US 3559247 A
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Feb. 2, 1971 I P. LARSSON 3,559,247
' MEANS FOR HYDRAULIC PRESSES Filed March 18, 1968 mvsuron. PAU L LA KS5 0A! United States Patent 3,559,247 MEANS FOR HYDRAULIC PRESSES Paul Larsson, Vasteras, Sweden, assignor to Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden Filed Mar. 18, 1968, Ser. No. 713,910 Claims priority, application Sweden, Mar. 22, 1967, 3,999/67 Int. Cl. B29c 3/00 U.S. Cl. 18-32 7 Claims ABSTRACT OF THE DISCLOSURE In hydraulic presses, preferably powder presses, there are arranged position measuring means, measuring the movements of upper and lower press heads or movements of inbuilt tools. The outputs of said means are modified in quota means and are combined into a comparison signal for controlling a hydraulic valve to control the movements of the heads.
BACKGROUND OF THE INVENTION (1) Field of the invention The present invention relates to a means for hydraulic presses, preferably powder presses, to control their press head movement and that of inbuilt tools.
(2) The prior art body, as well as movement of the powder material in the tool before it is compressed, and it is difiicult to avoid cracks in the material.
SUMMARY OF THE INVENTION The invention provides a solution of this and other similar problems and is characterised in that movements of both the upper and the lower press heads are sensed by individual position sensing means, the output signals of which, after modification in a quota means, are combined to a comparison signal arranged to control a hydraulic valve for checking the press head movement. Such an arrangement, in the case of cylindrical powder bodies, enables the mentioned weak point and other Weaknesses in the final product to be avoided by giving the press head movements a ratio other than 1:2. A certain required movement ratio can be set between the two press heads and, for example during pressing, gradual or linear adjustment of the movement ratio to the desired value can be automatically obtained. With both cylindrical and more irregular or conical bodies, it is thus possible, by correctly setting the movement ratio, to obtain compactness without cracks as well as the movement of the powder material before compression takes place.
In a preferred embodiment of the invention one press head is balanced, preferably the lower, by one or more hydraulic cylinders, the outlets of which are substantially throttled by the hydraulic valve. By means of suitable position sensing means, such as potentiometers, and suitable quota means, a potentiometer or a device according to the Swedish patent application No. 16,452/ 65 it is easy to obtain the desired adjustment signal for the hydraulic valve and thus the desired constant or variable movement ratio between upper and lower press heads in order to obtain good quality in the finished product.
BRIEF DESCRIPTION OF THE DRAWINGS The drawings show schematically a press control mechanism embodying the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is exemplified in the accompanying drawing which shows a powder press with an upper movable press head 11 and a lower movable press head 12. In the upper press head 11 is a pressure piston 13, arranged to be pressed down into a mold with a cavity 14 in the power press head 12. Powder material, for instance metal powder is poured into the mold. The lower press head 12 is balanced by two (or some other number) hydraulic cylinders 15 and the powder material in the lower press head 12 is pressed against a stamp 16 in the base. Two pistons (plungers) 17 from the lower press head 12 are inserted and sealed with respect to said balancing cylinders 15.
The control equipment used is in principle constructed as a position-servo means where the guiding magnitude is the position of the upper press head 11 and the controlling magnitude is the position of the lower press head 12. The position of the upper press head is sensed in a suitable manner, for example by means of a potentiometer 18, by photo-electric cells, or the like, and output signals from the potentiometer 18 will thus be a measurement of the position of the upper press head 11. The position of the lower press head is sensed in a similar way, for example by means of a potentiometer 19, and the output signal from this potentiometer will be a measurement of the position of the lower press head 12. In order to obtain the desired ratio between the positions of the upper and lower press heads, in series with one of the potentiometers 18 a second potentiometer 20 is connected and the output signals from this potentiometer will thus be k-l, where k is a certain factor set in the potentiometer 20 and if the output signals from potentiometers 20 and 19 are compared, a comparison signal proportional to said factor k is obtained. The device 20 is called a quota means.
Instead of making the quota means 20 as a potentiometer this means can be arranged with a radiation source and a radiation-sensitive emitter, between which is placed a curved screen of opaque material which influences the output signal from the emitter. The position of the curve is made dependent on the position of one of the press heads and it may be a straight line or be more complicated depending on the desired quota. The latter means is described in Swedish patent application No. 16,452/ 65.
Comparison signals are supplied to an integrating amplifier 21, the output side of which is connected to a regulator of suitable type, for example of the type described in Swedish Pat. No. 170,280. By means of a current coil 22 connected to said amplifier 21 the supply of pressure medium to a hydraulic valve 23 connected to the outlet from the hydraulic balancing cylinders 15 is controlled.
For more complicated shapes, the various press heads may be included in a larger tool and for each alteration in area of the body a new movable lower press head can be arranged and its position compared with that of the upper press head (not shown).
If, during the pressing, the friction of the powder against the walls of the cavity 14 in the tool is not sufiicient, additional pressure is exerted on the powder by auxiliary pressure cylinders 24. The press head can be lifted in a similar manner. The normal pressure is obtained in a main cylinder 25 in a manner known per se.
3 By means of boundary position stops or similar limiting means, the movements of the press heads can be limited as desired.
The device operates in the following way:
Powder is poured into the mold cavity 14 and pressure is applied on the upper press head 11 in the pressure chamber 25. If there were no control equipment and the powder body and thus the cavity were cylindrical, the lower balanced press head 12 would be pressed down half as far as the upper press head. However, on the potentiometer 20 the desired ratio, possibly adjustable during the operation, has been set between the movements of the upper and lower press heads, and even with more complicated bodies an output signal from the amplifier 21 is obtained which corresponds to the deviation from the desired, preset ratio. The hydraulic valve 23 is adjusted in accordance with this value and the balancing pressure in the cylinders is altered in such a way that the movement of the lower press head will be adjusted in relation to that of the upper press head to achieved the desired, preset ratio.
With more complicated bodies, guide equipment can be arranged and a movable lower press head within a larger tool for each alteration in area. As mentioned, the ratio can be set at a constant or variable value according to the shape and material of the powder body.
The device described above can of course be used in other types of pressing than powder pressing and can be varied in many ways within the scope of the following claims.
1. In a hydraulic press with inbuilt and movable upper and inbuilt and movable lower press parts, means to control the movements of these parts, comprising sensing means for sensing the movements of the upper part and sensing means for sensing the movements of the lower part, quota means, the output side of one of said sensing means being coupled to said quota means, to furnish an input signal thereto, said quota means including means to multiply the input signal by a constant K, whereby the output signal of said quota means is the product of said constant and the input signal, means to combine the output signal of said quota means and the output signal from the other sensing means, without passing through said quota means, to form a comparison signal, a hydraulic valve for controlling the feed rate of pressure medium to at least one of the movable press parts, control means for said valve, and means to furnish said comparison signal to said control means.
2. In a press according to claim 1, hydraulic means to balance the lower press parts, the pressure of which is balanced by said hydraulic valve.
3. In a press according to claim 1, the position sensing means comprising otentiometers.
4. In a press according to claim 1, the quota means comprising a potentiometer series-connected to one of the position sensing means.
5. In a press according to claim 1, the quota means comprising a source of radiation, a radiation sensing emitter and a radiation screening device movably arranged between the source and the emitter.
6. In a press according to claim 1, the upper press head being balanced and carrying press tools.
7. In a press according to claim 1, having associated with the upper press head a position measuring potentiometer, which is coupled to a potentiometer in the quota means, the output of which is at least indirectly connected to another position measuring potentiometer in order to provide a comparison signal which is fed to an integrating amplifier the output of which controls the feed of pressure medium to the hydraulic valve.
References Cited UNITED STATES PATENTS 2,926,412 3/1960 Stacy et al. 18l6(c) 3,190,261 6/1965 Ziffer 182(i) 3,401,425 9/1968 Fink 18--17(c) H. A. KILBY, JR., Primary Examiner US. Cl. X.R. 182