Advanced High-Strength Steel (AHSS) forming challenges can lead to issues with the precision of part formation and stamping line productivity. The stamping industry is developing more advanced die designs as well as advanced manufacturing techniques to help reduce fractures and scrap associated with AHSS stamped from traditional presses. A Servo-driven Press1 is a significant option that has promising results when forming AHSS.
Characteristics of Servo Presses
A Servo Press is a press machine that uses a servomotor as the drive source. The advantage of the servomotor is that it can control both the position and speed of the output shaft compared to a constant cycle speed. In conventional mechanical presses, the press cycles at constant speed and press loads develop slowly, building power to their maximum force at bottom dead center (180-degree crank position), and then they reverse direction. In comparison, the Servo Press uses software to control press speed and position, which is much more flexible.
Servo Presses have a closed-loop feedback system to more accurately control cycle rate and loads, hence delivering a key advantage, which is the application of very high forming loads early in the stamping stroke. New forming techniques have been developed utilizing these features, and automakers are finding that with the use of the Servo Press, more complex part geometries can be achieved while maintaining dimensional precision.
Servo Presses have only been around for about 10 to 15 years, and so are considered “new technology” for the automotive industry. Recent growth in the use of Servo Presses parallels the increased use of AHSS in the structure of new automobiles. A cut-away of a Servo Press is shown below in Figure 1 to give a better understanding on the drive mechanism.
Types of Servo Presses
1) Servo motor and ball screw driven press: These Servo Presses employ ball screws to reduce the friction on the screws. Unlike traditional screw presses, this press does not require a clutch/flywheel and the slide velocity can be altered throughout forming. A notable feature of the ball screw type Servo Press is that the maximum force and slide speed are available at any slide position. This can be applied to almost all the forming methods. This capability proves to be very useful for forming with a long working stroke such as an extrusion, and for forming that requires a high-speed motion at the end of the formation.
The torque capacity of the servo motor, the load carrying capacity of the ball screw, and the reduction of the belt drive limit the maximum load. One solution to increase the power of the ball screw driven Servo Press is to increase the number of motors and driving axles (spindles). The only problem with this solution is the cost increases tremendously. Figure 2 shows a mechanism of hybrid Servo Press.
2) Servomotor driven crank press: This press provides a lower-cost alternative compared to ball screw style Servo Presses. Crank presses have a high torque servomotor directly attached to the press drive shaft. A few press manufacturers developed C-Frame presses, which combine the servo driving mechanism with the conventional press structure. These presses are visually similar to mechanical presses, but the servo motor replaces the flywheel, clutch, and main motor. When the main shaft rotates at a constant speed, the stroke-time curve mirrors that of a conventional mechanical press. To increase power (for higher forming loads), crank presses are coupled with two connecting rods.
3) Servomotor driven linkage press: Linkage mechanisms are often used for presses to reduce the slide velocity and increase the load capacity of a given motor torque near the bottom dead center. Servomotor driven linkage presses prove to be advantageous in increasing the approaching and returning speeds by slowing down the slide speed in the working region within the stroke. They also have the advantage of maintaining high load capabilities through a considerably long working stroke.
4) Hybrid Servo Press: The knuckle joint and the linkage mechanisms are used to increase the press power with the crank shaft mechanism, and they can be combined with the ball screw mechanisms for Servo Presses.
5) Servomotor driven hydraulic press: These hydraulic presses have been developed because the high-power servomotors achieve higher slide speed compared with the conventional Servo Presses.
Advantages of Servo Presses
Servo Press technology has many advantages compared to mechanical presses when working with AHSS materials. Press manufacturers and users claim advantages in stroke, speed, energy usage, quality, tool life and uptime; these of course are dependent upon part shape and forming complexity.
1) Adjustable Stroke: The Servo Press has an adjustable stroke; the slide motion can be programmed to exert the required press load for deep draw stampings and then switch to different program routines to allow for shallow part stampings, or even blanking. This makes the Servo Press very versatile.
2) Speed: Compared to a standard mechanical press, Servo Presses manufacturers claim up to 37% increased cycle rates, which translates into better stamping plant productivity. Figure 3 shows cycle rates for comparable stroke heights for both a Servo Press and a traditional mechanical press.
3) Energy Savings: The Servo Press has no continuously driven flywheel, cutting the costs of energy consumption. This is especially true in large capacity presses. The installed motor power is greater than that of a mechanical press whose capacity is comparable. However, throughout the stamping operation, the servo-driven motor is used only while the press is moving since the Servo Press has no continuously driven flywheel. Also contributing to nominal energy savings is the dynamic braking operation of the servo driven motor. Through this operation, the braking energy is transferred back into the power system. It is also possible to install an external energy storage feature to make up for energy peaks while reducing the nominal power drawn from the local power supply system (in cases where it is economically justified).
Figure 4 shows a comparison between a Servo Press and a traditional mechanical press with respect to energy use and storage. In a Servo Press, energy from deceleration of the slide is stored in an external device and tapped when the press motion requires more than 235 HP for each motor. The stored energy (maximum of 470 HP) is thus used during peak power requirements, enabling the facility power load to remain nearly constant, around 70 HP.
4) Quality: Better forming stability translates into fewer part rejections.
5) Longer tool life: Decreasing the tool impact speed while simultaneously reducing the cycle time reduces impact loading, thus maximizing tool life. Increased lubrication effectiveness has been observed and using pulsating or oscillating slide motion can further extend tool life.
6) Uptime: Synchronized clutching and extended brake life allow for less frequent maintenance and better equipment uptime.
We have much more information we can share with you about these presses, more than we can fit into one blog. We encourage you to download your free copy of the AHSS Application Guidelines at worldautosteel.org and have a look at Section 3.C.6. Our next blog will provide a case study on Press Energy.
1 Dr. T. Altan, Professor at Ohio State University
2 Courtesy of AIDA America
3 Courtesy of P. Mooney archives
HEADER IMAGE SOURCE: Press line with ServoDirect Technology, courtesy of Schuler