Retrofit R_xfor automaker's aging press

Modular hydraulic cushion upgrade eases maintenance, improves performance

By Ricardo Pereira

October 9, 2007

An automotive OEM's 4,000-ton transfer press produces parts with difficult draws, such as pickup door panels, in the 1,600-ton first station. Facing problems finding obsolete parts for repair and also considering crucial performance requirements in that first drawing operation, the automaker had Schuler Inc. upgrade the press with a modular hydraulic bed cushion.

Aging presses can pose challenges, from maintenance to unavailability of obsolete spare parts to performance limitations on new or complex applications. For metal forming processes requiring deep or off-center draw operations, a bed cushion retrofit can resolve some of these challenges.

Old Cushion, Old Technology

An automotive OEM's 4,000-ton transfer press produces parts with difficult draws, such as pickup door panels, in the 1,600-ton first station.

Tight production schedules require reliable performance to keep pace with expected assembly rates, but as the 15-year-old press's original draw cushion aged, the press began to require more downtime for cushion maintenance. It became increasingly difficult for the OEM to source spare parts that were no longer "shelf" items. Not only did the company face problems finding obsolete parts for repairs, but also crucial performance requirements in the draw press.

The automaker consulted press manufacturer Schuler Inc., Canton, Mich., to see if a press upgrade would decrease maintenance downtime and improve productivity.

Although it had not manufactured the press, Schuler was able to retrofit it with a modular hydraulic bed cushion. The modularity of the cushion makes it applicable for most press brands. The upgrade was achieved with standard shelf components and a simplified design for easier troubleshooting than on common older, complex cushion designs. The new design also resulted in a smaller footprint with ample accessibility.

Upgrade Avails Digital Servo Control

The OEM's new cushion has four-point control, providing precise blank-holding force to the die. The blank holder force is adjustable for each corner. The maximum force of each cylinder is 89 tons, with an available drawing stroke programmable from 2 to 10 inches.

The double-acting cylinders, control blocks, and encoders are servo-controlled in hydraulic modules (see Figure 1).

The die data is programmable with a digital multiprocessor control system. Set values for each cylinder are given as a table of positions and related forces through the operator control screen. Up to 10 force points are programmable, and the control carries out linear interpolation between the force points.

The automaker's operators program a range of production parameters using a touchscreen with go-to and teach functions. System status and component fault messages available at the control screen help them simplify troubleshooting.

All of the cylinder modules are identical, intended to simplify the retrofit and facilitate maintenance. Using the same components in each module also minimizes the number of spare parts required for the system.

With a block manifold design, piping is minimal (only four lines connect to the cylinder units), which reduces opportunity for leaks and the system size.

The entire system, including control cabinets and the hydraulic unit, was supplied as modules on eight skids (see Figure 2).

Taking Advantage of Full Functionality

Typically, conventional cushions require auxiliary mechanical and hydraulic accessories to provide additional functions such as a fixed pickup position or pull-down; the modular cushion design does not. Within one complete cycle of the press slide, the die cushion performs a number of functions:

• Preacceleration. Calculated by the control, the cushion is accelerated before contact with the upper die. This function is based on the parameters of slide speed, crank angle, stroke rate, and draw depth to minimize the impact of the upper die against the blank holder. The OEM has been able to get better part quality and longer service life of the dies, press, and cushion system.
• Pressure Buildup. Based on the required force for the application, the cushion force is generated after the slide and upper die contact the blank holder.
• Drawing. The cushion forces are individually programmable for each of the four cylinders over the entire drawing stroke. The desired blank-holding force can be achieved by using any desired pattern of pressure pins, with force transmitted from the pressure box to the pins via the controlled forces of the hydraulic cylinders.
• Pull-down at Bottom Dead Center. When using spring-loaded dies, the OEM selects the BDC function to assist in preventing upward resilience, and upstroke into pickup and start positions.
• Pickup Position and Upstroke. The cushion lifts the part to the pickup position of the transfer and then the part is removed from the die. Then the cushion performs a controlled upstroke to starting position for the next press cycle. Both functions are programmable to optimize the motion sequence.

In operation for about a year, the new bed cushion has the maintenance department smiling and the daily production reports looking good. The OEM achieved its goal to reduce expensive and time-consuming repairs while making complex, quality parts with this upgrade.