US3662640A

US3662640A - Machine tool

Info

Publication number: US3662640A
Authority: US
Inventors: Theodore J Wrona
Original assignee: Press Systems Inc
Current assignee: Press Systems Inc
Priority date: 1969-10-09
Filing date: 1969-10-09
Publication date: 1972-05-16
Anticipated expiration: 1989-05-16

Abstract

A press having a C-shaped frame including a die support surface and slide means for moving a punch toward and away from the support surface. A Y-shaped pitman has upwardly extending arms eccentrically mounted upon shaft means for effecting vertical reciprocation of the stem at the lower end of the pitman which is pivotally connected through a pressure fluid actuator to the slide means. Thus, vertical movement of the slide means relative to the support surface can be effected both by the eccentrics acting through the pitman and by the pressure fluid actuator acting between the pitman and the slide means. The fluid actuator includes means for making extremely small adjustments in the stroke of the pressure fluid actuator.

Description

United States Patent Wrona [4 1 May 16, 1972 [54] MACHINE TOOL 3,005,404 10/1961 Tucker, Sr. 100/257 [72] Inventor: Theodore J. wrona, Sturgis, Mich. 3,334,581 I 8/1967 Bergman ..83/530 X [73] Assignee: Press Systems, Inc., Sturgis, Mich. Primary Examiner-James M. Meister [22] Filed: Oct 9, 1969 Attorney-Woodhams, Blanchard and Flynn [21] Appl. No.: 865,095 [57]- ABSTRACT A press having a C-shaped frame including a die support sur- [52] U.S.Cl ..83/530, 83/628, 100/257, fa e and slide means for moving a punch toward and away 7 100/292 from the support surface. A Y-shaped pitman has upwardly [5 I] lit. Cl. ..B26d 7/26, 826d 5/16 exte ding arms eccentrically mounted upon shaft means for [58] Field of Search ..83/530, 543, 628, 615; ff i vertical redpmcation f h Stem at the lower end f 100/257 292; 72/452 the pitman which is pivotally connected through a pressure fluid actuator to the slide means. Thus, vertical movement of [56] References cued the slide means relative to the support surface can be effected UNITED STATES PATENTS both by the eccentrics acting through the pitman and by the pressure flurd actuator actmg between the pltman and the 362,566 5/1887 Beaudry X slide mean5 The actuator includes means for making ex- 783,588 2/1905 Nixon ..100/292 X tremely mall adjustments in the stroke of the pressure fluid 1,009,298 1 1/191 1 George actuator. 1,716,414 6/1929 Beyer 2,438,951 4/1948 Stephens ..83/543 7 Claims, 14 Drawing Figures O O Q 7 1 1' a J 1 i 1 .1; 1 ll 1 "1 1 ii i i =1 22 0 1| i m; 1 I 22 9e 22 a I I l l i --7z 2 I l4 7;

II v

MACHINE TOOL BACKGROUND OF THE INVENTION This invention relates in general to presses and, more particularly, to a type thereof having a C-shaped frame which is positively held against gapping of the upper and lower plates and wherein extremely small adjustments can be made in the shut height of the upper platen.

Although tools for applying compressive forces have been in existence as far back as history records, a number of problems inherent in existing presses have persisted to evade solution. For example, it is well known that existing presses have either been unable to provide adequate provision for upper platen withdrawal (to permit die inspection, parts change, repair and so forth), or they have provided such withdrawal at the expense of accuracy in the shut height of the movable platen. That is, after withdrawal, it has been necessary for the machine operator to carefully check the shut height, and in many instances, make time-consuming adjustments therein.

One attempt to eliminate some of these problems is disclosed in my US. Pat. No. 3,400,625, assigned to the Assignee of this application. While the machine disclosed in said patent operates effectively and overcomes some of the previously existing problems, it is more complicated and, hence, more expensive than necessary for accommodating dies having relatively small work-engaging surfaces.

Accordingly, a primary object of this invention is the provision of an improved press having a minimum of moving parts, being capable of adequate withdrawal for repair and the like, being capable of accurately returning to its proper shut height after a withdrawal, making maximum use of ball bearings in order to permit high-speed operation, and having means for positively preventing gapping which has plagued previous attempts to provide a lightweight, light-duty machine capable of performing completely satisfactorily.

A further object of this invention is the provision of a machine, as aforesaid, using a fast-acting, pressure fluidoperated mechanism for effecting withdrawal and having an extremely accurate means of adjusting the shut height of the press after which the same shut height will be achieved following each withdrawal.

Other objects and purposes of the invention will become apparent to persons familiar with this type of equipment upon reading the following descriptive material and examining the accompanying drawings, in which:

FIG. 1 is a perspective view of a press embodying the invention.

FIG. 2 is a front-elevational view of said press.

FIG. 3 is a side-elevational view of said press.

FIG. 4 is a sectional view substantially as taken along the line lVlV in FIG. 2.

FIG. 5 is a broken, front-elevational view of a fragment of said press.

FIG. 6 is an exploded, perspective view of the upper platen support and its actuating mechanism.

FIG. 7 is an enlarged, sectional view taken along the line VII-VII in FIG. 2.

FIG. 8 is a sectional view taken along the line VIIIVIII in FIG. 5.

FIG. 9 is an enlarged, sectional view taken along the line IXIX in FIG. 7.

FIG. 10 is a sectional view taken along the line X-X in FIG. 5.

FIG. 11 is an enlarged fragment of FIG. 7.

FIG. 12 is a sectional view taken along the line XII-XII in FIG. 2.

FIG. 13 is an enlarged fragment of FIG. 12.

FIG. 14 is a sectional view taken along the line XIV-XIV in FIG. 9.

For convenience in description, the term upper, lower," left," right," front," rear and words of similar import will have reference to the press embodying the invention as appearing in FIG. 2 wherein the front side is shown. The terms inner, outer and derivatives thereof will have reference to the geometric center of said press and parts thereof.

SUMMARY OF THE INVENTION The objects and purposes of the invention, including those set forth above, have been met by providing a machine tool having a C-shaped frame with a rigid lower platen and an upper platen vertically reciprocably supported upon the head of the frame. A horizontal shaft is rotatably supported upon said head of the frame and eccentrically supports a Y-shaped pitman at two spaced points on the shaft. The pitman engages the upper platen through a pressure fluid actuator whereby vertical, reciprocal movement of the upper platen can be effected either by rotation of the shaft or by operation of the pressure fluid actuator. An accurate adjustment mechanism is provided between the pressure fluid actuator and the pitman for effecting small changes in the stroke of the actuator. Tie rods are connected between the head and base of the C- shaped frame in order to prevent gapping of the platens during normal operation.

DETAILED DESCRIPTION The press 10 (FIGS. 1, 2 and 3) which illustrates one preferred embodiment of the invention, is comprised of a substantially C-shaped frame 11 having a frontwardly projecting base 12 upon which is supported a lower platen 14. The frame 11 has an upright back member 16 and a head 17 which also projects frontwardly over the base 12.

A pair of

vertical tie rods

18 and 19 extend between and are rigidly secured to the head 17 and a support plate 20 having an integral pedestal 15 secured to the base 12. The

rods

18 and 19 are preferably located near the midpoints of the side edges of the lower platen 14. In this embodiment, the plate 20 provides the lower part of a rigid, C-shaped and frontwardly opening member 21 which is nested within and rigidly secured to the remainder of the C-shaped frame 11. The member 21 includes spaced back members 27 and spaced upper members 22 which are respectively integral.

As shown in FIGS. 4 and 5, two pair of spaced and integral bearing supports 23, 24, 25 and 26 are mounted on, and project frontwardly from the head 17.

Roller bearings

28, 29, 30 and 31 are mounted, by means such as bolts 34, upon the bearing supports 23, 24, 25 and 26, respectively, in axially aligned positions for the purpose of rotatably supporting the shaft 36 in a horizontal position and, in this embodiment, in a vertical plane defined by the axes of the

tie rods

18 and 19. A pair of eccentrics 37 and 38 (FIG. 5) are rigidly mounted upon the shaft 36, by means such as keys 39, between the

bearing pairs

28, 29 and 30, 31, respectively.

An upper platen support 42 (FIGS 5 and 7) is vertically elongated and has a pair of

upper slides

43 and 44 and lower slides 46 and 47 (FIG. 5) which are engaged, respectively, with the

upper slide guides

48 and 49 and

lower slide guides

52 and 53. The upper and lower slide guides are mounted on adjacent portions of the head 17 on opposite sides of the platen support 42, and they are preferably, but not necessarily, substantially identical.

The upper slide 43 (FIG. 11) is comprised of an integral, vertically elongated and lateral projection 54 having a V- shaped, vertically extending groove 56 in its side surface. A pair of flat, elongated bearing races 57 and 58 are secured by screws 59 to the two opposing surfaces of the groove 56. A pair of roller bearing

retainers

62 and 63 position a plurality of bearing rollers 64 between the bearing races 57, 58, respectively, and the adjacent faces 66 and 67 on the upper slide guide 48. The faces 66 and 67 are, accordingly, parallel with the opposing sides of the groove 56. The slide guide 48 is rigidly secured to the adjacent portion of the frame 1 1 by bolts 68 which extend through openings 69.

The upper slide 44 and the two

lower slides

46 and 47 are preferably constructed and arranged as described in detail with respect to the slide 43. Likewise, the

slide guides

49, 52 and 53 are fabricated and arranged substantially as described above with respect to the slide guide 48. The openings 69 in the slide guides, as shown with respect to the slide guide 48 in FIG. 11, are slightly oversized to permit minor but important, lateral adjustment of the slide guides transversely of their openings 69.

A pressure fluid-actuated cylinder 71 (FIG. is connected between the upper platen 74 and the pitman 83.

The upper platen support 42 has a substantially cylindrical lower portion 72 which defines a cylindrical cylinder chamber 73, the lower end of which is covered by a substantially rectangular upper platen 74 which preferably projects radially beyond all sides of the lower portion 72. A cylindrical cylinder 76 (FIGS. 5, 6 and 7) is snugly disposed within the cylindrical chamber 73. A piston 77 is slideable disposed within the cylinder 76 and conventional seal means, such as the O-ring 78, are placed in the adjoining sidewalls of the cylinder 76 and piston 77 to prevent the escape of pressure fluid therebetween. An O-ring 79 is located between the cylinder 76 and the lower portion 72 to prevent the escape of pressure fluid therebetween.

An externally threaded collar 82 is engaged within the upper internally threaded end of the cylinder 76. The collar 82 can be raised or lowered with respect to the cylinder 76 by rotating the collar relative to said cylinder 76. The upper end of the piston 77 is engageable with the lower edge of the collar 82 whereby upward movement of the piston relative to a cylinder 76 is positively and accurately limited. FIG. 5 illustrates the piston in the raised position and FIG. 9 illustrates the piston in the lowered position.

A Y-shaped pitman 83 has a pair of upwardly projecting, spaced

arms

84 and 85 with enlarged, annular upper ends 87 and 88 which encircle the

eccentrics

37 and 38, respectively. A pair of roller bearings 89 and 90 (FIG. 5) are disposed around the

eccentrics

37 and 38, respectively, within the annular upper ends 87 and 88 of the pitman 83. Said annular upper ends 87 and 88 are split diametrically and held together by bolts 93 for convenience in assembly and maintenance.

By using a Y-shaped pitman 83, the stress on the shaft 36 is minimized while, at the same time, the force applied to the upper platen support 42, hence, to the upper platen 74 is centralized to effect a smooth engagement between the punch and the die with substantially uniform pressure applied on all areas of engagement. Since there is a single hydraulic piston for effecting withdrawal, a single diaphragm can be used and set to burst at a specified pressure in excess of normal punching pressures.

An eccentric counterweight 94 is mounted upon the shaft 36 between the bearing supports 24 and 25 to counteract the throw of the

eccentrics

37 and 38.

The lower stem 96 on the pitman 83, which stem is midway between the

arms

84 and 85, has a transverse opening 97 that is axially parallel with the shaft 36 and rotatably receives a wrist pin 98 supported by a roller bearing assembly 99. The wrist pin 98 is rigidly secured to the upper end of the piston 77 by a pair of bolts 102.

The peripheral surface 103 at the upper end of the collar 82 may be provided with scale markings to indicate the amount of rotation thereof by reference to markings on the upper axial end 104 of the cylinder 76. This means permits accurate, predetermined adjustment of the shut height of the upper platen depending upon the dimensions of the workpiece.

The upper platen 74 and lower platen 14 may be connected to a punch 139 and die 140, respectively, in a conventional manner and for conventional purposes, such as stamping pieces from the sheet metal.

The upper end 106 of the upper platen support 42 (FIG. 5) is connected to the lower end of an upright rod 107 which slideably extends upwardly through an opening in a transverse plate 108 rigidly secured to the top of the frame 1 1. The upper end of the rod 107 extends through a disk 109, and a spiral spring 112 is held under compression between the disk 109 A large flywheel 114 is rotatably mounted upon the shaft 36 I and adapted for rotation therewith in response to operation of a clutch 115 mounted on the shaft 36. The clutch 1 15 (FIG. 5)

may be of a conventional, frictional type electrically energized for the purpose of engaging the shaft 36 with the flywheel 1 14 at the election of the machine operator. A belt 116 connects the flywheel 114 to a variable pitch sheave 1 17 mounted upon the shaft 118 of a motor 119. A motor pedestal 122 extends rearwardly and downwardly from the motor 119 and is pivotally supported by the pin 123 which is mounted upon the upper end of the head 17. The end of the pedestal 122 remote from the motor 119 is pivotally connected by a pin 124 through which an adjustment rod 126 is threadedly received. The rod 126 extends downwardly and frontwardly through a bearing assembly 127 in a cross plate 128 on the head 17.

The bearing assembly 127 includes a rotary bearing 129 of the sleeve type and a thrust bearing 132 of the roller type. The thrust bearing 132 is disposed between the rotary bearing 129 and the hub 133 of the control wheel 134 which is rigidly secured to the lower end of the adjustment rod 126. An adjustment rod lock 136 is composed of an L-shaped member threadedly received through a plate 137 on the frame 11 adjacent the rod 126 for engagement therewith. A brake 138 (FIG. 5) is mounted upon the frame 11 adjacent the flywheel 114 for stopping the shaft 36 quickly when the clutch 115 is disengaged. Overload protection is provided for the pressure cylinder 71 (FIG. 9) by a fracturable disk 143 (FIG. 14) located in conduit 144 in upper platen 74 between the piston chamber and an exhaust pipe 145.

OPERATION Although the operation of the press embodying the invention will be apparent to persons skilled in the art after reading the foregoing descriptive material, such operation will be summarized briefly hereinafter for convenience.

Prior to operation of the press 10, an appropriate die 140 and punch 139 are mounted upon the lower platen 14 and upper platen 74, respectively, in a substantially conventional manner and as indicated by broken lines in FIG. 3. The hydraulic fluid is withdrawn from within the cylinder 76 below the piston 77 whereby the platen support 42 and its attached punch 139 are raised, due to the spring 112, with respect to the lower die 140 to the withdrawal distance therefrom. The motor is then energized and caused to rotate the shaft 36 whereby the pitman 83 moves the upper platen support 42 into its lowermost position.

With the

eccentrics

37 and 38 thus in their bottom deadcenter positions, hydraulic fluid is now carefully directed into the lower end of the cylinder 76 whereby the upper platen support 42 is lowered slowly and carefully into proper engaging position with a workpiece, such as sheet metal, between the punch and the die. If the proper closed condition is not achieved, the collar 82 is rotated in the appropriate direction to adjust the stroke of the piston 77 for the proper height. Following this adjustment, the press is now ready for normal operation.

The spring 112 tends to counteract the effects of gravity upon the upper platen support 42 and, accordingly, permits a faster reciprocation of the upper platen support 42 than would be achieved without such spring.

By turning the control wheel 134 (FIGS. 3 and 12), the level of the motor 119 canbe adjusted whereby the pitch of the sheave 117 is altered to change the rotational speed of the shaft 36. The clutch provides for selective engagement between the flywheel 114 and the shaft 36 so that-the flywheel can continue to rotate when the shaft 36 is not rotating. The brake 138 is utilized to stop rotation of the shaft 36 when the clutch 115 is disengaged.

By spacing the upper and

lower guides

48, 49 and 52, 53, respectively, at a substantial vertical distance from each other, binding between the slides and the slide guides due to expansion and contraction is minimized. That is, if the slides and slide guides extended continuously from the upper to the lower ends, any distortions in either due to expansion, contraction or other distortive effects, would tend to bind the movement of the upper platen support 42 with respect to the frame 11. By virtue of the manner in which the slide guides are mounted on the frame 11, it is possible to make fine, accurate adjustments thereof to compensate for wear and the like. Moreover, since the upper slide guides are spaced substantial distances from the corresponding lower slide guides, the vertical movement of the upper platen support 42 is accurately controlled with an absolute minimum of racking effect and minimum frictional losses.

The vertical axes of the

tie rods

18 and 19 define a plane which also preferably includes the central vertical axis of the piston 77 and cylinder 76, as well as the central axis of the shaft 36. Thus, since the

tie rods

18 and 19 are in the vertical force plane between the upper and lower platens, they positively oppose gapping between the base 12 and the head 17. Yet, the platens and their supported punch and die are readily accessible from all four sides of the press, so that superior strength is achieved with a minimum of weight in the press frame and inconvenience.

Because withdrawal control of the upper platen 74 is concentrated in a single piston and cylinder arrangement, accurate shut height positioning can be effected with a single vemier scale provided on a collar 82.

While this machine is designed for high-speed, mechanical operation, primarily, it can be operated hydraulically to perform functions where slow speed and accurate control are desired.

If an overload develops in the piston chamber 80 (FIG. 14) of the pressure cylinder 71, as where a limit switch or valve fails to function properly, the disk 143, which ruptures at a predetermined maximum pressure, will release the overload to the exhaust pipe 145.

Although a particular preferred embodiment of the invention has been disclosed in detail for illustrative purposes, it will be recognized that variations or modifications of the disclosed apparatus, including the rearrangement of parts, lie within the scope of the present invention.

What is claimed is:

l. A machine for applying a compressive force to a workpiece, comprising:

frame means having a rigid planar support surface;

shaft means and bearing means rotatably supporting said shaft means on said frame means axially parallel with and spaced from said rigid surface;

a pair of identical, spaced eccentrics mounted upon and rotatable with said shaft means;

a Y-shaped pitman having a stem and a pair of arms rotatably mounted upon said eccentrics;

member means defining a surface opposed to and movable relative to said rigid surface, said rigid and movable surfaces being adapted for applying a compressive force to a workpiece therebetween;

pressure fluid-actuated means connecting said stem to said member means having said movable surface for effecting relative movement between said stem and said movable surface in a direction toward and away from said rigid surface; said pressure fluid-actuated means including cylinder means movably supported upon said frame means and fixedly interconnected to said member means for movement toward and away from said rigid surface, and piston means mounted upon said stem and slidably disposed within said cylinder means; and

rigid stop means adjustably mounted on one of said cylinder means and said piston means and coacting with the other of said cylinder means and said piston means for positively limiting relative movement between said cylinder means and said piston means toward said rigid surface for permitting the minimum spacing between said surfaces to be selectively varied, said stop means and said other of said cylinder means and said piston means having opposed stop surfaces disposed in abutting engagement when the minimum spacing exists between said rigid and movable surfaces.

2. A machine for applying a compressive force to a workpiece, comprising:

substantially C-shaped frame means having a head and a base projecting sidewardly in the same direction, the base having a substantially horizontal, upwardly facing support surface;

shaft means rotatably and horizontally supported upon said head directly above the central portion of said support surface;

a pair of spaced, identical eccentrics mounted upon and rotatable with said shaft means;

a Y-shapedpitman having a downwardly extending stem and a pair of upwardly extending arms rotatably mounted upon said eccentrics;

reciprocable means slidably supported upon said head and having a downwardly facing surface opposing said upwardly facing support surface, said reciprocable means being vertically movable toward and away from said support surface;

pressure fluid-actuated means connecting said stem to said reciprocable means whereby rotation of said shaft means effects vertical reciprocation of said reciprocable means, said pressure fluid-actuated means comprising piston means pivotally secured to said stem and cylinder means secured to said reciprocable means; and

rigid stop means adjustably mounted on said cylinder means and coacting with said piston means for positively limiting downward movement of said cylinder means relative to said piston means and for selectively adjusting the shut height between said upwardly and downwardly facing surfaces when said reciprocable means is in its lowermost position, said stop means and said piston means having opposed stop surfaces disposed in abutting engagement when the minimum shut height exists between said upwardly and downwardly facing surfaces.

3. A machine according to claim 2, including resiliently flexible means connected between said head and said reciprocable means for resiliently resisting downward movement of said reciprocable means.

4. A machine according to claim 2, including counterbalance means on said shaft means counteracting the throw of said eccentrics; and

drive means connected to said shaft means for rotating same, said drive means including adjustment means for effecting changes in the rotational speed of said shaft means.

5. A machine according to claim 2, wherein said stop means on said cylinder means comprises a collar threadedly engaged within said cylinder means and projecting upwardly therefrom; and

scale means on the peripheral surface at the upper end of said collar means for indicating the vertical movement of the collar means in response to rotation thereof.

6. A machine according to claim 2, wherein said cylinder means defines a piston chamber; and

said reciprocable means includes an exhaust passageway connected to said piston chamber and rupturable means in said passageway blocking the escape of pressure fluid from said piston chamber until the pressure in said piston chamber exceeds a predetermined amount.

7. A machine according to claim 1, wherein said piston means is mounted upon said stem for pivotal movement around an axis parallel with the axis of said shaft means, and said cylinder means being slideably supported with the longitudinal axis thereof substantially perpendicular to the axis of said shaft means.

Claims

1. A machine for applying a compressive force to a workpiece, comprising: frame means having a rigid planar support surface; shaft means and bearing means rotatably supporting said shaft means on said fRame means axially parallel with and spaced from said rigid surface; a pair of identical, spaced eccentrics mounted upon and rotatable with said shaft means; a Y-shaped pitman having a stem and a pair of arms rotatably mounted upon said eccentrics; member means defining a surface opposed to and movable relative to said rigid surface, said rigid and movable surfaces being adapted for applying a compressive force to a workpiece therebetween; pressure fluid-actuated means connecting said stem to said member means having said movable surface for effecting relative movement between said stem and said movable surface in a direction toward and away from said rigid surface; said pressure fluid-actuated means including cylinder means movably supported upon said frame means and fixedly interconnected to said member means for movement toward and away from said rigid surface, and piston means mounted upon said stem and slidably disposed within said cylinder means; and rigid stop means adjustably mounted on one of said cylinder means and said piston means and coacting with the other of said cylinder means and said piston means for positively limiting relative movement between said cylinder means and said piston means toward said rigid surface for permitting the minimum spacing between said surfaces to be selectively varied, said stop means and said other of said cylinder means and said piston means having opposed stop surfaces disposed in abutting engagement when the minimum spacing exists between said rigid and movable surfaces.

2. A machine for applying a compressive force to a workpiece, comprising: substantially C-shaped frame means having a head and a base projecting sidewardly in the same direction, the base having a substantially horizontal, upwardly facing support surface; shaft means rotatably and horizontally supported upon said head directly above the central portion of said support surface; a pair of spaced, identical eccentrics mounted upon and rotatable with said shaft means; a Y-shaped pitman having a downwardly extending stem and a pair of upwardly extending arms rotatably mounted upon said eccentrics; reciprocable means slidably supported upon said head and having a downwardly facing surface opposing said upwardly facing support surface, said reciprocable means being vertically movable toward and away from said support surface; pressure fluid-actuated means connecting said stem to said reciprocable means whereby rotation of said shaft means effects vertical reciprocation of said reciprocable means, said pressure fluid-actuated means comprising piston means pivotally secured to said stem and cylinder means secured to said reciprocable means; and rigid stop means adjustably mounted on said cylinder means and coacting with said piston means for positively limiting downward movement of said cylinder means relative to said piston means and for selectively adjusting the shut height between said upwardly and downwardly facing surfaces when said reciprocable means is in its lowermost position, said stop means and said piston means having opposed stop surfaces disposed in abutting engagement when the minimum shut height exists between said upwardly and downwardly facing surfaces.

4. A machine according to claim 2, including counterbalance means on said shaft means counteracting the throw of said eccentrics; and drive means connected to said shaft means for rotating same, said drive means including adjustment means for effecting changes in the rotational speed of said shaft means.

5. A machine according to claim 2, wherein said stop means on said cylinder means comprises a collar threadedly engaged within said cylinder means and projecting upwardly therefrom; and scale means on the peripheral surfaCe at the upper end of said collar means for indicating the vertical movement of the collar means in response to rotation thereof.

6. A machine according to claim 2, wherein said cylinder means defines a piston chamber; and said reciprocable means includes an exhaust passageway connected to said piston chamber and rupturable means in said passageway blocking the escape of pressure fluid from said piston chamber until the pressure in said piston chamber exceeds a predetermined amount.