3 Fundamentals of press design

Hydraulic presses
pumps suspended in the oil tank, or they are mounted on supports in the
press foundation. The motors drive the pumps via an elastic clutch. The
pumps are supplied by suction pipes, and the hydraulic fluid is conveyed
to the valve control by means of high-pressure pipes. The valve control
system guides the hydraulic fluid to the appropriate units. Depending on
the function in question, the hydraulic cylinders are extended or retracted,
moved actively or passively in the press. The return flow is guided
back to the oil tank. The consumers, cylinders or hydraulic motors, move
the slide or cushions, or adjust spindles. They generate forces, lift loads or
clamp the dies. The service unitoperates in a bypass circuit away from the
main hydraulic flow. In this unit, the fluid is recirculated by means of filters
and coolers in a separate circuit, purifying and cooling it to ensure
that the press remains ready for production at all times.
Pressure accumulator drive system
If the pump conveys the fluid to a pressure accumulator, it is possible to
smooth out the energy delivery from the main electrical net. Large
quantities of hydraulic oil can be drawn from the pressure accumulator
for a short time, which has to be replenished by the pump before the
next press cycle. If the extraction of hydraulic oil is followed by a long
idle period, the accumulator can be supplied by a small pump – for
example when loading and unloading the die after retraction of the
slide. However, for processes requiring large quantities of hydraulic fluid
to be drawn in quick succession, a higher delivery power must be available
– for example a drawing operation followed by the opening of the
press. For the second example, a pressure accumulator-type drive system
is not suitable, since not only a large accumulator system but also
a large pump is required. Draw presses involve short cycle and transport
times which often cannot be achieved using an accumulator drive system.
Table 3.3.1 demonstrates additional differences between pump
and accumulator drive systems.
3.3.2 Hydraulic oil
Hydraulic systems in presses are operated using standard hydraulic oil
(HLP) according to DIN 51524 Part 2. Hydraulic oils are required to
comply with strict standards:
Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998
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