3 Fundamentals of press design

74 Fundamentals of press design
Mechanical presses operate according to the throughfeed principle,
and the output capacity is determined by the drive speed. In the case of
hydraulic presses, in contrast, the flow direction of the hydraulic medium
is reversed to close and open the press.
The cycle time is made up of a number of variable components,
whereby there is an optimum press setting value for each die set. The
total stroke, the drawing stroke, the forming force and blank holder
force are optimized in accordance with the dies. Each of these setting
values has an effect on the press cycle time, and thus on the press output.
The drive power is calculated on the basis of the ram displacement
versus time curve (Fig. 3.3.1) that is related to time constants of the
press.
T
stroke
[mm] press cycle
part transport time
stroke
acceleration
lowering in rapid tra erse
mm s
braking
drawing drawing
embossing
Fig. 3.3.1 Displacement-time diagram of a hydraulic press
braking
before T
return stroke
mm s
acceleration
pressure holding time pressure relief
and re ersal
,
time [s]
, t , t2 t
33 t
4
,7 t
5
,
press time T
cycle time T= , s+T+T
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
pro rated
transport time T