For some motion control challenges, a simple solution might be the best. When movement is between only two points and is uncomplicated, then mechanical motion control often can be the smart solution. A look at several examples illustrates why, for some machine builders, keeping it simple isn't stupid.
Most colorful T-shirts sold at concerts or seen on the streets passed through a screen printing textile press. Years ago, multi-color presses used pneumatics with an indexing cylinder to rotate a carousel, with the number of stops dependent on the number of colors being printed. In an hour, a press might crank out 650–1,000 T-shirts, but that wasn't good enough.
"The market started to require more stable performance and quicker changeover of job setup," says Darek Tkacz, chief mechanical engineer at M&R Printing of Glen Ellyn, Ill. The company is a global supplier of screen printing textile and graphic presses.
M&R Printing's initial solution to this demand involved a direct-drive mechanism with a high-end servo motor and gearbox. The latter had less than 3 arc-min (0.05°) of backlash — an accuracy needed because it had to position a carousel within a stationary fork to lock it. For a 40 in. diameter carousel, the location had to be accurate within less than 0.002 in. The precision of the positioning directly determined the quality of the print, since the colors are layered atop one another.
Though the direct-drive approach worked and offered higher throughput, it was significantly more expensive than the pneumatic method. So M&R Printing opted to try another route — one that exploited mechanical motion control.
"Instead of a direct drive, we installed a crank-type index design," Tkacz says. "The crank is connected through a link to a capture fork, so that rotary motion is converted into linear motion back and forth."
This works for the company's application, he adds, because the machines require only a stop and start point, a simple motion that always bounces between fixed locations. M&R Printing uses a design that has the crank with a 10° starting point and a 170° stopping point (Figure 1), although these two locations can be adjusted if needed. Experience during development revealed that the inertia of the largest and highest-output carousels was too great for this approach. Thus, those machines remained direct drive.
For smaller carousels, which can produce 900+ T-shirts per hour, the crank technique offers some significant advantages. For one, it allows M&R Printing to switch from a pneumatic index to a more cost-effective electric system with servo motor, Tkacz says, explaining that the servo motor made it easier to set up different parameters for different conditions.
"Most important for us was that we have been able to pre-program the speed of the machine for the different sizes of the printing pallets," he says. "Customers very often are changing pallet sizes a few times a day. With the pre-programmed pallet size speed of the machine, changeover is a very easy and fast process."
Another benefit is that the crank design boosts the throughput of some presses by about 50%, with little increase in price over the older technology. That is possible because the crank arm method is more forgiving in what it demands from a gearbox, Tkacz says.
"Using this motion will allow us to go with a gearbox that's not as accurate and not as expensive," he explains. "We've been able to choose a gearbox that can have 10 arc-min backlash and we can still achieve our linear accuracy for the indexing."
The success of the approach can be seen in the fact that M&R Printing has converted its product line almost completely to a crank-based design and away from pneumatic indexing, he adds. In implementing its design, the company specified gearboxes from Lenze Americas.
The biggest trend Lenze has seen is a push for increased energy efficiency, says Steve Greene, central regional sales manager. That has prompted Lenze Americas to move from a worm gear design to a helical gear. Doing so ups gear efficiency from 60% to 98%, and brings other benefits. "You can run the helical gear faster and it's more accurate," Greene says.
Flush Away Problems
Tekno, Cave City, Ky., produces conveyors and material handling solutions for the automotive and other industries. Cost is one reason the company sometimes turns to mechanical motion control, says Larry Mustread, chief operating officer.
Not only is a mechanical solution cheaper initially, it also can cost less to operate because of lower support costs and quicker time-to-repair. That's a consequence of the simplicity of the system and the ability to spot and diagnose problems quickly, Mustread says. "I know that it's going from point to point. If it starts losing that movement from point to point, it's easy to pick up that I'm getting wear somewhere."