Some modern industrial hydraulic systems use closed-loop, variable-speed motors to drive fixed-displacement pumps, instead of using fixed-speed motors to drive variable-displacement pumps. Simple systems just use the motor to vary the flow from the pump in response to load demands. As with conventional HPUs, this setup could provide hydraulic power to several circuits. The higher-end systems use a pump driven by a bidirectional servomotor. The combination is called a servopump. The powerful motors have such quick response that they can drive a cylinder or motor directly — without having to go through a directional or servovalve. When the system idles, the drive becomes a torque motor, producing just enough flow to compensate for case drainage. Flow, direction, and pressure are monitored and controlled throughout machine cycles.
An application of this modern concept is Voith’s CSH die-cushion drive, which consists of a servopump and a double-acting die cushion mounted underneath the bolster plate. Lying at the heart of the drive is the servopump’s internal-gear pump that has high volumetric efficiency, permitting highly accurate control. This innovative drive for demanding forming processes needs no valves and reduces energy consumption of the press significantly.
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With conventional die-cushion drives that combine both a die cushion and pressure-relief valve, the die cushion’s work will become irretrievably lost. In relatively stark contrast, the Voith CSH system feeds the mechanical power of the servopump back to the process, through the motor, in the form of electric power.
This ultimately reduces energy consumption and can reach overall energy-efficiency levels that were previously unattainable. In addition, the servo-hybrid CSH die-cushion drive utilizes a smaller oil reservoir and less oil, and it doesn’t require any cooling.
Furthermore, the CSH is freely programmable in terms of speed, force and position control, and acceleration — making the drive particularly applicable to forming processes involving complex geometries. The accelerated die cushion adjusts to the position-time curve of the ram and, in contrast to a single-acting die cushion, reduces tool impact on the workpiece. This helps improve surface quality, protects both the machine and tool, and prevents damage such as cracks and creases on the workpieces.
Voith Turbo GmbH is located in Crailsheim, Germany.