GB2053080A - Charging system for the fast- approach motion of hydraulic presses with a direct pump drive - Google Patents

Abstract

A charging system for the fast- approach motion of at least one operating piston of a hydraulic press with retraction cylinders 5 and with a low-pressure charging pump 16 which is connected via a non-return charging valve 17 parallel with the main press drive comprises variable-delivery pumps 6, 7 whose delivery direction is reversible. The low pressure charging pump 16 is driven by a hydraulic motor 15 which is fed, during the fast-approach motion, by the main pumps 6, 7 adapted to draw from the retraction cylinders 4, 5. <IMAGE>

Description

SPECIFICATION Charging system for the fast-approach motion of hydraulic presses with a direct pump drive The invention relates to a charging system for the fast-approach motion of at least one operating piston of a hydraulic press with retraction cylinders and with a charging pump which is arranged via a non-return valve parallel with the main press drive which comprises variable-delivery pumps with a reversible delivery direction.

To perform the forward motion without load and to perform the retraction motion, hydraulic presses are generally provided with charging devices which permit increased no-load speeds, the so-called fast-approach motion, to be obtained. Two different systems are used for charging the operating cylinder or cylinders, namely a) the charging tank system and b) the charging pump system.

The operating cylinder or cylinders in the charging tank system communicate(s) with a charging tank which is provided with a gas cushion and has sufficient internal pressure to ensure that acceleration of the pressure medium in the pipelines and overcoming of the pipeline and valve resistances is allowed for during the no-load descent motion of the press. During the retraction motion of the press, during which the operating cylinder or cylinders must be rapidly emptied, it is necessary to apply approximately twice the pressure in order to overcome the pipeline and valve resistances in the reverse direction in addition to the existing charging pressure.

The known charging pump system operates with low-pressure charging pumps, usually centrifugal pumps, instead of charging tanks. As in the first-mentioned system, the delivery rate and pressure of these pumps are designed so that the pipeline resistance and valve resistances can be overcome and acceleration forces can be provided for the fastapproach motion of the press. During the retracting motion, the pump is then bypassed by a discharge valve and it is again necessary for the pressure in the press cylinder to be sufficiently large to overcome the pipeline and valve resistances as well as the acceleration forces in addition to the pump charging pressure, which is applied to the non-return charging valve disposed in the delivery duct of the charging pump.

The relatively high pressure retraction force required to overcome the charging pressure is therefore a disadvantage in both known systems. In the charging tank system, there is the additional disadvantage that, due to the gas cushion in the fluid tank, it is possible for gases to enter into the hydraulic fluid and subsequently give rise to cavitation problems at other places. This disadvantage does not arise with the charging pump system but instead it is necessary to tolerate the fact that the centrifugal pumps, which are generally employed as charging pumps, have a very poor efficiency, tend to heat the fluid and are responsible for a substantial deterioration of the overall efficiency, more particularly if they are in continuous operation.

It is the object of the invention to improve the charging pump system under the assumption that the press is provided with a so-called direct pump drive. Proceeding from a charging system in accordance with the preamble of claim 1, it is the object of the invention to ensure functioning of the press during the retraction thereof with a press retraction force which is lower than that used hitherto.

To solve this problem, the invention proposes that the low-pressure charging pump is driven by a hydraulic motor, driven by the main pumps which draw from the retraction cylinders during the fast-approach motion.

This ensures that the low pressure charging pump is stationary and, during retraction of the press, it is not necessary to overcome an applied charging pressure from the pump when the operation of the main pumps is reversed to retraction into the retraction cylinders and the pressure medium displaced from the operating cylinder or cylinders is discharged. It would not be very convenient if an electric drive for the charging pump is constantly switched on and then switched off during the fast-approach motion in order to solve the problem on which the invention is based, because the switching frequency would be excessive. Moreover, a charging pump, which is driven by a hydraulic motor, stops instantly if the feed to the hydraulic pump ceases.

in a further embodiment of the charging system according to the invention, the main pumps with reversible delivery, already known, are connected to at least one operating cylinder as well as to the retraction cylinders for the movable press member; a directional valve with a control position, in which the hydraulic motor for the charging pump is connected to the delivery side of the main pumps which draw pressure medium from the retraction cylinders, is then interposed in the delivery duct which extends to the operating cylinder. The switching frequency of a directional valve can be relatively high without inpairing operational reliability.

It is regarded as particularly advantageous if the directional valve can be set into the control position in which the main pumps are connected for compressing the operating cylinder or cylinders and are isolated from the hydraulic motor by means of a control unit which responds to a pressure drop in the retraction cylinders. The said pressure drop in the retraction cylinders occurs when the work pieces contact the tool at the end of the fastapproach motion. This means, the control offers the advantage that press operation at the high pumping pressure commences automatically and directly after the fast-approach motion.

An unlockable non-return valve, which is connected parallel with the charging pump, is closed during the press operation and during the fast-approach motion and is open during the retraction motion, is recommended in order to bypass the charging pump. The excess amount of pressure medium which is discharged from the operating cylinder or cylinders during the retraction motion and cannot be supplied via the reversed main pumps to the retraction cylinders, can therefore be discharged.

In principle, the invention can be applied to all hydraulic single-cylinder or multi-cylinder presses, including those with floor-mounted drives and underfloor drives. The invention can also be applied to presses with a horizontal operating direction. If specific operating cylinders or groups thereof in a multi-cylinder press can be controlled by means of a preselector valve for setting different power stages, it will be necessary for an unlockable nonreturn valve to be provided parallel to the charging pump for each operating cylinder or each group of operating cylinders which can be controlled or activated, because as many branch ducts extending from the charging pump via a non-return charging valve must be provided as there are operating cylinders or groups of operating cylinders which can be optionally controlled.

One embodiment of the charging system according to the invention is shown in the drawing in conjuction with a single-cylinder press with a floor-mounted drive. The press comprises the press frame 1, the movable press member 2, the operating cylinder 3 and the two retraction cylinders 4 and 5. The main drive of the press comprises the two variable main pumps 6 and 7, whose delivery direction is reversible and whose output is designed in accordance with the required press operating speed. In the illustrated position of the press, the main pumps 6 and 7 draw pressure fluid from the unpressurized tank 8 via a non-return valve 9 and simultaneously from the retraction cylinders 4, 5 and deliver the fluid via a directional valve 10 and via the delivery duct 11 into the operating cylinder 3. The directional valve 10 will then be set into the control position I.

At the end of the press operation the delivery rate of the pumps 6 and 7 is reduced to zero in known manner by hydraulic or electric control elements so that the press is stopped.

At the same time, an unlockable non-return valve 1 2 is opened during the stopping phase so that the operating cylinder 3 is in direct communication via the opened non-return valve with the tank 8. The pressure medium for the directional valve 1 3 associated with the non-return valve 1 2 and required for reversing the said non-return valve is supplied from a separate pump 14.

Retraction of the press commences with the reversal of the delivery direction of the main pumps 6 and 7, which will then draw off part of the fluid from the operating cylinder 3 to deliver it to the retraction cylinders 4 and 5, while the excess quantity of fluid displaced from the operating cylinder 3, exceeding the requirements of the pumps 6 and 7, is discharged via the open non-return valve 1 2 into the tank 8.

The end of the retraction motion is again obtained by reduction of the delivery rate of the main pumps 6 and 7 to zero so that the press comes to a stop at its top dead centre.

To obtain subsequent fast-approach motion of the movable press part 2 under no-load conditions, the directional valve 10 is set into the control position II, so that, while the delivery direction of the pumps 6, 7 is reversed to lower the movable press part 2, the pressure medium, which is obtained by the main pumps 6, 7 from the retraction cylinders 4 and 5 and defines the speed of descent, is supplied to a hydraulic motor 1 5 which drives a charging pump 1 6. The charging pump 1 6 communicates via a non-return charging valve 1 7 with the delivery duct 11 which extends to the operating cylinder 3.The charging pump 1 6 supplies the pressure medium required for charing the operating cylinder 3 and the operating pressure is only as high as necessary to overcome the resistances offered by the valve 1 7 and by the pipeline.

The pressure required for driving the hydraulic motor 1 5 is applied partially by the prime movers of the main pumps 6 and 7, but partially also by the pressure produced by the dead-weight of the moving parts of the press in the retraction cylinders 4 and 5. This results in a partial recovery of the energy expended for the retraction of the press. At this point it should be mentioned, that with the directional valve 10 in control position ll, the charging pump 1 6 is driven only for the fast-approach motion but is otherwise stationary. In addition to the dead-weight of the moving parts, the force to be applied for the retraction of the press is confined to that required for ejecting the fluid from the operating cylinder 3 via the pipeline 11 and the unlockable non-return valve 1 2 into the tank 8. If the charging pump 1 6 is stationary, it will not produce any charging pressure which has to be overcome on the outlet side of the non-return valve 1 7.

The descent motion of the press in fastapproach operation is interrupted and the pressing operation commences as soon as the directional valve 10 is moved into the control position I. This control function can be per formed either mechanically or hydraulically or electrically in dependence on distance. However, it is preferable if the directional valve 10 is changed in dependence of pressure to which end a control device 18, adapted to respond to a pressure drop in the retraction cylinders 4, 5, is provided in the exemplified embodiment and communicates via ducts 1 9 and 20 with the retraction cylinders. This pressure drop in the retraction cylinders occurs if the tool 21 touches the workpiece, not shown, and the pumps 6, 7 draw unpressurized pressure medium from the tank 8 via the opening non-return valve 9. The active connection between the control apparatus 1 8 and the directional valve 10 is indicated by a control line 22. It will be understood that the directional valve 10 is set into the control position ll for the fast-approach motion by a separate control and is only set into the control position I via the control apparatus 1 8.

Setting the directional valve 10 into the control position I for the pressing operation also causes the hydraulic motor 1 5 to stop and delivery of the centrifugal pump 1 6 therefore ceases and accordingly the pump does not consume any energy during the press operation and the retraction motion.

Claims (5)

1. A charging system for the fast-approach motion of at least one operating piston of a hydraulic press with retraction cylinders and with a low-pressure charging pump which is connected via a non-return charging valve parallel with the main press drive comprising variable-delivery pumps whose delivery direction is reversible, wherein the low pressure charging pump is driven by a hydraulic motor which is fed, during the fast-approach motion, by the main pumps adapted to draw from the retraction cylinders.

2. A charging system as claimed in claim 1 wherein the main pumps of reversible delivery direction are connected in known manner to at least one operating cylinder and to the retraction cylinders for the movable press member and a directional valve, disposed in the delivery duct extending through the operating cylinder, has a control position in which the hydraulic motor for the charging pump is connected to the delivery side of the main pumps which draw pressure medium from the retraction cylinders.

3. A charging system as claimed in claim 2 wherein the directional valve can be set by means of a control apparatus, adapted to respond to a pressure drop in the retraction cylinders, to a control position in which the main pumps are connected to the operating cylinder for the pressing operation and are controlled by the hydraulic motor.

4. A charging pump as claimed in claim 1 wherein an unlockable non-return valve, connected parallel with the charging pump is closed for the pressing operation and for the fast-approach motion and is opened during the retraction motion to bypass the charging pump.

5. A charging system substantially as herein described with reference to and as illustrated in the accompanying drawings.