Page 17 - MetalForming June 2009

where a high-protection class rating is called for. The output of a press, deter- mined by force, slide displacement and speed, including lost energy, is provid- ed by the main motor. Periodically occurring load peaks are compensated by the flywheel, which stores energy. The highly elastic DC drive system can compensate for a drop of the flywheel speed of as much as 20 percent in every press stroke and can replace the con- sumed energy by acceleration of the flywheel prior to the subsequent stroke.
Useful energy also should be avail- able during setup with a reduced stroke rate, 5 strokes/min. for example. In this case, a speed drop of 50 percent is admissible. An important criterion in configuring the energy balance is to ensure a short run-up time. Running up a large-panel press from its minimum to maximum production stroke rate gen- erally takes less than 1 min. under load.
Due to the required useful energy and the permissible flywheel slowdown of 20 percent, the flywheel generally is designed to operate under the most ener- gy-consuming condition—in the lower stroking range of the press. Although a high flywheel speed is beneficial, this is limited by the admissible speed of the clutch, brake and flywheel itself.
Clutch and Brake
Mechanical presses use a clutch to transmit motor and flywheel torque to the gear shaft. After clutch release, a brake decelerates the slide, the top die and the gear. Particularly when working in single-stroke mode, the masses in translational or rotational motion must be brought to a standstill after every stroke within an extremely short time: 200 to 300 msec. for large-panel press- es, 100 to 150 msec. in universal press- es. Conversely, after engaging the clutch, the same masses must be accelerated from zero to operating speed.
For safety reasons, braking is generat- ed mechanically by spring power. Clutch torque is calculated from the nominal press force and the required working dis- tance, generally 13 to 25 mm above BDC.
Pneumatic single-disk clutch and brake combinations with minimum
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