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.
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.
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.
| Fig. 2
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, straightside, or tie-rod-constructed press may lack the rigidity necessary to hold tight tolerances under longer dwells and draws, causing the frame to stretch and elongate. To prevent this, some press manufacturers employ a monoblock or honeycomb frame design. These presses are designed with low-impact speed in mind, and are intended to “box-in” horizontal and parallel support on the slide and bolster, located in the center of the frame. Thus, all support is targeted to where it’s needed most, reducing the risk of press damage, as well as floor space.
| Fig. 3
Speaking of press damage, you cannot overlook the importance of an overload-protection system. Most manufacturers offer an upgraded version of the traditional hydraulic overload, understanding that new forces are being entered into the forming process, which could alter the way a hydraulic overload works. Other manufacturers have done away with the hydraulic overload and replaced it with an electronic, programmable overload, which may perform better over longer dwells operating closer to rated tonnage.
Every manufacturer gives you a means to select and adjust preprogrammed stroke profiles or create freestyle profiles (Fig. 3). Beyond that, a servo-press controller should offer more than what you’re used to. You should have more standard ways to integrate and orchestrate peripherals, a robust back end for collecting and analyzing data, and a user-friendly front end to put that data to use. The controller is the key to making your servo investment pay off. If you’re not using the most up-to-date and scalable control technology, you’ll soon be upgrading and paying more. MF
Get more info at: Seyi-America, Inc., 843 Joint Park Blvd., Tullahoma, TN 37388, Tel: 931/455-7700, www.seyiamerica.com
See also: SEYI America, Inc.
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