US3771267A

US3771267A - Work-holding and advancing table

Info

Publication number: US3771267A
Application number: US00257194A
Authority: US
Inventors: A Fortunski
Original assignee: FARGO MACHINE AND TOOL Co
Current assignee: FARGO MACHINE AND TOOL Co
Priority date: 1972-05-26
Filing date: 1972-05-26
Publication date: 1973-11-13
Anticipated expiration: 1990-11-13

Abstract

A work-holding and advancing table, pneumatically or hydraulically operated, and characterized by a fluid cylinder of relatively low vertical height and substantially wide horizontal transverse width enables the table structure to have a low compact profile of substantially lesser thickness than is present in conventional work supporting tables. Tables of the hereindisclosed construction are particularly suitable for use on snagging grinders, bench grinders, surfae grinders, pedestal grinders, and hand-fed grinders, whereby the operator at the touch of a button causes the work piece to be advanced by the table directly into contact with the peripheral edge of a grinding wheel, to travel a predetermned distance toward the wheel, and to retract to initial position under spring bias, upon release of the pneumatic or hydraulic force, or by pneumatic or hydraulic action.

Description

United States Fortunski atent 1 Nov, 13, 1973 [75] Inventor: Anthony C. Fortunski, Grosse Pointe Park, Mich.

[73] Assignee: Fargo Machine & Tool Company,

Detroit, Mich.

[22] Filed: May 26, 1972 [2]] Appl. No.: 257,194

[52] US. Cl 51/215 R, 51/216 R, 51/238, 51/240 [51] Int. Cl B24b 41/02, B24b 41/06 [58] Field of Search 51/240 R, 238 S, 51/215 R, 216 R, 224

[5 6] References Cited UNITED STATES PATENTS 1,875,486 9/1932 Peaslee 51/238 S 3,369,328 Freerks et al. 51/216 R Primary Examiner-Othell M. Simpson Att0rneyBenjamin W. Colman [57] ABSTRACT A work-holding and advancing table, pneumatically or hydraulically operated, and characterized by a fluid cylinder of relatively low vertical height and substan tially wide horizontal transverse width enables the table structure to have a low compact profile of substantially lesser thickness than is present in conventional work supporting tables. Tables of the hereindisclosed construction are particularly suitable for use on snagging grinders, bench grinders, surfae grinders, pedestal grinders, and hand-fed grinders, whereby the operator at the touch of a button causes the work piece to be advanced by the table directly into contact with the peripheral edge of a grinding wheel, totravel a predetermned distance toward the wheel, and to retract to initial position under spring bias, upon release of the pneumatic or hydraulic force, or by pneumatic or hydraulic action.

25 Claims, 10 Drawing Figures PATENTEDNUVIBIHH v 3,771,267

sum 1 or a PATENTEDNUVIIHHH 3.771.267

SHEET 2 or 3 FIG. '5 78 54 6O :00 M m a 90 96 68 g, Q -HIGH PRESSURE I08 il AIR SOURCE 28 I06 WORK-HOLDING AND ADVANCING TABLE The invention involves a slidable pneumatically or hydraulically actuated work-supporting table having a fence complementarily adjustable from front to back of the table for accommodation of work pieces of varying dimensions. The table is supported upon the housing or associated members of a grinding machine, in front of the peripheral edge of a grinding wheel. The pneumatic system, or alternatively if desired, a self-contained hydraulic system, is cooperatively associated with the table to slidingly advance the same toward the edge of the wheel. Work-supporting tables embodying the inventive features hereindisclosed and claimed are provided with manually-operated controls for admitting air or hydraulic fluid to the piston for advancing the table toward the wheel. Upon completion of the travel, the fluid in the drive cylinder is discharged and exhausted to the atmosphere in the case of air or to an oil storage tank in the hydraulic system. In a hydraulic system, oil once admitted to the cylinder is returned to the tank or source for re-use when required, the hydraulic system being power-operated.

One important feature of the invention is the provision in the table base of a fluid drive cylinder of compact design, whereby the work-supporting table has a low compact profile. In one embodiment of the invention a cylinder 1 inch in height by about inches in width was used with an air pressure supply of 100 psi. The effective force upon the slidable table plate member was 500 pounds so long as the air pressure was applied. To have effected this same force in a cylinder of circular cross-section would have required a cylinder of more than 2-% diameter, more than two and one half times the height of the flat cylinder developed for the 'table. Not only was less housing material required to produce the table, but such material was used more efficiently and effectively.

Although the table is moved and advanced forwardly by pneumatic or hydraulic fluid action, the distance traveled being controlled by the length of the cylinder, return of the table to its initial position can be effected mechanically by springs contained within the housing for the cylinder and its piston, in one embodiment of the invention. In another modified form, the springs are eliminated and a passage into the cylinder is provided for pneumatic or hydraulic fluid to return the piston and the connected work-supporting slide plate to its initial at-rest position. The combination of a pneumatic or hydraulic fluid drive with mechanical return actuation for the slidable table provides highly effective and positive reciprocating movement at relatively low cost. The implementation of the table with an adjustably positionable fence to hold and drive the work piece forward into the cutting edge of the wheel further reduces the amount of physical effort required of the operator in the performance of his duties at the work grinding station.

It is a principal object of the invention to provide a manually operated work-supporting and advancingpneumatic or hydraulic drive means to return the slide plate to its initial at-rest position, upon manual actuation of a control device.

Various further and more specific objects, features and advantages of the invention will appear from the description given below, taken in connection with the accompanying drawings, illustrating by way of example preferred forms of the invention. Reference is here made to the drawings annexed hereto and forming an integral part of this specification, in which FIG. 1 is a perspective view of a grinding machine mounting a work table embodying a preferred form of the invention.

FIG. 2 is a top plan view taken substantially on the line 2-2 of FIG. 1.

FIG. 3 is a longitudinal vertical sectional view taken substantially on the line 3-3 of FIG. 2.

FIG. 4 is a transverse vertical sectional view taken substantially on the line 4-4 of FIG. 3.

FIG. 5 is a view substantially similar to that illustrated in FIG. 3, with the table advanced to a stop position adjacent the peripheral edge of the grinding wheel.

FIG. 6 is a somewhat diagrammatic and sectional view illustrating a pneumatic system utilized in the table construction.

FIG. 7 is a view similar to that illustrated in FIG. 6, for a table-actuating hydraulic system.

FIG. 8 is a plan view similar to that illustrated in FIG. 2, showing a modified form of the table device.

FIG. 9 is a view similar to FIG. 6, showing a modified pneumatic system for the device illustrated in FIG. 8.

FIG. 10 is a view similar to FIG. 7, showing a modified hydraulic system for the device illustrated in FIG. 8

As illustrated particularly in the several views of the drawing, the work-supporting table 10 is mounted upon a grinding machine 12, at a suitable position adjacent and forward of the peripheral edge 14 of a grinding wheel 15, the table 10 comprising a work-supporting plate member 16 slidable upon a base 18. The base 18 is adapted to be secured to a suitable mounting plate or member 20 that can be affixed to the grinding machine or other supporting element by any suitable means such .as the upright member 22 and brace 24, welded together as a mount for the base 18 or secured together by fasteners or any other suitable means. Various means of supporting the base 18 in fixed position can be utilized, as indicated by the nature and size of the grinding machine with which it is to be used. As illustrated particularly in FIGS. 3 and 4,,the base 18 is secured to the mounting plate 20 by screws 26 passed through the plate 20 and threaded into the body 28 of the base 18.

As will be seen more clearly in FIGS 3, 4 and 5, the slidable plate member 16 comprises a body 30 and lateral depending

guides

32,32 having inwardly directed

legs

34,34. The

guide members

32,32 may be integrally formed with the body 30 or made separately and fixedly secured thereto by welding or suitable fasteners,

and extend longitudinally substantially the length of the plate member 16. As will be seen more clearly in FIG. 2, the body 30 is provided at its lateral edges with notches or openings 36 cut vertically through the

lateral edges

38,38 of the body. The notches 36 are disposed at regularly or irregularly spaced intervals longitudinally of the

edges

38,38 to form intermediate laterally extending lugs 39 in those edges. The notches 36 are aligned transversely of the body 30 so as to accommodate the fence 40 as will be described more fully below.

The fence 40 comprises the body member 42 extending transversely of the plate member 16 and having vertically disposed

keys

44,44 at each end of the body secured fixedly thereto and extending below the lower edge of the body 42 so as to project therebelow for engagement in the notches 36 with and at each side of the supporting plate member body 30, the projecting portions 46 of the keys being formed to closely and slidingly fit within the notches so that a relative minimum play is present. The fit of the key portions 46 in the notches 36 should be such that the fence can be raised from and reinserted into the notches of the plate member body 30 with relative ease and a good slip fit, and further that the fence body will not tilt to any appreciable extent so that it can be forced out of the notches upon actuation of the plate member 16 toward the grinding wheel when the work piece is engaged therewith.

The base 18 comprises the body 28, the fluid cylinder 54 therewithin and threaded openings 56 to receive the threaded securing screws 26. The lateral edges of the body 28 are provided with grooves or

slots

58,58 to accommodate and complementarily receive the inwardly directed

legs

34,34 of the

plate member guides

32,32.

The fluid cylinder 54, as seen particularly in FIGS. 2 and 4, is of relatively small height and of substantial width. Although shown with lateral semi-circular crosssectional sides, the cylinder 54 can also be made with lateral straight planar sides to form an elongated relatively flat rectangular cross-section. A piston 60 is slidably mounted within the fluid cylinder 54 and conforms substantially to the same transverse cross-sectional configuration and dimension. The peripheral edges of the piston 60 are provided with piston sealing rings or loops 62 of suitable composition for the fluids used to drive the piston, be it a gas such as air, or a liquid such as hydraulic oil.

As shown particularly in FIG. 3, the piston 60 is further provided with blind counterbores or

seats

64,64 on its distal side to receive one end of the

springs

66,66 in each counterbore to bias the piston to its initial at-rest or retracted position. The piston, 60 is further provided with a thrust pin 68 threadedly secured at and to its distal side and projecting forwardly therefrom substantially axially of the piston for securement at the pins distal end to the flange 70 depending from the plate member body 30 adjacent its distal edge 72 by suitable fastening means such as a socket cap screw 74. The depending flange 70 may be made integrally with the plate member body 30, or it is preferably formed as a separate element and secured to that body by

screws

76, 76 or other suitable fasteners, or by welding. As seen particularly in FIG. 3, the length of the thrust pin 68 to the flange 70 is proportional to the free travel distance of the piston in the fluid cylinder 54.

The fluid cylinder 54 in body 28 is open at its distal end where it is closed by the overlying cap or plate member 78 fixedly secured to the body 28. The cap 78 is provided with

counterbores

80,80 to receive the distal ends of the

biasing springs

66,66, and with a bore 82 in a bearing 84 for passage of the thrust pin 68 therethrough.

The table 10 further comprises a manually operated control mechanism 90 for a pneumatic drive system 92 The control mechanism of conventional valve construction comprises a body 102, a manually actuated push button 104 connected at its distal end to the valve unit 106 in cylinder 108 and biased to its normally closed position by a spring 110 bearing compressively upon the distal end of the valve 106 and upon the valve closure plate 112 which is provided with an exhaust port 114 communicating with the cylinder 108. The control mechanism 90 can be afiixed to the sliding plate member body 30, as shown, or to the base 18, or to any other suitable supporting body or member in a position easily and readily accessible to the operator.

Conduits

96 and 98 are preferably made of flexible materials, such as suitable plastics or combinations of metal and rubber or other compositions.

Upon manual actuation of the push button 104, valve 106 is moved forwardly against the spring 110 to allow the

conduits

96 and 98 to communicate through cylinder 108 for translation of the high pressure air from its source 94 to the fluid cylinder 54 in base 18. Upon release of the push button 104, the valve 106 is biased to its initial retracted position as shown in FIG. 6, and the air that has substantially filled the fluid cylinder 54 and pushed the piston 60 forwardly is now forced back into conduit 98 by the biasing effect of the spring 66 through piston 60 to flow through that conduit into the control cylinder 108 and out the exhaust port 114.

When a hydraulic actuating system is employed for advancing the work supporting plate member 16, a hydraulic system such as diagrammatically illustrated in FIG. 7 can be used. In this hydraulic system 120, a tank 122 containing a supply of hydraulic fluid serves a powered hydraulic pump 124 through the conduit 126, the pump communicating with the control mechanism 90A by means of a conduit 130, the control mechanism 90A communicating with the fluid cylinder 54 by way of a conduit 132 and the fluid passage 100 in base body 28. In this construction where the hydraulic system is utilized, a fluid exhaust conduit 136 communicates with the exhaust port 138 of the manually operated control mechanism 128. Operation of the hydraulic actuating system illustrated in FIG. 7 is substantially as described above and as is well known in the arts in which this and similar systems have long been used.

A slightly modified form of the invention involves the elimination of the

springs

66,66 to mechanically return the slide plate 16 to its initial at-rest position, and the substitution therefor of the fluid passage 140 in the cylinder overlying plate member 78 which communicates with the conjoined pneumatic conduit 142 (FIGS. 8 and 9) or the hydraulic conduit 144 (FIG. 10) connected to and communicating with the control valve 90 (FIG. 9) in the one system for admission of pressurized air to the downstream side of the fluid cylinder 54, or in the other system with the control valve 90A (FIG. 10) for admission of hydraulic liquid to the fluid cylinder 54, through the fluid passage 140. In either case, pneumatic or hydraulic fluid pressure returns the piston 60 to its initial at-rest position upon release of the manually actuated control button 104. The pressurized air source is maintained at the required pressure and is constantly available to the fluid cylinder 54 through either of the

conduits

98 or 142 depending upon the position of the control valve unit 106 in its cylinder 108. Similarly, in the case of the hydraulic fluid system, the hydraulic pump 124 is in constant motion, powered by its source of energy such as an electric motor, to force hydraulic liquid through the control valve unit into either of the

conduits

132 or 144, for advance or retraction of the work-holding slide plate 16.

ln operation, the work-supporting and advancing table is adapted to support a work piece upon the upper surface of the plate member body 30 forward of the fence 40 so that the fence will bear against the work piece as the latter engages the rotating peripheral edge 14 of the grinding wheel 15. Actuation of the

push button mechanism

90 or 90A causes pneumatic 0r hydraulic fluid to flow into the fluid cylinder 54 of the base 18, pushing the piston 60 forwardly and advancing the slidable plate member 16 with its work piece thereon into engagement with the peripheral grinding wheel edge 14 under the strong biasing force of the fluid. Upon release by the operator of the push button 104, the valve 106 is retracted by spring 1 l0, releasing the fluid to the atmosphere through exhaust port 114 in the case of air, or hydraulic fluid to the exhaust conduit 136 where hydraulic oil is used.

The compactness of the work-supporting table 10 hereindisclosed and claimed enables the operator to set the work piece at a lower level than would normally be available to him in conventional fluid cylinder constructions, and more importantly the hereindisclosed compact fluid cylinder of substantially small height with substantially large transverse width, utilizing a very large proportion of the base body 28, provides a more efficient and a more effective drive component for the work-supporting plate member 16, with a substantial reduction in the cost of manufacturing a worksupporting and advancing table. In conventional worksupporting tables using circular or square crosssectional fluid cylinders, only a relatively small porportion of the body of the base member'is utilized, leaving a very large mass of material performing a supporting function. The compactness of the work table 10 hereindisclosed and claimed has these and other meritorious advantages in simplicity of construction, operation and utilization, making it readily adaptable to modern automated procedures for either manual or programmed control.

The components of the work table 10 are made of relatively hard metals such as steel, suitably machined to perform their specified functions. The mass of the base 18 is substantially large, but it is much smaller, by virtue of the compact fluid cylinder 54 and piston 60 that uses a substantial proportion of the base body 28, than is present in currently available or known worksupporting tables. This compactness enables a multiplication of force from the pneumatic or hydraulic system in a work-supporting table of much smaller dimensions than is presently available.

Although particular embodiments of the invention have been disclosed herein for purposes of explanation, further modifications or variations thereof, after study of this specification, will or may become apparent to those skilled in the art to which the invention pertains. Reference should be had to the appended claims in determining the scope of the invention.

I claim:

1. In a work-supporting table apparatus for advancing a work piece into contact with the abrading surface of a rotating grinding wheel, the improvements comprising in combination a work supporting plate member,

a low profile base for said plate member having a fluid drive cylinder and a complementary piston therein of relatively low height and of substantial transverse width greater than said height,

said plate member being slidable upon said base,

propulsion means connecting said slidable plate member and said piston to advance and retract said plate member responsive to movement of said piston,

a controlled pressurized fluid system, to propel and advance said piston and connected slidable plate member toward said grinding wheel abrading surface, communicating with and connected to said fluid drive cylinder,

and means adapted to bear upon and retrun said piston to its initial at-rest postion upon conclusion of fluid pressure application propelling said piston in said drive cylinder.

2. The apparatus defined in claim 1, wherein saidpressurized fluid system comprises a source of pressurized fluid,

a manually actuated control mechanism connected to and communicating with said source and said fluid drive cylinder, for selectively releasing said fluid to said drive cylinder.

3. The apparatus defined in claim 1, wherein said slidable plate member comprises a body, lateral depending guide members affixed to said body and extending longitudinally thereof,

said guide members having inturned legs engageable with said base.

4. The apparatus defined in claim 1, and including a fence on said plate member for holding and advancing said work piece.

5. The apparatus defined in claim 4, wherein said plate member body is provided with longitudinally spaced apart notches in each lateral edge thereof,

said notches being disposed in an attitude substantially normal to the plane of said body surface.

6. The apparatus defined in claim 5, wherein said spaced apart notches form lugs therebetween,

said fence being removably disposed transversely of and upon said plate member body and engageable with said lugs to restrict said fence against rearward displacement upon forward propulsion of said plate member, driving said supported work piece against the abrading surface of said grinding wheel.

7. The apparatus defined in claim 5, wherein said spaced apart notches are aligned transversely of said slidable plate member body.

8. The apparatus defined in claim 7, wherein said spaced apart notches are uniformly spaced apart longitudinally along each lateral side of said slidable plate member body.

9. The apparatus defined in claim 1, wherein said slidable plate member is provided with a depending flange affixed thereto adjacent its distal edge, said depending flange being connected to said propulsion means.

10. The apparatus defined in claim 3, wherein said low profile base comprises a body of relatively low height and of substantial transverse width greater than said height, said fluid drive cylinder being disposed within said body longitudinally and substantially axially thereof. 11. The apparatus defined in claim 10, wherein said base body is provided with longitudinally extending complementary guide member leg receiving means. 12. The apparatus defined in claim 11, wherein said guide member leg receiving means comprises lateral grooves complementary with said inturned legs for slidable movement of said plate member relative to said base. 13. The apparatus defined in claim 10, wherein said base body is provided with a passage therein communicating with said pressurized fluid system at its inlet end and with said fluid drive cylinder at its discharge end. 14. The apparatus defined in claim 10, and including means fixedly supporting said base body relative to the abrading surface of said grinding wheel. 15. The apparatus defined in claim 10, wherein said base body is provided with a cap member fixedly secured to said base body and closing said fluid drive cylinder at one end thereof, said piston return means comprising spring means within said fluid drive cylinder bearing upon said cap member and said piston to bias said piston to its initial retracted position upon conclusion of fluid pressure application to said piston. 16. The apparatus defined in claim 9, wherein said propulsion means comprises a thrust pin fixedly secured to said piston and to said plate member depending flange. l7. The apparatus defined in claim 5, wherein said fence comprises a body member extending transversely of said plate member body, said fence body member being adjustably positionable upon said plate member body. 18. The apparatus defined in claim 17, wherein said fence body member is provided adjacent its lateral ends with depending keys adapted to seat in said notches. 19. The apparatus defined in claim 1, wherein said controlled pressurized fluid system comprises a source of pressurized pneumatic fluid, and a manually operable control mechanism including a valve and conduits connected to and communicating with said source of pneumatic fluid and said fluid drive cylinder. 20. The apparatus defined in claim 1, wherein said controlled pressurized fluid system comprises a source of hydraulic fluid,a powered pump, and

a manually operable control mechanism including a valveand conduits connected to and communicating with said source of hydraulic fluid, said pump and said fluid drive cylinder. 21. The apparatus defined in claim 19, wherein said control mechanism is affixed to said slidable plate member or alternatively to said base. 22. The apparatus defined in claim 20, wherein said control mechanism is affixed to said slidable plate member or alternatively to said base. 23. The apparatus defined in claim 1, wherein said means adapted to bear upon and return said piston comprises resilient means in said fluid cylinder. 24. The apparatus defined in claim 19, wherein said control mechanism conduits communicate with said fluid cylinder at the upstream and downstream ends thereof. 25. The apparatus defined in claim 20, wherein said control mechanism conduits communicate with said fluid cylinder at the upstream and downstream ends thereof.

Claims

1. In a work-supporting table apparatus for advancing a work piece into contact with the abrading surface of a rotating grinding wheel, the improvements comprising in combination a work supporting plate member, a low profile base for said plate member having a fluid drive cylinder and a complementary piston therein of relatively low height and of substantial transverse width greater than said height, said plate member being slidable upon said base, propulsion means connecting said slidable plate member and said piston to advance and retract said plate member responsive to movement of said piston, a controlled pressurized fluid system, to propel and advance said piston and connected slidable plate member toward said grinding wheel abrading surface, communicating with and connected to said fluid drive cylinder, and means adapted to bear upon and retrun said piston to its initial at-rest postion upon conclusion of fluid pressure application propelling said piston in said drive cylinder.

2. The apparatus defined in claim 1, wherein said pressurized fluid syStem comprises a source of pressurized fluid, a manually actuated control mechanism connected to and communicating with said source and said fluid drive cylinder, for selectively releasing said fluid to said drive cylinder.

3. The apparatus defined in claim 1, wherein said slidable plate member comprises a body, lateral depending guide members affixed to said body and extending longitudinally thereof, said guide members having inturned legs engageable with said base.

5. The apparatus defined in claim 4, wherein said plate member body is provided with longitudinally spaced apart notches in each lateral edge thereof, said notches being disposed in an attitude substantially normal to the plane of said body surface.

6. The apparatus defined in claim 5, wherein said spaced apart notches form lugs therebetween, said fence being removably disposed transversely of and upon said plate member body and engageable with said lugs to restrict said fence against rearward displacement upon forward propulsion of said plate member, driving said supported work piece against the abrading surface of said grinding wheel.

10. The apparatus defined in claim 3, wherein said low profile base comprises a body of relatively low height and of substantial transverse width greater than said height, said fluid drive cylinder being disposed within said body longitudinally and substantially axially thereof.

11. The apparatus defined in claim 10, wherein said base body is provided with longitudinally extending complementary guide member leg receiving means.

12. The apparatus defined in claim 11, wherein said guide member leg receiving means comprises lateral grooves complementary with said inturned legs for slidable movement of said plate member relative to said base.

13. The apparatus defined in claim 10, wherein said base body is provided with a passage therein communicating with said pressurized fluid system at its inlet end and with said fluid drive cylinder at its discharge end.

14. The apparatus defined in claim 10, and including means fixedly supporting said base body relative to the abrading surface of said grinding wheel.

15. The apparatus defined in claim 10, wherein said base body is provided with a cap member fixedly secured to said base body and closing said fluid drive cylinder at one end thereof, said piston return means comprising spring means within said fluid drive cylinder bearing upon said cap member and said piston to bias said piston to its initial retracted position upon conclusion of fluid pressure application to said piston.

16. The apparatus defined in claim 9, wherein said propulsion means comprises a thrust pin fixedly secured to said piston and to said plate member depending flange.

17. The apparatus defined in claim 5, wherein said fence comprises a body member extending transversely of said plate member body, said fence body member being adjustably positionable upon said plate member body.

18. The apparatus defined in claim 17, wherein said fence body member is provided adjacent its lateral ends with depending keys adapted to seat in said notches.

19. The apparatus defined in claim 1, wherein said controlled pressurized flUid system comprises a source of pressurized pneumatic fluid, and a manually operable control mechanism including a valve and conduits connected to and communicating with said source of pneumatic fluid and said fluid drive cylinder.

20. The apparatus defined in claim 1, wherein said controlled pressurized fluid system comprises a source of hydraulic fluid, a powered pump, and a manually operable control mechanism including a valve and conduits connected to and communicating with said source of hydraulic fluid, said pump and said fluid drive cylinder.

21. The apparatus defined in claim 19, wherein said control mechanism is affixed to said slidable plate member or alternatively to said base.

22. The apparatus defined in claim 20, wherein said control mechanism is affixed to said slidable plate member or alternatively to said base.

23. The apparatus defined in claim 1, wherein said means adapted to bear upon and return said piston comprises resilient means in said fluid cylinder.

24. The apparatus defined in claim 19, wherein said control mechanism conduits communicate with said fluid cylinder at the upstream and downstream ends thereof.

25. The apparatus defined in claim 20, wherein said control mechanism conduits communicate with said fluid cylinder at the upstream and downstream ends thereof.