The ability to butt linear rails together quickly and easily in the field and be assured that journals would run over the joints without any loss of precision was one of the main reasons why the HepcoMotion SL2 Stainless Steel system was selected by OC Robotics for a nuclear application. The company is the world’s leading manufacturer of snake-arm robots for use in a wide range of industries including aerospace, security and nuclear.
Snake-arm robots are unique robotic devices which, as their name suggests, snake their way into cluttered environments. The operator typically drives the tip of the snake-arm whilst OC Robotics proprietary software controls the arm along the same path. This gives the company’s robots an exceptional capability to reach relatively inaccessible spaces whilst maintaining a manageable operator workload.
This particular project was for a system designed to deliver tools and fixtures to repair a leaking pipe situated beneath a nuclear reactor in Sweden. “We didn’t have a lot of time to set up the robot because radiation levels at the work site were high,” explained technical director, Andrew Graham. “Spending hours adjusting the linear system was simply not an option. We couldn’t have the installer wearing nuclear protective PPE in a hot, radioactive environment being worried about selecting the ‘wrong’ beam section.”
The bespoke manipulation unit was designed to work in the Common Insulation Room that houses 157 control rod drive mechanism (CRDM) pipes that are welded to the bottom of the reactor vessel. These hold the control rods which are driven up and down to maintain a stable nuclear reaction. Each of these has an associated, smaller diameter pipe, called a SCRAM pipe. Not surprisingly, the room is nicknamed ‘The Jungle’, and it is the job of the manipulation arm to snake its way around this mass of pipework to assist in the repair of a particular pipe section.
The HepcoMotion slide system provides the main horizontal motion for the machine. A number of un-matched rail sections were used, each mounted on the underside of a modular beam section, allowing OC Robotics to build up beams of varying lengths in the field. The 25kg robot that travels at a modest 200mm/sec hangs from the beam using four journal bearings.
The journal assembly comprises three standard types and one modified version whose internal bearings were removed to accommodate a drive shaft to create a friction drive. “The system was designed so that it could, in the event of journal failure, still hang off the other two journals without falling,” Graham continued. A second rail, mounted on top of the beam, was also specified to carry camera systems above the robot.
Consistency of manufacturer and tolerance of mounting variation were clearly important criteria in OC Robotics choice of HepcoMotion. The company knew through experience that the rail profile would be sufficiently similar from one rail section to the next, to allow the friction drive to work on different lengths of rail without adjustment. A further important factor was cleanliness. SL2 has no seals or inaccessible crevices in which contaminated particles could be trapped, and could be easily cleaned once the robot was removed from the work site.
“All the decisions on system design and choice of components and modifications were made in-house. We knew from our previous use of HepcoMotion systems that we could rely on the products to do their job,” Graham added. “Indeed, we weren’t aware of any other linear rail system that would have been suitable. A recirculating ball slide system would not have coped with the potential misalignment between the beam sections. OC Robotics has been using HepcoMotion rail systems for 15 years and they remain a natural choice for us.”