A multi-function valve usable in a hydrostatic transmission and having valve components mounted in a single valve body to provide pressure limiter and high pressure relief functions and with the structure also providing a bypass function and for supply of makeup fluid to the hydrostatic transmission. The multi-function valve has a charge check valve member and a relief valve member in associated relation and movable upon the occurrence of a predetermined pressure differential thereacross to perform control functions. One pressure differential results from a flow through an orifice and with the flow being controlled by a poppet valve member normally held closed by a heavy spring which establishes a system pressure at which the pressure limiter function will occur. With this structure, the pressure limiter function occurs in advance of the high pressure relief function and under the control of a single heavy spring. With two of the multi-function valves utilized with the hydrostatic transmission, various controls are operable with flow in either direction to the motor through pressure lines between the pump and motor defining a closed loop circuit.

TECHNICAL FIELD 
This invention pertains to a multi-function valve having particular utility 
in a hydrostatic transmission. A hydrostatic transmission has a pair of 
hydraulic displacement devices connected in a closed loop circuit, with 
one of the displacement devices normally operable as a pump and being of 
variable displacement. A pair of the multi-function valves are associated 
one with each of the pair of pressure lines interconnecting the 
displacement devices in the closed loop circuit to provide a number of 
previously known control functions. The multi-function valves provide a 
compact structure greatly reducing the number of drilled internal passages 
in a housing or external hoses as required for independent valves which 
achieve the various control functions. 
BACKGROUND ART 
Various controls for protecting a hydrostatic transmission from 
over-pressure are known. A hydrostatic transmission has a pair of 
hydraulic displacement devices connected in a closed loop circuit, with 
one of the displacement devices being of variable displacement and 
normally operating as a pump to supply fluid under pressure in a high 
pressure line side of the closed loop circuit to the other displacement 
device functioning as a motor. A low pressure line of the closed loop 
circuit directs fluid from the motor back to the pump. The controls for 
protecting the hydrostatic transmission from over-pressure have included 
pressure relief valves, pressure compensators and pressure limiters. 
The high pressure relief valve cross-ports fluid from the high pressure 
line to the low pressure line to relieve the over-pressure condition. The 
high pressure relief valve reacts quickly, preventing excessive overshoot 
in the pressure in the high pressure line. However, there is unnecessary 
power consumption and wasted heat by the pumped flow being recirculated 
without use in operation of the motor. 
The pressure compensator control, as typically used, limits a control 
pressure applied to servo means associated with the control of pump 
displacement when the system pressure in the high pressure line reaches a 
predetermined value. This limiting of control pressure for the servo means 
results in a reduction of pump stroke and the resulting limiting of system 
pressure. The pressure compensator results in less power consumption and 
heat generation than the high pressure relief valve control, but is 
inherently slow due to the required destroking of the pump. This results 
in significant system over-pressure under transient conditions where a 
load is applied quickly to the hydrostatic transmission. 
The pressure limiter control utilizes a pilot valve which senses system 
pressure in the high pressure line and, when that pressure exceeds a 
predetermined value, the pilot valve opens and ports fluid to the servo 
means for destroking the pump. The destroking of the pump results in a 
reduction of the volume of fluid being pumped and a limiting of the system 
pressure and, thereby, minimizes power consumption and heat generation. 
The pressure limiter does not have the fast response of the high pressure 
relief valve, but is an improvement over the older pressure compensator 
control, due to the fact that high pressure fluid in the high pressure 
line is used to destroke the pump and the destroking occurs more rapidly. 
It is known to use both high pressure relief valves and pressure limiters 
in the same hydrostatic transmission to overcome the problems inherent in 
use of only one or the other of such controls. However, such use of high 
pressure relief valves and pressure limiters has presented another 
difficulty due to the two controls being separate and the settings of the 
controls are subject to variations due to manufacturing practices and 
degradation from use. Normally, the setting of the pressure limiter is at 
a value lower than the setting of the high pressure relief valve, so that 
the pressure limiter will first function upon system pressure reaching a 
predetermined value and, if the pressure goes to a certain value higher, 
the high pressure relief valve will then operate. With the use of separate 
controls, the settings of the two control devices must be separated 
sufficiently to assure that the high pressure relief valve will not 
operate before the pressure limiter. The invention disclosed herein 
combines the high pressure relief valve and the pressure limiter valve 
into one valve assembly, resulting in a high pressure relief valve whose 
setting cannot go below that of the pressure limiter valve. 
A hydrostatic transmission has a charge pump for supplying make-up fluid to 
the closed loop circuit through charge check valves which open when charge 
pressure exceeds the pressure in the low pressure line of the hydrostatic 
transmission. It is also known to utilize a bypass valve with the 
hydrostatic transmission which cross-connects the two pressure lines of 
the closed circuit when a device, such as a vehicle utilizing the 
hydrostatic transmission in the drive for the vehicle, is being towed and 
the displacement device normally operating as a motor is functioning as a 
pump. These additional functions have normally been accomplished by valves 
independently positioned in the circuitry associated with the hydrostatic 
transmission. A multi-function valve embodying the invention disclosed 
herein incorporates these valve mechanisms into the same valve body which 
has the high pressure relief valve and the pressure limiter, resulting in 
a valve that replaces several independent valves and which are 
independently mounted to result in a reduction of passageways in an end 
cap of the variable displacement device and an elimination of external 
manifolds, hoses and pressure compensator control housings. 
Valve structures are available which incorporate components for performing 
plural control functions in association with a hydrostatic transmission. 
One of these valve structures provides high pressure relief as well as a 
charge check valve and adjustable mechanism provides the bypass function. 
This valve structure does not combine the functions of high pressure 
relief and pressure limiter control. Another available valve assembly has 
the charge check valve and the high pressure relief valve incorporated 
into the same structure, but does not provide structure functioning as a 
pressure limiter, nor does the structure provide the bypass function. 
DISCLOSURE OF THE INVENTION 
A feature of the invention is to provide a multi-function valve which 
incorporates a number of valve functions into a single valve body to 
provide improved control functions and which is more reliable in operation 
and which reduces the over-all manufacturing costs by the reduction in 
passageways, hoses, and housings required when the control functions are 
performed by independent valves. 
Another feature of the invention is to provide a multi-function valve 
having valve components providing for a pressure limiter function and a 
high pressure relief function, wherein the two controls are associated in 
a series relation which assures that the pressure limiter control will 
function to destroke the pump of the hydrostatic transmission prior to 
operation of the high pressure relief valve structure which cross-connects 
the high pressure and low pressure lines of the closed loop circuit of the 
hydrostatic transmission. 
Another feature of the invention is to associate with the multi-function 
valve structure as defined in the preceding paragraph a charge pressure 
check valve structure and enable a bypass function to cross-connect the 
two pressure lines of the closed loop circuit when the motor of the 
hydrostatic transmission is functioning as a pump. 
Still another feature of the invention is to provide new and improved 
structure for accomplishing the bypass function including adjustable 
mechanism and which can be adjusted without creating a leakage problem. 
In carrying out the foregoing features of the invention, the hydrostatic 
transmission has a pair of hydraulic displacement devices connected in a 
closed loop circuit with two parallel pressure lines and with one of the 
displacement devices being of variable displacement and normally 
functioning as a pump and the other displacement device functioning as a 
motor and a pair of the valve structures as referred to in the foregoing 
features are associated with the hydrostatic transmission. The pair of 
valve structures are interconnected by a charge pressure passage whereby 
charge pressure can be directed to the low pressure line of the closed 
loop circuit of the hydrostatic transmission and this same passage is used 
in the high pressure relief function to cross-connect the high and low 
pressure lines. 
The multi-function valve incorporates a pressure limiter valve member 
normally closed to block flow from one side of the closed loop circuit to 
servo means for controlling the displacement of the variable displacement 
device. A relatively heavy spring biases the pressure limiter valve member 
closed against system pressure and when system pressure exceeds a 
predetermined value the pressure limiter valve member opens to direct 
system pressure to the servo control means for destroking the variable 
displacement device. A high pressure relief valve member opens in response 
to a predetermined pressure differential resulting from flow to the servo 
means of the variable displacement device for cross-connecting the two 
pressure lines of the hydrostatic transmission. Thus, the two valve 
members are in series and the pressure limiter function must occur prior 
to the high pressure relief function. 
The bypass function is achieved by removing the bias on the pressure 
limiter valve member referred to in the preceding paragraph whereby the 
high pressure relief valve member is free to move when a pressure 
differential resulting from flow through the pressure limiter valve member 
creates a pressure differential adequate to overcome a relatively light 
spring associated with the high pressure relief valve member. Upon 
movement, pressure in one pressure line of the hydrostatic transmission 
can be applied through the charge pressure passage referred to above to 
the charge check valve of the other valve structure to open the latter and 
crossconnect the pressure lines of the hydrostatic transmission. An 
important feature of this bypass function resides in the use of the flow 
through the open pressure limiter valve to the servo means associated with 
the variable displacement device to enable the bypass function. Thus upon 
initiation of the bypass function, the variable displacement device, if in 
stroke, is destroked. The bypass function is commonly used when a vehicle 
having the hydrostatic transmission used in drive of the vehicle is towed 
and, if the variable displacement device has not been set in neutral prior 
to towing but is still in a stroke position, the towing of the vehicle in 
the bypass function will cause the variable displacement device to 
destroke and the pump and prime mover will not rotate. 
An object of the invention is to provide a hydrostatic transmission having 
a pair of hydraulic displacement units connected in a closed loop circuit 
and with one of the units having variable displacement comprising, 
pressure limiter means including a pressure limiter valve member and 
responsive to a predetermined pressure in the closed loop circuit for 
reducing the displacement of the variable displacement unit, means 
including a relief valve member providing high pressure relief for said 
closed loop circuit at a pressure in said closed loop circuit which 
exceeds said predetermined pressure, and said valve members being 
structurally related to require operation of said pressure limiter valve 
member before said relief valve member can operate. 
Another object of the invention is to provide a multi-function valve usable 
in a hydrostatic transmission having a pair of hydraulic displacement 
units connected in a closed loop circuit with one of the units being of 
variable displacement comprising, a valve body with a bore and a pair of 
valve seats, a charge check valve having a poppet valve member associated 
with one valve seat to block flow between said bore and a first port, a 
relief valve having a hollow relief valve spool movably positioned within 
said poppet valve member and having an end wall abutting a transverse wall 
of said poppet valve member, a first spring acting on said relief valve 
spool to urge said valve members into abutting relation and the poppet 
valve member against its valve seat, said poppet valve member having a 
passage in the transverse wall thereof communicating with said first port, 
said relief valve spool end wall having an orifice opening to said first 
port through said transverse wall passage, a charge pressure port in said 
valve body opening to said bore, passage means in the wall of said poppet 
valve member connecting the interior of said poppet valve member with the 
bore exteriorly of the poppet valve member, said relief valve spool 
blocking said passage means when the valve members are in abutting 
relation, a pressure limiter port, passage means connecting the interior 
of the hollow relief valve spool with the pressure limiter port, a second 
poppet valve member exposed to pressure in said interior coacting with a 
poppet seat to close said last-mentioned passage means, and a second 
spring urging the last-mentioned poppet valve member onto its poppet seat.

BEST MODE FOR CARRYING OUT THE INVENTION 
A typical hydrostatic transmission is shown in FIG. 1 wherein a pair of 
hydraulic displacement devices, indicated generally at 10 and 11, are 
connected in a closed loop circuit by a pair of pressure lines 15 and 16 
which can be hoses or passages in a housing for the displacement devices. 
The hydraulic displacement device 10 is an axial piston unit and, in 
normal operation, functions as a pump to deliver fluid under pressure to 
the displacement device 11 which normally functions as a motor. In one 
mode of operation, the pressure line 16 is a high pressure line and the 
pressure line 15 is a low pressure line and, in another mode of operation, 
the pressure conditions in the two pressure lines are reversed. 
The hydraulic displacement device 10 is shown as an axial piston unit 
having an input shaft 20 connected to a rotatable cylinder block 21. The 
cylinder block has axially-extending bores, each of which mounts a movable 
piston 22 and with the stroke of the pistons being under the control of a 
reversible swashplate 25. This structure is housed in a housing 26 having 
an end cap 27 having ports 28 and 29 which connect with pressure lines 15 
and 16, respectively, and a valve plate associated with the 
axially-extending bores in the cylinder block 21. The swashplate 25 is 
adjustable to provide variable displacement for the hydraulic displacement 
device 10 which, hereinafter, will be referred to as a pump. The position 
of the swashplate is controlled by servo means including a displacement 
control valve, indicated generally at 30, and a pair of servo cylinders 31 
and 32 which are connected to the displacement control valve 30 through 
the lines 35 and 36, respectively. The construction of the pump 10 is well 
known in the art and with the servo cylinders 31 and 32 each housing a 
movable piston 34 and 35, respectively, which is urged outwardly of the 
associated servo cylinder by a spring associated therewith. 
The swashplate 25 is positioned in one maximum displacement position, as 
shown in FIG. 1, as the result of charge pressure acting through the 
displacement control valve in the servo cylinder 32 against the piston 35 
which is connected through a link 40 associated with the swashplate. The 
swashplate 25 has a destroked neutral position wherein the pistons 22 are 
not stroked by the swashplate and also has a maximum displacement position 
at the opposite side of neutral from the position of the swashplate shown 
in FIG. 1. The opposite maximum displacement position results from 
pressure acting on the piston 34 in the servo cylinder 31. There can be 
intermediate displacement positions either side of the neutral destroked 
position and which is less than the full stroke position, as shown in FIG. 
1. 
The hydraulic displacement device 11 functions generally as a motor and 
will hereinafter be referred to as a motor. The motor has an output shaft 
42 splined to a cylinder block 43 rotatable in a housing 44 and with the 
cylinder block having a series of axially-extending bores, each of which 
receive a movable piston 45 which is associated with a fixed swashplate 
46. An end cap 47 has ports 48 and 49 associated with the pressure lines 
15 and 16, respectively, whereby fluid flowing from the pump through one 
of the pressure lines can be directed through a valve plate to the piston 
chambers for causing rotation of the cylinder block and the output shaft 
42. With the swashplate 25 of the pump 10 positioned as shown in FIG. 1, 
the output shaft 42 will rotate in one direction and when the swashplate 
is positioned the other side of neutral, the output shaft 42 will rotate 
in the opposite direction and with the pressure lines 15 and 16 
alternately being high pressure and low pressure lines in these alternate 
modes of operation. 
A charge pump 50, as well known in the art, supplies make-up fluid to the 
closed loop circuit and also supplies charge pressure for control 
functions. The charge pump has an inlet connected through a line 51 to a 
reservoir 52, with a filter 53 in the line. The charge pump has an outlet 
connected by lines 54 and 55 to the displacement control valve 30, with 
the line 55 having an orifice 56. A charge pressure relief valve 58 
connects to the outlet of the charge pump 50 through a line 59 and 
functions to limit charge pressure. 
The displacement control valve 30 has a housing in which a valve member 60 
can be positioned to control communication of full or partial charge 
pressure with one or the other of the pump servo cylinders 31 and 32 and 
with the other servo cylinder being connected to the reservoir. The 
connections to reservoir are through lines 61 and 62, each of which have a 
flow-restricting orifice therein. 
The valve member 60 can be positioned through a linkage operated either 
manually by a handle 65 or by other suitable means and with the linkage 
including a feedback connection from the swashplate 25 through an arm 66 
which is partially shown in FIG. 1 and with a broken line extension 
thereof to the swashplate. 
A line 68 interconnects the interiors of the pump 10 and the motor 11 
whereby the interiors are connected to the reservoir 52 through a line 69 
extending from the motor to the reservoir and having a heat exchanger 70 
therein. 
The foregoing description describes the well-known basic components of a 
hydrostatic transmission utilizing a variable displacement pump and 
wherein the angle of the swashplate 25 can be varied to either side of a 
neutral destroked position. The use of charge check valves whereby the 
charge pump 50 can supply make-up fluid to the pressure line 15 or 16 
which is at low pressure and which is conventional in a hydrostatic 
transmission has not yet been described because of the charge check valves 
being incorporated in the multi-function valve to be described. 
Various types of controls can be associated with a hydrostatic 
transmission, including pressure controls as previously discussed and the 
multi-function valve to be described provides a pressure limiter function 
and a high pressure relief function as well as having valve structure 
incorporated therein providing the charge check valve function. Also, the 
multi-function valve is constructed to enable bypass operation wherein the 
pressure lines 15 and 16 are cross-connected when the motor 11 is 
operating as a pump and the destroking of the pump occurs as part of the 
bypass operation. 
There are a pair of the multi-function valves. A first multi-function 
valve, indicated generally at 75, is associated with the pressure line 15 
and a second multi-function valve 76 is associated with the pressure line 
16. Each of these multi-function valves 75 and 76 is of the same 
construction and the multi-function valve 75 is shown particularly in FIG. 
2. A valve body 80, which may be part of the end cap 27, has a first port 
81 which is an inlet port connected by a passage 82 optionally in the end 
cap) to the pressure line 15. A charge pressure port 83 connects to a 
charge pressure passage 84 which connects with the charge pump 50 through 
a line 85 and which also connects to a similar port 86 in the 
multi-function valve 76. A servo port 90 is connected by a line 91 to the 
servo line 35 extending between the displacement control valve 30 and the 
servo cylinder 31. A branch line 92 connects the line 91 to a pressure 
limiter check valve structure 93 having a spring-loaded check valve acting 
against pressure in branch line 92 and openable to permit flow to a line 
94 connected to the charge pressure line 95 whereby there is an upper 
limit on the pressure in line 91. 
The multi-function valve 76, which may also be in end cap 27, has a first 
port 100 corresponding to the inlet port 81 which connects to the pressure 
line 16 and a servo port 101 which connects to the servo line 36 
associated with the servo cylinder 32. 
With the swashplate 25 in a fully-stroked position, as shown in FIG. 1, the 
pressure line 15 is the high pressure line and the pressure line 16 is the 
low pressure line and the multi-function valve 75 is the valve operative 
to provide pressure control functions in the hydrostatic transmission. 
When the pressure condition in the pressure lines 15 and 16 is reversed, 
the multi-function valve 76 is the valve operative to control pressure 
conditions. 
In the subsequent description, the multi-function valve will be described 
in detail and it will be understood that the structure of the 
multi-function valve 76 is the same and the functions thereof are the same 
when pressure line 16 is the high pressure line. 
The multi-function valve 75 has a charge check valve member 110 in the form 
of a tubular poppet movable within a sleeve 111 fitted in the valve body 
and which has a valve seat 112 formed thereon. An end wall of the tubular 
sleeve has an opening 115 aligned with the inlet port 81. The charge check 
valve member 110 is urged to a closed position on the valve seat 112 by a 
spring to be described, whereby communication between ports 81 and 83 is 
blocked. The charge check valve member 110 has a major diameter section 
which slidably mounts the valve member in the sleeve 111 and a reduced 
diameter section 117, both of which are subject to pressure in the charge 
pressure passage 84 by communication through openings 118 in the sleeve 
111. The charge check valve member 110 can move to the right away from the 
valve seat 112 and place the charge pressure passage 84 in communication 
with the inlet port 81 whereby fluid flows from the charge pressure 
passage 84 to the passage 82. 
The charge check valve member 110 is hollow to define a chamber which 
receives a relief valve member 120, in the form of a spool having an end 
wall 121 which abuts the interior of a transverse wall of the charge check 
valve member and with the valve members being urged into abutting relation 
by a light spring 122 positioned between the end wall 121 of the relief 
valve spool 120 and a valve seat member 125. The end wall 121 of the 
relief valve spool 120 has an orifice 127 aligned with an opening 128 in 
the transverse wall of the charge check valve member and with the opening 
115 in the sleeve 111 whereby the occurrence of flow through the orifice 
127 creates a flow-induced pressure differential across the end wall of 
the relief valve spool and a force acting in opposition to the spring 122. 
The interior of the relief valve spool 120 is always filled with oil, with 
there being a flow condition when the interior is open to communication 
with the port 90. This communication is controlled by a pilot valve having 
a poppet valve member 130 coacting with the valve seat member 125. The 
valve seat member 125 has a cylindrical extension 131 having a bore which 
receives a stem portion of the pilot poppet valve member 130 and which has 
a piston 132 loosely fitted therein and which merely functions as a 
loose-fitting damper piston which permits equal pressure on both sides of 
the piston in a static condition. When the pilot poppet valve member 130 
is in a dynamic condition, the motion produces a pressure differential 
across the piston 132 which tends to stabilize the movement. The interior 
of the relief valve member 120 communicates with the interior of the 
cylindrical extension 131 of the valve seat member through ports 135 of 
the latter, whereby pressure existing within the interior of the relief 
valve member acts on the poppet valve member 130. The poppet valve member 
130 is biased to a closed position against its valve seat by a heavy 
spring 140 and, in comparison, the spring 122, previously referred to, is 
a light spring. The spring 140 acts between a spring seat 141 engaging an 
end of the pilot poppet stem and an end cap 142 which is threadedly 
engaged with an outer end of the valve seat member 125 and sealed relative 
thereto by a seal member 143. When the poppet valve member 130 opens, 
fluid flows to lateral passages 145 in the valve seat member 125 which 
communicate with the port 90 and the passage 91. 
Additional structural features of the multi-function valve include lateral 
passages 150 formed in the charge check valve member which selectively 
connect the interior of the charge check valve member with the port 83 and 
the charge pressure passage 84. The valve seat member 125 is threaded into 
an open end of the valve body 80 at 155 and a seal member 156 provides a 
seal therebetween. 
The bias force of the strong spring 140 acting on the poppet valve member 
130 determines the system pressure at which a pressure limiter function 
will commence to limit the pressure in the high pressure line 15 by 
destroking the pump. This pressure is sensed at the pilot poppet valve 
member 130 by communication through the passage 82 and port 81, the 
openings in the sleeve 111 and the charge check valve member 117 and the 
orifice 127 in the relief valve member 120. This pressure exists within 
the interior of the relief valve member and acts through the openings 135 
to urge the poppet valve member 130 open. When the pressure overcomes the 
force of the spring 140, the poppet valve member 130 opens, whereby fluid 
can flow to the passage 91 and to the servo line 35 whereby pressure is 
applied at the servo cylinder 31 to move the swashplate 25 toward a 
destroked position. 
As the pump is destroked by movement of the swashplate 25, fluid is 
displaced from the servo cylinder 32 which flows through the servo line 36 
to the displacement control valve 30. Since the flow path through the 
displacement control valve 30 is restricted by the charge pressure passage 
orifice 56, there is also flow to the pressure limiter check valve 93 
through a line 160 whereby fluid can be expelled rapidly from the servo 
cylinder 32 to allow rapid destroking of the pump. The servo line 35, 
which is receiving pressure flow from the multi-function valve 75, is 
connected to the reservoir through an orifice associated with line 61 and 
the flow through the orifice results in an increase of pressure in the 
servo cylinder 31. 
The high pressure relief valve function is coordinated with the pressure 
limiter valve function by using the poppet valve member 130 as a pilot 
valve for the relief valve member 120. With there being flow through the 
orifice 127 during the pressure limiter function, there is a flow through 
the orifice 127. This flow creates a pressure differential acting to move 
the relief valve member to the right against the relatively light spring 
122. The movement of the relief valve member 120 eventually opens the high 
pressure line 15 to the charge pressure passage 84 through transverse 
openings 150 in the charge check valve member which has not moved. The 
sizing of the orifice 127 with respect to the size of the relief valve 
member 120, the force of the spring 122, and flow requirements for the 
pressure limiter function to the port 90 determine the coordination 
between the pressure limiter function and the relief valve function. In an 
embodiment of the control, the pressure limiter function requires a flow 
of less than 1 gpm and the orifice 127 is sized to open the relief valve 
member 120 to the charge pressure passage 84 at a flow of about 1.5 gpm or 
200-300 psi above the pressure limiter setting as set by the spring 140. 
Any degradation in the setting of the pressure limiter due to wear or 
contamination will also automatically reduce the high pressure relief 
valve setting by the same amount since the valve members for the two 
functions are arranged in series. 
The completion of the high pressure relief function is achieved by flow 
from the high pressure line 15 to the low pressure line 16 by use of the 
multi-function valve 76. When high pressure is delivered to the charge 
pressure passage 84, it communicates with the corresponding port 83 in the 
multi-function valve 76 whereby a pressure differential acting on the 
charge check valve member 117, as described hereinafter in connection with 
its conventional function, causes the latter valve member to open and 
fluid can flow to the low pressure line 16. The previously mentioned 
charge pressure relief valve 58 protects the charge circuit from excessive 
charge pressure in this operation. 
Referring to FIG. 1, if the swashplate 25 is in an opposite maximum stroke 
position from that shown, pressure line 16 is the high pressure line and 
the multi-function valve 76 is subject to pressure in the pressure line 16 
and controls the pressure limiter and high pressure relief functions. When 
the pressure in pressure line 16 exceeds that established by the force of 
the heavy spring 140, the poppet valve member 130 thereof opens to permit 
flow to servo line 36 with pressure acting in the servo cylinder 32 to 
urge the swashplate 25 to a neutral position. When the flow through the 
orifice 127 of the relief valve member is adequate to create a certain 
pressure differential, the latter valve member opens to the charge 
pressure passage 84 and the charge check valve member 110 of the 
multi-function valve 75 opens to permit flow from pressure line 16 to the 
low pressure line 15. 
The charge check valve member 117 performs the conventional function of 
supplying make-up fluid to the low pressure side of the closed loop 
circuit. The spring 122 is designed to keep the charge check valve member 
117 closed until charge pressure is approximately, in one embodiment, 
15-20 psi higher than pressure in the low pressure line. In further 
describing this operation in connection with the multi-function valve 75, 
it will be noted that the charge check function occurs when the pressure 
line 15 is the low pressure line. The charge check function is 
accomplished by having the pressure in the interior of the relief valve 
spool 120 at the same pressure as the low pressure in the pressure line 15 
by communication through the orifice 127. When a predetermined pressure 
differential between low side loop pressure and charge pressure occurs, 
with the latter being higher, the pressure acting on the sections 110 and 
117 of the charge check valve member causes the valve member to open by 
movement to the right, as shown in FIG. 2, against the action of spring 
122. When the pressure in the pressure line 15 exceeds charge pressure, 
this pressure, acting within the interior of the relief valve spool 120, 
functions to maintain the charge check valve closed. 
The bypass function cross-connects the two pressure lines of the closed 
loop circuit. This function is utilized when the hydrostatic transmission 
is used in the drive of a vehicle and the vehicle is being towed. When 
this occurs, the motor 11 functions as a pump and the pump 10 functions as 
a motor. 
In the multi-function valve of FIG. 2, the bypass function is achieved by 
unscrewing the end cap 142 a relatively few turns to relieve the bias of 
the heavy spring 140, placing the poppet valve member 130 in a neutral 
position. This places the interior of the relief valve member 120 in 
communication with the port 90 and passage 91 leading to the servo 
cylinder 31 to permit flow through the orifice 127 and with the previously 
mentioned flow therethrough creating a pressure differential sufficient to 
open the relief valve member 120 against the light spring 122. As a 
result, fluid can flow through the openings 150 to the charge pressure 
passage 84 and act on the charge check valve member of the other 
multi-function valve to permit flow to the other pressure line of the 
closed loop circuit. This general function is known in the art. However, 
an improvement provided by the multi-function valve resides in the fact 
that the relief valve member 120 is permitted to open only by flow through 
the orifice 127 which flows to the passage 91 and, therefore, fluid flows 
to a servo cylinder to destroke the pump 10, if the pump had not been 
previously placed in neutral. This is a desirable result since, during 
towing, the pump and the vehicle engine which normally drives the pump are 
not rotated. 
A modification of the multi-function valve is shown in FIG. 3 wherein the 
construction is the same except for the structure operable to achieve the 
bypass function. In the multi-function valve of FIG. 3, the parts which 
are the same as those shown in FIG. 2 have been given the same reference 
numeral. 
In the embodiment of FIG. 3, the bias of the heavy spring 140 is released 
by structure including a screw 170 threaded into an end cap 171 threaded 
into the valve seat member 125. A spring seat 175 loosely receives a 
reduced diameter section 176 of the screw 170 and the end thereof has an 
enlarged flange 177 which, in normal operation, is spaced from the walls 
of an interior opening in the spring seat 175 to define a lost-motion 
connection. This permits the spring seat 175 to move freely, without 
restraint by the screw 170 during normal operation. A retaining ring 180 
prevents the screw 170 from being threaded into the end cap 171 so far as 
to prevent free movement of the spring seat 175. When the bypass function 
is desired, the screw 170 is rotated several turns to cause the flange 177 
to move to the right and engage a wall of the interior chamber of the 
spring seat and move the spring seat to the right to compress the heavy 
spring 140 and, at the same time, disengage the contact between the spring 
seat 175 and the poppet valve member stem. When the bypass function 
occurs, the pressure lines in the closed loop circuit are cross-connected 
and the pump is destroked if necessary. 
The embodiment shown in FIG. 3 avoids problems that might occur in 
connection with the structure shown in FIG. 2. There is fluid in the 
chamber within the valve seat member 125 housing the heavy spring 140 and 
this chamber is sealed by the seal member 143. In FIG. 2, when the cap 142 
is backed out, the seal is broken and its capacity to seal is reduced, 
depending on the extent of backing-out of the cap and, therefore, a slow 
leak can occur. An additional problem results if the cap 142 is screwed 
out too far, which could result in disassembly of the structure associated 
with the poppet valve member 130. The leakage would increase and parts 
could be lost or improperly reassembled by persons unfamiliar with the 
valve assembly. These problems are avoided by the structure shown in the 
embodiment of FIG. 3 wherein the cavity housing the heavy spring 140 is 
always sealed, since the screw 170 is always sealed by a seal member 185. 
With the multi-function valve disclosed herein, a number of control 
functions for a hydrostatic transmission can be accomplished by use of two 
similar valve packages with resulting reduction in flow connections as 
provided by hoses or housing passages. Although both pressure limiter and 
high pressure relief functions are known in connection with the control of 
hydrostatic transmissions, the multi-function valve functions to place 
these two operations in a series relation whereby both functions are under 
the control of a single spring and the pressure limiting function must 
always occur in advance of the high pressure relief function. The bypass 
function is also previously known. However, the multi-function valve 
disclosed herein performs this function in a new manner to assure also 
that the pump is destroked, if not in neutral, during a bypass operation.