19.07.2005

Modular hydraulic cushion

Perfect draw

Göppingen - Schuler’s new modular hydraulic cushion improves the quality and, at the same time, reduces the complexity of the drawing process. In contrast to conventional systems, each cylinder of the modular cushion has complete functionality and carries out all active control and regulation functions of the drawing process. Previously, a separate actuator was required for each function. This new, extremely rigid and compact hydraulic system prevents vibration problems and has reduced the number of components. The system is therefore more reliable and easier to service and maintain.
At the same time, it achieves a highly dynamic control behavior. This provides the user with an extended force area and more precise blank holder force settings, adapted to the specific requirements of the workpiece. Softer motion sequences without hard mechanical impacts raise the reliability and productivity. Each cylinder module can be regarded as an independently functioning unit. The necessary drawing force is achieved by combining several units. Depending on the particular drawing process requirements, the modular cushion can be supplied in 1-, 2-, 4- or 6-point construction. An overriding control unit keeps the modules in sync with each other.

Design and components

The main components of the cushion include:

• Cylinder modules with main cylinder, control block and bubble memories
• Pressure pad
• 8-track guides
• Hydraulic unit
• Pipelines and control cabinets

The pressure pad transfers the force of the hydraulic cylinders to the pressure pins. The desired blank holder force is achieved by positioning the pressure pins as required. The pressure pad is led by an 8-track guide in the press bed. The cylinder modules are attached to stable cylinder bridges and transfer their forces to the bed with low and defined deflection.

Functions during the press stroke

The cushion has a considerable influence on part quality. Its most important function is the provision and flexible control of the holding force, with which the workpiece is fixed between the top die and the blank holder. The force must be exactly controlled to guarantee optimal material flow during the forming operation. The cushion therefore allows flexible adjustment of the blank holder force for every part during the drawing process. The use of a draw cushion in the lead-off press of a press line, for example, is particularly recommended for the production of high-quality parts in order to guarantee constant quality through reproducible production parameters.
The cushion carries out a number of functions over the entire slide stroke cycle of a press:

• Pre-acceleration in order to adapt to the slide speed,
• Pressure generation up to the required holding force,
• Drawing process with force progression which can be programmed over the entire draw depth and separately for each cylinder module,
• Retraction at BDC to prevent damage to the part,
• Part moved to pick-up position and controlled run-up to starting position.

Pre-acceleration

Pre-acceleration minimizes the impact when top die and blank holder meet. Shortly before the top die makes contact, the cushion is accelerated downwards and reaches about 70 to 90 percent of the slide speed. Adapting to the slide speed not only improves part quality, but also extends the service life of both press and die as well as reducing noise emission. The cushion control system calculates the required pre-acceleration based on slide kinematics, crank angle, stroke number and draw depth.

Pressure generation

The necessary drawing force is generated after the slide and top die have made contact with the blank holder. The elasticity of the overall system, and especially of the oil column in the lower cylinder area, determines the generation of the drawing force. Pressure generation is dependent on the drawing force and drawing path (height of the oil column). Pressure must always be generated, irrespective of whether the press uses pre-acceleration or not.

Drawing process

During the actual drawing process, the blank is formed between the top and bottom dies and held with a pre-determined force by the blank holder. The cushion provides this blank holder force by displacing oil out of the lower cylinder area. The process is controlled by pressure via one proportional valve per cylinder. This means that a pressure sensor constantly measures the oil pressure in the cylinder, compares it with the required value and opens the proportional valve accordingly. The blank holder force can either be kept constant or individually programmed for each cylinder in the form of force surges or constant force progression. Such force profiles can be individually programmed for each cylinder module with regard to the permissible pressure difference.

Retraction at BDC

The use of binders or ejectors in the top die can lead to a deformation of the fully drawn part, if the cushion springs back up when pressure is released at BDC. In order to effectively rule out this springback, the cushion is actively pulled down when it reaches BDC.

Pick-up position and run-up

The drawn part is stripped off the bottom die by the upward motion of the cushion and brought to the pick-up position. The timing and stroke of this motion to the pick-up position are programmable. Once the part has been removed from the die, the cushion returns to TDC in a controlled manner and without overshooting, ready for the next cycle (press stroke). The timing, or crank angle, of this run-up motion can also be programmed.

Summary

With the aid of a cushion, the blank holder force can be adapted precisely to the respective die and part requirements. The ability to program the force of each cylinder, independently of each other, along the drawing path is a decisive advantage. It enables the user to vary the force acting on the die over a wide range, by programming force surges and progressions.
The benefits:

• Exact control of blank holder force without stroke-dependent limitations
• Lower wear on die and press
• Less susceptible to breakdowns
• Extended service life of dies and press
• Same drawn part quality on single-action as on double-action presses