GB1558437A - Shock prevention means - Google Patents

Description

(54) SHOCK PREVENTION MEANS (71) We, L. SCHULER GMBH, a body corporate organised under the laws of West Germany of 7320 Göppingen, West Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a device for preventing cutting shock in blanking presses.

In a previously known device of this type (German laid open specification No.

2 350 378) the pressure medium chamber of the cylinder/piston unit is connected to a high pressure pressure medium source and a pressure limiting valve is provided on the outlet side. The vertical position of the impact surface of the cylinder piston unit is adjustable relative to the press ram so that the press ram comes into operative association with the impact surface of the cylinder piston unit after the cut has been made and as break-through commences.

Due to the fact that the pressure medium chamber of the cylinder piston unit is connected to the high pressure pressure medium source, the force required to prevent cutting shock, which corresponds to the cutting force of the press, is fully available immediately at the commencement of the breakthrough by the top tool through the material to be cut. A shock-like relief of the press frame and also of the driving parts of the press is prevented in this case.

The pressure limiting valve now ensures that the required pressure, and hence the force prescribed by the cutting force, is still maintained in the pressure medium chamber of the cylinder piston unit until a relief valve is opened at a point which is a function of the ram travel after the breach, whereby the press is relieved continuously and silently. During the return stroke of the press up to the commencement of a fresh cutting operation, the piston of the cylinder piston unit and hence the impact surface is returned into the initial position by the high pressure pressure medium source. The apparatus is then once more ready for use.

It is an object of the present invention to improve the reliability of the known device in its operation and to provide for adjustment of the pressure in the pressure medium chamber in order to cater for differences in the cutting forces employed which result from variations in the material strength of the work-pieces to be cut.

The present invention provides a device for preventing cutting shock in a blanking press comprising at least one cylinder/piston unit disposable, in use, between a tool base plate and a press ram of said blanking press so that an impact surface thereof is positionally adjustable relative to the press ram, the stroke of the piston of said cylinder/piston unit being limited at one end by the interior end wall of the cylinder opposite a facing end of the piston, and a pressure chamber between said interior end wall of the cylinder and the facing piston end being connected on an inlet side via a non-return valve to a high pressure pressure medium source and on an outlet side to a pressure limiting valve provided with a control chamber in communication with a high pressure pressure medium source, the pressure in said control chamber being adjustable by means of a pressure control valve in a control pipe connected to the control chamber.

Desirably where more than one cylinder piston unit is provided the adjustment of their operative pressures is effected by means of a central pressure control valve.

It is further ensured by said control of the operative pressure in the cylinder/piston units by a central pressure control valve, that by means of a single adjustment the operative pressure and hence the reaction force (corresponding to the cutting force) required to prevent cutting shock is applied equally. The use of a pressure controlled pressure limiting valve in the device of the present invention, as will be more fully explained in the description below, further produces a reduction in the danger of overloading the press, since forces of inertia and friction in the pressure limiting valve are largely excluded. By feeding the control chamber of the pressure limiting valve directly from the pressure medium chamber of the cylinder piston unit, an additional pressure pipe from the high pressure pressure medium source may be avoided.

Further, by using a throttle point in the connection between the pressure medium chamber of the cylinder/piston unit and the control chamber of the pressure limiting valve, reactions between the two pressure chambers may be largely prevented. Advantageously, the cylinder / piston unit is constructed as a single compact sub-assembly which includes the pressure limiting valve and optionally the throttle point, Zso that delays in reaction which result from the finite reaction times occurring when connecting pipes are used, are minimised.

A device according to the invention is more fully explained hereinbelow with reference to an exemplary embodiment illustrated in the accompanying drawings, in which Fig. 1 is a schematic circuit diagram of the hydraulic circuit, Fig. 2 is a graph of the essential operating parameters of a press with the device switched off, and Fig. 3 is a corresponding graph with the device switched on.

In Fig. 1 there is illustrated schematically a tool baseplate 10 upon which four simi larly constructed cylinder-piston units 11 are arranged symmetrically to each other Each cylinder/piston unit 11 has an impact surface 13 connected to a piston 12, which is adjustable in its vertical disposition relative to a press ram (not shown) or a top tool fixed to the latter. Between the end face 14 of the piston 12 and an interior wall 15 of the cylinder 16 there is a pressure medium chamber 17 which is connected on its inlet side, via a non-return valve 18, to a high-pressure pressure medium source 19. The pressure medium source 19 comprises a hydraulic pump 20 which is driven by means of an electric motor 21 and delivers pressure medium out of a tank 22 through a non-return valve 23 and pressure pipes 24 to the pressure medium chambers 17 or the cylinder/piston units 11. Each cylinder/piston unit 11 is also provided with a pressure limiting valve 25 which is arranged on the outlet side of the pressure medium chamber 17 and is connected through an associated return pipe 26 to the tank 22. The pressure limiting valves 25 are arranged for control by the pressure medium and each has a control chamber 27 which is also in communication with the high pressure pressure medium source 19.

The connection is conveniently established from the pressure medium chamber 17 of the cylinder piston unit 11 through a throttle point 28. The control chambers 27 of the pressure limiting valves 25 are connected by similar control pipes 29 of similar length to a central pressure control valve 30, the outlet of which terminates in the tank 22.

The cylinder 16, the cylinder chamber of which is closed at an upwardly disposed end by a collar and a downwards disposed end by the interior end wall 15, is mounted on a tool baseplate 10. The piston 12 is disposed in the cylinder chamber, while the pressure medium chamber 17 is defined by the end face 14 of the piston 12 and the interior end wall 15 of the piston 16. The portion of the piston 12 opposite the end face 14, which portion projects out through the collar of the cylinder 16, provides the vertically adjustable impact surface 13. The pressure limiting valve 25, which connects the pressure medium chamber 17 to the tank 22 on the outlet side of said chamber 17, is incorporated in the lower part of the cylinder piston unit 11. On the inlet side the non-return valve 18 is incorporated in the cylinder 16. The pressure medium chamber 17 is also connected by a connection 34 to a control chamber 27 through the throttle point 28.

The principal operation and functioning of the device will now be more fully explained with reference to the graphs illustrated in Figs. 2 and 3. The graphs show the essential parameters-ram travel S, connecting rod force F, and expansion D at the head piece of the blanking press and the pressure P in the cylinder piston units 11-as a function of time t. Only the essential part of the work cycle of the blanking press about the lower dead centre position is shown.

In Fig. 2 the cylinder/piston units 11 are not under pressure, so the pressure P is constant and the curve for the pressure P is linear, the device for preventing cutting shock being out of service. From the contact time 35 of the top tool or of the ram upon the work piece to be cut, the connecting rod force F and, in the same manner, the expansion D in the head piece of the press, rise steeply. The breach of the top tool through the work piece to be cut occurs in the region of the breakthrough time 36. Due to the sudden relaxation of the press frame-see expansion D-the ram is accelerated extremely sharply and, like the press frame, its vibrations die away slowly.

Due to this shock-like relaxation, there is produced the considerable noise characteristic of the cutting shock at breakthrough which is accompanied by heavy stressing of the tools and press.

The graph according to Fig. 3 shows the same work cycle section as Fig. 2, but with the cylinder piston units 11 prepressurised. The pressure P corresponds as far as possible to the required cutting force and may be adjusted by means of the pressure control valve 30 (Fig. 1) as will be explained hereinbelow.

At the contact moment 35 the top tool descends upon the work piece to be cut and the cutting operation commences.

Just before reaching the region of the break-through point 36 the ram descends upon the impact surfaces 13 of the cylinder piston units 11. The region of the breakthrough point 36 is now traversed without the top tool breaking through the work piece to be cut, because the resistance of the work piece to be cut which disappears abruptly at the break-through is immediately available at the commencement of breakthrough, by means of the cylinder piston units 11. Thus the blanking press cannot relax abruptly. In the further course of the work cycle, the press frame is relaxed continuously by reduction of the pressure in the pressure medium chamber 17 of the cylinder piston units 11, so that no cutting shock occurs. The noise level in this case is much lower than that when the device is not in operation. A reduction of wear is also achieved.

The pressure of the pressure medium in the pressure medium chambers 17 of the cylinder piston units 11 is generated by the pressure medium source 19 through the pressure pipes 24, and as previously mentioned it is adjusted simultaneously and centrally by the pressure control valve 30 and the pressure limiting valves 25 by adjusting the pressure in the control chambers 27 of the pressure limiting valves 25. The adjustment corresponds to the cutting forces to be anticipated depending upon the strength of material to be cut. Any overloading of the press can be simply and reliably prevented in this manner. Any pressure in the control chamber 27 which exceeds the adjustment of the pressure control valve 30 is dissipated through the pressure control valve 30, whilst the associated excess pressure in the pressure medium chambers 17 is dissipated through the pressure limiting valve 25 and the return pipes 26. The throttle points 28, through which the pressure medium chambers 17 are connected to the control chamber 27, serve merely to prevent reactions of slight pressure fluctuations in the said chambers 17, 27. As soon as the ram comes into operative association with the impact surfaces 13 of the cylinder piston units in the region of the break-through moment 36, the pressure in the pressure medium chambers 17 rises, the pressure limiting valve 25 opens, so that the adjusted equilibrium state between the pressure medium chambers 17 and the control chambers 27, and hence the adjusted pressure, is maintained.

Cutting shock is prevented, because a counterforce corresponding to the cutting force is now exerted by the cylinder piston unit 11. By a continuous pressure decay in the pressure medium chamber 17, this force is gradually dissipated without undesirable side effects such as noise, wear. In the example illustrated and described, the decay of pressure occurs after the press ram passes through the bottom dead centre position, by reduction of the pressure in the pressure medium chambers 17, which due to the relative dimensions of the parts of the device, is not fully compensated by the pressure medium source 19 (see Fig. 3). The effect produced might be compared to that which can be obtained by using pressure reduction valves (e.g. as represented at 21 in Fig. 2 of the above-mentioned German Specification No. 2 359 378) which are controlled as a function of the ram travel.

In order to ensure constant operating conditions for the device heating and cool ing units are desirably provided in the hydraulic circuit to ensure a constant temperature of the pressure medium and hence equal operating properties at all times. In order to damp any undesirable pulsatory pressure oscillations in the control chambers 27, further throttle points may also be interposed in the control pipes 29.

WHAT WE CLAIM IS: 1. A device for preventing cutting shock in a blanking press comprising at least one cylinder/piston unit disposable, in use, between a tool base plate and a press ram of said blanking press so that an impact surface thereof is positionally adjustable relative to the press ram, the stroke of the piston of said cylinder/piston unit being limited at one end by the interior end wall of the cylinder opposite a facing end of the piston and a pressure chamber between said interior end wall of the cylinder and the facing piston end being connected on an inlet side via a non-return valve to a high pressure pressure medium source and on an outlet side to a pressure limiting valve provided with a control chamber in communication with a high pressure pressure medium source, the pressure in said control chamber being adjustable by means of a pressure control valve in a control

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. D-the ram is accelerated extremely sharply and, like the press frame, its vibrations die away slowly. Due to this shock-like relaxation, there is produced the considerable noise characteristic of the cutting shock at breakthrough which is accompanied by heavy stressing of the tools and press. The graph according to Fig. 3 shows the same work cycle section as Fig. 2, but with the cylinder piston units 11 prepressurised. The pressure P corresponds as far as possible to the required cutting force and may be adjusted by means of the pressure control valve 30 (Fig. 1) as will be explained hereinbelow. At the contact moment 35 the top tool descends upon the work piece to be cut and the cutting operation commences. Just before reaching the region of the break-through point 36 the ram descends upon the impact surfaces 13 of the cylinder piston units 11. The region of the breakthrough point 36 is now traversed without the top tool breaking through the work piece to be cut, because the resistance of the work piece to be cut which disappears abruptly at the break-through is immediately available at the commencement of breakthrough, by means of the cylinder piston units 11. Thus the blanking press cannot relax abruptly. In the further course of the work cycle, the press frame is relaxed continuously by reduction of the pressure in the pressure medium chamber 17 of the cylinder piston units 11, so that no cutting shock occurs. The noise level in this case is much lower than that when the device is not in operation. A reduction of wear is also achieved. The pressure of the pressure medium in the pressure medium chambers 17 of the cylinder piston units 11 is generated by the pressure medium source 19 through the pressure pipes 24, and as previously mentioned it is adjusted simultaneously and centrally by the pressure control valve 30 and the pressure limiting valves 25 by adjusting the pressure in the control chambers 27 of the pressure limiting valves 25. The adjustment corresponds to the cutting forces to be anticipated depending upon the strength of material to be cut. Any overloading of the press can be simply and reliably prevented in this manner. Any pressure in the control chamber 27 which exceeds the adjustment of the pressure control valve 30 is dissipated through the pressure control valve 30, whilst the associated excess pressure in the pressure medium chambers 17 is dissipated through the pressure limiting valve 25 and the return pipes 26. The throttle points 28, through which the pressure medium chambers 17 are connected to the control chamber 27, serve merely to prevent reactions of slight pressure fluctuations in the said chambers 17, 27. As soon as the ram comes into operative association with the impact surfaces 13 of the cylinder piston units in the region of the break-through moment 36, the pressure in the pressure medium chambers 17 rises, the pressure limiting valve 25 opens, so that the adjusted equilibrium state between the pressure medium chambers 17 and the control chambers 27, and hence the adjusted pressure, is maintained. Cutting shock is prevented, because a counterforce corresponding to the cutting force is now exerted by the cylinder piston unit 11. By a continuous pressure decay in the pressure medium chamber 17, this force is gradually dissipated without undesirable side effects such as noise, wear. In the example illustrated and described, the decay of pressure occurs after the press ram passes through the bottom dead centre position, by reduction of the pressure in the pressure medium chambers 17, which due to the relative dimensions of the parts of the device, is not fully compensated by the pressure medium source 19 (see Fig. 3). The effect produced might be compared to that which can be obtained by using pressure reduction valves (e.g. as represented at 21 in Fig. 2 of the above-mentioned German Specification No. 2 359 378) which are controlled as a function of the ram travel. In order to ensure constant operating conditions for the device heating and cool ing units are desirably provided in the hydraulic circuit to ensure a constant temperature of the pressure medium and hence equal operating properties at all times. In order to damp any undesirable pulsatory pressure oscillations in the control chambers 27, further throttle points may also be interposed in the control pipes 29. WHAT WE CLAIM IS:

1. A device for preventing cutting shock in a blanking press comprising at least one cylinder/piston unit disposable, in use, between a tool base plate and a press ram of said blanking press so that an impact surface thereof is positionally adjustable relative to the press ram, the stroke of the piston of said cylinder/piston unit being limited at one end by the interior end wall of the cylinder opposite a facing end of the piston and a pressure chamber between said interior end wall of the cylinder and the facing piston end being connected on an inlet side via a non-return valve to a high pressure pressure medium source and on an outlet side to a pressure limiting valve provided with a control chamber in communication with a high pressure pressure medium source, the pressure in said control chamber being adjustable by means of a pressure control valve in a control

pipe connected to the control chamber.

2. A device as claimed in claim 1 wherein at least two cylinder/piston units are provided, the control pipes of each of said cylinderlpiston units being similarly dimensioned and connected to a single central pressure control valve.

3. A device as claimed in claim 1 or claim 2 wherein the pressure limiting valve is formed as part of the cylinder/piston unit.

4. A device as claimed in any of claims 1 to 3 wherein the control chamber is connected to the high-pressure pressure medium source through the pressure medium chamber of the cylinder/piston unit.

5. A device as claimed in claim 4 wherein a throttle means are provided between the control chamber and said pressure medium chamber of the cylinder/ piston unit.

6. A device for preventing cutting shock in a blanking press according to claim 1 substantially as described hereinbefore with particular reference to Figures 1 and 3 of the accompanying drawings.

7. A blanking press wherein is provided a device for preventing cutting shock according to any of the preceding claims.