Page 16 - MetalForming August 2011

14 MetalForming/August 2011 www.metalformingmagazine.com
In the Classroom
Servo Presses:
Breaking Down the Choices
The flexibility and efficiency of servo presses challenge tooling engineers and part designers to rethink the way they form. For any company hunting to gain a competitive advantage, having a servo press will be a mainstay.
When it comes to investing in a servo press, metalformers must take the time to understand the different technologies that the various press manufacturers bring to the table, and how these differences impact performance, reliability and cost. While you dream of new and creative ways to form, be sure to rethink power, construction and control.
Rethinking Power
Typical Mechanical Press
Servo Press
All servo presses replace the main motor, flywheel, clutch and brake with a servo motor (Fig. 1). What varies among the various types of servo presses on the market are the types of motors and drive trains used.
F/W: Flywheel
C/B: Clutch and Brake PG: Pinion Gear
MG: Main Gear
MG
SERVO MOTOR
FW
C/B
PG
MG
PG
MOTOR
Fig. 1
The first option is a hybrid drive configuration, which uses a knuckle or link-type drive train along with a standard high-RPM/torque AC servo motor. This drive configuration allows for a compact and efficient motor, but has drawbacks. The need to down-gear a high-RPM motor to generate torque requires additional drive components and belts, increasing maintenance and service requirements. Also, because of the knuckle or link-style drive train, the mechanical linkages generate the required thrust, which can negatively affect tonnage and torque, especially high in the stroke.
The second option is a direct-drive servo motor, which boasts a high torque/low RPM combination. In some cases the max RPM is only 340, with a torque rating of 14000 N-m. At such a ratio, less than 40 percent of the torque is required to accelerate the drive train, leaving 60 percent or more to accelerate or decelerate the slide or to develop press force. This configuration eliminates energy-consuming linkages, down-gearing and other mechanical obstacles. The pinion shaft connects directly from the motor and, in some instances, directly to the motor (part of the motor, actual rotor.) Only the pinion gear and main gear are needed. Because of this mechanical arrangement, the direct-drive configuration provides improved transmission, less maintenance and more consistent energy through the stroke. Some manufacturers include large capacitors to capture any unused energy, such as when the press is not under load, and use it when the time is right.
Rethinking Press Construction
A servo press’s programmable slide speed, position and dwell (Fig. 2) will open your mind to creative drawing processes and in-die activities such as insertion and welding. But this versatility introduces new strains, twists and forces on the press frame. A conventional gap-frame,