Method and apparatus for controlling a press plunger system

A drive system for use in a press in which a pneumatic cylinder and hydraulic pressure transformer are coupled together and connected by separate valves to respective gas and hydraulic sources. The valves are controlled by position transducers arranged along the path of movement of the output member of the transformer.

The present invention relates to a method of controlling the mechanical 
force or power produced by pneumatic or hydraulic means in a piston system 
comprising at least two cylinders, by selectively applying pressure to one 
or more pistons and by controlling the pressure of the medium used within 
the piston system. 
In addition to mechanical presses and purely hydraulically operated 
presses, pneumatic-hydraulic presses have been developed in recent years, 
which presses utilize compressed air as their motive energy, whereby the 
mechanical power provided by the compressed air is multiplied in a 
hydraulic converter or transducer portion. 
In the conventional press force multiplier shown in FIG. 1, the pressure 
p.sub.1 of the compressed air acts upon one or more pistons (1) arranged 
in series and the mechanical force or power of which is transmitted to a 
plunger piston (2) such that an oil pressure p.sub.2 is applied to a 
hydraulic piston (4) in the transducer portion. With increasing 
transmission ratio, the power increases and the stroke decreases in 
correspondence with the relation 
EQU F.sub.2 =U.multidot.F.sub.1 ; S.sub.2 =S.sub.1 /U; F.sub.1 
.multidot.S.sub.1 =F.sub.2 .multidot.S.sub.2 
wherein 
F.sub.2 =force developed by the press plunger (ram) 
F.sub.1 =force produced by the plunger piston 
U=transmission ratio 
S.sub.1 =stroke of the plunger piston 
S.sub.2 =stroke of the press plunger (ram). 
The use of compressed air as an elastic medium is advantageous when sudden 
and excessive loads are involved. Further, the force exerted by the 
compressed air can be readily conformed to the required shaping techniques 
(rapid impact--slow impact), and this force additionally lends itself to 
easy control by control elements provided in the hydraulic transducer 
portion. Still further and mainly, compressed air is storable and harmless 
to the environment, and compressed air is available at least in medium and 
large manufacturing plants. Add to this that compressed air permits a high 
frequency of strokes (e.g., 100 strokes per minute). On the other hand, 
the compression pressure of normal compressed air installations is too low 
to be useful for purely pneumatically operated presses. 
However, existing compressed air installations and conventional air 
pressure accumulators in general cannot be used for the afore mentioned 
pneumatic-hydraulic presses since the air flow capacity of such compressed 
air installations is too low. This applies not only to the capacity of the 
air compressor as such, but also, for example, to the cross-sectional 
areas and other parameters of the pipeline systems. Accordingly, it is a 
primary object of the present invention to provide a method and an 
apparatus which permit use of the abovementioned advantages of the 
combination of hydraulic presses with compressed air motive means in a 
structurally simple manner even if only a low or limited quantity of 
compressed air is available. This holds true particularly for medium to 
high press capacities and with respect to achieving particularly favorable 
force/travel diagrams in which the starting and terminal points of two 
lengths of travel are adapted to be set independently of each other, 
wherein two different forces are effective through such distances. 
In a method of the type as outlined at the beginning, the improvement 
provided by the present invention resides in the feature that the starting 
and terminal points of supplying two different pressurized media to at 
least two faces of jointly acting pistons are controlled independently by 
means of control elements the actuating members of which are adapted to be 
adjusted in the direction of plunger movement along the power stroke. 
A further object of the present invention is to provide pneumatic-hydraulic 
driving means for producing a variable output force, comprising a 
plurality of coaxial cylinder chambers of identical or different diameters 
slidably receiving pistons therein, the forces of different magnitudes of 
said pistons being produced through jointly acting piston rods either in 
timed succession or simultaneously, whereby the net output force produced 
is adapted to be varied either incrementally by selectively disconnecting 
and connecting individual ones of the cylinder chambers, or continuously 
by varying the pressures of the pressurized working fluids. The driving 
means in accordance with the present invention is characterized in that 
control elements cooperating with the output member of the driving means 
are distributed along the path thereof so as to be adjustable to any 
desired distance, said control elements initiating and terminating the 
supply of pressure fluids to the pistons, two different pressure fluids 
being controlled independently to correspondingly pressurize two faces of 
two jointly acting pistons. 
With the facilities provided by the present invention, pneumatic-hydraulic 
presses may be operated by a substantially smaller volume of air than 
prior art presses of the pneumatic hydraulic type. Thus, existing standard 
pressure air installations may be used. The interposition of an auxiliary 
hydraulic power source as taught by the present invention also allows 
minimizing pressure air since the positioning of the press plunger (ram) 
is effected by hydraulic oil instead of pressure air, while the pressure 
air is used only when the advantages offered thereby become particularly 
important, i.e., in the phase of shaping or deforming a workpiece. In this 
way, the consumption of compressed air is reduced to the phase of shaping 
the workpiece. 
From the law of energy conservation, the demand of pressure air is 
calculated as: 
EQU V=F.multidot.s/p, in liters per stroke 
wherein 
F=output force, e.g., in Mp 
s=travel of output member (e.g., in m) 
p=air pressure (e.g., in bar). If the air pressure p is considered a 
constant parameter, the air demand is directly proportional to the force F 
provided by the output member and the travel s thereof. According to the 
present invention, the reduction of the quantity of air required is 
achieved by splitting up either the force F to be applied or the travel s, 
or both factors, into a sum of a pair of separate quantities each; more 
particularly one term of the sum is always produced by pneumatic means and 
the other term of the sum is produced by hydraulic means. 
In a preferred embodiment of the present invention the control elements 
comprise initiators including an activating member assembly thereof 
forming part of a transducer as cooperating with a transducer connected to 
the output member the stationary portions of the transducers being adapted 
to be adjusted and fixed on a stationary bracket extending parallel to the 
axis of the output member. Pulses provided by the transducer arrangement 
are amplified and subsequently transmitted to servo-valves connected into 
the pressure fluid supply lines. 
The driving means according to the present invention allows control of the 
force provided by the output member, throughout the power stroke of the 
press, using a parallel or a series arrangement of pistons and associated 
cylinders. 
Furthermore, if a series arrangement of the pneumatic piston/cylinder units 
is used, one of the two power sources may be connected to a plurality of 
auxiliary hydraulic cylinders adapted to be controlled independently of 
each other.

As shown in FIG. 2, press drive means are operated by the pressure P.sub.1 
of a pressure air source 10. Plunger piston 2 (provides an effective force 
F.sub.1) provided by a plurality of serially arranged pneumatic pistons 1, 
which plunger piston produces a pressure P.sub.2 in an input chamber of a 
hydraulic pressure converter portion 3. A hydraulic piston 4 of the 
converter portion 3 is exposed to the pressure P.sub.2 of hydraulic liquid 
9 (e.g., oil). The piston 4 is directly connected to a press plunger 7 
providing a press force F.sub.2. On the opposite side of the hydraulic 
piston 4, an oil chamber 5 is provided which is connected to a control 
element 6 for controlling the movement or stroke (S.sub.2) of the press 
plunger 7. 
In the embodiment of the invention shown in FIG. 2, an auxiliary hydraulic 
power source comprising a hydraulic pump 8 is provided serving to move the 
piston 4 through a first portion of its stroke, wherein the press plunger 
2 is moved into engagement with the workpiece. To this end a third path 
between pump 8 and the inlet chamber of the converter portion 3 is 
established by control valve 12 and line 13. Subsequently, the actual 
press stroke, which is the second portion of the stroke of piston 4, is 
initiated by connecting the pressure air source 10 to the pneumatic 
pistons 1. The required volume of pressure air is reduced with respect to 
exclusive pressure air drive means by a factor corresponding to the second 
portion of the stroke of piston 4 over the total stroke thereof. Below the 
first portion of the stroke of piston 4 will be referred to also by the 
term positioning stroke. 
Further, the motive forces exerted by the hydraulic pump 8 are controlled 
by means of a return line 14 including a further control valve 15 and a 
pressure limiting valve 16. Furthermore, the oil chamber 5 is also 
connectable to pump 8 and the return line 14 via a control valve 6 and 
lines 17 and 18 respectively. An oil reservoir 19 is connected to pump 8 
via a check valve 20. A pressure relief valve 21 is connected between the 
feed side of pump 8 and reservoir 10. Control valves 12, 6 and 15 may be 
controlled by a central control unit (not shown). 
In an alternate embodiment shown in FIG. 3 the stroke of press plunger 7 is 
constant, since no hydraulic fluid is supplied to the inlet chamber of the 
converter portion 3. The net output force produced is obtained both from 
the hydraulic and the pneumatic system. Thus the system is a force adding 
one, while the system of FIG. 2 is a stroke adding one. The pressure oil 
is supplied by the hydraulic pump 8 through a control valve 23 and a power 
line 24 and acts upon an auxiliary plunger piston 22 connected to that 
side of the pneumatic piston 1 being remote from the press plunger 7. 
Thus, the pneumatic pressure and the pressure produced by the hydraulic 
pump act in tandem and simultaneously such that their effects sum up 
mechanically through the common piston rod to be finally applied to the 
plunger piston 2. The effective cross-sectional areas of both plunger 
pistons 2 and 22 may be chosen equal or different from each other. In a 
press drive means as shown in FIG. 3, the operating characteristics can be 
calculated as follows: 
##EQU1## 
wherein: 
F=force provided by the press plunger 
p=pressure of hydraulic liquid 9 
p.sub.1 =pneumatic pressure 
p.sub.2 =feed pressure of pump 8 
A.sub.1 =cross-sectional area of the various pneumatic pistons 1 
A'.sub.2 =cross-sectional area of plunger piston 2 
A".sub.2 =cross sectional area of the auxiliary plunger piston 22 
A.sub.3 =cross-sectional area of piston 4 
s.sub.1 =stroke of piston 1 
s.sub.2 =stroke of hydraulic piston 4. 
The value of the pressure air consumption appears in the square brackets of 
equation (II). 
The embodiment of FIG. 3 allows free selection of the relative 
contributions of the pneumatic and hydraulic energy sources, providing the 
output force F may be varied via the ratio of the contribution of either 
pressure source to the output power. 
In addition, the elasticity of the pressure force F produced is varied by 
the ratio of combination of both types of motive power. Therefore, in 
accordance with this invention it is possible to conform the elastic 
characteristic of the press to the specific working properties of the 
material and to the kind of forming used (drawing, cutting, stamping, 
etc.). 
The press drive means shown in FIG. 4 permits dividing both the power 
stroke (s=s.sub.1 +s.sub.2) and the force (F=F.sub.1 +F.sub.2) between the 
pneumatic and the hydraulic systems. The positioning stroke s.sub.1 is 
effected by one of a plurality of hydraulic pumps 8 through line 13 and 
control valve 12. The power stroke s.sub.2 of the press as such is 
assisted by the auxiliary plunger piston 22, receiving pressure oil 
through control valve 23 and power line 24. Thus the forces provided by 
plunger 22 and the pistons are added. So the embodiment of FIG. 4 provides 
for maximum economy in the consumption of pressure air. 
FIG. 5 shows further modified press drive means, wherein the contributions 
of the pneumatic and the hydraulic system to the net output power may be 
varied in accordance with the travel of the press plunger. Also in this 
system we have: 
EQU E=E.sub.1 +E.sub.2 
, wherein: 
E=total energy 
E.sub.1 =pneumatic energy 
E.sub.2 =hydraulic energy. 
If the stroke s is considered constant, the above equations may be 
rewritten 
EQU F=F.sub.1 +F.sub.2 
, wherein: 
F=total output force 
F.sub.1 =portion of the output force produced by the pneumatic system 
F.sub.2 =portion of the output force produced by the hydraulic system. 
In force adding drive systems as shown in FIG. 3 and FIG. 5, as well as in 
the force and stroke adding drive system of FIG. 4 providing of the force 
components F.sub.1 and F.sub.2 can be controlled, in accordance with 
travel of the press plunger. This will now be explained in more detail 
referring to FIGS. 5 to 13. 
In the pneumatic-hydraulic drive system of FIG. 5, three pneumatic 
cylinders 1' and two auxiliary hydraulic cylinders 23' and 24' are 
positioned in coaxial relation, whereby the mechanical forces produced in 
the pneumatic and hydraulic systems add to supply cumulative force to 
plunger 25 which in the embodiment shown is formed as a plunger piston 
having a hydraulic pressure converter 26 connected in series thereto. 
Pneumatic energy E.sub.1 and hydraulic energy E.sub.2 are fed separately at 
each of the cylinders 1' through electrically operated 3/2-way valves 27, 
28, 29 and 30, 31, respectively. With this arrangement, the following 
effects are obtained. 
(1) The maximum plunger force F=F.sub.1 +F.sub.2 produced by the pneumatic 
bias pressure p.sub.1 and the hydraulic bias pressure p.sub.2 may be 
reduced incrementally by blocking the pressure fluid supply to individual 
ones of the cylinders 1', 23', 24'. 
(2) With the hydraulic energy supply being completely blocked by valves 30 
and 31, the drive system will operate in a purely pneumatical mode; or 
alternatively, by fully blocking the pneumatic energy (power) supply 
(valves 27, 28, 29), the press motive means operate in a purely 
hydraulical mode. 
(3) By controlling the above-mentioned valves as a function of travel of 
plunger 25, both switching on and off of the pneumatic and/or hydraulic 
power may be effected as required (compare the diagrams of FIGS. 9 to 13). 
In addition to the incremental control of the force F provided by plunger 
37 as indicated above in item 1, this force may be controlled continuously 
by setting the switching point of pressure relief valves 32, 33 and 34 to 
values smaller than the maximum value of p.sub.1 and p.sub.2, such that, 
by making use of both incremental and continuous control, any desired 
value F&lt;F max. may be obtained. 
In the drive system of FIGS. 5 and 6, electrical actuation of the 3/2-way 
valves 27 to 31 in accordance with travel of plunger 37 (see supra item 3) 
is effected by contact free position transducers 35 adapted to be adjusted 
along the path of plunger 37, which can be replaced by mechanically 
operated limit switches if desired. Actuating pins 36 are secured to a 
hammer plate 38 secured to the press plunger 37. The transducers 35 are 
adjustably carried by brackets 39 fixed to a press frame 40. Transducers 
41 and 42 shown in FIG. 6 provide electrical command signals used in 
switching on and off the pneumatic pressure, while transducers 43 and 44 
control the supply of hydraulic power. With the transducer arrangement of 
FIG. 6 the force/travel diagram of FIG. 9 is obtained. 
Pneumatic power is supplied by a compressor, the operating pressure 
normally being set to 6 bar. Hydraulic energy is supplied by a hydraulic 
pump 45. The pressure oil is supplied to the hydraulic cylinders 23' and 
24' via a 4/2-way valve 46. A pressure relief valve 47 is provided for 
returning excess pressure oil. To initiate a return stroke, valve 46 is 
energized such that the pressure oil provided by pump 45 is directed to 
the lower face 49 of power piston 50 via line 48. At the same time, the 
3/2-way valves 27, 28 and 29 of the pneumatic system are switched into the 
vent position, and the 3/2-way valves 30 and 31 of the hydraulic system 
are moved into their vent positions wherein a fluid connection between the 
cylinders 23' and 24' and a return line 51 is established. Numerals I, II 
and III in FIGS. 7 to 13 indicate various points of travel of plunger 37 
where switching on or off at one of the pressure fluid sources is 
effected.