U.S. patent number 3,768,928 [Application Number 05/148,559] was granted by the patent office on 1973-10-30 for pump control system.
This patent grant is currently assigned to Borg-Warner Corporation. Invention is credited to Raymond E. Johnson, Wendell E. Miller.
United States Patent |
3,768,928 |
Miller , et al. |
October 30, 1973 |
PUMP CONTROL SYSTEM
Abstract
A hydraulic system including a variable displacement pump with a
displacement control mechanism, a sensing valve to control the
displacement control, and a logic system providing over-ride
control of the sensor valve and the displacement of the pump.
Specific embodiments include the over-riding of load or flow
compensated systems, torque over-ride by the use of a pressure and
displacement sensitive valve, and a sensor valve that includes a
bypass position which cooperates with the displacement control to
control the effective output of the pump.
Inventors: |
Miller; Wendell E. (Warsaw,
IN), Johnson; Raymond E. (Muncie, IN) |
Assignee: |
Borg-Warner Corporation
(Chicago, IL)
|
Family
ID: |
22526288 |
Appl.
No.: |
05/148,559 |
Filed: |
June 1, 1971 |
Current U.S.
Class: |
417/213; 60/451;
60/450; 417/222.1 |
Current CPC
Class: |
F04B
1/324 (20130101) |
Current International
Class: |
F04B
1/32 (20060101); F04B 1/12 (20060101); F04b
001/26 (); F04b 049/00 (); F15b 001/00 () |
Field of
Search: |
;91/505,506
;417/218,219,222,213 ;60/52VS,450 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Freeh; William L.
Assistant Examiner: LaPointe; Gregory
Claims
What is claimed is:
1. In a hydraulic system including a variable displacement pump as
a source of fluid pressure,
a sump, fluid conduits, and a fluid motor all operatively
connected;
a fluid actuated displacement control mechanism connected to said
pump and adapted to control the displacement of said pump in
response to fluid pressures applied to said displacement control
mechanism;
a sensor valve operatively connected to said displacement control
mechanism and to said hydraulic system and being adapted for being
moved by a force balance to displacement increasing and
displacement decreasing positions wherein connections are made
between said system and said displacement control mechanism to
control the fluid pressures applied to said displacement control
mechanism and to increase and to decrease the displacement of said
pump;
force balance means consisting of bias force means and fluid
responsive means and adapted to move said sensor valve to said
displacement increasing and displacement decreasing positions in
response to a control signal pressure;
control signal pressure supply means interconnecting said fluid
responsive means with a source of fluid pressure in said system
whereby said sensor valve is moved to said displacement increasing
position by said control signal pressure;
over-ride valve means consisting of a power amplifier means having
one port in fluid communication with said fluid responsive means
and having another port in fluid communication with said sump;
and
fluid restrictor means interposed into said control signal pressure
supply means and being adapted to reduce the fluid flow that is
required through said over-ride valve means to reduce said control
signal pressure applied to said fluid responsive means, to change
said force balance; and to move said sensor to said displacement
decreasing position; whereby actuation of any one of said over-ride
valves is effective to reduce the displacement of said pump.
2. The system as claimed in claim 1 in which said over-ride valve
means includes a pilot relief valve operatively connected to said
fluid responsive means and to said pump and being adapted to
control the force balance of said sensor valve in response to the
pressure produced by said pump; the displacement of said pump being
reduced when pump pressure exceeds a predetermined value.
3. The system as claimed in claim 1 in which said sensor valve
includes a position in which fluid is bypassed from said pump to
said sump, and said control signal pressure is effective to move
said sensor valve to said position, whereby position of said sensor
valve comprises a bypass valve means.
4. In a hydraulic system including a variable displacement pump as
a source of fluid pressure,
a sump, fluid conduits, and a fluid motor all operatively
connected;
a fluid actuated displacement control mechanism connected to said
pump and adapted to control the displacement of said pump in
response to fluid pressures applied to said displacement control
mechanism;
a sensor valve operatively connected to said displacement control
mechanism and to said hydraulic system and being adapted for being
moved by a force balance to displacement increasing and
displacement decreasing positions wherein connections are made
between said system and said displacement control mechanism to
control the fluid pressures applied to said displacement control
mechanism and to increase and to decrease the displacement of said
pump;
force balance means consisting of bias force means and fluid
responsive means and adapted to move said sensor valve to said
displacement increasing and displacement decreasing positions in
response to a control signal pressure;
control signal pressure supply means interconnecting said fluid
responsive means with a source of fluid pressure in said system
whereby said sensor valve is moved to said displacement increasing
position by said control signal pressure;
over-ride valve means consisting of a transducer which is pressure
and displacement sensitive and comprises; a pressure-to-position
transducer; a mechanical displacement reference mechanism; and a
variable fluid resistance means;
fluid restrictor means interposed into said control signal pressure
supply means and being adapted to reduce the fluid flow that is
required through said over-ride valve means to reduce said control
signal pressure applied to said fluid responsive means, to change
said force balance; and to move said sensor to said displacement
decreasing position; whereby actuation of any one of said over-ride
valves is effective to reduce the displacement of said pump.
5. The system as claimed in claim 4 in which said displacement
control mechanism includes a control piston; and said variable
fluid resistance means comprises two relatively movable elements,
one of said elements being connected to and coaxial with said
control piston in said displacement control mechanism whereby said
control piston provides said mechanical displacement reference.
Description
THE INVENTION
The present invention represents an advance in the concept of pump
control. In the hydraulic system, a sensing valve is provided to
control the displacement control of a variable displacement pump.
This sensing valve is actuated to pump displacement increasing and
pump displacement decreasing positions by a force balance
consisting of a bias force and a hydraulic force. The hydraulic
force is controlled by a plurality of manual or automatic valve
devices interconnected to form a logic system so that actuation of
any one of these valve devices is effective to change the force
balance on the sensor valve and to move the sensor valve to a
displacement decreasing position. Thus the displacement of the pump
can be controlled by a variety of combined signals.
One of the principal uses of the present invention is to achieve
over-ride control of flow compensated pump displacement. The
over-ride control can then be used to achieve manual or automatic
control of the pump displacement from any internal or external
source.
THE DRAWINGS
FIG. 1 is a schematic of a hydraulic system of this invention;
FIG. 2 is a schematic of a different embodiment of a hydraulic
system of this invention;
FIG. 3 is a schematic of a still further embodiment of a hydraulic
system of this invention;
FIG. 4A is a cross sectional view of a sensing valve that includes
a bypass position; and
FIG. 4B is a schematic of the sensing valve that includes a bypass
position .
DESCRIPTION
Attention is directed to FIG. 1 of the drawings wherein a hydraulic
system according to this invention is schematically illustrated.
There is shown a variable displacement pump 10 provided with a
mechanical power input shaft 11 connected to a source of mechanical
power (not shown) which rotates a cylinder barrel 12 causing
pistons 13 to reciprocate as they engage a swash plate 14. This
general type of variable displacement pump is known in the art.
The swash plate 14 is part of a fluid actuated displacement control
mechanism 15 which also includes trunions (not shown), a large
control piston 16, a small control piston 17 and a bias spring 18
biasing the control piston 17 toward the swash plate. A large
control cylinder 19 is associated with the piston 16 and a small
cylinder 20 is associated with the piston 17.
The inclination of the swash plate 14, the pumping stroke of the
pistons 13, and thus the displacement of the pump 10 are dependent
upon the pressure fluid in the large control cylinder 19 and in the
small control cylinder 20.
Pressurized fluid from a pump output pressure conduit 21 is fed
through a conduit 22 to the cylinder 20 where it exerts a force
against the small control piston 17, assisting the spring 18 in
urging the swash plate 14 toward the maximum displacement position,
as shown.
The pressure fluid in the cylinder 19 is the controlling factor in
determining the displacement of the pump 10 because the cylinder 20
will accept additional fluid from the conduits 21 and 22 or return
excess fluid to these conduits depending upon whether or not fluid
is added to or released from the larger area of the piston 16 and
the cylinder 19.
A differential pressure sensitive sensor valve 25 controls the
admission of pressure fluid into the cylinder 19 and the release of
fluid from the cylinder 19. The sensor valve 25 includes a valve
spool 26 in a bore 27, biased by a spring 28 toward a first
position toward the left from its neutral position shown.
The sensor valve 25 includes fluid receiving chambers 29 and 30,
which serve to apply fluid pressure against fluid responsive areas
31 and 32, respectively, provided on the projected ends of the
valve spool 26.
The chamber 29 also serves as a pressure inlet port for the sensor
valve 25 and cooperates in this manner with a spool land 33 and
with ports 34 and 35 to control fluid to and from the cylinder 19
and the large control piston 16.
The fluid responsive areas 31 and 32 of the valve spool 26 are made
responsive to pressures that vary in magnitude as a function of
flow across a variable orifice 39 by a connection with the chamber
29 through conduits 21 and 41 and a connection with the chamber 30
through conduits 21, 42, a restrictor 43, and conduits 44, 45, 46
and 47.
In operation, a fluid from a reservoir 36 enters the pump 10
through a conduit 37 and is delivered to a fluid motor 38 through
the conduit 21 and the variable orifice 39. Fluid exhausted from
the motor 38 is returned to the reservoir 36 by a return conduit
40. The variable orifice 39 controls fluid flow to motor 38.
Flow through the variable orifice 39 produces a differential
pressure signal to the chambers 29 and 30 in proportion to, or
indicative of the value of the variable orifice and the
instantaneous flow rate in the conduit 21.
The difference in the pressures applied to the fluid responsive
means consisting of fluid responsive areas 31 and 32 is effective
to oppose the bias force means or spring 28 of the sensor valve 25.
Differential pressures above a certain level are effective to
overcome the bias force of the spring 28 and to move the spool 26
toward the right as viewed in the drawing to a second or
displacement decreasing position whereas when lower differential
pressures exist the spring 28 moves the spool 26 to the left toward
a first or displacement increasing position.
Excessive flow rates, in relation to those preselected by the
variable orifice 39 and indicated by the pressure drop across the
orifice 39, will result in the valve spool 26 being moved to the
right to the second position. The valve spool 26 then establishes a
fluid communication path between the outlet conduit 21 and the
large control piston 16 via the conduit 41, the chamber 29, the
port 34 and the conduit 51. With such connection, the same value of
fluid pressure is applied to the piston 16 as is applied to the
piston 17 because both cylinder 19 and cylinder 20 are connected to
the pump outlet pressure conduit 21. The larger area of the piston
16 is effective to overcome the force exerted by the small control
piston 17, and the displacement of pump 10 is reduced until the
flow through the variable orifice 39 equals the desired value as
determined by the present restriction of the variable orifice
39.
When flow rates are smaller than selected by the variable orifice
39, the spring 28 will move the valve spool 26 to the left, to a
first position, establishing fluid communication between the
cylinder 19 and a sump 52 via the conduit 51, the port 34, the port
35 and the conduit 53.
The system as described in the preceding paragraphs is a demand
type of system. The system is controlled by the flow caused
pressure drop across variable orifice 39 so that the pump
displacement is controlled to provide the flow necessary to satisfy
the system demand as expressed by the flow area established through
variable orifice 39.
This demand system can also be thought of as being load sensitive
in that the displacement of the pump is controlled to provide a
flow that causes a pressure equal to the load pressure plus a small
pressure increment for flow across variable orifice 39.
The over-ride system of the present invention can be visualized as
performing a function that is opposite to a demand type of system.
Whereas the demand system can be adapted to provide an increase in
pump displacement from the signalling of one or a plurality of
fluid restrictions such as variable orifice 39; the logic system of
the present invention provides means whereby a number of manual or
automatic safety or control devices can be provided to over-ride
the demand system; and any one of the over-ride devices is
effective to decrease the pump displacement.
This over-ride concept can also be thought of as providing a
negative signal in that it negates the signal produced by the
demand system. As illustrated in FIG. 1, opening of variable
orifice 39 increases the fluid pressure applied to area 32 through
conduits 42, 44, 45, 46 and 47. Conversely, opening of valve means
76 is effective to negate the pressure applied to area 32 by
conducting pressure fluid from area 32 to a fluid sump via conduits
47, 46, 45 and valve means 76. Restrictor 43 is effective to
prevent an excessive rate of flow from conduit 21 to area 32 via
conduits 42, 44, etc., thereby cooperating with the flow through
valve means 76 to negate the fluid pressure applied to area 32.
This negative signal concept is utilized in several different ways
in the embodiment shown in FIG. 1. In each case, a pressure drop is
caused in the conduits 44, 45, 46 and 47 and in the chamber 30 by
exhausting fluid to sump; and (A) the restrictor 43 between the
conduits 42 and 44 limits the flow into the chamber 30, so that a
relatively small exhaust flow is effective to lower the pressure in
the chamber 30.
A first application of this negative signal concept is illustrated
by the pressure and displacement sensitive valve 54 (hereinafter
referred to as "pd" valve). The pump outlet pressure in the
cylinder 20 exerts a force against a spring 55 by acting against a
shoulder 56 of a valve plunger 57, making the plunger 57 and the
spring 55 a pressure-to-position mechanism.
The projected area of the valve plunger 57 in a bore 58 is in
communication with a sump (not shown) via a cross-hole 62 and the
pump housing (not shown). The projected end of the plunger 57 in a
spring chamber 63 is likewise in fluid communication to the sump
via a longitudinal hole 64. Thus only the area of the shoulder 56
in the cylinder 20 is subjected to fluid pressure.
The small end of the plunger 57 cooperates with the bore 58 and a
cross-hole 59 of the small control piston 17 to form a two-way
valve. If the piston 17 is considered to be stationary, the
pressure-to-position transforming action of the shoulder 56 and the
spring 55 combines with the valving action of the cooperating
portions of the piston 17 and the plunger 57 to form a pressure
sensitive valve. However, the piston 17 is not stationary, but
moves with the swash plate 14. Thus the valve is made to be swash
plate angle or displacement sensitive and so the valve is both
pressure and displacement sensitive.
The pd valve 54 opens when the combination of pump pressure and
displacement exceed design or preset combinations. That is,
pressure moves the plunger 57 to the right and an increase in
displacement moves the piston 17 and the cross-hole 59 to the
left.
When the plunger 57 uncovers the cross-hole 59, the pressure in the
chamber 30 is reduced by fluid exhausting through a conduit 60, an
annular groove 61, the cross-hole 59, the bore 58 and a cross-hole
62 to the pump housing and to a sump. This reduction of pressure in
chamber 30 causes a reduction in the displacement of pump 10.
Since input torque is directly related to output pressure and
displacement, the pd valve formed by the plunger 57 and the piston
17 functions as a torque compensator to limit the input torque that
is required to drive the pump.
The pd valve may be combined with a remote transducer such as an
acceleration transducer 65. The transducer 65 includes a shaft 66
and a body 67 mounted on the output shaft of the pump driving means
to receive support and rotation therefrom. A sensor mass 68 is
mounted on the shaft 66 for relative rotation with the shaft; and
the cooperating surfaces of the shaft 66 and the mass 68 combine
with cross-holes 69 and 70 to form a two-way valve.
The mass 68 is held against a stop 71 by springs 72 and 73 when the
velocity of the shaft 66 is constant or increasing. However, upon
deceleration of the shaft 66, the mass 68 rotates clockwise with
respect to the shaft 66, compressing the springs 72 and 73 and
opening communication from the chamber 30 to a sump via the
conduits 47, 46, 45 and 74 to a longitudinal hole 75, the
cross-hole 69 and the cross-hole 70.
Opening of this communication from the chamber 30 to a sump as a
function of pump drive engine deceleration affects a reduction of
pump displacement wherever the total load on the engine exceeds the
available torque. Thus the deceleration sensing system considers
not only pump drive torque, but all engine loads, and the engine's
ability to handle the loads.
The variable restrictors or over-ride valves 76, 77 and 78
schematically represent a logic system of transducer or manually
actuated devices.
An additonal pressure control for the chamber 30 is a pilot relief
valve 80 which cooperates with the restrictor 43 to limit the
pressure in chamber 30. Pressure values in the conduit 21 and the
chamber 29 which exceed the limited value, will move the valve
spool 26 to the right, compress the spring 28, exhausting fluid
through the valve 80 to a sump, and effect a reduction in the
displacement of the pump 10 by communicating the large control
piston 16 with the conduit 21.
It should be noticed that the over-ride portion of the system will
control the displacement of the pump without the necessity of the
inclusion of the variable orifice 39 and the flow compensation
feature. With the variable orifice 39 eliminated, fluid pressures
in the chamber 29 and 30 will equal each other and the fluid
pressure in the conduit 21 just as if the variable orifice 39 were
included but opened to its minimum restriction. Thus the spring 28
urges valve spool 26 to the left to a displacement increasing
position; and the over-ride system is effective to reduce the
displacement of the pump 10 to the desired value by the bleeding
off of fluid pressure from the chamber 30 to a sump.
A bypass valve 81 is provided in the system and is adapted to
cooperate with the displacement control mechanism 15 in the control
of the effective output of pump 10 by bypassing excess flow to sump
in response to the same control signal pressures that actuate the
sensor valve 25.
The bypass valve 81 includes a plunger 84 slidably fitted in a bore
92 of a body or housing (not shown). The plunger 84 cooperates with
the bore 92 and with a seat 86 to provide the chambers 87 and 89;
and plunger 84 provides substantially equal fluid pressure
responsive areas 82 and 83 in the chambers 87 and 89.
The area 82 in the chamber 87 is thus responsive to the fluid
pressures in the conduit 21 as is the area 31 of the sensor valve
25; since the chamber 29 and the area 31 are connected to the
conduit 21 by the conduit 41; and the chamber 87 and the area 82
are connected to the conduit 21 by the conduit 91.
In like manner, the area 32 of the sensor valve 25 and the area 82
of the bypass valve 81 are subjected to the fluid pressures in the
conduits 44, 45, 46 and 47.
Therefore the bypass valve 81 responds to excessive flows and the
resultant excessive pressure differentials across the variable
orifice 39 to bypass excess flow to sump; and the bypass valve 81
also responds to any of the negative or over-ride signals connected
to the conduits 44, 45, 46 and 47 to bypass excess pump flow to
sump. The result is that the bypass valve 81 cooperates with the
displacement control mechanism 15 to control the effective output
of the pump 10, the required response of the displacement control
15 is reduced, so that the stability of the system is enhanced.
Another embodiment of the invention is illustrated schematically in
FIG. 2 to which attention is now directed. In this embodiment, the
swash plate 14 inclines in the opposite direction from that in FIG.
1. This requires a slightly different sensor valve construction, so
that a reduction in pressure in the chamber 30 will result in the
large control piston 16 and the cylinder 19 being placed in
communication with the sump 52 rather than with the pump pressure
conduit 21.
In the FIG. 2 embodiment the valve spool 100 of the sensor valve
101 moves to the right when the pressure in the chamber 30 is below
that in the chamber 102 and the force on the fluid responsive area
103 is sufficient to overcome the spring 28 and the force generated
on the spool 100 by the fluid pressure in the chamber 30. The valve
spool 100 by moving to the right establishes fluid communication
between the large control piston 16 and the sump 52 via the conduit
51, the port 34, the port 104 and the conduit 53.
The pd valve 105 of FIG. 2 is somewhat different from that shown in
FIG. 1. The partition between the spring chamber 63 and the small
control piston cylinder 20 of the FIG. 1 embodiment has been
eliminated along with various manufacturing problems. Thus, the
valve plunger 106 is greatly simplified.
Fluid pressure supplied to the chamber 107 via the conduits 21 and
108 acts against the projected end of the plunger 106 to compress
the spring 109, moving the annular groove 110 of the plunger 106
closer to registering with the cross-hole 111 of the small control
piston 17.
An increase in displacement is effective to reduce the distance
between the annular groove 110 and the cross-hole 111 making the pd
valve 105 both pressure and displacement sensitive.
When the groove 110 registers with the cross-hole 111 fluid from
the chamber 30 is bled to the sump via the conduit 112, the annular
groove 113 and the cross-hole 114 and the longitudinal hole 115 in
the valve plunger 106, the bore 58 and the cross-hole 62 to the
pump housing and sump.
A power amplifier 118 replaces the acceleration transducer 65 of
FIG. 1. The power amplifier 118 includes a high pressure port 119,
an exhaust port 125, and a control port 120. A flexible diaphragm
121 presents a large working area to a pressure signal in a chamber
122 whereas a high pressure seat 123 presents a small pressure
responsive area. Thus, the control of small pressures in chamber
122 is effective to control the seating of a plunger 124 against
the seat 123.
With such an amplifier, compressed air can be used as a control
medium for use with remote transducers or manually actuated
over-ride valves as schematically respresented by variable
restrictors 126, 127 and 128. The use of air permits the transducer
or other over-ride valve means to exhaust directly to the
atmosphere. An air supply (not shown) is connected to the air
conduit 116; and the restrictor 117 serves to limit the supply of
air to the parallel connected over-ride valve or variable
restrictors 126, 127, and 128 thereby assuring the effectiveness of
any of the variable restrictors 126, 127, and 128 in the lowering
of the pressure in the chamber 122 and the opening of the seat
123.
Another embodiment of the invention is illustrated in FIG. 3. Here
the variable orifice 39 has been inserted in the return conduit 40
between the fluid motor 38 and the sump 36; and the upstream side
of the orifice 39 and the chamber 102 of the sensor valve 139 have
been interconnected. The portion of the conduit 40 between the
orifice 39 and the reservoir 36 is assumed to have a negligible
pressure; so it is not necessary to connect the chamber 30 of the
sensor valve 139 to the downstream side of the orifice 39. Instead,
the chamber 30 communicates with a sump.
The pd sensitive valve 130 of FIG. 3 operates in a manner similar
to the valve 105 of FIG. 1. Movement of the small control piston 17
to the right and/or movement of the valve plunger 131 to the left
establishes communication from the pump outlet port to the chamber
102 of the sensor valve 139 via the conduit 21, the conduit 108,
the chamber 107, reduced stem portion 132 of the plunger 131, the
cross-hole 133, an annular groove 134, a conduit 135 and a conduit
136. A reduction in pump displacement by the pd valve 130 is thus
achieved by increasing the pressure in the chamber 102 rather than
by reducing the pressure in the chamber 30, as was done in the
FIGS. 1 and 2 embodiments.
A restrictor 137 prevents an excessively large flow from the
conduit 136 to the return conduit 40 during torque compensation by
pd valve 130 without preventing the actuation of the sensor valve
139 by the pressure signal generated by the orifice 39.
Any of the parallel connected transducers or valve means of FIGS. 1
and 2 can be adapted to the FIG. 3 configuration by adapting these
valve means to apply fluid pressure from the conduit 21 to the
chamber 102 as does pd valve 130.
FIGS. 4A and 4B show a variation of the sensor valve of FIG. 1 in
which a third operating positon has been added to bypass excess
fluid.
FIG. 4A shows a preferred configuration of the bypass type sensor
valve whereas FIG. 4B illustrates the principles of operation
schematically.
In FIG. 4B, the bypass type sensor valve 150 is shown in a neutral
position, as represented by the box 151, in which fluid
communication is blocked between the conduit 51 and the pressure
conduit 165 and between the conduit 51 and the sump 52. Thus fluid
is blocked in the conduit 51 and in the cylinder 19 of FIG. 1.
The sensor valve 150 is actuated to its operating positions by the
operators 152 and 153 and the bias spring 154. The operators 152
and 153 correspond respectively to the areas 31 and 32 and to the
chambers 29 and 30 of FIG. 1.
The sensor valve 152 is moved to the left to its first or
displacement decreasing position by the bias spring 154 when the
fluid pressures in the operators 152 and 153 approach equality.
This first operating position is shown by the box 155. Pressure
fluid trapped in the cylinder 19 (FIG. 1) then is released via the
conduit 51, the valve communication path 156 and the exhaust port
157A to sump 52. The releasing of fluid from the cylinder 19 allows
an increase in pump displacement (FIG. 1) which results in an
increase in flow and an increase in pressure drop across the
orifice 39 thereby moving the sensor valve 150 back to its neutral
position.
In like manner, the sensor valve 150 is actuated to the second or
pump displacement decreasing position 158 by a fluid pressure in
the operator 152 that is able to overcome the combined force
produced by the operator 153 and the spring 154.
A third operating position, not previously described, is shown by
the box 159. An excess of pressure differential between conduits 41
and 47 (FIG. 1), beyond that necessary to force the sensor valve
150 to its second operating position, will be effective to move the
sensor valve 150 to third operating position 159.
In the third operating position, pressure from the conduit 165 is
applied to the valve passage 160 and is then applied to the
cylinder 19 (of FIG. 1) via the passage 161 to decrease the
displacement of the pump and to the passage 162 and the exhaust
port 157A to bypass excess fluid.
A fourth operating position may be utilized to obtain a more nearly
idealized combination of large flow area and short actuating
stroke. The box 163 shows this fourth operating position as having
an additional bypass passage 164. The value of this fourth
operating position will be more apparent from a study of the
configuration shown in FIG. 4A.
FIG. 4A shows bypass type sensor valve 170 comprising a body 171
having a bore 172 and a valve spool 173. The valve spool 173 is
spring pressed to the left by a spring 174 and a spring adapter 177
in the cavity 175. The cover 176 is attached to body 171 by any
suitable method to form an end of cavity 175 and to retain the
spring 174.
The valve spool 173 of the sensor valve 170 is shown with the valve
spool 173 in the neutral position so that fluid in the conduit 51
and in the cylinder 19 (FIG. 1) is blocked by the land 178 of spool
173.
Movement of the valve spool 173 to the left, to a first operating
position, is effective to connect the conduit 51 to the sump 52 via
the port 181, the reduced diameter portion 179, and the passage
180A.
In a second operating position, the land 178 is moved to the right
allowing communication between the conduit 41 and the conduit 51
via the port 181. Pressure fluid is thus added to the large
cylinder of the displacement control mechanism to decrease the
displacement of the pump.
Additional movement of the spool 173 to the right is effective to
bypass excess pump flow by communicating the conduit 41 with the
sump 52 via the longitudinal hole 182, the cross-hole 185, the
reduced diameter 183, and the passage 180. Only a limited flow can
be bypassed via longitudinal hole 182. However, this construction
has the advantage of requiring a very small distance between the
second and third operating positions.
A fourth operating position may be added in which an additional and
greatly increased bypass flow path may be achieved. In the fourth
operating position, flow from the conduit 41 is communicated
directly to the passage 180A by the land 178 moving to the right,
and opening the bore 172 to the passage 180A.
It is apparent that the bypass type sensor valve provides the same
function as does bypass valve 81 of FIG. 1; and that it is
responsive to the same signals; that is, flow generated pressure
drops across the variable orifice 39 or negative signals from the
over-ride logic circuit.
Although the pump has been shown to include a displacement control
mechanism having two control pistons, it is to be understood that
other types of pumps having a variable discharge and other types of
discharge or displacement control mechanisms can be used without
departing from the spirit of the invention. For example, it is not
unusual to build pump displacement control having only a single
control piston, and a bias spring or an offset trunnion to bias the
swash plate in one direction.
* * * * *