US3726184A

US3726184A - Structure of fluid pulse cylinder for numeric control and a method of making the same

Info

Publication number: US3726184A
Application number: US00092806A
Authority: US
Inventors: T Saito
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-08-27
Filing date: 1970-11-25
Publication date: 1973-04-10
Anticipated expiration: 1990-04-10
Also published as: DE2127375B2; BE758863A; GB1342249A; DE2127375C3; JPS5510761B1; FR2069630A5; DE2127375A1

Abstract

A fluid pulse cylinder for numeric control in an oil pressure or air pressure cylinder includes the combination of a pulse motor and a fluid cylinder. A slider having a pressurized fluid path, a return fluid path and a guide port is provided on the inside of a hollow cylindrical rod, the slider being driven by a pulse motor which rotates in either the forward or backward direction in response to an instruction pulse. An injection port and a return port are provided on opposite sides of a piston, the ports being opened or closed to instantaneously effect the forward or backward movement or stoppage of the piston, or to control either application or release of pressure by the same principle, to allow positioning of the piston.

Description

Unite tates atet [191 Saito [4 1 Apr. 10, 1973 STRUCTURE OF FLUID PULSE CYLINDER FOR NUMERIC CONTROL AND A METHOD OF MAKING THE [21] App1.N0.: 92,806

[30] Foreign Application Priority Data Aug. 27, 1970 Japan ..45/74792 [52] US. Cl. ..91/35, 91/376, 92/111,

92/113 [51] Int. Cl. ..FlSb 21/02, Fl5b 9/10 [58] Field of Search ..91/35, 39, 419, 422, 91/423, 459, 435, 378, 376, 380; 92/110, 111, 113, 114

[56] References Cited UNITED STATES PATENTS 2,941,514 6/1960 Jablonsky ..92/110 2,992,633 7/1961 Stiglic et a1... .....9l/376 3,296,936 l/1967 Wess et a1. ..91/376 3,621,762 1l/l97l Ikese et a1 ..91/380 2,926,635 3/1960 Leonard et a1. ...9l/378 2,930,360 3/1960 Yando ...91/459 3,195,575 7/1965 Sheppard ...9l/378 3,277,790 10/1966 Walters ..91/35 3,310,284 3/1967 Inaba et al.. ...91/378 2,755,779 7/1956 Muller ...91/422 2,705,940 4/1955 Edwards ..91/422 Primary ExaminerPau1 E. Maslousky Att0rneyWenderoth, Lind & Ponack 5 7 ABSTRACT A fluid pulse cylinder for numeric control in an oil pressure or air pressure cylinder includes the combination of a pulse motor and a fluid cylinder. A slider having a pressurized fluid path, a return fluid path and a guide port is provided on the inside of a hollow cylindrical rod, the slider being driven by a pulse motor which rotates in either the forward or backward direction in response to an instruction pulse. An injection port and a return port are provided on opposite sides of a piston, the ports being opened or closed to instantaneously effect the forward or backward movement or stoppage of the piston, or to control either application or release of pressure by the same principle, to allow positioning of the piston.

8 Claims, 13 Drawing Figures PATENTEDAPR 1 01975 SHEET 1 [IF 4 N R w JN NW R ADASHI SAITO Mal/14d. ZI'LM Attorneys PATENTED 1 @1973 3,726,184

SHEET 3 [IF 4 TADASHI SAITO,

I NVENTOR.

ATTORNEYS PATENTEBAPRIOIQTS 3,726,184

sum u 0F 4 INVENTOR.

TADASHI SAITO wamm Attorney STRUCTURE OF FLUID PULSE CYLWDER FOR NUMERIC CONTROL AND A METHOD OF MAKING THE SAME BACKGROUND OF THE INVENTION Typical examples of the prior art positioning method are numeric control by an electric pulse motor and numeric control by an electric oil-pressure pulse motor. Both of these are constructed by a similar circuit, the former being suitable for smaller output while the latter being selected for larger output. It appears that existing numeric control systems are limited to these two systems. When the electric pulse motor is rotated by input pulses, a four-side guide valve of the oil-pressure motor is rotated through a reduction system to inject pressurized oil into the oil-pressure motor, which, in turn, is rotated and stops after a predetermined number of rotations as instructed by a program or a program tape. The motor may be rotated in the reverse direction and a high output oil-pressure motor may be controlled by a low torque pulse motor. However, both the electric pulse motor and the electric oil pressure pulse motor are stepped and the electric pulse motor has a low output and hence a large coil and diameter are required to produce high output. Thus, energy loss becomes large and it is difficult from a practical standpoint to manufacture a motor of more than one-half horse power (HP). Furthermore, it is so heavy that there is a problem in mounting it, and the output is too low to apply it to heavy shearing operations. Thus, it may be used only in positioning of a board in a drilling machine. The cost of the motor is so expensive in comparison with that of a body of a driven machine that it is often found that manual operation is cheaper than motor operation. On the other hand, the electric oil pressure pulse motor can overcome the defects of the electric pulse motor and can provide low speed, high output so that it is used as means for controlling the rotation of the oil pressure motor through the pulse motor. However, since the oil pressure unit for numeric control is very expensive, most of the total cost is shared by the oil pressure units in a three-dimension (X, Y and Z axis) control such as in a milling machine. Both the electric pulse motor and the electric oil pressure pulse motor perform numeric control by translating the pulses to rotation which causes rotation of the screw. Therefore, it is very expensive to obtain high power output with such systems. Namely the electric pulse motor which can provide high output exhibits low efficiency, and the electric oil pressurepulse motor which can provide high output'is expensive because the oil pressure motor therefor is expensive. A feeding apparatus for a blade support of a machine tool or a table for securing an article to be machined is generally classified into a rotary feed type or a cylindrical feed type, the former permits numeric control by a pulse motor, while the latter is relatively less expensive but has low accuracy so that the sequence (movement) of the blade support and the table are instructed by a program while the position and the dimension are received by an external limit switch or other detector to block a drive source (oil pressure or air pressure source) to halt movement. This is identified as the so-called programmed sequence control, and numeric control using this system is still impossible. For the positioning of the cylinder many approaches such as the limit switch method or approximate switch method has been investigated, but no practical way of numeric control is known. In the prior art positioning system for the cylinder, a position specifying means must be provided exteriorly and hence accurate positioning is difficult and the position of a dock must be adjusted while trying shearing. This is the so-called cut and try method. For this purpose, however, a high level of skill and a great amount of adjustment time are required. This diminishes the merits of the system except when the system is designed for a single purpose machine or a machine for limited purposes with mass production.

The above is the summary of the construction, operation and advantages or disadvantages of the prior art electric pulse motor and electric oil pressure pulse motor. Drive by an oil pressure or air pressure cylinder is highly desired in all sorts of machine tools and numeric control by the cylinder is extensively expected in the art.

SUMMARY OF THE INVENTION With the above discussion in mind, it is an object of this invention to combine a pulse motor and a pressurized fluid cylinder in a simple manner so that a highly efficient numeric control mechanism may be provided, whereby smaller enterprises having less economic power can compete in the mass production of devices such as machine tools.

It is another object of this invention to accomplish highly efficient numeric control in a simple manner only by combining a pulse motor and a pressurized fluid cylinder mechanism without necessitating any external limit switch or other positioning device, or high level skill or time consuming adjustment.

This invention provides linear output control by the use of low-cost and simple construction of a pressurized fluid cylinder mechanism, in which in response to forward or backward rotation of a pulse motor a screw shaft fixed to a reduction gear rotates in backward or forward direction whereby a positioning slider in a cylindrical rod is moved toward or away from the pulse motor to selectively change the direction of flow of pressurized fluid to facilitate instantaneous forward or backward actuation of a piston. A detent for the slider may be positioned in a rectangular bore so that it does not follow the rotation of the screw whereby the rotation of the screw may be converted to a linear motion.

The construction, operation and advantages of the fluid pulse cylinder for numeric control in accordance with the present invention which is formed by the combination of a pulse motor and a pressurized fluid cylinder mechanism will now be described in detail with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal cross-sectional view of a first embodiment;

FIG. 2 is a longitudinal cross-sectional view illustrating a forward motion in the first embodiment;

FIG. 3 is a longitudinal cross-sectional view illustrating a backward motion in the first embodiment;

FIG. 4 is a longitudinal cross-sectional view illustrating a one-way control mechanism in a pressure system of a second embodiment;

H6. 5 is a partial longitudinal cross-sectional view illustrating a forward motion in the pressure system of the second embodiment.

FIG. 6 is a partial cross-sectional view illustrating a backward motion in the pressure system of the second embodiment;

FEG. 7 is a lateral cross-sectional view taken along the line A-A in FIG. 5;

FIG. 8 is a lateral cross-sectional view taken along the line B-B in FIG. 4;

FIG. 9 is a longitudinal cross-sectional view illustrating a one-way control mechanism in a decompression system of the second embodiment;

FIG. 10 is a partial longitudinal cross-sectional view illustrating a forward motion in the decompression system of the second embodiment;

FIG. 11 is a partial longitudinal cross-sectional view showing a backward motion in the decompression system of the second embodiment;

FIG. 12 is a lateral cross-sectional view taken along DETAILED DESCRIPTION OF THE INVENTION With reference now to FIGS. 1-3 of the drawings, a first embodiment of the present invention will be described in detail.

A housing 44 has therein a pulse motor 49, an output shaft of which has fixed thereon a pinion 39. A reducing gear 41 engages pinion 39 and is fixed to a hollow screw shaft 38. Screw shaft 38 extends through housing 44 and through a fixed hollow cylinder 21. The inner periphery of cylinder 21 is engaged by the outer periphery of a piston 22 which is fixed to the outer periphery of a hollow cylindrical rod 43. The outer periphery of screw shaft 38 has at the end opposite reducing gear 41 external threads which engage the internal threads of a hollow slider 36. It will be apparent that slider 36 slides within hollow cylinder rod 43 and that hollow cylindrical rod 43 and piston 22 slide within cylinder 21. A center oil return pipe 34 extends inwardly of hollow screw shaft 38, and a center oil supply pipe 33 extends inwardly of center oil return pipe 34. An oil supply slide pipe 35 which is integral with slider 36 extends inwardly of center oil supply pipe 33. Within slider 36 is an oil supply channel 32 which communicates with oil supply slide pipe 35 and front supply pilot 28 and rear supply pilot 29 formed in slider 36. Also formed within slider 36 is an oil return channel 27 which communicates with center oil return pipe 34 and which also communicates with front return pilot 23 and rear return pilot 24 formed in slider 36. Formed in hollow cylindrical rod 43 are front supply port 30 and rear supply port 31, as well as front return port 25 and rear return port 26. Depending upon the relatively longitudinal positioning of slider 36 and hollow cylindrical rod 43, communication may be open between rear supply pilot 29 and rear supply port 31 and front return pilot 23 and front return port 25 (as shown in FIG. 2), or between front supply pilot 28 and front supply port 30 and rear return pilot 24 and rear return port 26 (as shown in FIG. 3). Extending forwardly from slider 36 is a detent 37 having a noncylindrical cross-section which fits in a similarly formed detent slot 42. Interaction between detent 37 and slot 42 prevent rotation of slider 36. It will be apparent that as pinion 39 is rotated in one direction by pulse motor 40, screw shaft 38 is rotated by reducing gear 41 in the opposite direction. Depending upon the rotation of screw shaft 38, slider 36 is caused to move either leftwardly or rightwardly as viewed in the Figures with respect to hollow cylindrical rod 43. This will open either the left or right portions of cylinder 21 to the pressurized oil supplied through center oil supply pipe 33, thereby causing piston 22 and hollow cylindrical rod 43 to be moved.

The above described device is a typical embodiment of the present invention, and permits instantaneous forward and backward movements for controlling a curve under two-axis control or curved working under threeaxis control (see FIG. 1). When reduced to one tenth by pinion 39 and reducing gear 41, screw shaft 38 completes one revolution per, for example, 240 pulses X 10 2400 pulses. Thus, assuming that the feed pitch of the screw shaft 38 is 10mm, the linear length (move ment) per pulse is l0/2400mm 0.0042mm. This enables highly accurate control. It is difficult, however, for a pulse motor to provide high power output by itself from both construction and economical standpoints. Therefore, by incorporating the pulse motor in a cylinder to allow pulse rotation to be translated into a linear motion of high power output in accordance with the present invention provides a solution for the above difficulty. According to the present invention, as the pulse motor 40 rotates, the screw shaft 38 secured to the reducing gear 41 rotates, and the positioning slider 36 moves forwardly or backwardly in response to the direction of rotation of the pulse motor, whereby the flow of the pressurized oil selectively causes instantaneous forward or backward actuation of the piston 22 and simultaneous recovery of return oil.

The forward movement of piston 22 will first be described with reference to FIG. 2. When the pulse motor 40 rotates clockwise (as viewed from the left of FIG. 2) the screw shaft 38 rotates counterclockwise, and positioning slider 36 in cylindrical rod 43 is advanced (to the right). As the positioning slider 36 is advanced, communication is opened between rear supply pilot 29 and rear supply port 31, and pressurized oil is injected into cylinder 21 serially through pressurized center oil supply pipe 33, pressurized oil supply slide pipe 35, channel 32, rear supply pilot 29, rear supply port 31. Thus, the piston 22 and cylindrical rod 43 move to the right. The piston 22 follows this continuous movement until mrew shaft 38 is stopped by instructions from the program. When screw shaft 38 stops its rotation, the positioning slider 36 stops. However, pressurized oil still enters cylinder 21 through pilot 29 and port 31. Therefore, cylindrical rod 43 continues advancement until port 31 is blocked from pilot 29. When this occurs, advancement of cylindrical rod 43 and piston 22 is terminated.

Similarly, during the above rotation of screw shaft 38, front return port 25 and front return pilot 23 communicate with each other, and the return oil from the right side of cylinder 21 is forced therethrough and through channel 27 into center oil return pipe 34. The operation of the return oil circuit is continued until the screw shaft 38 stops and thereafter until communication between port 25 and pilot 23 is terminated.

The communication and blocking of the pressurized oil circuit and the return oil circuit are effected in synchronous fashion as described above, and piston 22 advances under the control of the number of pulses instructed by the program.

The backward movement of piston 22 will now be described. The piston 22 halted at the position instructed during the forward movement is caused to start its backward movement as the result of backward movement of positioning slider-36 due to the screw shaft 38 being rotated in a direction opposite to that described above by the pulse motor 40. By the backward movement of the positioning slider 36, the front supply port 30 and the front supply pilot 28 communicate with each other (as shown in FIG. 3) whereby pressurized oil is injected into the cylinder 21. Thus, movement of the piston 22 toward the left is caused by the pressure of the injected oil. Similarly, the rear return port 26 and the return rear pilot 24 in the return circuit communicate with each other, and the recovery of the return oil is commenced. The operation of the pressurized oil circuit and the return oil circuit is identical to those of the forward movement as described above. When the rotation of the pulse motor 40 is stopped by the instruction of the program, the screw shaft 38 also stops its rotation and hence the positioning slider 36 is stopped. However, piston 22 continues movement until communication between port 30 and pilot 28 and between port 26 and pilot 24 is blocked.

With reference now to FIGS. 4-13 of the drawings, two modifications of a second embodiment of the present invention will be described in more detail.

This embodiment is based on the same principle as the above described first embodiment and includes a cylinder mechanism for controlling either the pressure system or the decompression system shown in FIG. 4 and FIG. 9, respectively. .The system illustrated in FIG. 4 and FIG. 9 is inferior to the pulse cylinder of FIG. 1 in performance, but is simpler in construction and is most suitable for linear control because the production cost thereof is low.

The pressurized oil type pulse cylinder of FIG. 4 is essentially identical to the pulse cylinder shown in FIG.

1 except that only pressurized oil supply is provided in the positioning slider, and recovery of return oil is effected by the pressurized oil solenoids a or b. Therefore, the pressurized oil center pipe 33 and the return oil center pipe 34 of FIG. 1 are combined to form a screw pipe 0. The principle of operation of rightward movement of piston 22 of this embodiment is identical to that of the pressurized circuit shown in FIG. 2 except that the return oil is recovered through the solenoid b as shown by arrow e (see FIG. 5). FIG. 6 illustrates the principle of operation of leftward movement of piston 22, which is identical to that of the pressurized oil circuit shown in FIG. 3 except that the return oil is recovered by solenoid a. FIG. 7 is a cross-sectional view taken along the line A-A of FIG. 5 and illustrates the path f of the pressurized oil into cylinder 21. FIG. 8 is a cross-sectional view taken along the line BB of FIG. 4, in which g designates a lateral cross-section of a detent for the positioning slider corresponding to the detent 37 in FIG. 1.

With reference to FIG. 9, the device of FIG. 4 will be described as enployed in a decompression, rather than a pressure, system. In this arrangement solenoids a and b may be alternately switched to inject pressurized oil to the right or left of piston 22 in cylinder 21 in response to the direction of rotation of the screw pipe c to move the piston rightwardly or leftwardly. The principle of the operation is identical to that of the return oil circuit shown in FIG. 1. FIG. 10 illustrates the principle of operation of rightward movement of piston 22, wherein d designates pressurized oil and e designates return oil. FIG. 11 illustrates the principle of operation of leftward movement of piston 22, wherein d designates pressurized oil and e' designates return oil. FIG. 12 is a cross sectional view taken along the line AA' of FIG. 10, wherein f designates the path of the return oil. FIG. 13 is a cross sectional view taken along the line B'-B of FIG. 9, wherein g designates a lateral cross section of the slider detent as in the case of FIG. 8.

As will be apparent, in the cases of both FIGS. 10 and 1 1, return oil exits through screw pipe c.

As described above, FIGS. 1, 4 and 9 operate on the same principle. FIG. 1 shows a basic embodiment of the present invention and has the object of providing a pulse cylinder which is suitable for mass-production and which permits a rapid curve control in a very simple manner with low cost. FIGS. 4 and 9 relate to oneside control for pressurized oil or return oil. Since they do not necessitate curve control, low-cost massproduction is realized.

It should be understood from the foregoing description that the present invention has overcome many shortcomings that could not be solved by the prior art electric pulse motor or electric oil pressure pulse motor. When a pulse motor is used, a reduction mechanism and a screw feed mechanism must be provided on an output shaft, thus resulting in a remarkable increase in cost. In the pulse cylinder of the present invention, however, the provision of a slide mechanism directly coupled to the output shaft is sufficient, this being one of the most important distinctions of the present invention over the prior art.

I claim:

l. A numeric control system comprising a pulse motor having a predetermined rotational output; a fixed fluid cylinder; a screw shaft rotatable within said fluid cylinder and having external threads on one end thereof, said screw shaft being longitudinally fixed with respect to said cylinder; means connecting said rotational output to said screw shaft for causing said screw shaft to rotate; a positioning slider surrounding said screw shaft and having internal threads engaging said external threads, said slider being longitudinally movable with respect to said screw shaft; a hollow cylindrical rod surrounding said slider and being longitudinally movable with respect thereto; means interconnecting said slider and said cylindrical rod to prevent rotation of said slider; a piston fixed to the outer periphery of said cylindrical rod, the outer periphery of said piston slidably engaging the inner surface of said fluid cylinder; means for selectively supplying pressure fluid into said fluid cylinder on either side of said piston; 'and means for simultaneously discharging fluid from said fluid cylinder on the opposite side of said piston;

whereby pulses from said pulse motor cause selective clockwise or counterclockwise rotation of said screw shaft, thus causing forward or rearward movement of said slider, whereby said fluid is supplied to said fluid cylinder on either side of said piston to cause said piston to move forwardly or rearwardly.

2. A system as claimed in claim 1, wherein said pressure fluid supplying means comprises front and rear supply ports extending through said cylindrical rod into said fluid cylinder on either side of said piston; front and rear supply pilots in said slider and longitudinally offset-from said front and rear supply ports, respectively; and a center supply pipe extending through said screw shaft and in communication with said front and rear supply pilots.

3. A system as claimed in claim 2, wherein said fluid discharging means comprises front and rear return ports extending through said cylindrical rod into said fluid cylinder on either side of said piston; front and rear return pilots in said slider and longitudinally offset from said front and rear return ports, respectively; and a center return pipe extending through said screw shaft and in communication with said front and rear return pilots.

4. A system as claimed in claim 3, wherein said front and rear, return and supply, ports and pilots are dimensioned such that: as said slider is moved forwardly with respect to said cylindrical rod, said rear supply port and pilot and said front return port and pilot are in communication, but said front supply port and pilot and said rear return port and pilot are not in communication;

and as said slider is moved rearwardly with respect to said cylindrical rod, said front supply port and pilot and said rear return port and pilot are in communication, but said rear supply port and pilot and said front return port and pilot are not in communication.

5. A system as claimed in claim 1, wherein said pressure fluid supplying means comprises a rear plurality of ports extending through said cylindrical rod into said fluid cylinder on a first side of said piston; a rear plurality of pilots in said slider and longitudinally offset from said rear ports; a front plurality of ports extending through said cylindrical rod into said fluid cylinder on a second side of said piston; a front plurality of pilots in said slider and longitudinally offset from said front ports; and a center supply pipe extending through said screw shaft and in communication with said front and rear pilots; and wherein said fluid discharging means comprises a first selectively operable fluid return communicating with said fluid cylinder on said first side of said piston; and a second selectively operable fluid return communicating with said fluid cylinder on said second side of said piston.

6. A system as claimed in claim 5, wherein said rear ports and pilots and said front ports and pilots are dimensioned such that: as said slider is moved forwardly with respect to said cylindrical rod said rear ports are in communication with said rear pilots, but said front ports are not in communication with said front pilots; and as said slider is moved rearwardly with respect to said cylindrical rod, said front ports are in communication with said front pilots, but said rear ports are not in communication with said rear pilots.

7. A system as claimed in claim 1, wherein said fluid dischar in means com ri s a rear luraii of orts extend11ghrough said cyli ndrical rod into said fluid cylinder on a first side of said piston; a rear plurality of pilots in said slider and longitudinally offset from said rear ports; a front plurality of ports extending through said cylindrical rod into said fluid cylinder on a second side of said piston; a front plurality of pilots in said slider and longitudinally offset from said front ports;

and a center return pipe extending through said screw shaft and in communication with said front and rear pilots; and wherein said fluid supplying means comprises a first selectively operable fluid supply communicating with said fluid cylinder on said first side of said piston; and a second selectively operable fluid supply communicating with said fluid cylinder on said second side of said piston.

8. A svstem as claimed in claim 7, wherein said rear ports and pilots and said front ports and pilots are dimensioned such that: as said slider is moved forwardly with respect to said cylindrical rod said rear ports are in communication with said rear pilots, but said front ports are not in communication with said front pilots; and as said slider is moved rearwardly with respect to said cylindrical rod, said front ports are in communication with said front pilots, but said rear ports are not in communication with said rear pilots.

i i t

Claims

1. A numeric control system comprising a pulse motor having a predetermined rotational output; a fixed fluid cylinder; a screw shaft rotatable within said fluid cylinder and having external threads on one end thereof, said screw shaft being longitudinally fixed with respect to said cylinder; means connecting said rotational output to said screw shaft for causing said screw shaft to rotate; a positioning slider surrounding said screw shaft and having internal threads engaging said external threads, said slider being longitudinally movable with respect to said screw shaft; a hollow cylindrical rod surrounding said slider and being longitudinally movable with respect thereto; means interconnecting said slider and said cylindrical rod to prevent rotation of said slider; a piston fixed to the outer periphery of said cylindrical rod, the outer periphery of said piston slidably engaging the inner surface of said fluid cylinder; means for selectively supplying pressure fluid into said fluid cylinder on either side of said piston; and means for simultaneously discharging fluid from said fluid cylinder on the opposite side of said piston; whereby pulses from said pulse motor cause selective clockwise or counterclockwise rotation of said screw shaft, thus causing forward or rearward movement of said slider, whereby said fluid is supplied to said fluid cylinder on either side of said piston to cause said piston to move forwardly or rearwardly.

2. A system as claimed in claim 1, wherein said pressure fluid supplying means comprises front and rear supply ports extending through said cylindrical rod into said fluid cylinder on either side of said piston; front and rear supply pilots in said slider and longitudinally offset from said front and rear supply ports, respectively; and a center supply pipe extending through said screw shaft and in communication with said front and rear supply pilots.

4. A system as claimed in claim 3, wherein said front and rear, return and supply, ports and pilots are dimensioned such that: as said slider is moved forwardly with respect to said cylindrical rod, said rear supply port and pilot and said front return port and pilot are in communication, but said front supply port and pilot and said rear return port and pilot are not in communication; and as said slider is moved rearwardly with respect to said cylindrical rod, said front supply port and pilot and said rear return port and pilot are in communication, but said rear supply port and pilot and said front return port and pilot are not in communication.

7. A system as claimed in claim 1, wherein said fluid discharging means comprises a rear plurality of ports extending through said cylindrical rod into said fluid cylinder on a first side of said piston; a rear plurality of pilots in said slider and longitudinally offset from said rear ports; a front plurality of ports extending through said cylindrical rod into said fluid cylinder on a second side of said piston; a front plurality of pilots in said slider and longitudinally offset from said front ports; and a center return pipe extending through said screw shaft and in communication with said front and rear pilots; and wherein said fluid supplying means comprises a first selectively operable fluid supply communicating with said fluid cylinder on said first side of said piston; and a second selectively operable fluid supply communicating with said fluid cylinder on said second side of said piston.

8. A system as claimed in claim 7, wherein said rear ports and pilots and said front ports and pilots are dimensioned such that: as said slider is moved forwardly with respect to said cylindrical rod said rear ports are in communication with said rear pilots, but said front ports are not in communication with said front pilots; and as said slider is moved rearwardly with respect to said cylindrical rod, said front ports are in communication with said front pilots, but said rear ports are not in communication with said rear pilots.