Page 27 - MetalForming August 2013

Servo Presses— Why Pay More?
What makes a servo-driven stamping press different than a standard mechanical press,
and why would a metalformer pay more to get one? Here we present the four fundamental advantages provided by servo technology, and how they
pay off in the pressroom.
BY TODD WENZEL
Conventional mechanical presses come in different configurations of drivetrains to convert rotary flywheel motion into linear slide motion. This action closes and opens dies with enough tonnage and energy to add shape to otherwise featureless sheetmetal blanks. The various drive- train arrangements include crank- shafts, eccentric shafts, knuckle mech- anisms, link drives, direct drive, back-geared and double back-geared, and they all have been developed to provide tons of force; in.-tons of ener- gy; specific slide velocities; and dwell time with the die closed.
In the early history of press develop- ment, improvements started with the creation of new and better drivetrains and frames, driven by the need to create presses that delivered the required com- bination of force, energy, slide velocity and dwell time. The many types of drive- trains we are familiar with today have existed for more than 50 years. While machining accuracy has improved since that time, and therefore so has press accuracy, new drivetrains developed
Todd Wenzel is president, TCR Integrat- ed Stamping Systems, Wisconsin Rapids, WI; 800/676-2240; www.tcr-inc.com.
since then have resulted only in slight improvements on old designs.
Once built, a conventional mechan- ical press provides a fixed set of char- acteristics. Other than stroke length, which can be varied only with rare, special machine designs more com- mon in Europe than the United States, press characteristics are locked in. Met- alformers cannot change the drivetrain characteristics developed for a previous set of production needs to meet the needs of new parts being quoted.
Breaking Old Rules, Making New Rules
Skilled engineers have long under- stood mechanical-press technology and the built-in rules inherent in press design. The capabilities that servo- press technology brings have broken many of the old rules, giving metal- formers a new set of rules to play by. Those that understand and use these new rules can gain a significant com- petitive advantage.
The old rules:
1) You set flywheel speed before pro- duction, but slide velocity cannot be varied much with the press cycle. Slide speed relates directly to flywheel speed, stroke length and drivetrain design,
none of which can be changed once the stroke begins.
2) If you slow the flywheel you reduce the energy available for pro- duction. Energy relates directly to the motor and, more importantly, to the speed, geometry and mass of the fly- wheel, none of which can be changed once in motion.
3) You cannot stop the press and hold force on the part at a precise loca- tion, then restart. To stop the press, the clutch must be released and the brake applied. For an instant, neither is in control of the ram, during which time the force on the tool pushes the ram up. This causes the ram to lift dur- ing the transition. Restarting also is a problem since clutch torque is limited when first connecting.
4) Once in motion, the driveshaft must complete a full rotation. There is no easy or fast way to stop and reverse the slide—the motor and flywheel must be stopped and then started in reverse, too time consuming.
The new rules established by servo- press technology:
1) You can easily change slide veloc- ity during the stroke, multiple times if desired.
2) Full energy is available at speeds
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