US3530749A - Parallel-motion control mechanism - Google Patents

Description

United States Patent [72] Inventors Hans Riihmel',

Friedrichshafener Str. 26, Lindau, Germany;

Bozidar llabazin, Flur 1424, Berneck, Switzerland April 18, 1968 Sept. 29, 1970 April 18, 1967 Germany Appl. No. Filed Patented Priority PARALLEL-MOTION CONTROL MECHANISM 8 Claims, 8 Drawing Figs.

US. Cl 83/74, 83/529, 83/530, 83/624, 83/639 1nt. Cl 826d 5/12 Field of Search 83/74, 529,

530. 624. 639. 617: 72/(lnquired): lOO/(lnquired) [56] References Cited UNITED STATES PATENTS 3,129,625 4/1964 Jones, Jr. 83/624 3,183,756 5/1965 Dehn 83/624X Primary ExaminerFrank T. Yost Attorney-Tab T. Thein ABSTRACT: Parallel-motion control mechanism for doubleacting ram cylinders, particularly in trimming presses and the like machine tools, wherein a single valve is interposed between the pressure-fluid feed pipes and the pressure ducts to the individual cylinders, with a single piston, and having rotatable control disks at the ends of the valve body, the disks having ports which provide communication between a number of ducts so as to provide efficient compensation for asymmetrical loads.

Patented Se t. 29, 1970 Sheet n yenf Shoat IN V EN TORS: 75 'h vner PARALLEL-MOTION CONTROL MECHANISM The invention relates to a parallel-motion control mechanism for two or more, preferably double-acting, hydraulically actuated ram cylinders, in particular for operating edging or trimming presses and similar machine tools.

These machine tools have press plungers connected with the ram cylinders, in which any deviation from the parallel action of the working cylinders is corrected by the application of counter-pressure in the cylinders, controlled by rack bars adjustable in the direction of motion of the press plunger and meshing with gear wheels arranged on the ends and in the center ofa shaft or spindle extending over the whole length of the press plunger and supported thereon.

It is already known that to ensure the proper and accurate working of such edging or trimming presses it is important to maintain the press plunger strictly parallel to the surface of the work-table or platen of the machine tool throughout the entire working stroke thereof despite the presence of variable working pressure and unilateral pressure loads on said plunger.

It is further important to be able to arrest the motion of said press plunger in any required position, i.e., at any required distance from the table or platen and to hold it stopped for any length of time in such a manner that it does not change its attitude parallel to said machine table or platen under the influence of extraneous forces. This must be made possible also after said press plunger has performed any number of previous working strokes under varying pressure loads and at varying speeds.

At the present time this is usually attained by hydraulic means, preferably in association with mechanical elements, and by the provision of stops or arresting means provided in various positions. The known embodiments of this kind require for the respective power circuits, control circuits and regulating circuits a number of hydraulic valves, auxiliary valves and extensive electrical and electronic means for each such circuit; often, in conjunction with the above-mentioned, associated mechanical elements.

The disadvantages of such forms of embodiment consist in that, owing to inertia effects and the requisite response times, the various hydraulic elements as well as the electrical components require an appreciable reaction period for correcting any control or regulating errors, and give rise to inaccuracies in the control action. The errors arising cannot be sufficiently rapidly corrected since the time lag between the occurrence of the error and its transmission to the control means is subject to fluctuations,

One known form of embodiment relates to a hydraulic press with a plurality of cylinders having pistons which are coupled with a common press plunger, the skewing whereof, when the synchronization of the action of these pistons is disturbed, produces a variation in the quantity of power fluid supplied to these cylinders, counteracting the aforesaid skewing effect. For the purpose of restoring the parallelism of the press plunger, each such control element is associated with a fixed stop limiting the displacement of said valve element in the opening direction. In this form of embodiment, the action of the transmitting means altering the setting of said hydraulic control means is first required to override the restoring force of a spring which must be sufficiently high to act also as a holding force. In this regard, the necessity arises of providing sufficiently powerful and consequently expensive electrical control means or a servomotor system.

In other known presses the delivery of a pump coupled with each press cylinder is controlled in accordance with the amount of skewing of the press plunger. It is further known to vary the quantity of the power fluid admitted to the cylinder to counteract errors in parallelism by opening or closing the admission ducts for the power fluid connecting the individual cylinders with the power fluid supply. For this purpose a common control valve can be provided, controlling a plurality of such ducts opening into each cylinder or a number of control valves or flaps associated with each cylinder.

In all these forms of embodiment it has been found that the variation of the quantity of pressure fluid admitted to each of the press cylinders lacks the necessary precision for all requirements, which is principally due to the comparatively long communicating passages between each control valve and the corresponding cylinder.

The object of the present invention is consequently to eliminate the aforesaid disadvantages and provide an absolutely accurate synchronizing mechanism ensuring complete parallelism. I I I I According to one of the major features of the invention, this is effected in such a manner that a single valve is interposed between the pressure fluid feed pipes and the pressure ducts to the individual cylinders, with a single hydraulically actuated piston, and having at the ends of the valve body, looking in the direction of motion of said piston, rotatable control disks with notches or ports for communication between corresponding ducts or passages.

Preferably the valve piston is provided with a shaft or spindle which carries gear wheels associated with a press ram or plunger of the machine tool, the control mechanism having in the middle thereof a further gear wheel meshing with a resiliently biased rack bar.

In the parallel-motion mechanism according to this invention, consequently, only one control valve is required. The centrally placed rack bar according to the invention allows the levelling of the press plunger to be completed before the latter comes in contact with the workpiece. This arrangement can also be used with a suitable, inexpensive electrical control system.

The reduction in the number of valves required and the smaller number of pressure oil ducts decrease the friction in the oil pump, packing joints, etc., and consequently decrease the oil temperature so that these parts areless heavily loaded. In continuous operation of such presses, i.e., when the pump is kept running continuously, it becomes important to work with the smallest number of valves and the least amount of friction in the conduits in order to maintain a low temperature of the pressure oil. In the present invention, the oil temperature is held at a very low level.

The action of the mechanism is reversed by me'ansofthe single, axially working piston. Accurate control of the parallelism of the press plunger and the exact stopping thereof are thus assured on both the up and the down strokes of the plunger, and are effected by means of the two control disks, rotatable in either direction, in association with one or more cylinders each.

This form of construction reduces the flow resistance through the mechanism to a minimum whereby the overall hydraulic efficiency is substantially improved. The accurate and sensitive control by means of these, low-mass disks and the perfect hydraulic compensation in the system afford considerable advantages as compared with existing forms of embodiments.

Other objects, features and many of the attendant advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description, when considered with the accompanying drawings, wherein:

FIG. 1 illustrates in a somewhat schematic sectional view a synchronizing valve forming part of the inventive control mechanism;

FIG. 2 is a partly structural and partly schematic illustration of the synchronizing controls of the invention;

FIG. 3 is a transversal sectional view of the valve mechanism, taken along line III-III in FIG. 1;

FIG. 4 is an end view ofone of the control disks, taken along line IV-IV ofFIG. 1;

FIG. 5 is another sectional view of the valve mechanism, taken along line V-V in FIG. 1; 7

FIG. 6 is an end view of the other control disk, taken along line VI-VI in FIG. 1;

FIG. 7 is a side view of one of the spindle mechanisms with the corresponding rack bar, used in the inventive control mechanism; and

FIG. 8 shows a further rack bar as arranged between the two spindles, as shown in FIG. 7.

In the idle or rest position of the inventive control mechanism, a pump 1 delivers pressure fluid through a pipe or passage 2 to a valve 3, further flow being interrupted to other connections in the valve by way of passages 4 and 5. In this rest position the fluid feed is directed without pressure through a valve 6 to a discharge 7. The upper chamber or space of a press cylinder 9 is connected by way of a pipe 78 with a passage 8 in the valve 3, and the upper chamber of a second press cylinder 11 is similarly connected by way of a pipe 79 with a passage 10 in the valve.

It will be seen from the illustrations of FIGS. 1 and 2 that cylinder 9 is the left-hand element and cylinder 11 the righthand element of the inventive control system, as will be explained in more detail as the specification proceeds. The lower side of cylinder 9 is connected with a passage 12 and the lower side of cylinder 11 with a passage 15 in the valve 3, both by way of a pipe 76 as shown.

The valve 3 contains a piston 16 which in the rest position is held at both ends by springs 17 at the middle of its stroke. This closes the connection between the cylinders 9, 11 and the pump 1 as well as the discharge 7 by way of the respective narrower and wider piston sections, as shown in FIG. I. The piston 16 has the further purpose of connecting the pump 1 with either cylinder 9, 1 1 through the appropriate passages, or with a discharge pipe or return circuit 82.

The parallel motion is regulated by respective rotatable control disks l8 and 19 (FIGS. 4 and 6, respectively) associated with the working or power cylinders. These control disks are housed in respective covers 20 and 21 and easily rotatable in either direction until they leave axially a minimum amount of play between a casing or valve body 22 and the respective cover. Both end faces of the body 22 have control apertures in a particular arrangement with respect to each other, as will be seen from FIGS. 3 and 5.

Control apertures or ports 23a, 23b of disk 18, as shown in FIG. 3, communicate directly with the passage 4 of the valve 3 (FIG. 1). For the sake of clarity, pipes between FIGS. 3 and 5 as well as the valve of FIG. 1 have been omitted from the latter. Ports 24a, 24b are connected with a passage 25 while ports 26a, 26b communicate with a passage 27. Ports 28a, 28b are in communication with a passage 29. The ports 24a, 24b are radially extended by slits 30a, 30b. The ports 28a, 28b are similarly extended by slits 31a, 31b.

Ports 14a, 14b in the body 22 are connected with a port 13 of valve 3, while ports 32a, 32b, also in the body 22, shown in FIGS. 3 and 5, respectively, with the passage 5, similarly in the body 22, ports 33a, 33b with a passage 34, ports 35a, 35b with a passage 36, bores 37a, 37b with a passage 38, and ports 39a, 39b with a passage 40. The ports 33a, 33b are continued in the direction of the pitch circle by slits 41a, 41b. The ports 37a, 37b are also extended in the direction of the pitch circle by slits 42a, 42b.

Since the control disks 18, 19 provide communication between the difierent ducts or passages, as well as they perform the regulation of the synchronized parallel motion, both disks have on their end faces depressions or hollows arranged in a predetermined and particular manner.

The hollows on the end faces of the control disks 13, 19 on the side of the end faces of the body 22 afford communication between the individual control ports on said body 22, i.e., open or close them as may be necessary for correcting alignment errors in the parallel motion. The control disks are completely pressure balanced. They are connected by respective spindle portions 43 and 44 (FIG. 2) with corresponding gear wheels 45 and 46. These wheels mesh with corresponding rack bars 47 and 48, coupled by respective rods 49 with rack bars 50 and 51.

A press ram or plunger 52 (FIG. 2) carries a through shaft or spindle 53 in three bearings, the shaft being provided with gear wheels 54, 55 and 56. The rack bars 50, 51 mesh with the gear wheels 54, 55 as long as the wheel 56 is in mesh with a rack bar 57. The rack bars 50, 51 are freely suspended but held together by respective bearing rollers 58 and 59. The rack bar 57 is upwardly biased by a spring 60 (see FIG. 8) and is slidable only in the direction of the pressure or power stroke of the press plunger 52. This locking of the rack bar 57 also holds the rack bars 50, 51.

Valves 71, one each for the cylin ders7 and 9 are only schematically represented in FIG. 2, and serve as arresting valves during the downstroke of the press plunger 52 (see FIG. 2) until they give free passage in the opposite direction to the lower spaces of respective cylinders 9, l 1.

The action of the synchronizing parallel motion control mechanism according to the invention is as follows, reference being also had to certain structural elements which were not described so far.

When the valve 6 (see FIG. 2) is set for a press stroke, it interrupts the pressure free flow from the pump 1 to an intermediate section of a pipe or passage 61 and thence into the return pipe or discharge 7. In this manner, the pump 1 is placed in communication with a pipe or passage 62, and the pipe 2, while a pipe or passage is now connected with the discharge 7. Consequently, pressure is admitted by way of passage 62 to a chamber 64 in the valve 3 (see FIG. 1) while a chamber 63 therein is connected with the discharge pipe 7 by way of passage 65. The piston 16 necessarily moves to the left, by the pressure applied in the chamber 64, and stops at the end of its stroke.

The delivery from the pump 1 passes through pipe 2 and thence through a passage 66 and ports 67 and 68, respectively, to the control disks 18, 19 (FIG. 1). It the error in parallelism between the press ram 52 and a platen 52' equals zero, and the rack bars 50, 51 are not moved, the spindle portions 43, 44, and with them the control disks l8, 19 remain in mid-position.

The control disk 18 is also shown in FIG. 4. The hollows or ports are located on both sides of the end face of the control disk, those on the one side being for regulating and those on the other side only for compensating the pressure. All the hollows are connected by bores 69, as represented in FIG. 4 (and also in FIG. 6, for disk 19).

The control disk is considered to be in the zero or neutral position when the respective passage cross sections 70a, 70b, 72a, 72b from the ports 23a, 23b to the control disk 18 are of identical area. The same applies for the control disk 19 which is also shown in FIG. 6, and will be explained later in more detail.

In the press or working stroke, the oil or hydraulic fluid passes through the passage 66 and the port 67 in the control disk 18, to the ports 26a, 26b and 28a, 28b, respectively. The ports 26a, 26b lead to the passage 27 and through the free opening thereof, the pressure fluid reaches the passage 8 and the upper chamber of cylinder 9, respectively.

The ports 28a, 28b lead into the passage 29 wherein the further flow to the other passages is blocked. Through the port 68, the oil passes through the control disk 19 and into the ports 35a, 35b or the passage 36, respectively, and further into the upper chamber of cylinder 11.

The lower side of cylinder 9 (FIG. 1) is placed in communication by the valve 71 with one of the passages 12 and 13, respectively. The passage 13 leads to the ports 14a, 14b and through the control disk 18 to the ports 24a, 24b. The latter lead into the passage 25. This passage is connected with the previously mentioned discharge pipe or passage 82.

The lower space of cylinder 11 is similarly connected, through the other valve 71, with the passage 15, or with the passage 40, respectively. The latter communicates with the ports 39a, 39b. These ports are connected through the control disk 18 with the ports 33a, 33b, and the latter communicate with the passage 34 which is in communication with the discharge pipe 82.

In the manner described, the press plunger 52 moves downwards (refer to FIG. 2). If there is no error in parallelism,

the gear wheel 56 meshes with the rack bar 57, and the gears 54, 55 similarly mesh with the rack bars 50, 51. Since the rack bar 57 is pretensioned upwards by the spring 60, the rack bars 50, 51 are now immobilized. If the left-hand cylinder 9 leads with reference to the right hand cylinder 11, that is, when the press beam or plunger 52 becomes skewed, the rack bar 51 is drawn downwards; and since the gear 56 is in mesh with the rack 57, which is upwardly pretensioned, the opposite rack bar 51 is imparted an identical, oppositely directed motion.

This causes the spindle portion 43 and with it the control disk 18 to rotate counterclockwise, which reduces the cross sections 70b, 72b of the ports 23a, 23b with reference to the area of the ports 26a, 26b, i,e., the feed to the upper, left-hand cylinder space is throttled. It should be noted that the cross sections 70a, 72a will be described somewhat later. At the same time, cross sections 73a, 73b (FIG. 3) are reduced and the flow from the lower space of cylinder 9 toward the discharge 82 is likewise throttled.

Since the control disk 19 is similarly rotated, but in the opposite direction, sections 77a, 77b (FIG. 5) to the upper space of cylinder 11 are increased, whereby cross-sections 75a, 75b of the communication between the lower space of cylinder 11 and the return flow 82 are likewise enlarged. Consequently, the cylinder 9 acts more slowly, and the cylinder 11 more quickly.

This provides a correcting action on the parallel-motion control gear and the error is compensated. The control disks 18, 19 move into the starting positions. Should the cylinder 11 lead the cylinder 9, the error in the parallel-motion gear is likewise compensated in the same manner as described above but in the opposite direction.

For instance, if edge trimming is proceeding at full power on the right-hand side of the press plunger 52, i.e., maximum asymmetrical load is being applied, the left-hand side of the press plunger 52 will tend to lead. Although the error in parallelism is being corrected, the press plunger will still continue to lead. In such case, as already described, the vertical displacement and rotation of the spindle will drive the left-hand rack bar downwards, and the right-hand rack bar upwards.

The left-hand, leading side is drawn down by the vertical displacement of the rack bar, and the spindle 43 will rotate clockwise; similarly, the control disk 18 will be rotated by the action of the spindle 44 in the opposite direction. Since the left-hand side is continuously in the lead, the passage cross sections 72a, 72b, 73a, 73b will also be reduced, as well as the connections between the pump and the upper cylinder space, and between the lower cylinder space and the return circuit.

If the error in parallelism on the left-hand side further increases, the rotation of the control disk 18 will further interrupt the communication from the ports 23a, 23b through the hollows in the control disk 18, with the ports 26a, 26b; and thus, also the connection between the pump and the upper cylinder space.

If, by increasing error, the control disk 18 is further rotated, even very slightly, the ports 26a, 26b are placed in communication with the slits 30a, 30b through the hollows in the control disk 18. Since these slits are in communication with the ports 24a, 24b, and the latter communicate with the passage 25, a connection is established from the upper side of the cylinder 9 to the return circuit 82.

Simultaneously, with rotation of the control disk 18 the communications between the ports 24a, 24b and the ports 14a, 14b are interrupted, and thereby the discharge from the lower side of the cylinder 9 to the return circuit is closed. By the rotation of the control disk 18 the hollows therein will simultaneously interrupt the communication between the ports 14a, 14b and the return circuit, while communication is opened with the slits 31a, 31b.

This means that at the limit of the permissible error, the leading piston in the cylinder is relieved of pressure from above and receives pressure underneath. In such manner it is possible to obtain a satisfactory, steady parallel motion, however asymmetrical the loading may be, which is most convenient for edging over a born or beak when the forces in the cylinder are alternating.

If, conversely, the valve 6 is set for a return stroke, the pressure fluid flows through the pipe 65 into the chamber 63, and the chamber 64 is relieved of load by the pipe 62. The piston 16 moves to the right, to the end of its stroke. The pressure fluid passes through the ports 23a, 23b and the bores or cross sections 70a, 7b into the ports 28a, 28b. These ports communicate with the passage 29 and thence with the passage 12 which in turn communicates through the pipe 76 and one of the valves 71 with the lower space of cylinder 9. The upper space thereof is connected with the passage 8 and the latter with the passage 25 leading to the discharge pipe 82. Another pipe 76 is provided for cylinder 11, as shown and explained before.

The ports 26a, 26b as well as the ports 14a, 14b lead to the corresponding passages in the valve body 22 and are not operating. Similarly, the ports 32a, 32b are connected by apertures 74a, 74b with the ports 37a, 37b and thence with the passage 38 continuing further into the passage 15. From the latter, through the pipe 76 and the other valve 71, the pressure fluid finally reaches the lower space of cylinder 11.

The upper space of the same communicates through the pipe 79 with the passage 10, and the latter with the passage 34, leading furtherinto the return pipe 82. The ports 35a, 35b as well as the ports 39a, 39b lead into the respective passages 36, 40. The further connections are closed. The press plunger 52 moves upwardsf I If the right-hand side of the press plunger leads the left-hand side, the rack bar 51 is moved for the same distance upwards by the central bearing (gear wheel 56 with rack bar 57) while the rack bar necessarily moves downwards. Consequently, the rods 49 and the rack bar 48 meshing with the gear wheel 56, as well as the spindle 44, are rotated in clockwise sense. The spindle 43 rotates through the same angle but in the opposite direction.

By this means, the rotation of the control disk 19 causes throttling of the cross sections or apertures 74a, 74b while similar rotation of the control disk causes the cross sections or bores 70a, 70b to be increased. The cylinder 11 slows down its motion while the cylinder 9 correspondingly speeds up. This corrects the error in parallelism.

If repeated, accurate stopping of the top press platen is required, the length of stroke can be adjusted by a setting screw and a bearing nut 81, fitted on the press platen 52' associated with the press ram 52 (see FIG. 8). Accurate stopping is possible only in the pressing direction. This is attained in the following manner: The press plunger 52 moves downwards, and with it the setting screw 80. The rack bar 57 with the spring 60 are exactly in the mid-position. During the free stroke, the rack bar 57 remains stationary and the gear wheel 56 rotates therealong.

After a specified distance run, the setting screw 80 approaches the rack bar 57, overcoming the force of the spring 60. The rack bar 57 is thus constrained to move with the same speed as the press plunger 52 through a corresponding distance downwards, and by this action of the rack bar and the meshing of the gear wheel 56 with the rack bar, the spindle 53 is similarly moved downwards, without rotation. This causes the rack bars 50, 51 on the spindle 53 similarly to move downwards, and the rack bars 47, 48 rotate the respective spindle portions 43, 44. The portion 43 is rotated clockwise and the portion 44 counterclockwise. This causes the control disks 18, 19 to rotate so that the cross sections apertures or 72a, 72b, 73a, 73b throttle or close the cylinder 9, and the cross sections or apertures 75a, 75b, 77a, 77b act similarly with regard to the cylinder 1 1.

At this instance the pump feed to the two cylinders 9, 11 is closed and simultaneously the pressure is released and con nected with the return circuit, on the upper sides of the cylinders. If the press plunger 52 continues to move by the action of the accelerated masses, the control disks 18, 19 are correspondingly rotated in the closing direction whereby the connection between the pump and the lower sections of the cylinders is restored.

Pressure from underneath causes instantaneous stopping of the top press platen. In this manner, hydraulic equilibrium is established between the pressure on the upper and the lower sides of the cylinders 9, 11. By this means, the upper press platen is accurately stopped and held in the desired position.

It is of course also possible to separate the valve mechanism so that each control disk has a separate control valve, associated therewith in separate assemblies, each of which functioning through the corresponding piston or cylinder and control disks, to perform the combined function of the single valve as described herein.

Owing to the involved details of the structure described hereinabove, it was not possible to allot to the structural elements a unitary system of reference numerals. In the way of additional clarification, the following is therefore submitted:

A. The elements designated by numerals l to 3, 6, 7, 16, 22, 52, 52', 53, 56, 57, 60, 61, 70 as well as 80 to 82 relate to the entire control mechanism.

B. The following elements cooperate with, or are provided for, both sides of the valve system (left and right, as viewed in FIGS. 1 and 2): 17, 49, 58,59, 66, 69, 71 and 76.

C. Finally, a concordance between leftand right-side elements of the mechanism can be given as follows, for ready identification of the corresponding separate (but not systematically numbered) structural elements:

a. Certain pairs of leftand right-side elements can be listed as follows: 4, 5; 8, 10; 9, ll; 12, 15; 13, 40; l8, 19; 20, 21; 25, 34; 27, 36; 29, 38; 43, 44; 45, 46; 47, 48; 50, 51; 54, 55; 63, 64; 65, 62; 67, 68; and 78, 79.

b. The remaining pairs of reference numerals all have a and b suffixes, like 14a and 14b, corresponding to 39a and 39b, respectively; in the following, the suffixes will be omitted for the sake of simplification: 23, 32; 24, 33; 26, 28, 37; 30, 41; 31, 42; 70, 74; 72, 77; and 73, 75.

It should be understood, of course, that the foregoing disclosure relates only to perferred embodiments of the invention and that it is intended to cover all changes and modifications of the example described which do not constitute departures from the spirit and scope of the invention as set forth in the appended claims.

We claim:

1. In a parallel-motion control mechanism for at least two double-acting, hydraulically actuated cylinders, in particular for the operation of edging and trimming presses and the like machine tools, the latter having a press plunger 52; two press cylinders 9, 11 having respective power pistons operatively coupled with said plunger; means 43 to 57 for correcting errors in parallelism between said cylinders by the application of counter-pressure in the latter; said means including rack bars 47, 48, 50, 51 adjustable in the direction of motion of said pistons, and at least one pair of gear wheels 45, 46, 54, 55 meshing with said rack bars and arranged at the ends of at least one spindle 43, 44, 53 extending substantially over the entire length of said plunger and carried thereby, the improvement comprising, in combination, pressure pipes 76, 78, 79 leading to said cylinders; pressure-fluid feed lines 2, 7, 61, 62, 65, 82 associated with said pipes; and valve means 3 for controlling an actuating piston 16 therein, said pressure pipes coacting with said piston; said valve means including two rotatable control disks 18, 19 disposed on the ends of a body portion 22 of said valve means, being located in the direction of motion of said valve piston, and having depressions therein adapted to connect corresponding passages 4, 5, l3, 15, 25, 27, 29, 34, 36, 38, 40, 67, 68 leading to said feed lines; said spindle having substantially in the center between said pair of gear wheels a further gear wheel 56 meshing with a further rack bar 57 associated with said plunger.

.2. The improvement in a control mechanism as defined in claim 1, wherein said disks 18, 19 are fitted on end portions of said spindle 43, 44, 53, each coacting by means of one of said gear wheels 45, 46, 54, 55 with a respective one of said rack bars 47, 48, 50, 51, the latter being slidable in the direction of motion of said plunger 52.

3. The improvement in a control mechanism as defined in claim 1, further comprising backing rollers 58, 59 arranged adjacent said rack bars 47, 48, 50, 51 opposite their sides meshing with said gear wheels 45, 46, 54, 55.

4. The improvement in a control mechanism as defined in claim 1, wherein the range of motion of said further rack bar 57 is restricted by a stop provided on a frame portion of the machine tool, and said further rack bar is movable only in the direction of advance of said plunger 52.

5. The improvement in a control mechanism as defined in claim 1, further comprising a spring 60 for holding said further rack bar 57 in an extreme position and biased in a direction opposite to the direction of motion of said plunger 52.

6. The improvement in a control mechanism as defined in claim 1, further comprising a stroke-limiting, adjustable stop for said plunger 52, coacting with said further rack bar 57 so as to limit the working stroke of said plunger.

7. The improvement in a control mechanism as defined in claim 1, wherein said valve means 3 and said disks I8, 19 constitute independent control means performing substantially identical functions.

8. The improvement in a control mechanism as defined in claim 1, further comprising spindle portions 43, 44 for carrying respective ones of said disks 18, 19, said spindle portions coacting, by means of said gear wheels 45, 46 carried thereby, with said rack bars 47, 48, slidable in the direction of motion of said plunger 52.

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