There are many reasons to add a linear axis your articulated industrial robots. For example, the robots may need to handle large parts and rather than moving that part around, you need a way to bring the robot to the work area. Or, you may want the robot to operate multiple workstations and need an efficient method for getting the robot to each location. You may want to synchronize your automation with conveyor systems and moving assembly lines. Perhaps, you simply want to optimize your throughput and gain a competitive advantage.
Whatever the specific reason or application, the two primary goals of adding a linear axis to industrial robots are to increase your automation’s reach and to expand its work envelope.
For these things to happen, the robot needs to be mounted on a track that smoothly guides it to its destination.
It might all sound straightforward, but in harsh production environments, these tracks are exposed to dust, soot, abrasives and fine contaminants. This can cause the track’s roller bearings to stop rotating, leading to malfunction and costly robot downtime. Moreover, applications like welding can create additional hazards, such as spatter that the track system must be able to handle.
Guides and followers
The two main types of linear axis (a.k.a., 7th axis) track technology are profile guides and cam followers. The crucial design difference between the two is the anti-friction bearing system used to support and guide the tracks.
Profile guides, also referred to as linear bearings, typically consist of two rows of small, cylindrical bearings. Each row rides on an opposing and angled surface of a hardened, ground steel guide rail, which provides your robot with both vertical (load) support and lateral guidance. Profile guides are a well-established technology and they provide accurate, stable and smooth linear guidance but even with scrapers, the bearings are still prone to contamination.
The newer cam follower, or track follower, technology consists of a cylindrical roller bearing mounted at the end of a stud. The cam follower rides on a guide rail and uses a simple block configuration.
Several cam followers can be arranged on the top and sides of a rail to provide support for the robot in all directions. Meanwhile, a wiper on the block scrapes the rail clean of debris, including paint and weld spatter. Crucially, the bearings are completely enclosed in the block to keep out contamination.
Major differences
The cylindrical rolling elements of cam followers tend to be larger than those found in profile guides. The bearings of profile guides directly contact the guide rail, whereas cam followers have an outer ring that provides a more robust point of contact between the bearing and rail.
With larger rolling elements and an outer ring, cam followers typically have a larger overall geometry compared to profile guides. In most applications, this size difference is of no consequence.
However, the contact angle of the anti-friction bearing and guide rail is of importance. For profile guide-based tracks, it is essential that any replacement components, either the guide itself or the guide rail, have precisely matched angles.
Cam followers are deployed on simple rectangular guide rails, so this is not an issue. Moreover, the rollers and rail can be replaced independently of each other with no performance issues. When required, some cam follower systems can incorporate profiled, slightly curved contact surfaces that provide added performance and lifecycle advantages.
The all-metal construction and larger contact surfaces found on cam followers typically make them more suitable than profile guides for supporting larger robots. This sturdy design also provides more reliable performance in high-inertia applications created by rapid reversals of linear motion.
Cam followers are adjustable because they allow you to set the preload torque. Higher preloads provide greater stiffness and precision, while excessive preload adds stress that increases wear on system components. You can easily adjust a cam follower to achieve the desired balance between precision and the overall lifespan of the system.
Because of the larger rolling elements, cam followers also have an advantage over profile guides in applications where faster motion is required. Profile guides use smaller cylindrical elements, making them more susceptible to damage when operating at relatively high speeds.
Every application has unique precision requirements for both robots and their supporting 7th axis tracks. You can specify the required level of precision or stiffness required regardless of which anti-friction technology or track type is used. Cam followers and profile guides offer a range of precision options, with a motion repeatability of ±0.2 mm easily achievable. Keep in mind that this accuracy is as dependent on the guide rail as it is on the rollers.
Harsh environments
Cam followers and profile guides incorporate design elements like shields, seals, scrapers and covers to protect internal rolling elements from the contamination caused by the debris and microparticles found in many production environments. Dust, soot and other abrasives find their way into the bearings, where the abrasives consume lubricant and accelerate wear on contact surfaces.
When debris enters a profile guide, its rolling elements typically stop rotating. It then slides along the guide rail, accelerating rail and bearing wear. And in a worst-case scenario, one or more rolling elements can dislodge from the support block with potentially catastrophic results.
With their fully enclosed bearings and larger rolling elements, cam followers are much more resilient to these types of harsh industrial environments. Their larger internal cylindrical rollers and large outer race enable them to simply roll over such debris.
Recent examples of successful implementation of cam followers in harsh environments include several of JR Automation’s customers using the technology when welding large objects, such as bleacher seats and large metal tanks. These types of applications are where a dirty welding operating environment can wreak havoc with regular profile bearings.
“JR Automation has utilized cam followers for durability and also for the ease of maintenance and repair,” says Tom Moneypenny, manager of welding engineering at JR Automation, a global automation integrator headquartered in Holland, Mich. “We’ve experienced effective, quality service that helps keep our customers’ automation solutions running smoothly.”
Servicing benefit
When manufacturers need access to service the roller bearings on a profile guide system, they typically have to remove the entire carriage, robot or payload away from the track. It is a time-consuming process that often requires a crane and can take an entire production shift to complete. It also means the robot is down. If you are replacing a profile guide bearing, after you have pulled the robot and carriage off the track, you will need to re-teach all the robot points and paths.
To service the newer cam follower systems, all you have to do is slide out the cartridge containing the bearings on the side of the rail. It takes just 10 min. to remove, replace and adjust the cam follower.
The linear axis track technology the robot runs on has a huge impact on your maintenance costs, overall efficiency, adaptability and how well it handles contaminants. With their fully enclosed bearings and larger rolling elements, cam followers are much more resilient to harsh industrial environments than linear profile guides.
