Charge pressure priority valve

A charge pressure priority valve for supplying a flow of sufficient pressure to energize an auxiliary implement while supplying a nearly constant pressure flow to a hydrostatic fluid circuit includes a valve body having an interior bore with a pump inlet passage for communicating a flow from a hydraulic pump thereto. A main flow outlet passage extends from the interior bore for supplying fluid to the hydraulic fluid circuit and has a main flow feedback passage interconnecting the main flow outlet passage and the interior bore. An auxiliary flow outlet passage extends from the interior bore for supplying a flow of sufficient pressure to energize an auxiliary implement. A valve spool is disposed within the interior bore and defines a central chamber intermediate a pair of spaced apart end chambers in the bore, with one of the pair of end chambers being isolated from the other of the pair of end chambers.

TECHNICAL FIELD 
This invention relates to a valve for use in a hydrostatic transmission 
and, more specifically, to a valve for maintaining constant system 
pressure within a hydrostatic transmission fluid closed loop while 
independently providing a pressurized flow to energize an auxiliary 
implement. 
BACKGROUND ART 
Many present hydrostatic transmissions include a variable displacement 
hydraulic pump driven by an input shaft. A hydraulic fluid is pumped from 
the hydraulic pump to a hydraulic motor for driving an output shaft. In 
these devices, there is no mechanical linkage between the input shaft 
which drives the hydraulic pump and the output shaft which is driven by 
the hydraulic motor. The hydraulic pumps and motors used in hydrostatic 
transmissions are typically axial piston devices which use a small amount 
of fluid for internal lubrication, which in turn results in fluid being 
lost from the hydrostatic circuit. In order to replenish fluid lost from 
the hydrostatic loop during operation of the transmission, a fixed 
displacement charge pump is driven by the variable displacement pump to 
communicate a supply of fluid in a charge flow path from a fluid reservoir 
to the hydrostatic circuit. 
A desirable feature in the transmission of a working vehicle is the 
capability of transmitting fluid power for driving auxiliary implements, 
such as lift cylinders, steering valves, and the like. In some instances, 
this is accomplished by means of a dedicated hydraulic pump external to 
the hydrostatic transmission for supplying a pressurized auxiliary flow to 
an auxiliary implement fluid circuit. This approach is costly and 
mechanically redundant. More commonly, the fixed displacement charge pump 
which replenishes fluid in the hydrostatic circuit is used also to supply 
the auxiliary flow. This type of arrangement eliminates the expense, 
manufacturing, and maintenance detriments associated with the provision of 
an additional pump. 
While the above approach minimizes the mechanical complexity of the system, 
additional problems exist which detrimentally impact the overall 
efficiency of the transmission. Today, there exists two commonly employed 
arrangements for a hydrostatic transmission in which a single pump is 
utilized to provide system charge pressure as well as providing auxiliary 
implement flow. Each of these arrangements have significant drawbacks 
which affect hydrostatic transmission performance and hydrostatic 
transmission life. 
In one arrangement, the charge flow path supplies oil for auxiliary 
functions, i.e., to the auxiliary implement circuit, before it is 
available to the hydrostatic closed loop. At high auxiliary pressure 
requirements the charge pump leaks substantially, and when the leakage is 
great enough that the charge pump cannot maintain enough flow to replace 
the leakage in the hydrostatic closed loop, the hydrostatic transmission 
component life is severely affected and may cause premature failure. 
To circumvent this problem, and as proposed in the second currently used 
arrangement, the charge flow path supplies fluid first to the hydrostatic 
closed loop and then the auxiliary circuit. This creates, however, two 
additional problems. At high auxiliary function pressure requirements the 
charge pressure on the side of the hydrostatic loop which communicates 
with the auxiliary circuit, traditionally the "low" side of the loop, is a 
sum of the auxiliary pressure and the pressure setting of a charge relief 
valve. The addition of auxiliary pressure to the low side of the 
hydrostatic loop increases loading on shaft bearings and rotated 
components, thereby decreasing transmission life. 
An additional problem lies in the fact that added auxiliary pressure on the 
low side of the hydrostatic loop tends to act as a brake. The braking 
action relates to an increased torque necessary to generate working 
pressure in the pump and the motor, and, in turn, lowers the torque 
efficiency of the pump and motor thereby increasing horsepower loss and 
heat generation. 
The present invention is directed toward overcoming one or more of the 
problems set forth above. 
SUMMARY OF THE INVENTION 
An of object of the present invention is to provide a charge pressure 
priority valve adapted to provide a sufficient flow of hydraulic fluid to 
the charge pressure side of a hydrostatic closed loop to maintain a near 
constant pressure independent of auxiliary function requirements. 
In the exemplary embodiment of the invention, a charge pressure priority 
valve includes a valve body having an interior bore with a pump inlet 
passage for receiving a flow from the hydraulic pump to the interior bore. 
A main flow outlet passage extends from the interior bore for supplying 
fluid to the hydrostatic fluid circuit, and an auxiliary flow outlet 
passage extends from the interior bore for supplying a flow of sufficient 
pressure to energize an auxiliary implement. 
A valve spool having a pair of end-mounted pistons is disposed within the 
interior bore and defines a central chamber intermediate first and second 
end chambers in the bore. A feedback passage communicates hydrostatic 
charge feedback pressure to one of the end chambers, such that one of the 
pistons communicates with the hydrostatic feedback pressure. When the 
feedback pressure reaches a predetermined level, the valve spool is 
displaced to limit flow to the hydrostatic loop and thereby maintain a 
nearly constant hydrostatic charge pressure. 
A biasing spring is provided in the end chamber opposite the end chamber 
which receives the feedback flow. The spring cooperates with the valve 
spool to limit flow to the auxiliary implement when the hydrostatic 
circuit pressure falls below a predetermined pressure. 
Other objects, features and advantages of the invention will be apparent 
from the following detailed description taken in connection with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
A typical hydrostatic transmission is shown schematically in FIG. 1 and 
generally at 10 wherein a pair of hydraulic displacement units 12 and 14 
are connected in a closed hydrostatic loop 16 by a pair of pressure lines 
18 and 20 which can be hoses or passages in a housing for the displacement 
devices. Hydraulic device 12 is a variable displacement axial piston unit 
which normally functions as a pump to deliver fluid under pressure to 
displacement device 14 which also is a variable displacement axial piston 
unit and normally functions as a motor. Pump 12 is driven at a pump input 
shaft 12a as by a prime mover, such as an internal combustion engine (not 
shown), and is operable by means of substantially incompressible flow 
through hydrostatic loop 16 to transmit power to motor output shaft 14a. A 
pair of multi-function valve 22 and 24 are associated with pressure lines 
18 and 20, respectively, and are further described herein. 
A charge pump 26 is driven by axial piston pump 12 and supplies makeup 
fluid to the hydrostatic loop 16 and to an auxiliary fluid circuit 27. The 
charge pump has an inlet connected through a line 28 to a tank reservoir 
30, with a filter 32 in the line, and an outlet connected by a line 34 to 
a charge pressure priority valve, shown generally at 36. Charge pressure 
priority valve 36 apportions a pressurized flow from charge pump 26 among 
a pressure line 38 for maintaining a desired system pressure in 
hydrostatic loop 16 and a pressure line 40 for supplying flow in auxiliary 
circuit 27. 
Multi-function valves 22 and 24 comprise known structures providing both 
over-pressure relief functions and pressure check functions for providing 
a supply of makeup fluid to the hydrostatic loop. Multi-function valve 22 
is connected to pressure line 18 by a line 42 and multi-function valve 24 
is connected to pressure line 20 by a line 44. The multi-function valves 
are cross-connected by a line 46 which also communicates with pressure 
line 38. With this construction, pressurized fluid is pumped by charge 
pump 26 over charge pressure priority valve 36 and can be directed through 
the multi-function valves to supply hydraulic fluid to either of pressure 
lines 18 and 20 in the hydrostatic loop. 
Multi-function valves 22 and 24 comprise identical structures and each 
includes a charge pressure check valve 48 and 50, respectively, and a 
system pressure relief valve, 52 and 54, respectively. Check valves 48 and 
50 are of a ball-type in which fluid pressure normally acts against a ball 
48a and 50a, respectively, within a seat 48b and 50b, respectively, to 
keep fluid under charge pressure in line 46 isolate from the hydrostatic 
loop. When the pressure acting on the seat side of the check valve, i.e., 
the charge pressure exceeds the pressure on the ball side of the check 
valve (i.e, the hydrostatic loop pressure), the ball is unseated and the 
valve is opened until the pressure on opposite side of the check valve is 
equalized. 
System pressure relief valves 52 and 54 are of a spring biased shuttle-type 
in which fluid pressure normally acts against a movable valve spool, 
represented schematically at 52a and 54a, respectively. When the fluid 
pressure is insufficient to overcome a biasing spring 52b and 54b, 
respectively, the valve is in a closed position and fluid is prevented 
from passing over the valve. When the fluid pressure exceeds the spring 
force, the valve spool shuttles to an open position and flow passes 
therethrough. 
In normal operation of the transmission, one of pressure lines 18 and 20 
operates as a high pressure, "high side" supply line for supplying 
pressurized fluid from pump 12 to motor 14, and the other of the pressure 
lines 18 and 20 functions as a low pressure, "low side" return line for 
delivering fluid from pump 14 to charge pump 26. Depending on the nature 
of the work being performed by motor shaft 14a, pressure in the high side 
increases as the torque requirements of the motor increase. In the event 
that the pressure in the high side line exceeds a predetermined value, the 
multi-function valves are operable to relieve the over-pressure condition 
and divert flow to the low side line. 
Particularly, in an exemplary condition in which pressure line 18 is a high 
pressure line, excessive line pressure acts against spring biased spool 
54a to overcome the charge pressure in line 46 and the biasing force of 
spring 54b to shuttle the spool across the valve and divert over-pressure 
flow to multi-function valve 24. The pressure of the relief flow exceeds 
the low side pressure acting against check valve 50 and, as a result, the 
check valve opens and communicates over-pressure flow with return line 20. 
An analogous situation arises when pressure line 20 is a high pressure 
line. Excessive line pressure acts against movable spool 52a to overcome 
the pressure level in line 46 and the biasing force of spring 52b to shift 
the spool and divert over-pressure flow to multi-function valve 22. The 
flow is of sufficient force to open check valve 48 and communicates with 
return pressure line 18. 
The above description of the operation of the over-pressure relief function 
of multi-function valves 22 and 24 illustrates one function of check 
valves 48 and 50. Namely, when a high side relief valve opens to divert 
fluid to the low side of the loop, pressure in line 46 equals the sum of 
the charge pressure and the relief pressure. The low side check valve is 
forced open and the pressure of the low side of the loop is increased. In 
this way, the difference in pressure between the high side of the 
hydrostatic loop and the low side of the hydrostatic loop is maintained at 
a constant value when the high side of the loop is at an over-pressure, or 
maximum pressure, condition. 
The check valves also serve a fluid make-up function in the hydrostatic 
loop. In the event that the system pressure in the low side line falls 
below the fluid pressure in line 46, one of the check valves 48 or 50 is 
unseated to permit make-up fluid to be supplied to the loop and increase 
the low side pressure. When low side system pressure falls below the 
charge pressure in line 46, as by loss of fluid in the hydrostatic loop 
due to leakage or lubrication requirements of the pump 12 or motor 14, the 
check valves permit fluid to replenish the supply in the hydrostatic loop. 
It can be understood, therefore, that the low side system pressure 
normally is equivalent to the system charge pressure. 
Auxiliary circuit 27 includes an open-center auxiliary implement valve 56 
for energizing auxiliary implements (not shown) and communicating with 
charge pressure priority valve 36 by means of line 40. Fluid is returned 
from the implement valve to tank reservoir 30 through a pressure return 
line 58. An implement pressure relief valve 60, of a spring biased 
spool-type, communicates with pressure line 40 by means of a line 62. When 
fluid pressure in line 40, and hence line 62, exceeds a predefined value, 
pressure relief valve 60 is activated to divert over-pressure fluid to 
tank reservoir 30. 
The foregoing description describes the basic components of a hydrostatic 
transmission utilizing a common charge pump for supplying make-up fluid to 
a hydrostatic circuit and for performing auxiliary functions. The 
construction of charge pressure priority valve 36, whereby the flow 
displaced by charge pump 26 can be regulated to maintain a desired charge 
pressure within the hydrostatic loop independently of an auxiliary flow, 
now will be described. 
Referring to FIGS. 2 and 3, charge pressure priority valve 26 includes a 
valve body 64 having a cylindrical interior bore 66 which movably mounts a 
valve spool 68 between spaced apart end walls 70 and 72. The valve spool 
has a pair of cylindrical pistons 74 and 76 on axially-opposed spool ends 
78 and 80, respectively, of a spool shaft 82, which are disposed in sealed 
engagement with an annular sidewall 84 of the bore to partition the 
interior bore into three internal chambers An annular central chamber 86 
is formed in the space between the pistons and adjacent spool shaft 82. An 
end chamber 88 is formed between an outer face 90 of piston 74 and bore 
end wall 70, and an end chamber 92 is formed between an outer face 94 of 
piston 76 and bore end wall 72. It can be understood that the volumes of 
the annular central chamber 86 and the opposite end chamber 88 and 92 are 
variable and are established by the axial position of valve spool 68 
within the interior bore. 
A number of ports are formed along the length of bore sidewall 84. Main 
flow outlet passage 96 connects the interior bore with pressure line 38. 
Auxiliary flow outlet passage 98 is axially spaced from main flow outlet 
passage 96 and connects the interior bore with pressure line 40. A pump 
inlet passage 100 communicates with central chamber 86 and receives a flow 
from line 34. When valve spool 68 is positioned such that the pistons do 
not completely obstruct main flow outlet passage 96 or auxiliary flow 
outlet passage 98, as illustrated in FIG. 3, charge pump 26 (not shown in 
FIG. 3) is operable to direct a pressurized flow toward hydrostatic loop 
16 and auxiliary implement valve 56. When the valve spool is disposed such 
that piston 76 blocks auxiliary flow outlet passage 98, as shown in FIG. 
2, flow is directed only toward the hydraulic loop through pressure line 
38. 
A helical spring 102 is disposed in end chamber 92 and cooperates with 
piston face 94 to bias movement of the valve spool toward and away from 
end wall 72. A drain passage 104 interconnects the end chamber with tank 
reservoir 30 and purges excess fluid from the interior bore. Due to the 
sealed engagement of piston 76 with the interior bore sidewall 84, end 
chamber 92 continually is isolated from central chamber 86 and opposite 
end chamber 88. 
A main flow feedback passage 106 interconnects pressure line 38 and end 
chamber 88 such that a charge pressure equivalent to that supplied to the 
hydrostatic loop by means of line 38 is directed against face 90 of piston 
74. As will be described below, pressure acting against piston face 90 
normally urges valve spool 68 to the right as shown in FIGS. 2 and 3 and 
is opposed by the biasing force developed by helical spring 102 in the 
opposite end chamber 92. 
It is believed the operation of the charge pressure priority valve will be 
readily understood from the foregoing description and may be summarized as 
follows. Initially, charge pressure within lines 34 and 38 is low and the 
charge pressure priority valve assumes a position as illustrated in FIG. 
2, with the valve spool positioned such that flow is prevented from 
traveling toward auxiliary circuit 27 and there is no pressure drop across 
main flow outlet passage 96. As charge pump 26 continuously pumps fluid 
toward the hydrostatic loop, charge pressure begins to rise. Charge 
pressure also rises within end chamber 88, but because opposite end 
chamber 92 is isolated from the central chamber and end chamber 88, no 
fluid enters end chamber 92 and the charge pressure force acting against 
piston face 90 is reacted only by spring 102 As long as the force 
developed by charge pressure within pressure line 38, and therefore within 
end chamber 88, does not exceed the preload stiffness of spring 92, the 
valve spool remains in this configuration, with pressurized fluid being 
directed solely toward hydrostatic loop 16. 
When the charge pressure reaches a level at which the resultant force 
acting against piston face 90 overcomes the preload of spring 92, valve 
spool 68 shuttles to the right as shown in FIG. 3. Piston 72 shifts across 
auxiliary flow outlet passage 98 to permit an auxiliary flow to the 
auxiliary circuit, and piston 74 shifts across main flow outlet passage 96 
to create a pressure drop between central chamber 86 and pressure line 38. 
By appropriately selecting the stiffness of spring 102, the charge 
pressure, and thereby the low side system pressure, at which flow becomes 
available for auxiliary functions can be predetermined. 
When auxiliary pressure requirements are increased, as by work being 
performed by an auxiliary implement, the fluid pressure in passage 34 and 
central chamber 86 also increases. While it is desirable to provide a high 
pressure flow to the auxiliary implement circuit, it is important that the 
hydrostatic loop is not subjected to an equivalent and potentially 
damaging high pressure. To this end, the charge pressure priority valve 
maintains a constant relatively low pressure within the hydrostatic loop 
during increased pressurization of the auxiliary circuit. As auxiliary 
pressure rises, flow entering end chamber 88 through main flow feedback 
passage 106 continuously shuttles valve spool 68 further to the right. 
Displacement of piston 74 increasingly shunts flow to line 38 and thereby 
increases the pressure drop across outlet passage 96. The increased 
pressure drop provides that a constant pressure is maintained in line 38 
and, therefore, the low side of the hydrostatic loop. The hydrostatic loop 
pressure is thereby regulated independently of increased auxiliary 
pressure in the central chamber. 
Due to the above mentioned leakage and lubrication requirements of 
hydraulic pump 12 and motor 14, a volume of fluid is consumed during 
operation of the transmission, which in turn results in an increase in the 
charge flow requirements of the hydrostatic loop. The pressure priority 
valve is responsive to the changed loop charge flow requirements. When the 
hydrostatic loop is operating at a reduced pressure, the low side check 
valve opens to permit make-up fluid to enter the loop. As a result, charge 
pressure in line 38, and therefore end chamber 88, is reduced and the 
force acting against piston face 90 is overcome by the elastic restoring 
force of spring 102. Valve spool 68 shifts to the left as shown in FIGS. 2 
and 3 under the force of the spring to move piston 74 across main flow 
outlet passage 96 to reduce the pressure drop until sufficient charge 
pressure is developed within line 38 to satisfy the requirements of the 
hydrostatic loop. Once the hydrostatic loop is recharged, charge pressure 
again rises until the valve spool shifts to increase flow through passage 
98 for performing auxiliary functions in a manner as described above. 
It will be understood that the invention may be embodied in other specific 
forms without departing from the spirit or central characteristics 
thereof. The present examples and embodiments, therefore, are to be 
considered in all respects as illustrative and not restrictive, and the 
invention is not to be limited to the details given herein.