US3784333A

US3784333A - Piston air compressor for air assist shock absorber

Info

Publication number: US3784333A
Application number: US00262086A
Authority: US
Inventors: J Whelan
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1972-06-12
Filing date: 1972-06-12
Publication date: 1974-01-08
Anticipated expiration: 1991-01-08

Abstract

A fluid control system for a diaphragm type air compressor including a spring biased shuttle valve rapidly moved between a first position supplying pressurized fluid to the diaphragm and a second position exhausting the fluid from the diaphragm for intermittently actuating the diaphragm to compress air. The shuttle valve is spring biased to a first position and is moved against the spring to a second position intermittently directing the pressurized fluid against one end of a spring biased piston which in turn pumps fluid against one side of the air compressing diaphragm. Valve means are associated with the piston and are responsive to movement thereof to direct the pressurized fluid against one end of the shuttle valve maintaining it in the second position while the pressurized fluid is exhausted from the cylinder containing the diaphragm actuating piston and its spring then returns the shuttle to the first position. The fluid control system of this invention automatically applies the pressurized fluid to the piston to rapidly pulse the diaphragm thereby providing a source of compressed air.

Description

United States Patent [191 Whelan [4 1 Jan. 8, 1974 1 PISTON AIR COMPRESSOR FOR AIR ASSIST SHOCK ABSORBER [75] Inventor: James E. Whelan, Dayton, Ohio [73] Assignee: General Motors Corporation,

Detroit, Mich.

[22] Filed: June 12, 1972 [21] Appl. No.: 262,086

[52] US. Cl 417/383, 91/290, 417/401 [51] Int. Cl. F04b 9/10, F011 25/04, F04b 35/02 [58] Field of Search 91/290, 293, 297,

Primary Examiner--A1 Lawrence Smith Assistant Examiner-Richard Sher Attorney-Frank J. Soucek et a1.

[57] ABSTRACT A fluid control system for a diaphragm type air compressor including a spring biased shuttle valve rapidly moved between a first position supplying pressurized fluid to the diaphragm and a second position exhausting the fluid from the diaphragm for intermittently actuating the diaphragm to compress air. The shuttle valve is spring biased to a first position and is moved against the spring to a second position intermittently directing the pressurized fluid against one end of a spring biased piston which in turn pumps fluid against one side of the air compressing diaphragm. Valve means are associated with the piston and are responsive to movement thereof to direct the pressurized fluid against one end of the shuttle valve maintaining it in the second position while the pressurized fluid is exhausted from the cylinder containing the diaphragm 3 Claims, 8 Drawing Figures PATENTEDJAN 81914- SHEEI 1 OF 2 JIIEEEJ W PATENTED 81974 3.784.333

SHIZEI 20$ 2 1 PISTON AIR COMPRESSOR FOR AIR ASSIST SHOCK ABSORBER This invention relates to a fluid control system automatically actuating a diaphragm air compressor and more specifically pertains to a system including a rapidly cycled shuttle valve intermittently applying pressurized fluid against a piston rapidly pulsing the diaphragm as the shuttle valve responds to a biasing means for movement in one direction and to the pressurized fluid for movement in an opposite direction.

Heretofore, various types of systems have been employed to actuate a diaphragm type compressor assem bly. Cam actuated mechanisms, for example, have been used to move a cylinder and pump fluid against one side of a diaphragm compressing air in a cavity on the other side thereof. A typical example of such an arrangement is shown in a copending application Ser. No. 73,845, filed Sept. 21, 1970 wherein a vehicle engine camshaft has a cam element mounted thereon engaging a pivotally mounted lever that is rotated moving a piston flexing a diaphragm of an air compressor assembly. The disclosure of this application is incorporated into the subject disclousre by reference.

In view of the prior art system, it is a purpose of the subject invention to provide a fluid control system utilizing a source of pressurized fluid as the sole means for actuating the control system and providing the forces required to flex the diaphragm in an air compressor assembly. By using such a fluid control system responsive to an existing source of pressurized fluid it is not necessary to incorporate any external driving means to accomplish actuation of the air compressor, and for this reason the subject system is an improvement over the prior art.

Accordingly, a general object of the subject invention is to provide a fluid control system automatically rapidly cycling a shuttle valve directing fluid against a piston flexing a diaphragm in an air compressor assembly.

Another object of the subject invention is the provision of a fluid control system utilizing a source of pressurized fluid as the sole means for actuating components in the system rapidly flexing a diaphragm in an air compressor automatically supplying a source of compressed air.

A further object of the subject invention is to provide a fluid actuated piston responsive to pulses of pressurized fluid supplied by a rapidly reciprocating shuttle valve regulating application of a source of pressfrized fluid against thepiston which in turn rapidly pulses a diaphragm in an air compressor assembly.

A still further object of the subject invention is to provide a piston reciprocating within a cylinder and controlling valve means by movement therein to direct pressurized fluid back to a shuttle control valve initially directing pressurized fluid to the piston, the application of the pressurized fluid against the valve moving the shuttle valve against a biasing means whereby the shut tle valve is rapidly reciprocated within a valve cavity providing pulses of pressurized fluid against the piston which in turn pulses a diaphragm in an air compressor assembly.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims.

My invention itself, however, both as to its organization and method of operation, will best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic view partly in section illustrating a specific embodiment of the subject fluid control system.

FIG. 2 is an elevational view with parts broken away illustrating the subject fluid control assembly connected with a diaphragm air compressor assembly.

FIG. 3 is a fragmentary sectional view taken on line 3-3 of FIG. 2.

FIG. 4 is. an elevational view of the subject fluid control system and air compressor assembly as viewed in the direction indicated by line 4-4 of FIG. 2.

FIG. 5 is a sectional view takne on line 5-5 of FIG. 2.

FIG. 6 is a partial sectional view taken on line 6-6 of FIG. 2.

FIG. 7 is a partial sectional view taken on line 7-7 of FIG. 2.

FIG. 8 is a sectional view with portions broken away taken on line 8-8 of FIG. 2.

Referring now to FIG. 1, a source of pressurized fluid, such as that available in the usual vehicle power steering system, is supplied through a conduit 20 to a high pressure relief valve 22 and a branch conduit 24, the latter connecting with a valve cavity 26 in a fluid control system housing 28. A shuttle control valve 30 is disposed within the valve cavity 26 and is biased to a first upward position by the spring 32. The valve cavity 26 contains lands 34, 36 and 38 engaged by

lands

40 and 42 on the valve 30 as it reciprocated within the cavity 26. A conduit 44 connects a chamber 45 defined between the

lands

40 and 42 of the valve 30 with a closed end 46 of a cylinder 48 also formed within the fluid control system housing 28. A piston 50. including an end 51, is biased toward the closed end 46 of the cylinder 48 by a spring 52. The piston 50 contains an annular groove 54 which cooperates with

annular grooves

56, 58 and 60 defining lands 62 and 64 therebetween, the grooves and lands all cooperating to form a valve means controlled by movement ofpiston 50 within cylinder 48. A conduit 66 is connected to groove 56 and directsfluid to a reservoir 68 for a purpose later described. A conduit 70 connects the groove 58 with a signal chamber 72 at one end of the valve 30 defined by valve land 40 and end 74 of valve cavity 26. A conduit 76 connects with the conduit 20 receiving the pressurized fluid from the external source which is directed through the conduit 76 to annular groove 60 in the cylinder 48. The signal chamber 72 is also connected with a low pressure relief valve 78 by a conduit 80, the valve 78 regulating flow to the reservoir 68 through a conduit 82.

With reference now to FIG. 2, the fluid control system housing 28 can be mounted directly to a diaphragm compressor assembly 84 which includes a first housing portion 86 and a second housing portion 88 secured together by a plurality of machine bolts 90 retaining a diaphragm 92 therebetween. The housing portion 88 has an air inlet fitting 96 and an air outlet fitting 94 secured thereto. With this arrangement the cylinder 48 is in fluid communication with a fluid chamber 98 in the air compressor assembly 84 as best illustrated in FIG. 3.

As illustrated in FIG. 4, the fluid control system housing 28 includes an oil charging port 100 which is in fluid communication wijh cylinder 48 and fluid chamber 98 in the diaphragm compressor assembly 84 so that oil may be initially supplied thereto. The pressurized fluid supply conduit 20 connects with a fitting 102 on the housing 28 as shown. The shuttle valve 30, as best illustrated in FIG. 1, also regulates flow of fluid through an exhaust conduit 104 to the reservoir 68 through the connecting conduit 62. The conduit 104 connects with a port 106 in the housing 28 as shown in FIG. 4. The conduit 20, as previously mentioned with reference to FIG. 1, also connects with a high pressure relief valve 22 in the housing 28 which in turn controls flow of fluid through a conduit 108 to the reservoir 68, the conduit 108 connecting with a port 110 in the housing 28. The fluid control housing 28 also has a cover 112 secured to a portion thereof by a plurality of bolts 114 as shown in FIG. 2, the cover 112 being removed to expose the

ports

106, 110, and the valve cavity 26 in FIG. 4.

FIG. 5 illustrates the valve 30 biased to its first position by spring 32 in valve cavity 26. In this position the pressurized fluid flows around a reduced central portion 118 in chamber 45 and through conduit 44 for application against the piston 50 in cylinder 48. The low pressure relief valve 78 is biased by a spring 122 closing passage 80 in the housing 28 thereby regulating the pressure in signal cavity 72.

With reference to FIG. 6, the high pressure relief valve 22 is biased by a spring 124 closing passage which supplies the pressurized fluid to the assembly and upon presence of fluid at pressures in excess of those designed for the system, the valve 22 opens and connects supply passage 20 with the sump 68 through the conduit 108. The

conduits

108, 66 and 82 join to transmit fluid to sump 68 through a single conduit 120.

In FIG. 7, a fluid pressure sensing valve 126 is provided to regulate the maximum fluid pressures applied to diaphragm 92 in the air compressor assembly 84. The valve 126 is biased closed by a spring 128 and bypasses fluid from the fluid chamber 98 when excessive pressures exist by providing flow through passage 130 to the sump 68 in a manner not shown.

Housing portion 86 of the air compressor assembly 84 as illustrated in FIG. 8, includes an oil intake valve 132 regulating flow of oil into the fluid chamber 98 preventing excessive negative pressures developing in the fluid chamber by virtue of the valve 132 opening and supplying additional oil to the chamber during air pressurizing flexing of the diaphragm 92. The passage 130 is shown in the upper portions of the housing 86 wherein the pressurized fluid existing in chamber 98 is subjected to the valve 126 so as to prevent excessive positive pressures in the chamber 98 as previously mentioned with reference to FIG. 7.

In operation, the fluid control system housing 28 is connected with a source of pressurized fluid, such as that existing in a vehicle power steering system for purposes of illustration, by coupling conduit 20 with a line conducting pressurized fluid in the power steering system. The pressurized fluid in conduit 20 enters the valve cavity 26 between

lands

40 and 42 of the shuttle valve 30 in chamber 45. The pressurized fluid exits the valve cavity 28 and is applied against end 51 of piston 50 which is normally biased to engage piston end 51 with end 46 of cylinder 48. The pressurized fluid moves piston 50 against the force of spring 52 pumping fluid in fluid chamber 98 against the diaphragm 92 flexing it into air chamber 99 pressurizing the air therein for discharge through outlet 94. Sufflcient movement of the piston 50 positions annular groove 54 to connect conduit 76 conducting pressurized fluid. from the supply line 20 through the groove 54 to conduit supplying pressurized oil to signal cavity 72. Receipt of the pressurized oil in cavity 72 moves the shuttle valve 30 against the force of spring 32 so that the reduced portion 118 of the valve connects conduit 44 with exhaust conduit 104 thereby removing the pressurized fluid from cylinder 48 to sump 68 through conduit 66. At this time the piston 50 begins to retract within cylinder 48 under the influence of spring 52. Sufficient movement in a retracted direction closes groove 60 connecting with conduit 70 and eventually moves piston groove 54 connecting conduit 70 and groove 58 with groove 56 and conduit 66 thereby removing the pressurized fluid in signal cavity 72 as the pressurized fluid flows to the sump 68. As the pressurized fluid is discharged from signal cavity 72, the valve 30 is again biased to its first position by spring 32 thereby completing a cylce of operation. Retracting movement of the piston 50 also withdraws fluid from fluid chamber 98 flexing the diaphragm 92 in a direction toward the fluid chamber. This movement of the diaphragm 92 draws air through the inlet passage 96 into the air chamber 99 on the remote side of diaphragm 92 for later pressurization.

A high pressure relief valve 22 is provided in the conduit 20 and is biased by a spring 124 so as to regulate the maximum fluid pressure in the fluid control system. Pressures in excess of those within design limits of the system result in the valve 22 opening and connecting the source line 20, through conduit 108 to the sump 68 thereby preventing damage to the system. A low pressure valve 78 is provided in conduit so as to regulate the pressures existing in signal cavity 72, the function of the pressure fluid in cavity 72 being that ofa holding circuit retaining the valve in a position compressing the spring 32 so that fluid may be positively exhausted from cylinder 48 while the piston 50 is movedretracted by spring 52. The low pressure valve 78 is biased closed by a spring 122 so as to maintain a desirable pressure in the cavity 72 which is in the range of 40 P.S.I.

As is more specifically described in copending application Ser. No. 73,845, the air compressor assembly 84 includes the

valves

126 and 132 shown in FIGS. 7 and 8. These valvesrespectively function to limit the maximum oil pressures applied to the diaphragm in fluid chamber 98 as well as limiting the maximum negative pressure applied therein so as to prevent damage to the diaphragm 92.

From the above description of a preferred embodiment of the subject invention it is apparent that this invention provides a shuttle valve rapidly moving within a valve cavity which is effective to rapidly cycle a diaphragm air compressor. More specifically, this invention utilizes a source of substantially constant pressure fluid and transforms the pressurized fluid into intermittent pulses that are applied to a piston pumping fluid against the air compressing diaphragm in response to the pulses. The valve 30 is operable to rapidly pulse the pressurized fluid received from the power steering system as previously described. It is significant that the control system of the subject invention does not rely upon utilization of any mechanically actuated devices or any outside power sources other than the source of pressurized fluid to actuate the diaphragm in the air compressor assembly 84. Utilization of a source of pressurized fluid already provided in a vehicle system results in an efficient method of providing compressed air to, for example, a vehicle leveling system.

While I have shown and described a specific applica tion of an embodiment of the present invention it will, of course, be understood that many modifications and alternative constructions may be made without departing from the true spirit and scope of the invention. I, therefore, intend by the appended claims to cover all such modifications and alternative constructions as fall within the true spirit and scope of this invention.

I claim:

1. A mechanism to compress air oil free from a source of oil under pressure, comprising; a housing, a diaphragm dividing a cavity in said housing into a fluid chamber and an air chamber, said housing containing a cylinder connecting with said fluid chamber, a piston in said cylinder, a spring normally biasing said piston retracted, valve means controlled by said piston as it moves between retracted and extended positions, a valve cavity in said housing, passage means between said valve cavity and said cylinder, passage means connecting said source of pressurized oil to said valve cavity, a shuttle control valve in said valve cavity, means normally biasing said valve extended directing pressurized oil against said piston for extension thereof in said cylinder pumping fluid into said fluid chamber flexing said diaphragm into said air chamber pressurizing air therein, sufflcient extended movement of said piston also positioning its associated valve means directing pressure fluid to a signal cavity on one end of said shuttle valve moving said valve against said biasing means to a position exhausting pressure fluid from said piston, and a pressure relief valve exhausting said signal cavity in response to pressure fluid from said source thereby reducing fluid resistance against said shuttle valve and against said source while said piston is being retracted, said biasing spring retracting said piston, sufficient retracting movement of said piston controlling said valve means exhausting the pressure fluid from said shuttle valve whereupon it returns to the normal biased extended position thereby initiating a new air compressing cycle.

2. In combination a diaphragm aircompressor assembly and a fluid control system intermittently applying a source of pressurized fluid to the air compressor and exhausting it therefrom pulsating the diaphragm to compress air, the combination comprising; an air compressor housing, a diaphragm in said housing defining a fluid chamber on one side and an air chamber on the other side thereof, a fluid control system housing, said housing defining a cylinder, a piston reciprocally received within said cylinder, said piston moving between extended and retracted positions, means biasing said piston to the retracted position, valve means actuated by said piston as it moves between the extended and retracted positions, means connecting said cylinder with said diaphragm fluid chamber, said fluid control system housing also containing a valve cavity, a shuttle valve slidably mounted within said cavity, means biasing said valve to a first position, means connecting said valve cavity to said cylinder at one end of said piston, a fluid reservoir, means connecting said cylinder to said reservoir, valve and said valve cavity defining a signal chamher on one end of said valve, means connecting said cylinder to said signal chamber, means connecting a source of fluid under pressure to said valve cavity whereby said shuttle valve directs pressurized fluid against said piston for extension within said cylinder pumping fluid to said diaphragm air compressor pressurizing air in the air chamber, movement of said piston a sufficient distance actuating its associated valve means supplying pressurized fluid to the signal chamber at one end of said shuttle valve, said shuttle valve moving to a second position exhausting fluid from said cylinder to said reservoir, allowing said piston biasing means to become retracting movement thereof, sufficient movement of the piston retracted actuating its associated valve means to exhuast the pressurized fluid from said signal chamber to said reservoir allowing said valve biasing means to return said shuttle valve to its first position thereby initiating a new air compressing cycle and a pressure relief valve connected between said signal chamber and said reservoir exhausting fluid from said signal chamber reducing the fluid pressure resistance against said valve and said source of pressurized fluid.

3. A mechanism to compress air oil free from a source of oil under pressure by automatically reciproeating a shuttle valve controlling flow of the pressurized fluid to a piston rapidly pulsating an air compressing diaphragm comprising; a housing, a diaphragm in said housing cooperating with portions thereof defining a fluid chamber on one side and an air comressing chamber on the other side, said housing including a cylinder having a closed end, said cylinder containing fluid and being in fluid communication with said fluid chamber, a piston reciprocally received within said cylinder for movement between extended and retracted positions, a spring biasing said piston to the retracted position, valve means on said piston, fluid flow controlling lands formed in said cylinder cooperating with said piston valve means directing fluid flow as said piston reciprocates in said cylinder, an inlet check valve controlling flow of air into said air compressing chamber,

an outlet check valve controlling discharge of pressurized air from said air compressing chamber, said housing also containing a shuttle valve cavity having fluid flow controlling valve lands therein, a shuttle valve having fluid controlling lands cooperating with said lands in said cavity and being reciprocally received within said valve cavity, a spring biasing said valve to a first positon, a conduit supplying pressurized oil from said source to said valve cavity, a conduit conveying pressurized oil from said valve cavity to and from saidcylinder closed end for application against and removal from said piston, a conduit supplying pressurized oil to said cylinder adjacent said fluid flow controlling lands, a conduit connecting said cylinder with a signal cavity on one end of said shuttle valve, said shuttle valve initially directing pressurized fluid to said cylinder closed end moving said piston against its spring pumping fluid into said fluid chamber against said diaphragm flexing it compressing air for discharge through said outlet check valve, sufficient movement of said piston positioning said piston valve means relative to said cylinder flow controlling lands directing pressurized fluid to said signal cavity and moving said shuttle valve against its biasing spring exhausting pressurized fluid from said cylinder closed end, said piston moving retracted by its biasing spring first closing the pressurized fluid in said cavity and being effective to exhaust pressure fluid from said cavity while said shuttle valve is moved against its biasing spring and fluid from said source flows directly into said signal cavity thereby reducing fluid resistance against said valve and said source of pressurized oil.

Claims

1. A mechanism to compress air oil free from a source of oil under pressure, comprising; a housing, a diaphragm dividing a cavity in said housing into a fluid chamber and an air chamber, said housing containing a cylinder connecting with said fluid chamber, a piston in said cylinder, a spring normally biasing said piston retracted, valve means controlled by said piston as it moves between retracted and extended positions, a valve cavity in said housing, passage means between said valve cavity and said cylinder, passage means connecting said source of pressurized oil to said valve cavity, a shuttle control valve in said valve cavity, means normally biasing said valve extended directing pressurized oil against said piston for extension thereof in said cylinder pumping fluid into said fluid chamber flexing said diaphragm into said air chamber pressurizing air therein, sufficient extended movement of said piston also positioning its associated valve means directing pressure fluid to a signal cavity on one end of said shuttle valve moving said valve against said biasing means to a position exhausting pressure fluid from said piston, and a pressure relief valve exhausting said signal cavity in response to pressure fluid from said source thereby reducing fluid resistance against said shuttle valve and against said source while said piston is being retracted, said biasing spring retracting said piston, sufficient retracting movement of said piston controlling said valve means exhausting the pressure fluid from said shuttle valve whereupon it returns to the normal biased extended position thereby initiating a new air compressing cycle.

2. In combination a diaphragm air compressor assembly and a fluid control system intermittently applying a source of pressurized fluid to the air compressor and exhausting it therefrom pulsating the diaphragm to compress air, the combination comprising; an air compressor housing, a diaphragm in said housing defining a fluid chamber on one side and an air chamber on the other side thereof, a fluid control system housing, said housing defining a cylinder, a piston reciprocally received within said cylinder, said piston moving between extended and retracted positions, means biasing said piston to the retracted position, valve means actuated by said piston as it moves between the extended and retracted positions, means connecting said cylinder with said diaphragm fluid chamber, said fluid control system housing also containing a valve cavity, a shuttle valve slidably mounted within said cavity, means biasing said valve to a first position, means connecting said valve cavity to said cylinder at one end of said piston, a fluid reservoir, means connecting said cylinder to said reservoir, valve and said valve cavity defining a signal chamber on one end of said valve, means connecting said cylinder to said signal chamber, means connecting a source of fluid under pressure to said valve cavity whereby said shuttle valve directs pressurized fluid against said piston for extension within said cylinder pumping fluid to said diaphragm air compressor pressurizing air in the air chamber, movement of said piston a sufficient distance actuating its associated valve means supplying pressurized fluid to the signal chamber at one end of said shuttle valve, said shuttle valve moving to a second position exhausting fluid from said cylinder to said reservoir, allowing said piston biasing means to become retracting movement thereof, sufficient movement of the piston retracted actuating its associated valve means to exhuast the pressurized fluid from said signal chamber to said reservoir allowing said valve biasing means to return said shuttle valve to its first position thereby initiating a new air compressing cycle and a pressure relief valve connected between Said signal chamber and said reservoir exhausting fluid from said signal chamber reducing the fluid pressure resistance against said valve and said source of pressurized fluid.

3. A mechanism to compress air oil free from a source of oil under pressure by automatically reciprocating a shuttle valve controlling flow of the pressurized fluid to a piston rapidly pulsating an air compressing diaphragm comprising; a housing, a diaphragm in said housing cooperating with portions thereof defining a fluid chamber on one side and an air comressing chamber on the other side, said housing including a cylinder having a closed end, said cylinder containing fluid and being in fluid communication with said fluid chamber, a piston reciprocally received within said cylinder for movement between extended and retracted positions, a spring biasing said piston to the retracted position, valve means on said piston, fluid flow controlling lands formed in said cylinder cooperating with said piston valve means directing fluid flow as said piston reciprocates in said cylinder, an inlet check valve controlling flow of air into said air compressing chamber, an outlet check valve controlling discharge of pressurized air from said air compressing chamber, said housing also containing a shuttle valve cavity having fluid flow controlling valve lands therein, a shuttle valve having fluid controlling lands cooperating with said lands in said cavity and being reciprocally received within said valve cavity, a spring biasing said valve to a first positon, a conduit supplying pressurized oil from said source to said valve cavity, a conduit conveying pressurized oil from said valve cavity to and from said cylinder closed end for application against and removal from said piston, a conduit supplying pressurized oil to said cylinder adjacent said fluid flow controlling lands, a conduit connecting said cylinder with a signal cavity on one end of said shuttle valve, said shuttle valve initially directing pressurized fluid to said cylinder closed end moving said piston against its spring pumping fluid into said fluid chamber against said diaphragm flexing it compressing air for discharge through said outlet check valve, sufficient movement of said piston positioning said piston valve means relative to said cylinder flow controlling lands directing pressurized fluid to said signal cavity and moving said shuttle valve against its biasing spring exhausting pressurized fluid from said cylinder closed end, said piston moving retracted by its biasing spring first closing the pressurized fluid in said signal cavity maintaining the cylinder closed end connected to exhaust until said piston is fully retracted, said piston valve means and said cylinder lands then cooperating to exhaust pressure fluid from said signal cavity and said spring returning said shuttle valve to the first biased position for initiation of a new air compressing cycle, and a relief valve connected with said signal cavity and being effective to exhaust pressure fluid from said cavity while said shuttle valve is moved against its biasing spring and fluid from said source flows directly into said signal cavity thereby reducing fluid resistance against said valve and said source of pressurized oil.