GB2071538A

GB2071538A - An installation for controlling the clamping force of a hydraulically operated clamping device

Info

Publication number: GB2071538A
Application number: GB8105196A
Authority: GB
Original assignee: Werkzeugmaschinenkombinat 7 Oktober VEB
Current assignee: Werkzeugmaschinenkombinat 7 Oktober VEB
Priority date: 1980-03-14
Filing date: 1981-02-19
Publication date: 1981-09-23
Also published as: GB2071538B; IT1170798B; DD149628A1; FR2477938A1; DE3043837A1; DD149628B1; JPS56139874A; IT8148011A0; FR2477938B1

Abstract

The installation comprises a double-acting hydraulic cylinder 1 having a piston 2 which forms chambers A and B and the rod of which is connected to a clamping device, for example a chuck of a grinding machine. For actuating the clamping cylinder 1 for closing the clamping device, pressurised hydraulic fluid is fed via line 12, a non-return valve 10, a directional valve VW1 and line 15 to chamber B of the clamping cylinder 1. Hydraulic fluid displaced from chamber A is conducted via line 16 to a pressure-adjusting valve 3' which pre-stresses the displaced fluid to vary the clamping force. The valve 3' has a piston 4 which is loaded by a compression spring 5 the force of which is adjustable by means of a setting spindle 6 carried in a further piston 22 whose outward stroke is limited by a further setting spindle 23. The excess pressure fluid from the valve 3' is conducted via lines 17 and 18 and directional valve VW1 to a collecting tank 19. To open the clamping device the valve VW1 is operated so that the hydraulic fluid from chamber B is conducted to the collecting tank 19 and the chamber A is pressurised via line 18, non-return valve 11 and line 16. To provide a two-stage clamping, i.e. a reduced clamping force during finish machining, the hydraulic fluid is also conducted via an appropriately operated second directional valve VW2 and an adjustable throttle VDr1 to a chamber C of the pressure- adjusting valve 3', to additionally load the piston 4 so as to increase the prestressing of the fluid displaced from chamber A and thus reducing the clamping force. <IMAGE>

Description

SPECIFICATION An installation for controlling the clamping force of a hydraulically operated clamping device.

This invention relates to a clamping forcecontrolling installation for a hydraulically operated clamping device of a machine tool, for example an internal grinding machine.

On modern high-performance grinding machines it is nowadays necessary to subdivide the grinding process into coarse grinding and finished grinding, followed by so-called sparkingout. The coarse grinding operations are as a rule carried out with high feed speeds and low workpiece revolutions. The result is heavy forces on the workpiece, which require high clamping forces.

However, for finishing grinding such high clamping forces are unsuitable, since at the moment the clamping device is released, deformations occur on the workpiece which as a rule make it unusable. The clamping forces must therefore be reduced before the finishing operation. Such reduction can be elected either abruptly, or it follows a course in accordance with a time function. According to German Offenlegungsschrift 2 157 268 it is known, for example, to control the clamping force in the operating process by the following circuit arrangement. A clamping cylinder is connected to a main hydraulic feed line. A clamping force changeover valve is incorporated in the main hydraulic feed line. During coarse grinding the clamping force changeover valve is opened. The full hydraulic pressure acts on the clamping cylinder.The excess hydraulic liquid is discharged via an adjustable throttle. The clamping force changeover valve is closed for the finishing operation. The hydraulic liquid is fed via a subsidiary circuit, the clamping pressure being controlled via a further throttle. The clamping force is changed abruptly by the operation of the clamping force changeover valve. If changeover is to be effected in accordance with a time function, an additional accumulator is coupled to the main line. This accumulator is filled during coarse grinding with hydraulic liquid which is at an appropriate pressure. In the accumulator a piston is displaced against the force of a spring. The force of the spring can be adjusted as required. When the clamping force changeover valve is closed, there is no abrupt reduction of the clamping force but an equalisation in accordance with a time function.

It is true that such known circuit arrangement is of simple construction, but it nevertheless has appreciable disadvantages, since the operation of the throttles seems uncertain. The heat flux in the throttle gap produces a change in clamping force, since oil heating results in a change in viscosity, and this is the same thing as a change in the flow rate with a constant throttle gap. Consequently, the pressure drop via the throttle is changed, and this corresponds to a change in clamping force.

Moreoever, a separate pressure flow generator is required to maintain the pressure throughout the system. Furthermore, this circuit arrangement is suitable only for use on jaw chucks. It also remains a completely open question what happens if the pressure in the pressure medium feed line suddenly fails.

It is known from German Offenlegungsschrift 2 458 958 that the side of the piston operated to reduce the clamping force can be connected to a pressure medium accumulator via a valve which can be temporarily opened during the transition from one clamping force to the other. In the transition from one clamping force to the other, a valve control system so supplies the quantity of pressure medium contained in the accumulator to the cylinder of the pressure medium piston that the latter relieves the clamping system. However, the relief is not complete, since the stored quantity of pressure medium is just enough to reduce the clamping force to below the value which is required for the finishing of the workpiece and which guarantees its precise clamping length. A finely-adjustable pressure-adjusting valve enables the reducible clamping force to be automatically pre-selected.

Such system calls for very expensive apparatus.

It requires motor-driven pressure-adjusting valves which are very expensive. Moreover, a separate pressure flow generator with a control system is needed. As regards application, as disclosed the system can be used only for jaw chucks. For use on a diaphragm chuck, alterations are necessary to enable the clamping force to be controlled.

It is also known from German Offenlegungsschrift 2 245 796 that a doubleacting pressure medium cylinder can be disposed on the rotating spindle of a machine tool. A supply line with non-return valves is associated with each cylinder chamber. A control system controls the discharge and delivery of the pressure medium.

The control system enables the non-return valves to be so controlled that, while they retain their functional reliability, the pressure in the pressureoperated cylinder chamber can be reduced steplessly. This construction is very expensive to manufacture, since it comprises a very large number of members which demand highprecision production. Furthermore, inevitably reliability is adversely affected by wear occurring on the valves and other continuously moving parts. Considerable attention must be devoted to the centrifugal forces on the valves. It should also be mentioned that such clamping-forcecontrolling installation is suitable only for rotating clamping cylinders. Moreover, such installation also cannot dispense without its own pressure flow generator.

It is an object of the invention to provide for hydraulic clamping devices a clamping forcecontrolling installation which requires no separate pressure flow generator, continues to operate reliably for a reasonable amount of time if the pressure medium supply fails, and is suitable for the operation of jaw chucks with hydraulically produced clamping force, and also for the operation of clamping means which themselves produce the clamping force in the clamping medium.

The problem which the invention aims at solving is to provide an installation which can be connected to the hydraulic system of a machine tool, can be used universally for all clamping means, and at the minimum expense for apparatus enables the clamping force to be steplessly adjusted and automatically changed.

Accordingly, the present invention consists in an installation for controlling the clamping force of a hydraulically operated clamping device, comprising a double-acting hydraulic cylinder having a piston which forms first and second chambers in said cylinder and the piston rod of which is connectable to the clamping device, a pressure-adjusting valve and a directional valve which is in conduit connection with said first and second chambers and which is operable for alternately connecting said first chamber either to a source of pressurised hydraulic fluid for actuating said piston to close the clamping device or to a pressureless return line and for connecting the second chamber to the return line via the pressure-adjusting valve interposed between said second chamber and said directional valve or to said source of pressurised fluid for actuating said piston to open the clamping device, said pressure adjusting valve being adjustable so as to prestress the hydraulic fluid displaced from the second chamber to vary the clamping force exerted on the clamping device by said piston.

The clamping force is obtained from the product of the difference in pressure (constant clamping pressure minus adjustable counterpressure) multiplied by the piston surface.

The pressure control valve may be so designed that the counterpressure can be adjusted in two stages independently. The second pressure stage may be operated hydraulically in accordance with technological demands, and the transition can take place abruptly or gradually, in accordance with a time function. For use on clamping means in which the clamping force is produced in the clamping medium itself, an additional directional valve can pre-stress via a workpiece-adjusting valve the oil displaced from the working cylinder during the opening of the chuck. This enables the clamping force of the chuck to be reduced at a given moment with a reduced opening force.

In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically and by way of example embodiments thereof, and in which: Fig. 1 is a hydraulic circuit diagram of an embodiment of the installation with a pressure stage in the clamping phase, Fig. 2 is a hydraulic circuit diagram of an embodiment with automatic force changing, and Fig. 3 is a hydraulic circuit diagram of an embodiment with automatic force changing and optional use also for clamping means in which the clamping force is produced in the clamping medium.

The installation illustrated in Fig. 1 operates in principle as follows. A line 12 is connected to the main pressure line (not shown) of the hydraulic circuit of a machine tool. The pressurised oil opens a non-return valve 10, and then flows via lines 13 and 14 to a pressure accumulator 7 which it fills.

A manometer 9 indicates the pressure in the circuit. After the workpiece has been changed, a directional valve '('All is switched over and assumes the rest position illustrated. The pressurised oil passes via the directional valve VW1 and line 1 5 into the cylinder chamber B of a clamping cylinder 1 and moves its piston 2 to the left left (as viewed in Fig. 1), closing a clamping device connected to the piston rod. The oil in the cylinder chamber A of the clamping cylinder 1 is displaced by the piston 2 via a line 16 to a pressureadjusting valve 3, since the direct discharge via a line 18 is blocked by a non-return valve 11. The displaced oil is pre-stressed by the pressureadjusting valve 3.During the movement of the piston 2 during the clamping operation, the oil displaced from the cylinder chamber A of the clamping clamping cylinder 1 acts on a piston 4 of the pressure-adjusting valve 3, which can be separately adjusted to a required pressure level via a spring 5 by means of a setting spindle 6. The pressure adjustment is indicated on a manometer 8. When the adjusted pressure is reached, the piston 4 opens a line 17 and the pressurised oil can flow via lines 17, 18, directional valve VW1 and line 19 into a collecting tank. When the clamping operation is terminated, i.e. no further oil is displaced from the cylinder chamber A of the clamping cylinder 1, the piston 4 of the pressurecontrol valve 3 immediately closes the line 17, thereby maintaining the adjusted pressure.The difference in pressures between the cylinder chambers A and B, when multiplied by the piston surface, produces the clamping force acting on the clamping device in the machine tool.

With a sudden drop in pressure in the machine tool hydraulic circuit, the non-return valve 10 closes. The pressure accumulator 7 maintains the pressure over a prolonged period and compensates for leakage oil losses. As a result the clamping force also remains at the level prevailing at that moment.

To open the clamping device the directional vlave VW1 is operated. The pressurised oil flows via VW1, line 18, non-return valve 1 1 and line 16 into the cylinder chamber A of the clamping cylinder 1. The piston 2 is moved to the right and in known manner opens the clamping device via its piston rod. The oil in the cylinder chamber B of the clamping cylinder 1 flows unpressurised via the line 15, the directional valve VW1 and the line 19 into the collecting tank.

Fig. 2 illustrates basically the same hydraulic circuit as Fig. 1, except that it is extended by a second adjustable pressure stage to change the clamping force in the final phase of the machining.

For this purpose a pressure-adjusting valve 3' is provided which is extended by a pressure chamber C. Disposed in the pressure chamber C is a piston 22 which is limited in its stroke by a further setting spindle 23. Also provided for hydraulic operation are a further directional valve VW2, two nonreturn valves 20; 21 and an adjustable throttle VDr1. For operation with only one force stage, the directional valve VW2 is not operated, i.e. remains in the position of rest (not shown). As a result the piston 22 of the pressure-adjusting valve 3' continuously bears against the setting spindle 23.

The counterpressure is adjusted via setscrew 6, which is mounted in the piston 22 for movement therewith.

To clamp the workpiece, the directional valves VW1 and VW2 are moved into the rest position (only VW1 thus shown). The clamping operation takes place as described in connection with Fig. 1.

The pressurised oil arising in a line 24 from line 15 cannot pass beyond the non-return valve 21. The clamping force is determined in accordance with the counterpressure adjusted via setting spindle 6, spring 5 and piston 4. With the start of finish machining (the final phase of machining) the directional valve VW2 is operated (Fig. 2). The pressurised oil arising in line 24 passes via directional valve VW2, lines 25; 26, throttle VDr1 and line 27 into chamber C of the pressure adjusting valve 3', and moves the piston 22 to the left (as viewed in Fig. 2) by the travel adjusted by the setting spindle 23. The spring 5 is more heavily stressed, and a higher counterpressure is set up.As a result the difference in pressures in cylinder chambers A and B of the clamping cylinder 1 drops, and the clamping force is reduced. This operation takes place abruptly when the throttle VDr1 is completely opened. In accordance with the decreasing chip-removing forces during the finish machining and sparkingout process, the transition from the higher clamping force to the lower force can take place in accordance with a time function. The throttle gap of the throttle VDr1 is then adjusted in accordance with the required time function.

To open the clamping means, the directional valve VW1 is operated and the directional valve VW2 brought into the position of rest. The operation takes place as described in relation to Fig. 1. In addition, the cylinder chamber C of the pressure-adjusting valve 3' is now connected via line 27, non-return valve 20, directional valve VW2, non-return valve 21 and line 24 to the line 15, and therefore connected to the discharge to the collecting tank. The piston 22 is brought into abutment with the setting spindle 23 by spring 5, which bears against the piston 4.

In Fig. 3 the highest extension stage of the clamping force controlling installation is also shown in the form of a hydraulic circuit diagram.

With this construction use can be made optionally of clamping means with pressure generation via a clamping cylinder, or clamping means such as, for example, diaphragm clamping chucks, in which the clamping force is produced in the clamping medium itself. For this purpose the hydraulic circuit diagram shown in Fig. 2 is extended by a further directional valve VW3 and a further throttle VDr2. If a clamping means is used in which pressure is produced via a clamping cylinder, operation is identical with that illustrated in Fig. 2.

The directional valve VW3 continuously remains in the position of rest (as shown) and therefore has no effect. If a diaphragm clamping chuck is used, the course of operation is as follows.

For the "open the chuck" operation the directional valve VW1 is operated (not shown), the two other directional valves VW2 and VW3 remaining in the rest position (as shown). The pressure oil from line 13 passes via directional valve VW1, line 18, non-return valve 11, line 30, directional valve VW3 and line 1 6 into chamber A of the clamping cylinder 1 and moves the piston 2 to the right (as viewed in Fig. 3). Since the oil can flow from the chamber B of the clamping cylinder 1 unpressurised via line 15, directional vlave VW3, line 31, directional valve VW1 and line 19 into the collecting tank, the right-hand piston rod of the piston 2 bears with full force against the diaphragm end (force-producing element) of the clamping means and thus opens the chuck.The oil in chamber C of the pressure-adjusting valve 3' can also flow away unpressurised via line 27, nonreturn valve 20, line 25, directional valve VW2, line 24, line 15, etc., to the collecting tank. The piston 22 is moved to the right by the spring 5 until it abuts against the setting spindle 23. The pressure oil arising from line 18 via line 17, line 29, throttle VDr2, line 28, is blocked by directional valve VW2.

After a workpiece has been inserted in the clamping means, the directional valve VW3 is operated, '('All remains in its operated position and VW2 remains in the rest position (as shown).

The pressure oil from line 13 then passes via directional valve VW1, line 18, non-return valve 11, line 30, directional valve VW3 and line 15 into chamber B of the clamping cylinder 1 and moves the piston 2 rapidly to the left, since the pressure oil can flow from chamber A of the clamping cylinder 1 unpressurised via line 16 directional valve VW3, line 31, directional valve VW1 and line 19 into the collecting tank. At the same time, via the line 24 connected to line 15, directional valve VW2, line 25, line 26, throttle VDr1 (completely opened) and line 27, pressure oil can pass into the chamber C of the pressure-adjusting valve 3'. As a result the piston 22 is moved to the left (as shown in Fig. 3). At the same time the spring 5 is stressed to a value pre-selected via the setting spindle 6.

The pressure oil arising from line 18 via line 17 and line 29, throttle VDr2 and line 28 is still blocked by directional valve VW2. If during the finish machining, with decreasing chip-removing force, the clamping force acting on the workpiece is to be reduced, then with the start of the finish machining directional valve VW2 is operated (not shown) and directional valve VW1 is moved into the rest position (as shown). The pressure oil from line 13 passes via directional valve VW1, line 31, directional valve VW3 and line 16 into the chamber A of the clamping cylinder 1 and moves the piston 2 to the right in the "open the chuck" direction. The oil flowing out of the chamber B of the clamping cylinder passes via line 15, directional valve VW3, line 30 into the pressureadjusting valve 3' and acts on the piston 4.As a result this oil column is stressed to the value adjusted via the setting spindle 6. The piston 22 is still in the left-hand end position, from the clamping operation. After the adjusted value for the counterpressure has been reached, the excess oil can flow unpressurised via line 17; 18, directional valve VW1 and line 19 into the collecting tank. The piston rod of the piston 2 therefore acts on the diaphragm end with only a low force, which corresponds to the difference in pressures between the two sides of the cylinder, and cancels out the clamping force of the diaphragm clamping chuck to a predetermined, pre-selectable extent.The oil in chamber C of the pressure-adjusting valve 3' can flow via line 27, non-return valve 20, line 25, directional valve VW2, line 28, throttle VDr2, line 29, line 17, line 18, directional valve VWI and line 19 to the collecting tank. As a result the piston 22 in the pressure-adjusting valve 3' moves to the right and expands the spring 5 until the piston 22 abuts the setting spindle 23. The counterpressure adjusted for chamber B of the cylinder 1 therefore continually declines, while the difference in pressure and therefore the force acting on the diaphragm end increases. The clamping force is reduced to a necessary minimum which is adjusted by the travel limitation of the piston 22 via the setting spindle 23.The throttle gap adjusted at the throttle VDr2 determines the course in time of the reduction in clamping force.

As can be gathered from the embodiments, the clamping force controlling system according to the invention can be used both for diaphragm clamping chucks and also for clamping means in which the clamping force is produced via a cylinder. This is an important advantage over the prior art. Moreover, the installation can be directly connected to the hydraulic system of a machine tool. This therefore eliminates a separate control pump. In general the installation is characterised by inexpensive apparatus, since only one pressureadjusting valve and the maximum of three directional valves are used. The prior art methods require an adjusting valve with a corresponding operating system for each pressure stage.

It should also be pointed out that if the pressure suddenly drops due to possible damage to the pressure medium supply, the clamping pressure is maintained, due to the construction disclosed.

Claims

1. An installation for controlling the clamping force of a hydraulically operated clamping device, comprising a double-acting hydraulic cylinder having a piston which forms first and second chambers in said cylinder and the piston rod of which is connectable to the clamping device, a pressure-adjusting valve, and a directional valve which is in conduit connection with said first and second chambers and which is operable for alternately connecting said first chamber either to a source of pressurised hydraulic fluid for actuating said piston to close the clamping device or to a pressureless return line and for connecting the second chamber to the return line via the pressure-adjusting valve interposed between said second chamber and said directional valve or to said source of pressurised fluid for actuating said piston to open the clamping device, said pressureadjusting valve being adjustable so as to prestress the hydraulic fluid displaced from the second chamber to vary the clamping force exerted on the clamping device by said piston.

2. An installation as claimed in claim 1, wherein the pressure-adjusting valve comprises a piston which is movable in a cylinder and which at one end is loadable by the hydraulic fluid displaced from the second chamber and at the other end is acted on by a compression spring whose force is adjustable by means of a setting spindle.

3. An installation as claimed in claim 2, wherein the cylinder is axially extended to provide a chamber for a further piston which carries the setting spindle and which is loadable against the force of the spring by a hydraulic fluid from the directional valve by way of a second directional valve and a first throttle to provide a two-stage clamping, the stroke of said further piston being adjustable by means of a further setting spindle mounted on the cylinder, and the further piston being relieved of pressure via a non-return valve arranged in a duct between the second directional valve and the valve cylinder.

4. An installation as claimed in claim 3, wherein for reversing the force ratios of the hydraulic cylinder a third directional valve is arranged between the first mentioned directional valve and the hydraulic cylinder and a second throttle is provided in a duct extending direct from the second directional valve to said first mentioned directional valve.

5. An installation for controlling the clamping force of a hydraulically operated clamping device, substantially as herein described with reference to and as shown in Fig. 1,2 or 3 of the accompanying drawings.