Page 40 - MetalForming September 2014

Servos Seeing More Service
link-assisted, on the market for the past 3 yr.
In general, any servo press replaces the standard motors, flywheels and clutch/brakes found in traditional mechanical presses with servo motors, pro- viding full working energy at any speed. Although, like any mechanical press, which at heart, servo-driven presses still are, it won’t provide its full rated tonnage throughout the entire stroke length. The payoff is flexibility in slide-motion profiles and, consequently, flexibility in applications. Servo presses can perform primary operations such as blanking, drawing, restriking and even warm forming, as well as sec- ondary processes such as drilling, tapping and staking.
Direct-drive presses provide
a shorter stroke length if need-
ed, in pendulum motion, according to Lee Ellard, Stamtec national sales manager. A 12-in.-stroke press with direct drive can run a 2-in. stroke if need be, making it effective for progressive-die applications, for exam- ple. Other advantages include a higher torque off bottom dead center (BDC) than a comparable link-assisted servo press.
An economical alternative to the direct-drive presses are Stamtec’s link- assisted servo presses, which employ standard servo motors. A link or toggle mechanism couples with the motors to provide servo advantages while con- serving energy. Less costly and more readily available servo motors augment the mechanical link drive to provide servo capabilities such as slide-speed adjustment, dwelling and bumping and reversing, though once BDC is reached, a link-assisted drive must complete the full stroke length without reversing. So, link-assisted presses won’t provide varied stroke lengths/ pendulum motion, and provide less torque and energy at rating points as compared to direct-drive units.
“Many applications involve more
exotic materials that are difficult to form and require dwelling or restrik- ing,” says Ellard, citing an example where servo presses shine. “For a die with a form in it, a servo press can slow down the slide speed through that form and dwell so that the material flows more effectively and sets. With a tradi- tional mechanical press, your slide- motion profile cannot be changed dur- ing the press cycle. So, a form that requires a slower slide velocity means that the press must run the entire cycle at that slower speed.
“Servo presses allow increased speed to the point of the form and after, with a slow speed and/or dwell through the form,” Ellard continues. “The result is improved forming and increased productivity.”
Find the Right Fit
Drawing, high-tensile-strength stamping and blanking all offer oppor-
Economical link-assisted servo presses, such as this model from Stamtec, use
links or toggle mechanisms that com- bine with servo motors and conserve energy.
tunities for improvement via servo- press technology. But the decision is not so clear cut, according to Jim Schulte, general manager of sales and administration at Nidec Minster Corp., Minster, OH.
“Suppose that in a specific appli- cation the use of a servo press can minimize impact forces and the subsequent snapthrough forces, by slowing down the ram at the point where the punch contacts the mate- rial,” he explains. “That is a benefit. But the tradeoff might be that the time consumed during the work- ing part of the stroke results in less net output, because a stamper can’t feed the part as quickly due to a slowdown in the ram speed.”
In these cases, an integrated servo solution may be the answer. Or, perhaps a traditional mechanical press is the safest and most economical route. There are other factors to con- sider as well.
“Servo-driven presses are more complex than mechanical presses,” Schulte says. “You have the servo motor and you have eliminated some of the mechanical components, but on the other hand you have a much more sophisticated electrical control system that routes more of your investment dollars into the controls and less into the mechanical systems. So you have to be prepared to support that.”
But as Nidec-Minster and others continue to offer updated servo tech- nology, metalformers will continue seeking servo solutions.
With that in mind, Nidec Minster has been developing servo-press tech- nology for several years. The company installed its first servo-driven press about three years ago, a P2H-FX model. That line offers capacities of 63, 100 and 160 tons and provides high speed —upwards of 250 strokes/min.
38 MetalForming/September 2014
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