Servo Presses in Cold, Warm and Hot Forging
This article was originally published in August 2014.
Servo press technology, long-used in the forming of sheet metal, has a future in the forging industry. The energy consumption per production stroke of a servo press is lower than that of a conventional press. Schuler’s Servo-Direct Technology (SDT) presses have been used in forging operations with favorable results. The company believes it is a technology that will improve the ability of the forging industry to deliver its products competitively.
The use of servo presses in sheet-metal forming has rapidly increased in recent years because many automotive OEMs and their tier stampers use them for reliable and efficient production. New stamping systems are now designed almost exclusively as servo presses, the key economic drivers for which are higher throughput, more production flexibility and better quality. Meanwhile, in the forging world, Schuler has delivered the first servo-driven production lines for applications in cold, warm and hot forging. The early experiences in these forging facilities show very positive results.
The main difference in servo press design relative to conventional presses is that servo presses do not use a steady flywheel speed and motor. These are eliminated in favor of one or more torque motors with variable speed. With highly dynamic torque motors, the power is transmitted directly from the motors to the drive shaft (Figures 1 and 2), which explains Schuler’s brand name: Servo-Direct Technology (SDT).
SDT allows the kinematics of the press slide to be programmed to the requirements of a forming cycle. Think of a Formula 1 race. To maintain control, the driver slows before the curve and then accelerates out of the curve. If the car was only able to maintain the slowest speed required for the curve over the entire track, it would lose the race. This is what happens with a conventional drive on a press. Because it operates at a steady speed, the entire forging cycle can only run at the speed of the slowest requirement in the entire process. With a servo-driven forging process, the torque motor can accelerate the ram up before the part transport and then accelerate the ram back down to reduce the time for transporting parts. Then the ram speed can slow to allow the forging operation to take place at the required speed to achieve the desired quality. This delivers an optimized forging operation where the overall cycle time is reduced and productivity increases.
Experience from Sheet Metal Forming
Since 2007, Schuler has supplied more than 300 SDT presses to automobile manufacturers and components suppliers for use in sheet-metal forming. In many cases, they are used as multi-purpose presses for a broad range of applications. This allows the manufacturers high flexibility to quickly change jobs with pre-programmed, optimized forming cycles. The capacities of these machines range from 275-4,000 tons (2,500-35,000 kN).
Depending on the application and practical experience, SDT systems can increase productivity from 30% to more than 150% relative to conventional presses. The ability to operate with a “pendular stroke” reduces the cycle time even more. Instead of complete 360-degree rotations, the eccentric gears in the drive rotate only partially, to perform a forwards/backwards rotational motion like a pendulum. For example, it is possible to reduce the “normal” stroke of a 630 metric ton press from 15 inches to 3 inches (400 mm to 80 mm) with a pendular stroke. This allows productivity to be increased significantly.
Sample Application 1: Vertical One-Point Servo Press
The newly developed MSL1-500 is a vertical single-point servo knuckle-joint press (i.e. the slide is driven by one connecting rod). This machine can be used for cold or warm forging. Due to the compact dimensions of the bed and slide, the press is used for one-station or multi-station dies in which the main forming station is located in the middle of the press. At present, machines are built with a nominal force of 560 and 700 tons (5,000 and 6,300 kN), and additional size offerings are in the planning stages.
The main drive of the MSL1-500 consists of two servomotors with a rated power of 165 kW each and a nominal speed of 300 rpm. The two motors are attached to a shared drive shaft and are directly flange-mounted on the press body at opposite ends (Figures 3 and 4). The lower connecting rod is connected to the slide, which is guided on eight-way roller guides, with the roller guides set using eccentric pins to eliminate play in the system.
Sample Application 2: Horizontal Multi-Station Press
The FormMaster series (FM) is a horizontal multi-station press for cold-heading wire material. With many new developments, one of the significant design improvements is that the main drives are now servomotors (Figure 5). One machine with a rated force of 560 tons (5,000 kN) has been in reliable commercial operation for several months, and a second 700 ton (6,300 kN) machine will go into production in the summer of 2014. Both presses are used to manufacture a variety of automotive components.
The FM500-6 SDT is driven by two servomotors, both of which are flange-mounted next to one another on the rear of the press body. The motors have a rated power of 377 kW each and a rated speed of 480 rpm. In addition, a separate AC motor acts as an energy accumulator. The torque motors drive not only the ram but also the countershaft, helical and bevel gears of all other ancillary drives. All gears have helical or spiral gearing for noise reduction.
The wire is fed and then sheared at a shearing station. This is done in a closed shearing knife driven by an advance and return cam, cam lever and shearing bar. The shearing socket and shearing knife are part-specific and can easily be interchanged. The sheared sections are then moved into the first transfer tong by means of a pusher.
In the FM series, the forming stations are arranged one above the other, and the parts are transported from station to station using individual gripper arms. The NC transfer for each station is driven by two separately controllable servo drives for “transport” and “gripping.” The FM500-6 SDT has six transfer tongs and one placement tong. The complete NC transfer can be swiveled by a motor to allow the die space to be easily accessed, and it is locked hydraulically during operation.
Other recent commercial applications of SDT in forging include:
- Two-point servo presses with 900 or 2,250 U.S. tons (8,000 or 20,000 kN) press force for cold forging
- A crank forging press with 1,800 U.S. tons (16,000 kN) press force for hot forging
- A horizontal upsetting machine with two mutually independent servo drives with 350 and 560 U.S. tons (3,150 and 5,000 kN) press force for hot forging
- A 3,500 U.S. ton (31,500 kN) machine for hot forging equipped with both a servo drive and conventional drive, which is only used during the forging operation. The servo drive enables the ram speed to be programmed as required before and after the forging operation.
Initial Experience from Commercial Production
The horizontal multi-station press FM500-6 SDT has been in operation for several months and has produced a dozen different parts. From the start of operation, even with just a conventional sinusoidal motion curve programmed, the parts run much more smoothly in the SDT press. This can be explained in that the loading is more even and there is no sudden energy transfer from a flywheel to the slide.
One forged part that was previously produced on a conventional machine at up to 60 strokes per minute can now be produced at 67 strokes per minute – a 12% productivity increase – when operating with the programmed SDT slide motion. Because it is in early stages of production, it should be noted that all of the capabilities offered by servo technology have not yet been explored, and further performance improvements are expected for this forging operation.
Initial results are also very promising in terms of improved die life, though it is not yet possible to provide verifiable statements since no long-term results are available. Another advantage of SDT is that in the die setup mode, the press can apply the full load at almost any time, and a soft run-through is possible during forging. Furthermore, the slide carrying the dies can be moved toward the parts in the bed die, and then moved back away without difficulty before the die makes contact with the parts. This is not possible at all with conventional presses.
Energy Management and Energy Balance/Power Consumption
Servo presses have a brief high-energy demand during forming, which means they have a connected load that is higher than conventional presses, in general. Typically, an electrical-energy storage system is used with servo presses to reduce the connected load that is actually required by the overall machine. The specific energy consumption per production stroke of a servo press is lower than in a conventional machine because of the reduced mechanical losses and as a result of the generally higher productivity.
Outlook for Servo Drive in Forging
The potential of servo press technology for forging is about as large as it is for sheet-metal forming. Because of the vast experience on the sheet-metal side, the mechanical design of the servo-drive concept is already proven to be reliable and deliver results. The forging industry is faced with the same competitive pressures as the stamping industry, and it is a technology that will improve the ability of the industry to deliver its products competitively.
Author Andreas Kress is the deputy director forging technology business unit at Schuler Pressen GmbH, Göppingen, Germany. For additional information please visit http://www.schulerinc.com.
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