and large workpieces common in automotive applications, for example — even while holding accuracy to within tight tolerances. Here, usually a support carriage complements the rack-and-pinion drive. Where both the rack-and-pinion set and ruggedized slides are modular, they mount end-to-end to get unlimited travel lengths.

Accuracy and load ratings of RTUs and tool-tray transfer systems

Choosing an appropriate 7th-axis robotic slide or transfer system includes careful determination of required accuracy or repeatability. Overall accuracy depends in part on the gearing integrated into the linear system’s drive — whether an RTU or a tooling transfer. Maintaining accuracy over long strokes further increases design costs, even by many thousands of dollars for an average setup. But a typical rack-and-pinion system with accuracy to 0.036 mm per meter (to give one example) can exhibit 0.240 mm more positioning error over a 15-meter length than a setup with accuracy of 0.02 mm or better per meter.

Accuracy of RTUs and transfer systems ultimately depends on:

• (As mentioned) backlash in the axis’ gear reducer — typically from less than 1 arc minute to more than 18 arc minutes, and the backlash between the rack and pinion in the drive system
• The amount of preload and installation alignment of the rack-pinion system
• The motor encoder’s counts per revolution — as well as proper commissioning of the servo or ac motor controlling drive parameters
• The system’s linear guides.

In fact, linear guides (and their specification to maintain accuracy and support the payload) are key to RRU and transfer-system design. Some suppliers offer integration of linear-bearing guides in versions with parallelism from 9 µm to more than 30 µm over 3,000 mm for a given guide size. Other guide-related parameters include the overall weight to be transported; the speed with which the load must move (often expressed in m/sec); the moving mass’ center of gravity; presence and degree of any tilting moment; and overall duty cycle and expected life and environment. Proper sizing of the RTU or transfer-system guides depends on accurate estimations of these factors.

All with all motion components, differences exist between manufacturers’ linear bearings; some of these affect the performance and maintenance requirements of RTUs and linear-transfer systems. So here are some questions to ask during the specification process:

• What are the linear bearings’ runout specification? Can sections of rail be changed out independently?
• Are any of the linear-guide options self-lubricating? Does the manufacturer offer protective bellows where the RTU or linear-transfer system operates in environments where painting or coating is taking place?

Some RTUs and transfer systems include linear bearings that are ground in a way that allows change-out of individual sections — and not entire spans of bearings that some manufacturers insist are ground as matching pairs. The latter can be expensive and time-consuming … so look for RTUs and linear transfer systems that incorporate quality bearing assemblies that allow partial track change-outs if damaged (to shorten MTTR).

Of course, uses for RTUs and linear transfer systems abound — and include material handling, welding, inspection, and painting, to name a few disparate application examples. The robotic functions on an installation ultimately determine what types of safety or specialty features the RTU or transfer system will need — whether in the form of engineered cable carriers (on dynamic material-handling axes) or explosion-proof motors and full bellows (for painting applications).

Where automating a specialty setup, partner with manufacturers’ sales engineers that work to understand end processes and suggest features specific to the application. Also look for integrators supply custom RTUs and transfer systems to deliver on accuracy while keeping costs down — with application-specific lengths, transfer speeds, payload ratings, and safety factors — though the latter should be at least 2:1.

More on selecting linear tool-tray transfer systems

As the motion of production lines becomes increasingly precise and repeatable, there’s been a rise in automation to replace large-scale manual handling and assembly tasks — especially with the use of linear tool-tray transfer systems. Some transfer systems have load capacities exceeding 100,000 lb … and some are suitable alternatives to rotary index tables. Plus like RTUs, linear tool-tray transfer systems often integrate with robotic controls to effectively communicate (and coordinate tasks) with collaborating robots.

When choosing a linear tool-tray transfer system for an installation, engineers begin by collecting information about the sizes and shapes and weights of workpieces to be handled, current operations, the allotted time for assembly or other processes, and available plant-floor real estate. But there are other design parameters relating to the linear tool-tray transfer itself.

First of all, are the design’s variable frequency drives (VFD) mounted in a control panel or controlled directly via a PLC? VFDs boost motor-output efficiency and transfer-system performance. What motors does the potential tool-tray transfer use — servomotors or ac induction motors? Designs with servomotors and motion systems compatible with robotics can network into larger controls for advanced connectivity and IoT functionality. That’s especially true of systems that incorporate precision sensor and switch feedback. Yet another question to ask: Can the linear tool-tray transfer system integrate with tooling fixtures, welding robots, and other machinery controlled with PLC logic?

Related feature on sister site therobotreport.com:
Automated linear-transfer tool trays: LazerArc commentary

Note only some linear tool-tray transfer systems have the accuracy to support the assembly of complicated workpieces. Those that do include precision guide rails and rack-and-pinion systems plus closed-loop motor controls and programmability, and fast indexing speeds. Transfer units from some manufacturers also include tubular steel and machine-welded construction for ruggedness … and the ability to accommodate customer trays and fixtures. Accommodating tooling trays in turn quickens transfer times.

Other tool-tray transfer features to appeal to end users include surface coatings for eliminating the need for external lubrication; internally mounted cable-management trays; and hard stops in case of overtravel.

LazerArc by Motion Index Drives | www.lazerarc.com