Combined fixed and variable displacement pump system

A hydraulic pump arrangement and control circuit for supplying fluid at high volume and a high pressure to a work circuit. The system utilizes a fixed displacement pump, a variable displcement pump, a dual pressure compensator, and an unloading valve working together to provide high pressure fluid delivery while reducing the horsepower requirements of the fixed displacement pump and the variable displacement pump. High flow is provided through the utilization of the combined pumps and high pressure is attained through the unloading of the fixed displacement pump through the unloading valve and destroking of the variable displacement pump in response to signals provided by the dual pressure compensator.

BACKGROUND OF THE INVENTION 1. Field of the Invention 
The invention relates to fluid control systems having a fixed displacement 
pump and a variable displacement pump working in unison and controlled by 
a regulating circuit normally for use on construction loaders, tractors, 
or other mofile implement carrying equipment. 
2. Description of the Prior Art 
It is known to mount two fixed displacement pumps in tandem in a hydraulic 
circuit and control the output of these pumps through the use of various 
valving arrangements. 
However, the use of a fixed displacement pump system of one or more than 
one pump becomes uneconomical in an application that requires high 
pressure but negligible fluid flow for any period of time as the fixed 
displacement pump will continue to attempt to deliver full flow even when 
only minimal flow is needed. This waste of pump capacity draws horsepower 
from the prime mover supplying the pump and if continued wil eventually 
stall the prime mover. In cases where the work circuit has a relief valve 
the relief valve will allow the dumping of fluid as it is being pumped by 
the fixed displacement pump thus generating undesirable heat in the fluid 
of the work circuit. Multiple fixed displacement pump systems are designed 
to reduce this wasteful pump operation by having different displacement 
pumps that can be called upon in various operating or load situations to 
deliver the required fluid volume or pressure. Unfortunately multiple 
fixed displacement pump systems still have the high pressure deficiencies 
previously mentioned. 
It is also known to use a variable displacement pump to satisfy the needs 
of a hydraulic fluid control system. It is not usual, however, to find two 
or more variable displacement pumps in tandem as is found with fixed 
displacement pumps as the variable displacement pump supplies fluid at the 
pressure and the volume needed by the hydraulic system to which it is 
responsive. 
The primary drawback of a variable displacement pump is the cost of the 
pump. An investigation of available hydraulic pumps and their respective 
costs will indicate advantages other than efficiency. The least expensive 
pumps presently used in hydraulic systems are gear type fixed displacement 
pumps. These pumps have limited pressure potential but are available in a 
wide variety of displacements. Variable volume piston pumps are 
comparatively expensive (especially in larger displacement sizes) and have 
limited availability in different displacements. Also a drawback is that 
there may be a slight lag in the stroking or destroking operation of the 
variable displacement piston pump. 
A fixed displacement pump will generate relatively instant pressure and 
flow to a work circuit when the circuit valving is opened due to the 
positive displacement characteristic of the fixed displacement pump. The 
combination system proposed in this invention utilizes the best 
characteristics of each type pump. THe variable displacement piston pump 
is used alone for high pressure requirements while a combination of the 
variable displacement piston pump and the fixed displacement gear pump 
will meet the high flow demands of the system. Therefore, the combination 
of a fixed and variable displacement pump in a regulated system such as 
the dual pressure compensator controlled system of this invention will 
have the advantage of the instant acting fluid delivery inherent in the 
fixed displacement pump and the high pressure, low horsepower consuming 
characteristics available with a variable displacement pump. A combination 
pump system therefore maximizes performance, efficiency, and dependability 
while minimizing costs. 
Therefore it is an object of the invention to provide a hydraulic pump 
system that provides high pressure and high flow when needed and yet does 
not waste pump driving horsepower when only high pressure and low flow is 
needed. 
Further it is object of the invention to provide a pump system that will 
not "bog down" the vehicle engine when high pressure work loads are 
imposed on the system. Another object is to provide a pump system that can 
provide pressure to a work circuit immediately upon request. A further 
object of the invention is to provide a compensator which is responsive to 
a dual pressure input to signal stroking or destroking of a variable 
displacement pump. A further object of the invention is to provide a 
hydraulic pump system that can maintain high work circuit pressure at a 
low horsepower requirement. Another object of the invention is to provide 
a hydraulic circuit that doesn't cause excessive heat generation in the 
fluid thereof. Also an object of the invention is to provide a combination 
pump system that delivers good performance at a reasonably moderate cost. 
SUMMARY OF THE INVENTION 
A multiple pressure hydraulic fluid system for use on agricultural or 
industrial equipment that is designed to deliver fluid flow at high volume 
or fluid at high pressure depending on the needs of circuit involved. 
A dual pump arrangement having a fixed displacement pump and a variable 
displacement pump is so controlled by a dual pressure compensator 
receiving a signal from a control line communicating with each of the 
output ports of the aforementioned pumps and an unloading valve which has 
the ability to dump the output of the fixed displacement pump such that 
the desired results mentioned above are made possible. 
More specifically this is accomplished through the use of a fluid circuit 
regulated by a dual pressure compensating system which has the following 
significant components. 
A variable displacement normally piston type pump which is driven by a 
prime mover has an inlet port communicating with the source of fluid such 
as a reservoir and an outlet port providing fluid delivery from the 
variable displacement pump. A fixed displacement pump which is driven by 
the same prime mover has an inlet communicating with a source of fluid 
such as a reservoir and an outlet port providing fluid under pressure to a 
work circuit. The variable displacement pump mentioned above has a 
pressure responsive actuator common to variable displacement piston type 
pumps. The output of both the variable displacement pump and the fixed 
displacement pump communicate with an incremental pressure compensating 
device which is initially responsive to the combined fluid output pressure 
of both pumps and secondarily responsive to the fluid output pressure of 
the variable displacement pump alone. This compensating device will signal 
the pressure responsive actuator of the variable displacement pump to 
either stroke or destroke as necessary to accommodate the requirements of 
the work system. Also an integral part of the system is a pressure 
responsive unloading valve which is responsive to the combined fluid 
output pressure of the fixed displacement pump and the variable 
displacement pump which when activated will divert output from the fixed 
displacement pump to a reservoir. There is a check valve in the conduit 
communicating with both the fixed and variable displacement pumps that 
prevents output from the variable displacement pump from being passed in a 
reverse direction through the fixed displacement pump and thence to the 
reservoir.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIGS. 1 and 2 of the drawings the invention and its 
application is shown by the schematic presentations wherein a prime mover 
10 is provided to communicate a driving force to the fixed displacement 
pump 12 of the positive displacement gear type, generally but not 
exclusively, and a variable displacement pump 14 generally of the axial 
piston type positive displacement pump. Hereinafter, the fixed 
displacement pump will be referred to as the PF and the variable 
displacement pump will be referred to as the PV. These being generally 
recognized abreviations for each of these respective pumps. The PF 12 has 
an inlet 16 communicating with fluid reservoir 18 by means of conduit 20. 
The output or outlet port 22 of the PF 12 is connected to conduit means 24 
and the fluid pumped by the PF 12 normally passes through a one way check 
valve 26 before joining the output of the PV 14 and conduit means 28 which 
connects the pump system with the work circuit 30. An alternative pass for 
the fluid output of the PF 12 is possible through conduit means 32 which 
intercepts conduit means 24. Conduit means 32 progresses from conduit 
means 24 through an unloading valve 34 of the pilot operated type thence 
to fluid reservoir 18 through conduit means 46. Fluid will pass from the 
outlet port 22 of the PF 12 to the fluid reservoir 18 upon activation of 
unloading valve 34 which is responsive to a pressure signal from conduit 
48 by means of conduit 28. This unloading valve shown as 34 in FIG. 1 is 
of the pilot operated type having a progressive opening means, however, 
this valve could be of a more conventional type if desired. 
The variable displacement pump or PV 14 has an inlet port 36 communicating 
with the fluid reservoir 18 by means of conduit 38. The PV 14 is driven by 
a prime mover 19 represented as driving both the PF 12 and the PV 14. 
However, this is only a schematic representation and alternative methods 
of pump driving would be to use two prime movers or other reasonable 
alternatives. Conduit means 28 connects the outlet port 40 of the PV 14 
with the work circuit 30. The PV, being of the axial piston positive 
displacement type pump having a swash plate and a hydraulic actuator, is 
not shown in detail as pumps of this type are well known in the art. 
A dual pressure compensator generally depicted as 50 is shown schematically 
in operative communication with the PV 14. Conduit means 42, generally a 
pilot or signal line, provides fluid communication between the conduit 
means 24 of the PF outlet port 22 and the dual pressure compensator 50. A 
second conduit means 44 provides fluid communication between the dual 
pressure compensator 50 and the conduit means 28 connecting the outlet 
port 40 of the PV 14 to the work circuit 30. This conduit means (44) is 
also a pilot or signal type line. 
The work circuit generally depicted as 30, is not specified in detail but 
could be, for example, the hydraulic system of a construction duty tractor 
having a backhoe and a forward carried bucket scoop. This system would 
have operating cylinders for moving the work implements through a wide 
range of digging, delivering, transferring and holding postures. Also, the 
work circuit may include hydrostatic or hydraulic transmission means, 
direction control means and additionally any other unspecified fluid 
operated apparatus as can be imagined. 
Turning now to FIG. 3 a detailed description of the dual pressure 
compensator, generally depicted as 50, will be given. 
The dual pressure compensator housing 52 has been provided with a bore 54 
therethrough having several concentric diameters. 
The first end 62 of the bore 54 acts as a first inlet port 64 of the dual 
pressure compensator 50. The housing is also equipped with a second inlet 
port 66, a first outlet port 68, and a second outlet port 70 which all 
communicate independently with the bore 54. 
As shown by the schematic presentation of FIG. 1 first inlet port 64 
communicates through conduit means 42 and 24 to the outlet port 22 of the 
the PF 12. Second inlet port 66 communicates with outlet port 40 of the PV 
14 through conduit means 44 and 28. First outlet port 68 communicates with 
the destroking actuator 58 by means of conduit 60. Second outlet port 70 
communicates with the bore 54 and the fluid reservoir 18 by means of 
conduit 56. 
Looking again at FIG. 3 wherein slidably carried inside bore 54 is a spool 
80 held in its normal position by a biasing assembly 72 carried in the 
larger second end 74 of bore 54. The biasing assembly 72 has a pressure 
plate 76 locating a biasing means represented by coil spring 78 which is 
further guided by a retainer plate 82. A retainer plate adjustment screw 
84 is threadably mounted in the second end 74 of bore 54 such that 
adjustment of the retainer plate adjustment screw 84 will result in 
varying the pressure exerted on the spool 80 by the biasing apparatus 72. 
The pressure plate 76 is further distinguished by having an aperture 86 
formed therein coincidental to the minor axis of the plate 76. Also formed 
on the innermost surface of the pressure plate 76 is an arcuate concave 
spool receiver 90 and the outermost surface of pressure plate 76 is 
contact surface 92 for aligning and guiding the coil spring 78. 
The retainer plate 82 is equipped with a groove 94 for holding an O-ring 96 
against the walls of the bore 54 and minimizing leakage of fluid which 
will be present in this section of bore 54. 
Still considering FIG. 3 it is noted that spool 80, formed on a core 81, 
has a first end 98 of somewhat smaller diameter than the second end 100 or 
the core 81. The second end 100 has a diameter greater than the core 81. 
Further the second end 100 has an arcuate portion which is compatible with 
the concave spool receiver 90 mentioned above. 
A land 104 is formed on the spool 80 dividing the midsection of the spool, 
which has a diameter greater than the diameter of the first end 98 of the 
spool and of the core 81, into two portions. The land 104 has a groove 108 
circumferentially formed on it. This groove 108 provides a galley or 
channel for directing fluid to an internal passage 110 running through the 
spool 80 from the land 104 to the arcuate portion 102 thereof. 
Spool alignment is further ensured through the use of spool alignment 
collar 112 encompassing first end 98 of the spool 80. The alignment collar 
112 carries an O-ring 114 and a washer 116 in a circumferential groove 118 
formed in the alignment collar 112. 
Two chambers are formed in the bore 54 by the surrounding components. These 
are shown as first chamber 120 at the first end 62 of the bore 54 and 
second chamber 122 formed between land 104 and the alignment collar 112. 
Regardless of spool position the first chamber 120 can only communicate 
with first inlet port 64. Second chamber 122 communicates with the second 
inlet port 66 until the spool 80 is shifted (laterally) against the 
biasing assembly 72 sufficiently far to enable communication between the 
second inlet port 66 and the first outlet port 68. First outlet port 68 
can communicate with second outlet port 70 when spool land 104 is shifted 
fully to the left as depicted by solid line position A. 
The normal operation of the combined fixed and variable displacement pump 
system with the dual pressure compensator just described will be explained 
in conjunction with FIG. 4 when considered with the explanations of FIGS. 
1, 2 and 3 previously presented. 
When there is no load presented to the combined pump fluid delivery system 
from the work circuit 30 both pumps 12 and 14 would be providing fluid to 
the work circuit which would direct this fluid to the reservoir as no work 
would be required. This situation is represented by point A on the graph 
which communicates that there is little horsepower being used to generate 
just enough pressure to ensure fluid flow. 
Note that the Horsepower scale does not indicate raw horsepower being used 
by the prime mover, which may also be driving a host vehicle, but rather 
net horsepower to drive the dual pump system and provide pressure in the 
fluid system under consideration. 
The fixed displacement pump, PF 12, is at full flow delivery as is the 
variable displacement pump PV 14. The unloading valve is in a closed mode 
as represented in FIG. 3 as solid line position A. The spool 80 of the 
dual pressure compensator 50 is biased as far as possible toward the first 
end 62 of the bore 54 preventing fluid flow between the second inlet port 
66 and the first outlet port 68. The check valve 26 is allowing passage of 
fluid through conduit means 24. 
Upon system demand requiring ever increasing pressure the following cycle 
will be encountered. 
Increasing pressure, resultant of work circuit load, is represented on FIG. 
4 as line A-B. Both pumps are working together and as pressure needed to 
operate the work circuit increases the horsepower to drive the pumps 
increases. The situation will progress until point B when the fixed 
displacement gear pump will be at full pressure capacity. At point B the 
variable displacement pump is signaled to commence destroking. The term 
destroking is used in the vernacular to define the change in a piston type 
variable displacement pump when it goes from full stroke to partial 
stroke. In other words, the displacement of the pump decreases in terms of 
fluid output, however, it increases in terms of pressure generation. This 
is accomplished through the mechanisms of the dual pressure compensator. 
Pressure in the first chamber 120 has increased with the pressure generated 
by the PF pump. This pressure increase was communicated between conduit 
means 24 and the dual pressure compensator 50 by conduit means 42 (see 
FIG. 1). A portion of the first chamber's boundry is the first end 98 of 
the spool 80. Thus, spool 80 is biased against the biasing assembly 72 by 
any significant pressure in the first chamber 120. 
Pressure in the second chamber 122 has also been increasing. The second 
chamber 122 gets a fluid pressure signal from conduit means 28 through 
conduit means 44. 
To reiterate, point B represents the pressure at which the pressure in the 
first chamber 120 and point in the second chamber 122 have combined to 
override the pressure on spool 80 imposed by biasing assembly 72. This 
forces the piston land 104 to slide past the first outlet port 68. This 
action then allows passage of fluid from the second inlet port 66 to the 
first outlet port 68 which communicates with the actuator 58 of the PV 14. 
The actuator of the PV will destroke the pump as necessary as system 
pressure continues to increase. 
At point C the PV will have been at least partially destroked and at this 
point the PF 12 will be unloaded to reservoir 18 by means of unloading 
valve 34. 
The pressure in conduit means 28 is communicated to the unloading valve 34 
by means provided by conduit 48 which is a pilot line for the unloading 
valve. The unloading valve 34 is set to open at system pressure that would 
start to bog down the prime mover due to the positive displacement 
characteristics of both the fixed and variable displacement pumps. Since 
the PF 12 can no longer provide increasing pressure to the system it is 
effectively relieved of this task by the unloading valve 34. 
When the unloading valve 34 is opened fluid will flow from the outlet port 
22 of the PF 12 through the conduit means 24 then through conduit means 
32, through valve 34, conduit 46, to the fluid reservoir 18. Check valve 
26 will prevent fluid conduit means 28 from reversing through the 
unloading valve or the dual pressure compensator. This unloading of the 
fixed displacement pump or PF 12 will decrease the horsepower consumed by 
the PF and make available more horsepower to drive the variable 
displacement pump PV to higher pressure. Line C-E represents the drop in 
horsepower being utilized as a consequence of the PF going to dump. 
The fixed displacement pump is now at full flow delivery to the reservoir. 
The variable displacement pump is at a stroked displacement providing 
pressure and flow as necessary. The unloading valve is opened allowing the 
output of the PF to pass to reservoir 18. 
The PV has been allowed to stroke due to the drop in pressure at the first 
chamber 120 of the dual pressure compensator 50 resulting from the absence 
of pressure being delivered by the PF. 
At point F of FIG. 4 the dual pressure compensator again starts to see high 
enough pressure to initiate further destroking of the PV. This is 
signalled by the pressure in the second chamber 122 getting high enough to 
force the spool 80 against the biasing assembly 72 far enough to have land 
104 clear the first outlet port 68. The destroking of PV is accomplished 
at point F as earlier described. 
At point G the PV is fully destroked. The PF is unloaded to reservoir and 
the check valve is closed. This point represents the maximum pressure 
available to the work circuit. The horsepower is low because the PF is 
going to the reservoir and imposing only minimal horsepower drain on the 
prime mover and the PV is fully destroked again which also imposes only 
marginal horsepower drain on the prime mover. 
The benefit here is that the work circuit has not bogged down the prime 
mover even under full pressure. The work circuit, for instance the backhoe 
previously mentioned, would have its maximum digging force at this point 
yet the engine would be at a normal RPM level. 
Upon release of system pressure requirements the pressure in pilot means 44 
would drop off causing the pressure in the second chamber 122 to decrease 
allowing the spool 80 in the dual pressure compensator 50 to be urged back 
to a closed position by biasing assembly 72. This is shown by spool 
position A of FIG. 3. 
When the spool 80 is back in the unloaded position passage between the 
first outlet port 68 and the second outlet port 70 is possible. This 
allows fluid to flow from the destroking actuator 58 of the PV to the 
fluid reservoir 18. The PV is of course fully stroked when this is done. 
The system is now effectively back to point A of the graph in FIG. 4 and 
ready to begin the increasing pressure pump compensation cycle again. It 
should be pointed out that the cycle need not be followed to optimum 
pressure every time it is utilized. The system can be used incrementally 
as required by the demand put on the pumps by the work circuit load. In 
other words the frequency of the highest pressure delivery of the system 
will occur sporadically depending on the type of work being done by the 
work circuit. 
Thus it is apparent that there has been provided in accordance with the 
invention a dual pressure compensator working with a fixed and variable 
displacement pump system that fully satisfies the objects, aims and 
advantages set forth above. While the invention has been described in 
conjunction with specific embodiments thereof it is evident that many 
alternative modifications and variations will be apparent to those skilled 
in the art in light of the foregoing description. Accordingly this 
invention is intended to embrace all such alternatives and modifications 
and variations as fall within the spirit and broad scope of the appended 
claims.