Abstract:
A fluid pump has a housing ( 4 ), a main output port ( 10 ), an auxiliary output port ( 14 ) and a priority pressure regulating valve contained with the housing ( 4 ). The priority pressure regulating valve has a spool ( 20 ) to direct fluid to one or both of the of the output ports ( 10,14 ), a force means ( 34,36 ) associated with the spool to bias the spool ( 20 ) to a position where it causes fluid to flow to the main output port ( 10 ) exclusively, and a pressure release means ( 40 ) which enables the spool ( 20 ) to move to a position where it permits fluid to flow to the auxiliary output port ( 14 ) when the pressure at the main output port ( 10 ) is at or greater than a predetermined pressure.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to fluid pumps, and in particular to gear pumps and other positive displacement hydraulic pumps, which can be used to deliver hydraulic fluid to two different sets of hydraulic loads. A priority valve is needed to distinguish between the two loads, and deliver hydraulic fluid preferentially to a first load up to a first working pressure, and only then deliver hydraulic fluid to the second load which is a non-preferential load. 
     2. Description of the Related Art 
     Priority valves are known which divide the flow from a hydraulic pump into preferred and non-preferred flows for servicing two loads as indicated above. The majority of such priority valves have been connected in series with the pump output, being connected to the pump by conduits and to the first and second loads by further conduits. 
     In GB 2298902, the present Applicant discloses a pump incorporating an integral priority pressure regulating valve. The valve is spring biased at one end face in a direction to permit fluid communication between a high pressure chamber of the pump and a main port connected to the preferential load only. An opposing end face of the valve is supplied with hydraulic fluid from the main port in such a manner to counter the spring bias. When the main port receives hydraulic fluid of a predetermined pressure, the pressure on the opposing end face is sufficient to overcome both the compressive force developed by the spring and the static friction associated with the valve, thereby enabling the movement of the valve to a position where it permits fluid communication between the high pressure chamber and an auxiliary port which is connected to the non-preferential load. This pump is much simpler to install than one requiring a separate priority pressure regulating valve to be inserted in the pipeline or conduit between the pump and the main and auxiliary loads, and there is a much lesser tendency for fluid leakage. 
     In this arrangement, the spring provided to bias the valve must be of sufficient strength so as to meet the total reaction developed against it when fluid of the predetermined pressure is applied to the non-spring end face of the valve. Quite often, the predetermined pressure selected is relatively high and hence the loading on the spring can be excessive. 
     The characteristics of the spring are extremely important since the spring must be compressed to a depth equal to the length through which the valve is require to travel without exhibiting substantial changes in its reaction against the valve, otherwise the reaction developed by the spring against the valve will increase significantly as the spring is compressed. 
     Furthermore, whereas pressure is uniformly applied to the non-spring end face of the valve from the main port, the force exerted by the spring on the valve is localized through the points of contact between the spring and the valve. This may induce distortion of the valve profile. 
     Additionally, as the spring is an integral component to the pump, it is a relatively difficult operation to adjust or replace the spring so as to provide the pump with a new predetermined pressure setting. 
     Therefore, it is an objective of the present invention to significantly reduce the problems identified above in relation to the prior art. This is achieved by means of pilot operation. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a fluid pump having a housing, a main output port, an auxiliary output port and a priority pressure regulating valve contained within the pump housing. The priority pressure regulating valve includes a spool having two opposing end faces. Each of these end faces is disposed within a chamber which is in fluid communication with the main output port. A force means is also included in association with one of the spool end faces to bias the spool to a position where it causes fluid developed by the pump to flow to the main output port exclusively. A pressure release means is provided in fluid communication with one of the chambers to enable fluid to flow from said one of the chambers when the pressure at the main output port is at a predetermined working pressure thereby establishing a pressure differential across the two end faces of the spool which is sufficient to overcome the bias developed by the force means thus causing the spool to move to a position where it permits fluid developed by the pump to flow to the auxiliary output port. 
     In a preferred embodiment, that chamber which is in fluid communication with the pressure release means houses the force means. In this arrangement, the force means is preferably a coil spring in compression. 
     Alternatively, the force means may be provided in the chamber which is remote from that which is in fluid communication with the pressure release means. In these circumstances, the force means may be a coil spring in tension. 
     Preferably, the pressure release means includes a poppet, a regulating spring and an adjuster, wherein one face of the poppet is in fluid communication with one of the chambers and the regulating spring is disposed to resist the force exerted on the face of the poppet by the pressurized fluid contained in said chamber. The poppet may be located between said chamber and a channel such that when the pressure exerted on the poppet by the fluid in said chamber overcomes the opposing force exerted on the poppet by the regulating spring, fluid communication is established between said chamber and the channel. 
     In a preferred embodiment of the invention, the channel drains to an inlet of the pump. 
     The regulating spring may be a helical spring in compression which engages that face of the poppet which opposes the face which is in fluid communication with said chamber. Additionally, that end of the regulating spring which is remote from the poppet may abut the adjuster in a manner such that the adjuster can be moved along the axis of the regulating spring to adjust the compressive force exerted on the poppet by the regulating spring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a transverse section through a gear pump according to the invention, taken along the axis of the priority pressure regulating valve of the pump, showing a spool of the valve in a position in which it delivers hydraulic output fluid to a main or priority outlet port at a pressure below a predetermined level; 
     FIG. 2 shows the pump of FIG. 1 in a condition wherein the pressure of the hydraulic fluid developed by the pump just equals the predetermined pressure required at the main or priority outlet port; and 
     FIG. 3 shows the pump of FIG. 1 in a condition wherein the pressure of the hydraulic fluid at the main or priority outlet port has just been reduced to a level slightly less than the predetermined pressure. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a gear pump  2  according to a preferred embodiment of the invention. The pump  2  has a housing  4  within which is disposed the pumping elements  6  of the pump  2 , an accurately machined bore  17 , a priority outlet port  10  and an auxiliary outlet port  14 . The housing  4  and pumping elements  6  can be of types as used in relation to any conventional positive displacement hydraulic fluid pumps. 
     As shown in FIG. 1, the bore  17  extends throughout the entire transverse length of the pump  2  and is hydraulically sealed at either end. On the left, the seal is achieved by a washer  19  which is mounted over a threaded bolt  18  in a conventional manner. The threaded portion of the bolt  18  engages with threads provided on the circumferential wall of the bore  17 . On the right, the seal is achieved by an O-ring seal  32  disposed on a screw end cap  30  which also is removably engaged with the housing  4 . 
     High pressure fluid developed by the pumping elements  6  is delivered through a supply channel  8  in the housing  4  to a supply annulus  9  machined into the wall of the bore  17 . A spool  20  is provided within the bore  17  and is capable of axial movement along the length of the bore  17 . Depending on its position, the spool  20  is capable of permitting high pressure fluid to flow from the supply annulus  9  to one or both of a first  12  and a second  16  output annulus provided on the wall of the bore  17 . The first output annulus  12  communicates directly with the priority outlet port  10 , while the second output annulus  16  communicates directly with the auxiliary outlet port  14 . 
     A first orifice and blind axial drilling  2 . 2  and a second orifice and blind axial drilling  24  are provided in the spool  20 . These are constantly in fluid communication with the priority outlet port  10 . The first blind drilling  22  delivers fluid to the right hand end face of the spool  20 . The second blind drilling  24  delivers fluid to the left hand end face of the spool  20  where it communicates with a second pressure chamber C 2  defined by the wall of the bore  17 , the threaded bolt  18  and the left hand end face of the spool  20 . 
     The screw end cap  30  has a first pressure chamber C 1  which contains a frictional spring  34 . This is a compressed helical spring which, during operation, is used to bias the spool  20  to the left as shown in FIG. 1 . The frictional spring  34  is mounted on a cylindrical spring carrier  36  (shown in FIG. 2 ). The spring carrier  36  extends from the screw end cap  30  into the bore  17  so as to abut the right hand end face of the spool  20 . The spring carrier  36  has an axial channel to permit fluid communication between the first orifice and blind drilling  22  and the first pressure chamber C 1 . As such, the spool  20 , through the spring carrier  36 , is biased by the frictional spring  34  to the left end of the bore  17 . 
     In addition to the first pressure chamber C 1 , the screw end cap  30  also houses a pilot  40 . The pilot  40  consists of a poppet  42 , a regulating spring  44 , a threaded adjuster  46  and two lock nuts  48 . The regulating spring  44  is in compression and biases the poppet  42  to the left. 
     Depending upon the pressure of the hydraulic fluid in the first pressure chamber C 1  and the biasing force exerted by the regulating spring  44 , the poppet  42  can prevent or permit fluid to flow from the first pressure chamber C 1  through a drain channel  50  to a tank or, preferably, to an inlet of the pumping elements  6 . Once the pressure of the fluid in the first pressure chamber C 1  is sufficient to overcome the opposing compressive force developed by the regulating spring  44 , the poppet  42  lifts against the spring  44  and thereby allows fluid to flow from the first pressure chamber C 1  to the drain channel  50 . The screw end cap  30  is provided with a removable plate  38  which enables the user to access the lock nuts  48  and the threaded adjuster  36 . By rotating the threaded adjuster  36 , the user changes the compressive force exerted by the regulating spring  44  on the poppet  46 , and hence changes the predetermined pressure setting at with the poppet  42  lifts. 
     In comparison to the pump disclosed in GB 2298902, the regulating spring  44  of the present invention can be made substantially stiffer since it is only compressed slightly and is not required to be compressed to the extent to which the spool moves along the bore as in the prior art. Indeed, the regulating spring  44  is only required to generate relatively low loads compared with the single spring design of the prior art. Additionally, in the prior art pump, when the predetermined pressure is established, the spool commences to compress the regulating spring but as the spring is compressed the reaction that it exerts on the spool progressively increases and therefore the pressure required to counteract the spring&#39;s reaction is required to increase. Hence, as the spool traverses along the bore the predetermined pressure changes. In the present invention, use of the regulating spring  44  in the pilot  40  gives a more definite predetermined pressure throughout operation as it is used to counteract the pressure only and not the movement of the spool  20 . 
     In the present embodiment, on start-up, and at all other instances when the pressure of the fluid developed by the pumping elements  6  is less than the predetermined pressure, the spool  20  is biased to the position as shown in FIG. 1 by the frictional spring  34 . Thus, fluid in the supply annulus  9  is delivered initially past a first land  26  (see FIG.2) to the first output annulus  12  which communicates with the priority outlet port  10 . At this stage a second land  28  (FIG.2) provided on the spool  20  blocks hydraulic flow to the auxiliary output port  14 . The pressure of the fluid at the priority outlet port  10  is communicated to the first and second pressure chambers C 1 ,C 2  by the respective orifices and blind drillings  22 , 24 . Since the pressure of the fluid is not sufficient to lift the poppet  42  of the pilot  40  against the regulating spring  44 , the spool  20  is pressure balanced across its end faces and the frictional spring  34  exerts a slight force on the spool  20  through the spring carrier  36  ensuring that the spool  20  remains in the same position to the left of the bore  17 . 
     In FIG. 2, the pressure of the fluid developed by the pumping elements  6  has just reached the predetermined level. Under these conditions, the pressure of the fluid at the priority outlet port  10 , in the first pressure chamber C 1  and in the second pressure chamber C 2  is at the predetermined pressure. Therefore, the pressure of the fluid in the first pressure chamber C 1  is sufficient to lift the poppet  42  against the regulating spring  44  and fluid is allowed to flow from the first pressure chamber C 1  through the drain channel  50  to the inlet of the pumping elements  6 . This produces a pressure drop in the first pressure chamber, and thereby a pressure differential is established across the two end faces of the spool  20 . The differential is more than sufficient to overcome the slight reaction exerted by the frictional spring  34  and hence the spool  20  moves to the right enabling fluid in the supply annulus  9  to be communicated to the auxiliary outlet port  14  as well as the priority outlet port  10 . 
     If the pressure of the fluid developed by the pumping elements  6  continues to be maintained at or above the predetermined level, the spool  20  continues to move until it reaches the extreme right hand position as shown in FIG. 3 in which a shoulder portion of the spool  20  abuts a stop washer that is retained in position by the screw end cap  30 . In this position, fluid communication between the supply annulus  9  and the priority outlet port  10  is interrupted by the first land  26  provided on the spool  20 , and fluid communication is exclusively established between the supply annulus  9  and the auxiliary outlet port  14 . If at this instance, the pressure at the priority outlet port  10  is greater than the predetermined level, the excess fluid is permitted to flow from the priority outlet port  10  through the first orifice and blind axial drilling  22 , through the channel provided in the spring carrier  36  and through the first pressure chamber C 1  to the drain channel  50 . Thereby the pressure at the priority outlet port  10  is reduced until the predetermined level is achieved, at which point the poppet  42  blocks fluid from flowing from the first pressure chamber C 1  to the drain channel  50  (as shown in FIG. 3 ). This establishes a pressure balance across the respective end faces of the spool  20  and the frictional spring  34  moves the spool  20  back to the left. If the pressure of the fluid developed by the pumping elements  6  is still greater than the predetermined level, the spool  20  moves back to the right, otherwise it moves to the position shown in FIGS. 1 and 2. 
     Thus the spool  20  preferentially feeds the priority outlet port  10  with a regulated pressure supply. When the supply is satisfied so that the pressure in the priority outlet port  10  reaches a predetermined working pressure, the spool  20  moves so that hydraulic fluid delivered by the pump  2  continues to be delivered, but to the auxiliary outlet port  14  rather than exclusively to the priority outlet port  10 . 
     The pressure at the auxiliary outlet port  14  can be greater than or less that the pressure at the priority outlet port  10 . If the predetermined working pressure, which is the pressure required at the priority outlet port  10 , is less than the working pressure at the auxiliary outlet port  14  the latter pressure can be allowed to rise until it reaches a maximum rated output pressure of the pump  2 . Alternatively, the working pressure at the auxiliary outlet port  14  can be limited by a pressure relief valve (not shown in the drawings) with excess hydraulic fluid being returned to drain.