3 Fundamentals of press design

54 Fundamentals of press design
over small distances, for instance during blanking or coining. The great
danger is that such overloading may go undetected. For this reason, overloading
safety devices must be used to protect the press (cf.Sect. 3.2.8).
Another form of overloading results from taking too much energy
from the flywheel. Such overloading can result in extremely high press
forces if the displacement during deformation is too small. However, if
the energy is applied over a large displacement, this type of overload is
much less dangerous. For example, if the press described above is
brought to a stop during a working distance of h = 100mm, the entire
flywheel energy of W = 15,600 Nm is utilized. Assuming that no peak
loads have occurred, the mean press force exerted is only
F = W/ h = 15, 600 Nm/ 01 . m = 156, 000 N =
156 kN
The press is therefore by no means overloaded although the flywheel
has been brought to a standstill. In this case, only the drive motor
suffers a very large slowdown and it is necessary to use a press of a larger
flywheel energy capacity, although the permissible press force of 1,000 kN
is more than adequate. Overloads of this type occur more frequently if
deformation is done over a large distance, e. g. during deep drawing, open
or closed die extrusion.
3.2.2 Types of drive system
Eccentric or crank drive
For a long time, eccentric or crank drive systems were the only type of
drive mechanisms used in mechanical presses. The sinusoidal slide displacement
of an eccentric press is seen in (Fig.3.2.3).The relatively high
impact speed on die closure and the reduction of slide speed during the
forming processes are drawbacks which often preclude the use of this
type of press for deep drawing at high stroking rates.
However, in presses with capacities up to a nominal force of 5,000 kN,
such as universal or blanking presses, eccentric or crank drive is still the
most effective drive system. This is especially true when using automated
systems where the eccentric drive offers a good compromise between
time necessary for processing and that required for part transport. Even
in the latest crossbar transfer presses, eccentric drive systems used in
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998