The International Manufacturing Technology Show, held last month in Chicago, puts the latest and greatest machine tools on display. But the show was also a great place to see the motion control components that make these machines work in the first place. Consider linear motion, for example. The show highlighted developments ranging from tiny new linear motors to improvements on the time-tested ball screw.
Ball Screw Improvements Promised
The good news is that Steinmeyer Inc. has come up with a breakthrough ball screw design that promises to eliminate reversing error, reduce friction and improve stiffness. The bad news is the company is not saying a word about how it achieved these performance improvements. “It’s a secret, and that’s all I’ll say,” says George Jaffe, the company’s executive vice president. Though, he did say a bit more. This new ball screw, called ETA+, contains “very subtle” design changes that have resulted in a 50 percent increase in rigidity and a 67 percent decrease in friction compared to conventional ball screws. “Usually friction increases with higher stiffnesses,” Jaffe notes. At the same time, he says, the new ball screws have no reversing error whatsoever. He backs up this claim with ball-bar test results that show reductions in quadrant changing error — an overshoot related to overcompensation of backlash on servo driven axes. The new ETA+ screw, which sells for a 10 to 15 percent price premium, is intended for any high-speed positioning application that can benefit from improved ball screw dynamics.
Tiny Linear Motors
A truly tiny linear motor from THK America Inc. made its debut at the show. These new motors come in 3-, 5- and 7-mm axial pitches. All three sizes feature a ceramic body that serves both as the housing and as the insulation for the coil. The ceramics have a high thermal conductivity, which allows them to remove heat from the motor. According to Hiroshi Yamaguchi, a senior market analyst with THK in Japan, the small size of these new motors allows them to be “clustered” together in a more compact space than would be possible by combining a linear guide and ball screw. For example, he says, multiple linear motors could be put side-by-side to create multiple vertical axes for an electronics assembly system, such as those used to populate PC boards. Another potential application for these linear motors would be precision liquid dispensing systems.
Honing In On Ball Screw Performance
Ball screws continue to turn up in challenging machine tool applications. Sunnen Products Co., for one, has adopted Bosch Rexroth ball screws for precision honing machines. These machines traditionally used either hydraulics or four-bar linkages to handle the stroke of the honing tool. And at first, ball screws didn’t seem like a good way to handle that honing motion. “We initially thought we wouldn’t be able to use ball screws,” says Russell Jacobsmeyer, Sunnen’s manager of product design and development. He explains that the repetitious duty cycle of a honing machine causes excessive wear on those portions of ball screws corresponding to the start and finish of the machine’s repeated strokes into a bore. Sunnen’s research, however, proved that the ball screw could take the abuse — and provide much better positioning accuracy than hydraulics or linkages. Jacobsmeyer reports that Sunnen’s latest honing machines are about 400 percent more accurate than earlier models. They also allow better surface finishes because the improved motion control allows better optimization of how the honing tool and grit are applied within the bore. Ball screws play a big role in both of these process improvements, though the modern honing machines also have new gauging systems and controls that contribute to the accuracy gains.
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