Page 20 - MetalForming June 2009

Mechanical-Press Design
achieve high stiffness. Here, slide adjust- ment is unnecessary.
Nominal press force is limited and safeguarded by a hydraulic overload safe- ty device in the slide. If this press force is exceeded, the pressure exerted on a hydraulic cushion integrated into the pres- sure point is quickly relieved, allowing an overload displacement. For blankholders used in double-acting mechanical presses, the overload safety function is coupled to the adjustment function of the blankhold- er force. The overload safety function is activated only when the deflection of the four blankholder pressure points exceeds a permissible magnitude of approximate- ly 3 mm. The safety function is actuated by an air-hydraulic pressure scale. By chang- ing the compressed-air setting, blankhold- er force can be adjusted to the require- ments of the die at the four pressure points, within a range of 40 to 100 percent.
Slide weight is compensated for by a slide counterbalance system. Pneumatic cylinders balance the weight of all mov- ing parts—joints, connecting rods, slide
and upper die. As a result, the drive sys- tem is largely free of gravity forces—no stresses act on the slide adjustment, and there is an additional safeguard against downward travel of the slide caused by gravity. Pneumatic weight compensa- tion guarantees low-noise vibration-free operation, smooth loading of the motor and short braking distances.
To counterbalance weights to 200 tons at 10-bar air pressure, cylinder size must be as great as 900-mm dia. Long slide strokes, to 1300 mm, and permis- sible pressure spikes to 25 percent require substantial surge tanks. The arrangement of cylinders and tanks depends on the type of press and safe- ty issues. In the case of counterbalance cylinders not externally enclosed, dou- ble piston rods often are required to safeguard against the risk of breakage. Conversely, integrating the system into the press crown makes this additional safety feature unnecessary.
With every die change, the weight- compensation system must be auto-
matically adjusted to the new die weights. Program-controlled automat- ic systems used in transfer presses auto- matically set the program inherent pres- sure value when changing dies, to minimize setup time.
Pneumatic & Hydraulic Systems
The major pneumatic functions of mechanical presses are the clutch, brake, flywheel brake, slide counterbalance, slide cushion and bed cushion. With transfer presses, pneumatic functions also include the application of air pres- sure to the transfer cam followers and workpiece transport by suction cups. Large volumes of air require compressed air tanks with a monitoring safety func- tion and drainage system. The maxi- mum pneumatic pressure is generally 6 bar (90 psi), although high-pressure networks or compressors producing a maximum of 16 bar (230 psi) increas- ingly find use.
The use of hydraulics in mechanical- press construction is relatively minimal,
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