Stainless linear guides selected for nuclear robot project

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A HepcoMotion SL2 stainless steel linear guidance system has been used by OC Robotics to guide its snake-arm robot in a nuclear industry application.

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 described as 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 robotic devices that, as their name suggests, snake their way into cluttered environments. An operator typically drives the tip of the snake-arm while OC Robotics' proprietary software controls the arm to move along the same path. This gives the robots exceptional capability to reach relatively inaccessible spaces while maintaining a manageable operator workload.

A recent project was for a system to deliver tools and fixtures to repair a leaking pipe situated beneath a nuclear reactor in Sweden. Andrew Graham, technical director at OC Robotics, explains: "We did not have a lot of time to set up the robot because radiation levels at the work site were high. Spending hours adjusting the linear system was simply not an option! We could not have the installer wearing nuclear protective PPE in a hot, radioactive environment being worried about selecting the 'wrong' beam section."

Snake in the jungle

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 that 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 nick-named '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.

A 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 to allow OC Robotics to build up beams of varying lengths in the field. The 25kg robot that travels at 200mm/s 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. Andrew Graham says: "The system was designed so that it could, in the event of journal failure, still hang off the other two journals without falling." A second rail, mounted on top of the beam, was also specified to carry camera systems above the robot.

Easy to clean

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; the SL2 has no seals or inaccessible crevices in which contaminated particles could be trapped, thereby ensuring that the guidance system could be easily cleaned once the robot was removed from the work site.

Andrew Graham concludes: "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. Indeed we were not 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."

To date OC Robotics has won three DTI Smart awards as well as the 2005 IEE Award for Innovation in Engineering. The company was short-listed for the Royal Academy of Engineering MacRobert Award, and directors Dr Rob Buckingham and Andrew Graham were also nominated for the 2006 and 2007 IEEE/IFR Innovation and Entrepreneurship Award.

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