3 Fundamentals of press design

Mechanical presses
Overload safety devices
The nominal force of the press is limited and safeguarded by the
hydraulic overload safety device in the slide. If the nominal press force
is exceeded, the pressure exerted on a hydraulic cushion integrated in
the pressure point is quickly relieved, allowing an overload displacement.
For blank holders used in double-acting mechanical presses, the overload
safety function is coupled to the adjustment function of the blank
holder force. The overload safety function is activated only when the
deflection of the four blank holder pressure points has exceeded a permissible
magnitude of approx. 3 mm. The safety function is actuated by
an air-hydraulic pressure scale. By changing the compressed air setting,
the blank holder force can be adjusted to the requirements of the die at
the four pressure points within a range of 40 to 100%.
Slide counterbalance
The slide weight is compensated by the slide counterbalance system.
Thus, the weight of all moving parts such as joints, connecting rods,
the slide and upper die, is balanced out by pneumatic cylinders
(Fig. 3.2.11). As a result, the drive system is largely free of gravity forces.
There are no stresses acting on the slide adjustment and there is an addi-
slide
adjustment
device
connecting rod
wrist pin
adjustment nut
overload bolt
oil cushion
pressure pad
adjustment spindle
adjustment device
Fig. 3.2.10 Spindle (left) and pressure point (right)
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
connecting rod
wrist pin
adjustment nut
adjustment pad
overload bolt
oil cushion
67