Nearly all discrete manufacturing processes incorporate linear motion, and as manufacturers become familiar with the flexibility and simplicity of modular linear motion, these systemswhether one, two or complete three-axis Cartesian robotics systemsare finding their way into more and more areas of production.
Component selection among ball screws, linear guides, and actuators begins to make sense when some basic application considerations are applied.
How to Handle Stress
Machines known as laners organize packaging line products for the next downstream operation by rapidly dividing the line and diverting products left and right as needed, creating multiple lanes for case packing, shrink wrapping, or further processing.
Hartness International, Greenville, S.C., recently introduced a laner that pushes the limits of line speed, while protecting product packaging along the path (See Figure 1 below). The GlobalLaner 2260s ambitious design called for a linear belt drive module with an acceleration rate of 1 m/s2 to keep pace with the line speed of 200 ft/min. To give the laner the ability to relocate a section of bottles to any point in the grid, Hartness incorporated three linear modules on each machine. Two X-axis linear modules are connected to a common shaft and driven by one servo motor. The third linear module also is servo-driven and acts as the Y axis.
|FIGURE 1: KEEP PRODUCT TRAFFIC MOVING|
The GlobalLaner 2260 uses a linear belt drive module with an acceleration rate of 1 m/s2 to keep pace with the laners line speed of 200 ft/min.
The MKR 2080 module supplied by Bosch Rexroth features an AC servo motor and a pre-tensioned toothed belt drive for demanding speed and load requirements from orange juice jugs running at 550 bottles per minute. The modules are designed with anodized aluminum frames and carriages with low-maintenance, one-point lubrication. Each modules cover is constructed using a corrosion-resistant steel strip that performs to the DIN EN 10088 standard.
The GlobalLaner 2260 prototype included linear modules that couldnt handle the speed, says Coulomb. With the high acceleration and deceleration rates of 1 m/s2, the previous modules began losing position.
Bosch Rexroth has experience helping users make the right selection. A common mistake that engineers and designers make when sizing and selecting linear motion systems is to overlook critical application requirements in the final system, believes Danielle Collins, systems product manager, Bosch Rexroth Linear Motion and Assembly Technologies. This can lead to costly redesigns and re-works in the worst case, but also might result in an over-engineered system that is more costly and less effective than desired.
How much load will the system need to handle? How fast will it need to move? What is the most cost-effective design? We considered all of these questions when our group developed a guide to specify the appropriate linear motion components or modules in any given application, says Collins. LOSTPED is an acronym that stands for load, orientation, speed, travel, precision, environment and duty cycle. Each is a factor to be considered when sizing and selecting a linear motion system.
Rexroth engineers determined that each factor must be considered individually as well as in conjunction with the others to ensure the best overall system performance. For example, the load imposes different demands on the bearing system during acceleration and deceleration than during constant speed movements, says Collins. As more linear motion solutions move from individual components to complete linear module systems, the interactions between linear bearing guides and ball screw, belt, or linear motor drives become more complex. The system can help designers avoid mistakes by simply reminding them to consider all of the interrelated factors during system development and specification.
We wanted to upgrade our Classic SL-18 Micro shrink bundler, says Devandra Shenge, product development specialist at Omega Design Corp., Exton, Pa. The pneumatically-driven, PLC-controlled machine required frequent maintenance and was expensive to operate.
Speed was a critical factor in designing the new machine. Omega needed a machine that could process multiple quantities of varying sized and shaped personal care items such as shampoo and deodorant bottles, says Shenge. With the pneumatic system, products had a tendency to get turned in the wrong direction and jam up the machine, resulting in downtime and a loss of profits.
Omega ultimately used a new linear slide in a new machine design. As a result, the company has been able to reduce maintenance, the number of sensors needed, air leaks and labor costs.
Almost every piece of industrial machinery today is designed for long life with minimum maintenance. A big contributor to premature end of life is lubrication failure, says Tom Solon, applications and sales engineer at Kerk Motion Products, which supplied the rapid guide screw RGS10000 to Omega. Now many manufacturers have jumped on the PTFE bandwagon. When used with high performance polymers, PTFE coatings can make slides and lead screws truly maintenance free. The challenge is protecting the comparatively soft plastic parts from premature wear. Solon says Kerk has spent many years refining its Kerkote TFE coating for this task.