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Cushions Part of the Servo Revolution

By: Lou Kren

Thursday, November 01, 2012
 

This illustration depicts a servo-controlled hydraulic cushion with double-acting cylinders, linear transducers and pressure transducers. The unit can be stud-mounted to the bottom of a press bolster.
A decade after arriving on North American shores, servo-drive mechanical presses truly are taking over. Automotive OEMs are installing the machines in plants across the land, with their tier suppliers, and even job shops, following suit.

To complement servo presses, manufacturers have unveiled servo-controlled cushions. The cushions provide flexibility and controllability, allowing metalformers to dial-in and fluctuate pressure on downstrokes and upstrokes, thus aiding in precision forming and drawing while nearly eliminating the evils of reverse tonnage.

“Automotive manufacturers and press builders see servo technology as the next big innovation in the industry and have embraced it,” says applications engineer Darrell Quander Jr., of Hyson, Brecksville, OH. “The flexibility and controllability in servo-controlled hydraulic cushions complement servo-press flexibility and controllability —the idea with servo presses is to fine-tune the process and servo-controlled cushions do the same. Stampers can dial force up or down to move the cushion prior to the die actually contacting it, decreasing press wear. And, built-in adjustable-force profiles add flexibility where, before, only one cushion pressure was available. Now we can change pressures on the fly as the servo press changes characteristics on the fly.”

New hydraulic cushions employing servo control have already entered the field.

“One automotive OEM recently switched to servo presses to avoid having to tweak each press after a die change, spending weeks making it right and scrapping a lot of material in the process,” says Michael Culbertson, Hyson senior project engineer. “Combining the presses with the new cushions allows even greater force control to ease job setup and increase production of quality parts. For example, during forming, the cushion can adjust pressure by a percentage point or two to eliminate crinkling and tearing while maintaining the same press force.”

Servo-controlled hydraulic cushions offer this and other benefits in response to metalforming trends.

Assist in Blank-Size Reduction

 A screenshot from a press-cushion control panel shows how operators and management can monitor and control cushion functions to best assist the press during part forming.
As more metalforming companies embrace lean manufacturing, so too have needs increased for more efficient tooling, presses, cushions and force generation. Waste elimination is another major goal. Stamped-parts producers task their engineers to reduce blank sizes to combat scrap generation. Bringing more control into the stamping process through the use of a servo press and accompanying cushion, and their combination of variable high and low forces and speeds, can eliminate the need for draw beads, resulting in reduced blank sizes, according to Hyson officials.

“Most servo presses can provide full tonnage on a downward stroke, but once the ram stops that force can be held but not changed,” explains Culbertson. “However, a servo-controlled cushion can vary the force while the ram remains stationary.”

That feature can enable forming without draw beads.

“If you can just hold the material and then vary the force with a cushion as you draw, you will save material as compared to using a draw bead, because there is less scrap to trim,” says Steve Reilly, Hyson’s manager of product engineering. “The result is a smaller blank size, and the varying-force feature of these new servo-controlled cushions requires less redraws, where the ram has to come back up and return to draw to a greater depth. Stampers attempting a deep draw would want to vary the holding force through the stroke to produce the long draw. With servo-controlled cushions, stampers can program the varying forces right into the cushion to accomplish that draw quickly and more accurately. Blankholding force can be reduced, held constant or increased through the draw. Whatever is needed, the stamper just programs it in.”

Ideal for Advanced High-Strength Steel

 This graph displays the force of a servo-controlled hydraulic press cushion as well as a position curve. Note how the cushion ramps up force at the bottom of the stroke to ease shock on the part, tooling and press.
Forming advanced high-strength steel (AHSS) requires increased press energy and higher cushion tonnages. Nitrogen cushions ramp up process force using pressures surpassing 2000 psi, according to Quander Jr. However, hydraulic cushions operate at pressures greater than 4000 psi, with the added benefit of delaying or eliminating reverse tonnage.

“While air and even nitrogen no longer suffice with the pressures used for forming AHSS,” he says, “servo-controlled hydraulic cushions provide much more force. That force, in traditional systems, has a drawback: reverse tonnage. However, this cushion technology delays that force and controls the return, eliminating the tonnage spike that ordinarily would filter back through the press.

“Also,” continues Quander Jr., “forming AHSS sometimes requires dynamic adjustment of force and speed in the press. The level of controllability with servo-controlled hydraulic cushions allows the stamper to fine-tune part profiles, as well as save energy and reduce wasted work.”

In essence, the cushion matches the speed of the press and dissipates the force to eliminate reverse tonnage, reining in that force to enable stamping of more precise parts.

Increased Productivity and Part Complexity

The advent of servo-controlled hydraulic cushions also provides for increased productivity and greater part complexity, according to the Hyson team.

Advantages of
Servo-Controlled Hydraulic Cushions
• Adjustable/programmable force profile—Force can be controlled and programmed throughout the stroke.

• Pre-acceleration—This capability allows the cushion to start moving (pre-accelerating) in order to minimize impact load.

• Delay function—The system can delay at the bottom of the stroke to reduce reverse tonnage into the part and press.

• Standalone force—Variable forming force can be applied as the ram remains stationary.

“Stampers are trying to get the most usage out of their presses,” says Quander Jr. “With the trends of increased part complexity and condensed dies/ stations, stampers need more controllability in their cushions. This new technology can be programmed to know each part and the tooling required, with the controllability to vary the force in real time.”

The technology ultimately allows for increased production, as Reilly explains.

“Suppose a servo-controlled press operates in pendulum mode where the stroke height varies,” Reilly says. “The servo-controlled hydraulic cushion can vary the return height of the system. For parts requiring a longer or shorter stroke height, the cushion is programmed to make that chzange. Rather than going through a full stroke with every hit, the cushion is programmed to return to a shorter stroke height. This allows for more parts per minute since wasted movement is eliminated.”

Simplified Installation, Retrofit and Programmability

New technology often comes with a dreaded steep learning curve and apprehension about its incorporation. To address those issues, stampers will want to look for servo-controlled hydraulic cushions with their own interfaces and touchscreen controls, independent of press controls. This standalone design eliminates difficulties in tying in with press controls. The cushions can be run by remote LCD control panels that operate via local or plant WiFi.

“Stampers will be able to control the cushion with a keypad,” Quander Jr. explains. “They can change heights, pre-accelerations and pressures. Another screen will display the force curves to allow users to see the movement of the system, cushion position and the force employed.”

For retrofits, such systems can be installed by tapping holes into the press bed to allow bolts to hold a cushion. A linear transducer is added to provide feedback on ram movement. The hydraulic power unit slides in and connects to the control system and it is ready to go. MF

 

See also: Hyson Products

Related Enterprise Zones: Tool & Die

 


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