Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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1 BACKGROUND OF THE INVENTION
This application is a division of our copending Cana-
dian patent application Serial No.220,025 filed February 13,1975.
This invention pertains to hydrostatic transmission
controls for obtaining precise displacement of the variable dis-
placement pump and motor of the transmission and, additionally, for
providing accurate, uniform opera~ion as well as steering control of
a dual path hydrostatic transmission. The controls are usable in
many applications, including track-type vehicles for obtaining
both propulsion and steering thereof.
The assignee of this application has a number of prior --
art patents relating to hydrostatic transmissions. In considering
the appropriate prior art, it should be noted that the disclosure
herein relates to a hydrostatic transmission control, and in one
embodiment to a dual path hydrostatic transmission control wherein
a master circuit including a pressure-reducing valve for setting
the control pressure and a directional control valve for establish-
ing the direction of operation of two hydrostatic transmissions as
well as a horsepower control valve for preventing stalling of the
prime mover connect into two branch circuits with a branch circuit
for each of the hydrostatic transmissions and each branch circuit
having a steering control valve. The steering control valves pro~
vide for modification of the control pressure delivered to the dis-
placement control means of a particular hydrostatic transmission
and with a staging action in each transmission whereby the pump of
the transmission reaches substantially full displacement prior to
reduction in the initial maximum displacement of the motor.
Moon Patent No. 3,187l509 discloses a displacement
control valve in Fig. 2 which functions as a directional valve
and which is in circuit with a manually-controllable torque
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1 valve 130. The latter valve functions as a manually-adjustable
pressure-reducing valve. This patent does not disclose a dual
path control with a master circuit having speed and direction
establishing components operable by a single handle and a pair
of branch circuits each having a manually-operable steering
control valve nor a number of specific features in the displace-
ment control for a transmission providing for precise displace-
ment control.
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The Hann et al Patent No. 3,230,699 discloses a hydrostatic trans-
mission wherein the reduction in motor displacement is staged to follow the
increase in displacement of the pump by having a spring associated with the -~
control valve for the motor stronger than a similar spring in the control
valve for the pump. This patent also shows a rotatable shaft with cams for
modifying the action of valves.
The Hann Patent ~o. 3,247,669 discloses the use of springs of dif-
ferent strength in the displacement controls for the motor and pump of the
hydrostatic transmission in order to stage the operation of the motor after ;
the operation of the pump.
The Hann Patent No. 3,411,297 discloses the use of a staging valve
21 for causing change in the displacement of the motor after the pump is
adjusted to maximum displacement. This control does not include a shuttle
valve for shift in response to a higher control pressure in one of two lines
for assuring application of the higher control pressure at all times to one
end of the control valve for the motor displacement control. Additionally,
this patent shows a feedback linkage associated with the pump.
The Moon Patent No. 3,247,919 and Ross Patent No. 3,349,860 relate
to dual hydrostatic transmissions. In the Moon patent, one handle provides
for speed control of both transmissions and a second handle provides for
steering. The handles operate control cams for mechanically-modifying the
operation of the transmissions. The control cams which set the speed and
direction of operation have their actions modified when steering control is
desired. In the Ross patent, steering is obtained by use of a separate hy-
drostatic transmission which operates to provide an input into a differential.
Additionally, the assignee of this application has a previous dual-
path hydrostatic transmission control wherein each hydrostatic transmission
is manually controlled by a separate control circuit and with each control
having a pressure-reducing valve to set a control pressure and a directional
control valve for determining the direction of operation of the associated
hydrostatic transmission. A horsepower or anti-stall control valve is con-
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nected into the control circuits for both of the hydrostatictransmissions, whereby overloading of the engine will result
in a uniform reduction in control pressure in the controls for
both of the hydrostatic transmissions. In this prior art
control, there are two control handles, with each handle in-
dependently setting a speed for a transmission, whereby there is
no positive direct relation between the speeds of the two
transmissions.
An additional prior art patent is Lauck, Re. No. 27,48
wherein two distinct controls, such as described in the
preceding paragraph, are operated by a single master control
handle. In this patent, the control for each hydrostatic
transmission has its own speed control and direction-establish-
ing means as well as a horsepower control valve.
SUMMARY OF THE INVENTION
-
The present invention provides a control for a
hydrostatic transmission having a variable displacement pump
and a variable displacement motor, a pair of displacement co`ntrols
associated one with each of the pump and motor and each control
including a valve connected to a source of operating fluid under
pressure and a pair of operating cylinders subjected to operating
fluid dependent upon the position of the valve, means for
applying a control pressure to the valves, means for applying
a control pressure to the valves of the pump and motor dis-
placement controls, feedback means for each of said valves and
each of said means including, an arm connected to the valve,
a pair of springs acting oppositely on said arm, a linkage for
indicating displacement and for varying the force applied to one
of said springs, and means for adjusting the force on one of
said springs to provide a null position for the valve when the
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1 swashplate of the associated pump or motor is at a predetermined
position.
Another feature of the invention resides in the dual
path control for a pair of hydrostatic transmissions wherein a
master control circuit operable from a single control handle sets a
control pressure to establish a speed common to both hydro-
static transmissions and also a direction of operation thereof
and with two branch circuits associated one with each of the
hydrostatic transmissions and each having a manually-operable
steering control valve for varying the speed of one transmission
with respect to the other for steering of a vehicle, such as
a double-track vehicle, with there being a track driven by each
of the hydrostatic transmissions.
Another feature of the invention provides for operation - -
of a steering control valve in a branch circuit to modify a
control pressure for the associated hydrostatic transmissions
for speed variation to cause a turn, with the additional
feature of valve operation reversing the application of control
pressure to a hydrostatic transmission whereby the vehicle may
be given a spin turn about a center disposed within the space
between the tracks.
- The control of the speed of the hydrostatic transmissions
is controlled by a pressure-reducing valve in the master
control circuit, with this valve including means settable to deter-
mine the desired control pressure for setting speed. A
manually-operable handle has a cam associated therewith
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coacting with the settable means and with the cam surface having two simi-
lar sections extending oppositely from a neutral center position whereby ro-
tation of the handle in either direction to establish a direction of opera-
tion of the hydrostatic transmissions will effectively set a control pres-
sure to establish the speed of the transmissions dependent upon the amount
of handle rotation.
Additional features of the invention include the use of a known
horsepower control valve to also function as a relief valve with respect to
pressure generated by a control pump operated by the prime mover engine, as
well as improvements in the displacement control means for the swashplates
of the variable displacement pump and variable displacement motor of the
transmission, including a pair of springs acting in opposition against a
control arm connected to a control valve of the displacement control means
with adjustable structure for the spring seat of one of the springs in order
to obtain a null position for the control valve when the associated swash-
plate is in neutral, and the use of a shuttle valve between the displace-
ment controls for the pump and motor which are in series. The shuttle valve
operates to direct the highest control pressure existing in either of two
lines ahead of the shuttle valve to one end of a control valve for motor
displacement whereby the motor is always adjusted in the same direction.
A further feature of the invention resides in the utilization of
feedback means for the control valve in the displacement control for the
motor which provides for a certain non-linear relation between transmission
speed and motor displacement in order to obtain a linear relation between
transmission speed in response to control pressure. This assures a uniform ~`~
control of speed as a control pressure is varied by a steering control valve.
BRIEF DESCRIPTION OF THE DRAI~NG
Fig. 1 is a diagrammatic illustration of a dual-path hydrostatic
transmission with two complete hydrostatic transmissions;
Fig. 2A is a diagrammatic view of a part of the control system for
the dual-path hydrostatic transmission;
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Fig. 2B is a disgrammatic view of the displacement controls for
one of the hydrostatic transmissions and which connects into the left-hand
side of Fig. 2A;
Fig. 3 is a graph illustrating the speed setting and direction of
operation of the hydrostatic transmissions in response to positioning of
the manual control handle;
Fig. 4 is a graph showing the staging action of the variable dis-
placement pump and motor of a hydrostatic transmission;
Fig. 5 is a graph showing the desired linear relationship between
control pressure and output speed for a hydrostatic transmission, and
Fig. 6 is a graph showing a non-linear relation between displace-
ment of the motor of the transmission with respect to output speed and con-
trol pressure thereof in order to obtain a linear speed-control pressure
relation.
DESCRIPTION OF THE PREFERRED EMBODIME~T
A dual path hydrostatic transmission is shown in Fig. 1. These
transmissions, in one embodiment, may be used to each Individually operate
a track of a dual track vehicle. Referring to the hydrostatic transmission,
illustrated in the upper half of Fig. 1, a variable displacement pump 10 is
connected by a pair of lines 11 and 12 to a variable displacement motor 15.
The variable displacement pump 10 has an input shaft (not shown) driven
from a power source, such as the engine of the vehicle, with the motor 15
having an output shaft (not shown) connectable to a drive train, such as for
a track of the vehicle.
Preferably, both the pump 10 and motor 15 are of the axial piston
type having rotatable cylinder blocks, each with a plurality of cylinders in
annualar array and with pistons 30 and 31, respectively, having ends recip-
rocable in the cylinders. The pump 10 has a variable angle reversible
swashplate 32 engaged by the projecting ends of the pistons 30 for initially
controlling the speed of the transmission as well as the direction of ro-
tation thereof. The motor 15 has a variable angle swashplate 33 movable
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from a maximum displacement position shown to a minimum displacement po-
sition somewhat greater than zero. The swashplate 33 engages pistons 31
causing reciprocation thereof and rotation of the cylinder block and the
output shaft of the motor connected thereto as high pressure fluid is supp-
lied to the motor from the pump.
A pair of pump control cylinders 36 and 38 are provided for posi-
tioning the pump swashplate 32. Each has a spring construction, indicated
diagrammatically at 39 and 40, respectively, acting against pistons 41 and
42. The pistons 41 and 42 pivot the swashplate 32 through links 45 and 46
connected to the pistons. The control cylinders 36 and 38 position the
swashplate in response to the supply of operating fluid to the cylinders by
means of conduits 48 and 50, respectively. The centering springs 39 and 40
normally act through the pistons to position the pump swashplate as shown
in its neutral position when no operating fluid is delivered through the
conduits. In this position, there is no positive output from the pump.
The control cylinders are of a conventional, single-action type so that the
entry of fluid under pressure into one cylinder will cause the swashplate
to tilt or pivot in one direction, and the influx of fluid under pressure
in the opposite cylinder will cause reverse pivoting of the swashplate,
thus permitting the flow from the pump 10 to reverse and, consequently, the
output of the motor is reversed.
A positive displacement charge pressure pump 53 is provided and is
driven through suitable means by the engine, or prime mover, of the vehicle- ;
or other device using the transmission. The pump 53 is in communication
; with a reservoir 54 through an intake conduit 55 and has a discharge con-
duit 56. The capacity of the charge pump 53 is sufficient to replace leak-
age fluid, and to supply operating fluid to a pair of control valves of the ~;
; displacement control for the pump and motor (hereinafter described) and to
supply cooling fluid to the hydrostatic transmission in excess of that re-
quired for the aforementioned purposes in order to maintain the transmission
cool. Additionally, the pump develops a pressure sufficient whereby the ~ -
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force in the cylinders for positioning the swashplates of the pump and
motor may exceed the forces tending to move the swashplate as created in-
ternally by the action of the pump and motor.
A pair of spring-biased check valves 57 and 58 are in communica-
tion with the conduit 56 and wi~h the main lines 11 and 12, respectively,
interconnecting the pump and the motor. The check valves 57 and 58 permit
the supply of replenishing and cooling fluid to the low pressure side of
the transmission circuit through one check valve while pressure in the high
pressure line will maintain the other check valve closed. A spring-biased,
make-up relief valve 63 communicates with the conduit 56 and serves to re-
lieve excess fluid. For establishing a circuit between the main lines 11
and 12 that is at low pressure and a low pressure relief valve 68, a shut-
tle valve 70 is provided. The shuttle valve is in communication with the
lines 11 and 12 by means of the conduits 72, 74, 76, and 78 and provides a
means for removing heated oil displaced by cooling oil supplied by the
charge pump 53. The fluid pressure in the lines 11 and 12 acts through ;
; the conduits 72 and 76, respectively, to appropriately position the shuttle
valve 70 so that communication is established from the low pressure relief
valve through a conduit 80 to the low pressure side of the transmission
circuit, by means of either the conduit 74 or the conduit 78, so that the
heated fluid may be drained to the reservoir, passing through the heat ex-
changer 7~. Shuttle valve 70 is spring-centered to a closed position so
that during the transition of reversing of pressure in the main lines, none
of the high pressure oil is lost from the circuit.
The transmission includes over-pressure relief valves 81 and 82 in
communication with each of the main lines by the conduits 83 and 84. The
valves serve to prevent abnormally high pressure in either of the two main
lines 11 and 12 by relieving the circuit of surge pressures which may occur
during rapid acceleration or abrupt braking. In response to this high
pressure, the over-pressure relief valves shift to dump the excess oil to
the low pressure side of the transmissiOn circuit. For example, when
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excessive high pressure exists in line 11, fluid pressure in conduit 83
~ill cause the valve ~l to shunt the fluid to line 12 through the conduit
84.
A bypass valve 90 is connected into conduits 83 and 84 by a pair
of conduits 91 and 92 and, by a conduit 93, is connected to the outlet of
the charge pump 53. In the event of loss of pressure from the charge
pump, the bypass valve shifts to connect conduits 91 and 92 and short-cir-
cuit the system to result in a free-wheeling condition of operation.
The motor 15 has a pair of operating cylinders 94 and 95 contain-
ing piston and link elements 96 and 97 pivotally connected to the swash-
plate 33 and with conduits 98 and 99 connected to their respective cylin-
ders or delivery of operating fluid thereto.
The second hydrostatic transmission, shown in the lower half of
Fig. 1, is of the same construction as that just described and is supplied
with fluid from the reservoir 54 through the conduit 55. Corresponding
structural elements of the second hydrostatic transmissibn have been given
the same reference numeral with a prime affixed thereto.
The dual path control system is shown primarily in Fig. 2A and,
more specifically, the master control circuit for setting the speed and
direction of operation of the two hydrostatic transmissions shown in Fig. 1
Fig. 2A additionally shows a pair of branch circuits extending from the
master control circuit including structure to provide steering capability. ~ -
The control system is operated by means of three control handles. A con-
trol handle 100 is operable to set the speed and direction of operation.
A control handle 101 is operable to establish a modification in speed of
one transmission or direction of operation thereof for steering capability
in one direction of turn and a third handle 102 is operable to control the
speed and direction of operation of the other hydrostatic transmission for
steering capability in the other direction of turn.
The master control circuit includes a con~rol pump 110 suitably
driven by a prime mover, such as an engine of a vehicle, to deliver control
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fluid used to establish a control pressure for the system. The output of
the control pump 110 is directed through a conduit 111 to a variable ori- -
fice valve 112 having a rotatable member 115 with a variable orifice 116.
The position of this valve is set through a connection 117 to the throttle
linkage of the engine (not shown) to provide a pressure differential across
the valve dependent upon the setting of the engine and with the variable
; orifice opening varying directly relative to the throttle setting. The
pressure differential is constant for each engine setting at rated speed.
The valve functions to signal speed of the engine and, thus, if the engine
speed decreases from that normally obtained by the throttle setting, there
is a lowered output flow fTom the control pump 110 which results in a low-
ering of the pressure differential across the variable orificeO A conduit
117 extends from the variable orifice valve 112 to a pressure regulator
valve 120 which assures adequate back pressure downstream of the variable
orifice valve 112 in order to operate the control system, and further pro-
vides a regulated constant pressure in a conduit 118 and which is designa-
ted PR. The conduit 118 extends to a pressure-reducing valve 125 (ratio
control valve), with this valve functioning to establish a control pressure
for obtaining a uniform speed control of both hydrostatic transmissions.
The pressure-reducing valve 125 includes a valve spool 126 which controls
communication between the inlet conduit 118 and an outlet conduit 127. The
valve spool 126, at one end, is subjected to outlet pressure therefrom
through a conduit 128 having a flow restriction as well as an adjustable
spring 126a and, at the opposite end, is acted upon by a spring 129. The
spring is partially positioned within a cylindrical member 130 movable with-
in the housing of the valve and having an end engageable by a speed-setting
cam 131 connected to a shaft 132 which is rotatably positioned by the man-
ually operable handle 100.
The cam 131 has a pair of oppositely-inclined cam surface sections
160 and 161 extending upwardly in opposite directions from the neutral por-
; tion of the cam which is shown in engagement with the member 130 in Fig. 2A.
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The control pressure fluid delivered from the pressure-reducing valve 125
by conduit 127 is directed to a horsepower control valve 140 which has, as
a primary function, prevention of stalling of the prime mover, such as the
engine of a vehicle, and, thus, also may be referred to as an anti-stall
valve. A conduit 141 extends from the anti-stall valve to a directional
control valve 142 having a valve spool 143 positionable by a linkage 144
operatively connected to the shaft 132 rotatably positionable by the manù-
ally-operable handle 100. The position of the valve spool 143 determines
the normal direction of operation of the hydrostatic transmissions.
The foregoing structure shown in Fig. 2A constitutes the master
control circuit whereby positioning of the handle 100 determines the nor-
mal direction of operation and a basic c = on and uniform speed for the
two hydrostatic transmissions. A first outlet conduit 150 from the direc-
tional control valve 142 branches into a pair of conduits 151 and 152 lead-
ing to branch circuits associated, one with each of the hydrostatic trans~
missions. A second conduit 153, extending from ~he directional control
valve, has two branch conduits 154 and 155 also extending to the respective
hydrostatic transmissions. In the illustrated position of stem 143, line
~141 is blocked and lines 150, 153 are drained.
. . ~,, :
When the shaft 132 is rotated from the position shown in Fig. 2A - ~Y
by force applied to the handle 100, the valve spool 143 of the directional
control valve is shlfted from the position shown to direct control pressure
~ fluid at a set pressure to one of the conduits 150 and 153 and operate the
- transmissions in one direction, or flow may be directed to the other con-
duit causing operation in the opposite direction, The two sections 160 :
and 161 of the cam surface of cam 131 are mirror images of each other,
whereby rotation of the shaft 132 in either direction will result in com-
pressing of the spring 129 of the pressure-reducing valve 125 to a degree
to set a control pressure of a desired value for speed of operation of the
hydrostatic transmissions.
The branch conduits 151 and 154 from the master control circuit ;~
connect to the housing 170 of a steering control valve, indicated generally
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at 171, while the correspondin~ branch conduits 152 and 155 connect into
the housing 172 of a steering control valve, indicated generally at 173.
The steering contro~ valve 171 has a pair of conduits 175 and 176 extending
therefrom and to the pump and motor displacement control shown in Fig. 2B.
The steering control valve 173 has a similar pair of conduits 177 and 178
which extend to the displacement control (not shown) for the pump and mo-
tor of the other transmission. Each of the steering control valves is of
the same construction, as are the displacement controls for the pump and
motor of the hydrostatic transmissions and, therefore, the steering control
valve 171 and the displacement control shown in Fig. 2B for the transmission
shown in the upper part of Fig. 1 will be described in detail with the un-
derstanding that there is the same structure for the other hydrostatic
transmission.
The control pressure signal is delivered to one or the other of
the branch conduits 151, 154, dependent upon the position of the direction-
al control valve 142, with these conduits communicating with a bore in the
valve housing 170 which movably mounts a valve spool 180. The position of
the valve spool 180 is controlled by the handle 101, with the handle being
connected to a rotatable shaft 181 which carries an eccentrically pivoted
link 182 which is pivotally connected to the upper end of the valve spool
180. In the position of the valve, as shown in Fig. 2A, and assuming con-
trol pressure is supplied to branch conduit 151, the control pressure
fluid enters the valve bore and, by flow around a reduced portion of the
valve spool, flows to the conduit 176 leading to the displace~ent control. -~
This supplies full control pressure to the displacement control. Supply
of full control pressure provides for operation of the associated hydro-
static transmission at a desired speed, as determined by the setting of
the control pressure by the pressure-reducing valve 125. In one direction
of vehicle turning, the speed of the hydrostatic transmission associated
with the steering control valve 171 is reduced. lhe handle 101 is opera-
ted to move the valve spool 180 downwardly, as shown in Fig. 2A, with the
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result that a certain amount of the control pressure fluid is metered or
bypassed to the branch conduit 154 which connects to a tank port 185
associated with the directional control valve 142 because of the latter
valve having its spool 143 in an upshifted condition from that shown in
Fig. 2A. This reduces the control pressure to reduce the speed. This
metering flow is across a land of the valve spool 180 which is provided
with external slots 186 on the land which function as a variable orifice,
with the slots increasing in size as the valve spool 180 is lowered from
the position shown in Fig. 2A to gradually reduce the control pressure
delivered through the conduit 176 to the displacement control. These
slots do not extend for the full length of the land, whereby the land is
continuous at the lower part thereof as viewed in Fig. 2A.
The valve spool 180 has two limit positions, with the upper limit -~
position shown in Fig. 2A, and with an opposite limit position being in a -~
full lowered position of the valve spool against the action of a spring
: :,
187 in the valve housing. In the fully-lowered position, there is a re-
versal of flow of the control pressure fluid with respect to output con-
duits 175 and 176 whereby direction of operation of the associated hydro-
static transmission is reversed. This action occurs by connecting an in-
ternal passage 188 in the valve spool 180 between the branch conduit 151
and the conduit 175 by means of a pair of radial openings in the valve
spool which extend the internal passage 188 to the surface of the valve
spool, with tkese openings being indicated at 189 and 190. This-results
in reversing the direction of operation of the associated hydrostatic
transmission whereby a spin-turn of the vehicle will result, with the ve-
hicle turning about an axis between the two tracks thereof.
If the directional control valve 142 is downshifted to obtain an
opposite direction of operation of the hydrostatic transmission, then -
pressure is supplied to the branch conduit 154 leading to the steering
; 30 control valve 171 and in the position of the latter valve, as shown in
Fig. 2A, full control pressure is delivered through the conduit 175 lead-
ing to the displacement control. Similarly, the control pressure may be
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reduced by metering or bypass as the valve spool 1~0 is lowered by the
slots 186 on the land of the valve spool and the direction of operation
may be reversed by further downward movement of the valve spool 180 until
the land having the slots 186 blocks the branch conduit 154 from the con-
duit 175 whereby the control pressure signal will then be delivered to
the conduit 176 to obtain the opposite direction of operation of the hydro-
static transmission.
The magnitude of control pressure reduction depends upon the rel-
ative size of an orifice 195 in the branch conduit 151 and the porting
characteristics of the valve 171. A sleeve 196 positioned in the bore of
the valve housing and which movably receives the valve spool 1~0 is pro-
vided with suitable flow passages and preferably an orifice passage 197 in
communication with branch conduit 154 which reduces the tolerance effects
of orifice size.
As shown in Fig. 2B, the conduits 175 and 176 extending from the
steering control valve 171 connect into opposite ends of a displacement
~ control valve 200 to effect the positioning of a control valve spool 201
- movable within the housing of the valve. The control valve 200 addition-
ally has a pair of tank connections 202, 203 as well as connections to
the conduits 48 and 50 leading to the pump control cylinders 36 and 38,
previously described. Operating fluid is delivered to the control valve
through a conduit 205 (Fig. 1) which is supplied by the charge pump 53 of
Fig. 1 with the outlet conduit 56 thereof connecting into the conduit 205.
The swashplate 32 for the pump is shown in a centered condition
in Fig. 2B as caused by the springs 39 and 40 within the cylinders. Assum-
ing that a control pressure signal directed to conduit 176 provides for op-
eration of the hydrostatic transmission in a forward direction, this will
result from control pressure shifting the valve spool 201 to the left from
the position shown in Fig. 2B whereby operating fluid from conduit 205 is
directed to the conduit 4~ and the piston 41 is caused to move to shift
the swashplate 32. Alternatively, if the control pressure signal is
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delivered to conduit 175, the control pressure signal is applied to the
left-hand end of the valve spool 201 to shift it to the right, as viewed
in Fig. 2B, whereby operating fluid from conduit 205 is applied to the
piston 42 to shift the swashplate in the opposite direction. The conduits
175 and 176, in addition to connecting into the control valve 200, extend
to a shuttle valve 210. The shuttle valve 210 functions to port the high-
est control pressure existing in either of conduits 175 or 176 to one end
of the displacement control valve for the swashplate of the motor whereby
the motor swashplate always adjusts in the same direction.
A control valve 220 of the motor displacement control has a valve
spool 221, with the right-hand end thereof always being subjected to the
highest control pressure through a conduit 222 extending from the shuttle
valve 210. The opposite end of the control valve has a conduit 223 with
branch conduits 224 and 225 connected to the shuttle valve 210. The con-
trol valve 220 is of the same construction as the control valve 200, with
there being a pair of tank connections and also a conduit 226 which branch-
es off from the conduit 205 connected into the outlet from the charge pump
53 to provide operating fluid for opera~ion of the displacement control for
the motor swashplate. '
Referring to the displacement control for the pump, the valve
spool 201 is shown in a neutral position with the swashplate 32 in neutral
position and with the position of the valve spool partially being controlled
by a pivoted arm 230 which is connected to the valve spool for movement
with it and which has a pair of springs 231 and 232 engaging against oppo-
site sides of the arm and acting in opposition to each other. The null po-
sition of the valve spool with the swashplate 32 in neutral is set by ro- -
tatable adjustment of a threaded seat member 233 for the spring 232, with
the rotatable adjustment either advancing or retracting the spring seat
233 to vary the compression of the spring 232 in action against the spring
231 to obtain the desired null position. Additionally, the displacement
control has feedback means including a pivoted linkage 240 including an
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!` arm 241 on a fi~ed pivot 242 and having a seat at an end thereof for the
spring 231. As the valve spool 201 is shifted, the control a~m 230 is
similarly shifted to create an imbalance in the springs 231 and 232. The
shift of the spool results in a pivo~ing of the swashplate 32, which is
indicated through the feedback linkage 240 to vary the compressive force
on spring 231, whereby when there is again a balance in the spring forces
the valve spool 20I will be in a neutral flow-blocking position. This ac-
tion will be the same regardless of the direction in which the swashplate
32 is pivoted.
As stated previously, the highest control pressure is always dir-
ected through conduit 222 to the valve 220 of the motor displacement con-
trol whereby the pressure acts to shift the valve spool 221 toward the left
as viewed in Fig. 2B. This valve spool has a control arm 250 movable there-
with and with a pair of springs 251 and 252 engageable against opposite
sides of the control arm and acting in opposition. The spring 251 has an
adjustable, threaded spring seat member 253 on a fixed mounting in order to
obtain a null position for the valve spool 221. Additionally, the spring
251 is stronger than the springs 231, 232 and 252 whereby the valve spool
221 is normally open to conduit 98 and will not shift in response to a con-
trol pressure until the value of that pressure exceeds a control pressure
which acts to substantially fully shift the valve spool 201 of the pump
displacement control valve. This provides for staging of the co~ponents
of the transmission whereby the swashplate 32 of the pump which has been
at neutral will move to a substantially maximum displacement prior to any
movement of the motor swashplate 33. Following maximum displacement of the
pump swashplate, the displacement of the motor will be reduced as the con-
trol pressure increases.
This action is shown by the graph of Fig. 4 wherein the control
pressure obtained by the setting of the pressure reducing valve 125 is
plotted as the ordinate and the displacement of thP pump and motor are
plotted as the abscissa As the control pressure increases from (a) to (b),
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the displacement o~ the pump changes fro~n neutral to maximum, as indica-
ted by symbols beneath the abscissa line and at approximately the time
the pump achieves full displacement, the displacement of the motor reduces -
~ progressively from a maximum to a minimum as the control pressure moves
'! from value (b) to (c) and as shown by markings above the abscissa line. -
The shuttle valve 210 has a valve spool 260 and a pair of inter-
nal passages opening to opposite ends thereof and extending to the surface
of the valve spool by a pair of radial passages 261 and 262. The last-
mentioned passages provide for communication of pressure in conduits 175
and 176 to the respective opposite ends of the shuttle valve spool 260
whereby the greater control pressure will act to shift the shuttle valve.
Assuming that the greater control pressure is in conduit 175, the shuttle
valve spool 260 will be shifted upwardly whereby the conduit 175 is placed
in communication with the conduit 222. If the greater control pressure is
in conduit 176, this will be directed to the upper end of the shuttle valve
spool to shift the valve downwardly, with the result that conduit 176 will
... :
be placed in communication with conduit 222 leading to the control valve
;' 220 of the motor displacement control. In either of the above situations,
one or the other of the branch conduits 224 or 225 will be connected to
; 20 the other of conduits 175 and 176 which will extend back to the steering
control valve 171 ~or suitable connection to tank at the directional con-
trol valve 142.
In considering the feedback means of the motor displacement con-
trol, reference shall first be made to Figs. 5 and 6. In order to maintain
the same steer radius of a vehicle when changing ground speed while in a
turn, there should be a linear relationship between the control pressure x
and the output speed of the transmission, as indicated in the graph of
Fig. 5.
The feedback means for the motor displacement control has a cam
to provide the relationship indicated by the graph of Fig. 6 between trans-
mission speed and displacement of the motor in order to result in the
- 16 -
1~35t~2~
; linear relation between control pressure and transmission speed shown in
the graph of Fig. 5. ~ithout the cam, the relationship between pressure
and speed shown in Fig. 5 would not be linear at the higher control pres-
sures whereby it would be possible to have an indirect relation between the
speeds of the two hydrostatic transmissions when one of the steering control
- valves 171 or 173 might be operated to have one transmission operating in a
speed range different from the other. This would result in an uncontrolled
relation in a turn of the véhicle.
- Referring specifically ~o the feedback means for the motor dis-
placement control shown in Fig. 2B, a linkage connected to the swashplate
has an arm 270 connected to the swashplate which, by a link 271, pivots an
arm 272 pivotally-connected to a fixed bracket 273 with a cam roller 274 on
the end of the arm 272. A cam 275 pivoted on a fixed mount 276 carries a
spring seat for the spring 252 and has a cam surface coacting with the cam
roller 274.
- The action of the cam 275 results in obtaining a relatively small
change in the compression exerted on spring 252 when displacement of the
` motor varies near the maximum displacement range of operation of the motor,
with there being a greater rate of change of compression of the spring 252
in response to a given control pressure signal when the motor is operating
toward the minimal displacement range of operation. Thus as the swashplate
33 pivots counterclockwise as viewed in Fig. 2B, the cam 275 also pivots
counterclockwise to reduce the compression on the spring 252 until there is
' a balance obtained to bring the control spool 221 to a neutral position
wherein operating fluid from conduit 226 is blocked from communication with
the operating cylinders 94 and 95. This balance is obtained when the con-
trol pressure acting on the right-hand end of the control spool 221 plus
the force of spring 252 equals the force of the stronger ~pring 251 and any
force resulting from pressure acting against the left-hand end of the con-
trol spool 221.
It will be obvious that the cam and cam roller relation shown in
1C~ 6Z~
Fig. 2 could be reversed ~lereby the cam could be associated with the feed-
back linkage and the follo~er associated with the spring seatD
~ cam, such as cam 275, designed for a hydrostatic transmission
using a single variable displacement motor with a variable displacement
pump, will provide the desired relation shown in Fig. 5 regardless of pump
size or input speed. The only requirement is that the actual control pres-
sure to produce full pump displacement be similar for different pump sizes.
Similarly, the same cam can be used for different motor sizes as long as
suitable feedback linkage is used in order to produce the relationship
shown in Fig. 6.
In the control system as now described, a single control handle
100 controls components of the master control circuit to establish a uni-
form control pressure and, therefore, a uniform speed for both of the hydro- -
static transmissions as well as the direction of operation thereof. With ~ -
operation of the control handle 100 and rotation of the shaft 132, the con- -
trol valve 142 is appropriately positioned and either of the cam surface
sections 160 and 161 of the speed-setting cam 131 controls the force of the
spring of the pressure-reducing valve 125. This applies the same control
pressure to both branch circuits, with each circuit having a steering con-
... :,.
trol valve, namely valves 171 and 173. When it is desired to obtain non- -
. uniform operation of the transmissions, either of the handles 101 or 102 v
may be operated to vary the control pressures applied to the displacement
controls for the associated hydrostatic transmission. The relation of -~
rotation of the handle 100 to the establishment of control pressures by ;
the pressure-reducing valve 125 is shown in the graph of Fig. 3 wherein ~-`
the control pressure values (a), (b) and (c) correspond to those given in
Fig. 4. It will be noted that initial rotation of the handle determines ~ -
the direction of operation with the magnitude of rotation of the handle ~ -
~` determining the control pressure and, therefore, the speed of the hydro-
static transmissions. ~ `
As described previously, the steering control valve 171 is shown
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. . . ~ . .
`"` 10396Zl
in one limit position and, as the handle is moved, the valve spool progres-
sively may be moved toward the other limit position and during mov~ent
will meter part of the control pressure fluid to tank through connection
;` back to the directional control valve 142, with full movement of the steer- ~ -
ing control valve to an opposite limit position actually reversing the dir-
ection of control pressure fluid to the displacement controls.
.~:
~ When the steering control valve spool 180 is shifted to meter the
:.~
control pressure, there is a pressure establlshed in either of conduits
3 175 or 176 which is connected to tank by directional control valve 142 be-
-10 cause of the orifices 197 and 195, respectively, which restrict flow to tank.
This establishment of pressure along with a reduction of the control pres-
sure upsets the balance on the displacement control to have the hydrostatic
s!
'~ transmission operate at a reduced speed.
The steering control valve 173 has the output conduits 177 and 178,
referred to previously, which correspond to the conduits 175 and 176 for the
steering control valve 171. The conduits 177 and 178 connect into a dis- -
~ placement control, the same as shown in Fig. 2B for the pump and motor of
Y'~ the second hydrostatic transmission. Corresponding parts of the two
steering control valves have been given the same reference numerals.
The borsepower control valve 140, shown in Fig. 2A, is a normally-
open, three-way modulating valve in series with the pressure-reducing
valve 125. The valve is biased closed by an adjustable spring 300 acting
; against the valve spool 301. Pressures existing at opposite sides of the
~,.
~; variable orifice valve 112 are applied to opposite ends of the valve spool
301 with the pressure in advance of the variable orifice valve being applied
-~ thereto through a conduit 305 connecting to the conduit 111. The pressure
downstream of the variable orifice valve is applied to the opposite end of
il;~ the valve spool 301 through a conduit 306 connected into conduit 118 which
senses the regulated pressure PR. When the engine is operating at, or near,
rated speed for any selected throttle setting, the pressure drop through
-;.J
the variable orifice valve exceeds the force of the spring 300 to maintain
valve spool 301 in a raised position as viewed in Fig. 2A, allowing the
,
19
....
:~ i , .. , , , . . . . . _
103~36Zl
control pressure to pass therethrough ~ith no pressure loss. If the engine
begins to operate below rated speed, the pressure drop across the variable
orifice valve is reduced below the setting of the spring 300 which causes
the valve spool 301 to move downwardly and restrict the flow path to the
conduit 141 leading to the directional control valve and begin exhausting
the control pressure fluid to tank. This result~ in reducing the control
pressure signal to resultingly reduce the speed of the hydrostatic trans-
missions and cause the vehicle speed to match the power capability of the
engine. If the vehicle is moving straight ahead, the action of the horse-
; 10 power control valve will result in equal, but reduced, control pressures
- on both hydrostatic transmissions to keep the vehicle moving on a straight
path, but at a lower ground speed. If the vehir~le is in a steer mode, a
reduction of control pressure by the valve will reduce the control pres- ;
sures at each hydrostatic transmission proportionally. A prior art construc-
tion of applicants' assignee had a valve to provide the same antistall
: result.
.. :,.. ..
The horsepower control valve 140 is now modified to also provide
over-pressure relief protection for the control pump 110. This action occurs
by an over-pressure in conduit 305 acting to shift the valve spool 301 up-
wardly against the spring 300 and pressure PR to connect conduit 305 to tank. -~
A condition causing such action would be when the oil used in the system is
quite cold and will not properly flow through the variable orifice valve 112
whereby an over-pressure condition would exist in the conduit 305.
.:
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