The flexibility and efficiency of servo presses are challenging 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 a servo press investment, there are varying differences between manufacturers to consider that will impact performance, reliability and cost. While you dream of new and creative ways to form, be sure to rethink power, construction and control.
All servo presses replace the main motor, flywheel, clutch and break with a servo motor. The differences are the types of motors and drive trains. The first option is a “hybrid” drive configuration. With a hybrid, a knuckle or link-type drive train is used with a standard high rmp/torque AC servo motor. This hybrid drive configuration allows for a compact and efficient motor, but has drawbacks. For one, the need to “down-gear” a high-rpm motor to generate torque requires more parts, belts and mechanical components, increasing maintenance and service. Because of the knuckle or link-style, you still rely on mechanical linkages to generate thrust which can negatively affect tonnage and torque, especially high in the stroke.
The second option is a “direct drive” servo motor. This motor has a dramatic hi torque/low rpm ratio. In some cases the max rpm is only 340, with a torque rating of 14000 N-m. At such a ratio, less than 40% of the torque is required to accelerate the drive train, leaving 60% or more to accelerate or decelerate the slide or develop press force. This configuration eliminates energy-consuming linkages, down-gearing and other mechanical obstacles. The pinion shaft is connected 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. This configuration provides better transmission, less maintenance and more consistent energy thru the stroke. Some manufacturers have large capacitors to capture any unused energy, like 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 will open your mind to creative metal drawing and in-die processes. But this versatility introduces new strains, twists and forces on the press frame. Most servo press manufacturers offer conventional gap frame, straight side, or tie-rod constructions. However, other manufacturers argue that conventional structures are not rigid enough to hold tight tolerances under longer dwells and draws without stretching, and elongating the frame. These manufacturers employ a completely different design. Some call it monoblock, others call it honeycomb. In essence, both design the press with low-impact speed in mind and are intended to “box-in” horizontal and parallel support on the slide and bolster, which are 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.
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.
As you weigh the options in servo press technology, you have more choices than ever. With these choices come more information and a better understanding of what to expect, which should help you make the right decision.