Note: Descriptions are shown in the official language in which they were submitted.
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POWER TRANSMISSION
This application discloses and claims matter dis-
closed in copending United States patent application
Serial No. 024,058, filed March 26, 1979, having a common
assignee with the present patent application.
BACKGROUND AND SUMMARY
This invention relates to a hydraulic flow metering
circuit and a metering valve therefor and particularly to
such circuits and valves having multiple functions.
In the control of return fluid flow from loads applied
to hydraulic actuators such as cylinders on earthmoving and
construction vehicles, it is customary to provide spool-
type valve elements with each valve element serving various
individual functions such as speed control of lowering
loads, limiting excessive pressure, preventing or minimiz-
ing cavitation in the hydraulic cylinders, and holding a
load stationary in a preselected position.
However, use of spool-type valve elements having a
movable spool member lead to certain disadvantages, such
- 20 as instability of the spool member while throttling fluid
flow to decelerate a lowering load resulting in erratic
or ~erking movements of the load, and drifting of a sta-
tionary load from a preselected position due to leakage
of fluid past clearances necessary to proper operation of
the spool member.
- The present invention is directed to a circuit com-
prising a single hydraulic flow metering valve which will
function as a proportional speed control valve for lower-
ing loads, a pressure limiting relief valve, and an
anti-cavitation check valve.
In accordance with the present invention, a poppet
type main valve meters return fluid flow from loaded
cylinders in response to a variable orifice formed in the
main valve, which is controlled by an integral servo
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valve, and to fluid flow through a bleed flow orifice
which is controlled by a pilot valve. The metering valve
is designed so that the following three different valve
functions can be accomplished:
When the main valve is controlled by fluid flow
through the variable orifice, the servo valve provides a
machine operator with a proportional speed control for
lowering loads.
If cylinder fluid pressure drops below return fluid
pressure in the system, the main valve will open allowing
fluid from the return to flow to the cylinder thereby
operating as an anti-cavitation check valve.
The pilot valve controls bleed flow from the bleed
flow orifice through the main valve to provide operation
as a pressure limiting relief valve.
A fuller understanding of the invention may be had
from consideration of the following description and claims
taken together with the accompanying drawings.
DESCRIPTION OF T~E DRAWINGS
Fig. 1 is a schematic of the hydraulic circuit embody-
ing the invention;
Fi~. 2 is a diagramatic sectional view of a preferred
embodiment of the invention;
Fig. 3 is another embodiment of the hydraulic circuit
of Fig. 1.
DETAILED DESCRIPTION
Referring to Fig. 1, the hydraulic control valve
circuit embodying the invention includes a load pressure
line 10 adapted to be connected to a source of load
pressure and a return flow line 12 adapted to be connected
to the return flow of a pump or other source of fluid. A
main valve 14 is spring loaded to yieldingly shut off fluid
flow between lines 10 and 12. Main valve 14 is controlled
by a bleed flow through a bleed flow orifice 18 positioned
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between pressure line 10 and bleed flow end 16 of valve
14, and a variable orifice, more fully described below,
formed between bleed flow orifice 18 and return flow line
12.
A pilot valve 20 controls the bleed flow and thereby
the opening of main valve 14. Pilot valve 20 includes a
pilot piston 22 rnounted for movement in a pilot bore 24.
Pilot bore 24 includes a pressure end 26 connected to load
pressure through line lO, a bleed flow chamber 28 connected
to bleed flow end 16 of valve 14, a drain chamber 30 con-
nected to return flow, and a spring end 32 also connected
to return flow. Pilot piston 22 is yieldingly urged toward
pressure end 26 to shut off flow between bleed flow chamber
28 and drain chamber 30 by a spring member 34 positioned in
spring end 32 of pilot bore 24.
Main valve 14 is mounted for movement in a main bore
36. Main bore 36 includes a return chamber 38 formed in
one end thereof, a load pressure chamber 40 adjacent return
chamber 38, a bleed chamber 42 spaced from return chamber
38 by pressure chamber 40, and a control pressure chamber
44 formed at the opposite end of main bore 36.
Return chamber 38 is connected to the return flow
through line 12 with pressure chamber 40 connected to the
load pressure source through line 10, bleed chamber 42 is
connected to bleed flow chamber 28 of pilot bore 24 and
through bleed flow orifice 18 through bleed flow line 46,
and control pressure chamber 44 is connected to a source
of control pressure through a control line 48.
Main valve 14 includes a main piston 50 mounted for
movement in main bore 36. Main piston 50 is formed with
a head end 51 having a tapered portion 52 extending into
return chamber 38. Tapered portion 52 is adapted for
engagement with a main valve seat 54 formed at the juncture
of load pressure chamber 40 and main bore 36. A spring
member 56 yieldingly urges main piston 50 in the direction
of return chamber 38 to seat tapered portion 52 in engage-
ment with main valve seat 54 to shut off fluid flow between
load pressure chamber 40 and return chamber 38.
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Main piston 50 includes a large area or rear end 58
and head end 51 contacts valve seat 54 at a diameter less
than that of rear end 58 thereby forming a first differ-
ential area which is acted on by fluid pressure in load
pressure chamber 40. The portion of head end 51 extending
into return chamber 38 forms a second differential area
which is acted on by fluid pressure in return chamber 38.
Main piston 50 further includes a counterbore 60
formed in rear end 58 and which together with the walls of
main bore 36 define bleed chamber 42. A metering passage
62 formed through head end 51 provides a path for fluid
flow between bleed chamber 42 and return chamber 38.
A poppet type servo valve 64 having a cone shaped end
65 extends from control charnber 44 in the direction of
main piston 50 and is yieldingly urged by spring member 56
into seated engagement with metering passage 62 to shut
off fluid flow therethrough. Movement of cone shaped end
65 into or out of engagement with metering passage 62
generates a variable orifice therebetween for metering
fluid flow between bleed chamber 42 and return chamber 38
A servo piston 66 associated with servo valve 64 is
slidably mounted for movement relative thereto in control
chamber 44. Servo piston 66 is yieldingly urged by a spring
member 68 to resist movement thereof by control pressure in
control chamber 44. Under the urging of control pressure
in chamber 44 servo piston 66 acts on servo valve 64 against
the force of spring members 56 and 68 to unseat the engage-
ment of the servo valve 64 with metering passage 62.
METER~NG OPERATION AS A FLOW CONTROL VALVE
Metered flow, from load pressure to return, will be
proportional to the applied control pressure, with this
proportionality achieved in the following manner. Control
pressure, acting on the exposed area of the servo piston
66, generates a force that moves the servo piston 66 and
poppet valve 64 in a direction away from main piston 50
against the force of springs 56 and 68. Motion stops
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when the spring force becomes equal to the control pressure-
generated force. Because the springs have a linear rate,
displacement of the servo valve 64 will be proportional to
the applied control pressure.
As servo valve 64 moves, it separates from main
piston 50 allowing fluid flow through the varlable orifice
so formed and through metering passage 62 into return
chamber 38. The resulting drop in pressure across metering
passage 62 due to the variable orifice reduces the closing
force on large area end 58 of main piston 50 to a lesser
value than the opening force developed on the first differ-
ential area of piston 50 by the load pressure. The main
piston 50, therefore, will move following the motion of the
servo valve 64. As a steady-state condition, the flow area
between the servo valve 64 and the variable orifice must
result in a bleed flow rate through the variable orifice
that develops a closing pressure on large area end 58 of
main piston 50 that exactly balances the opening force on
the first differential area of main piston 50. Since the
gain ratio between the servo valve motion and bleed chamber
42 pressure is very high, the positional difference between
the servo valve 64 and main piston 50 is negligible, and
main piston 50 displacement may be considered as being
directly proportional to applied control pressure. If the
variable metering orifice in the main piston 50 is of con-
stant width, metered flow will be proportional to piston
displacement and to applied control pressure, assuming a
constant pressure differential between load and return.
OPERATION AS A RELIEF VALVE
If the load pressure exceeds a predetermined "cracking~
pressure, the pilot valve 20 will be displaced to open a
flow path from the load pressure through bleed flow orifice
18, to return. This bleed flow reduces the pressure and
closing force on large area end 58 of the main piston 50,
and in the same manner as described above for flow control,
the main piston will open, allowing flow from load to
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return. If load pressure tends to drop below the cracking
pressure, pilot valve 20 will close, causing an increase
in the pressure and closing force on the main piston 50.
- OPERATION AS AN ANTI-CA~ITATION VALVE
In the type of system for which this valve is intended,
load pressure may drop below return line pressure as the
result of an overhauling load. To prevent cavitation, the
valve is designed to open in response to such a pressure
differential, allowing fluid flow from the return line into
the load circuit.
If return line pressure in return chamber 38, acting
on the second differential area of main piston 50, generates
a force exceeding the sum of the forces developed by load
pre~sure acting on an equal area and spring 56, main piston
50 will move away from valve seat 54, opening a flow path
from return to load. A free sliding fit between servo
piston 66, and servo valve 64 and the use of two springs 56
and 68 allows the closing force exerted by servo valve 64
on main piston 50 to be held to a minimum and, therefore,
requiring a relatively small pressure differential to open
the main valve 14. Note that if servo valve 64 and servo
piston 66 were one piece and only one spring having a force
equal to the sum of forces of springs 56 and 68 were used,
the other two functions of the valve, as described above,
would not be significantly affected, but the higher spring
force of the single spring would require an unacceptably high
pressure differential to overcome it and open the valve.
Fig. 2 shows a preferred embodiment of the hydraulic
control circuit of the instant invention as a unitary
multiple function control valve wherein corresponding
elements shown in Fig. 1 are provided with a suffix a.
The control valve circuit of Fig. 1 is shown housed
in a body 70 which includes a load pressure passage lOa
spaced from a return passage 12a. A main bore 36a formed
in body 70 extends from return chamber or passage 12a
through pressure passage lOa and terminates at a bearing
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member 72 held in position in main bore 36a by an end cap
74 portion of body 70. End cap portion 74 of bod~ 70
includes a servo bore 76 spaced from main bore 36a by
bearing member 72 and terminates at a distal end 78. A
pilot bore 24a is formed in body 70 spaced from main bore
36a and includes a pressure end 26a in communication with
and extending transverse of pressure passage lOa, a drain
chamber 30a, and a spring end 32a spaced from pressure end
26a by drain flow chamber 30a.
Body 70 further includes a bleed flow passage 46a
extending between main bore 36a adjacent bearing member 72
and in communication with pressure end 26a of pilot bore
24a through bleed flow orifice 18a adjacent drain chamber
30a, a drain passage 80 interconnecting distal end 78 of
servo bore 76 and drain chamber 30a of pilot bore 24a with
return passage 12a, and a control passage 48a in communi-
cation with servo bore 76 adjacent bearing member 72.
A pilot valve 20a is mounted for movement in pilot
bore 24a and is yieldingly urged to shut off fluid flow
between bleed flow passage 46a and metered flow passage
80. Pilot valve 20a includes a pilot piston 22a extending
through pilot bore 24a between spring end 32a and pressure
end 26a. Pilot piston 22a is of two-piece construction and
includes a metering section 82 and spring end section 83.
Section 82 is formed with a tapered portion 84 positioned
in drain chamber 30a and seats in bore 24a to form a leak-
proof seal when system pressure is below the cracking
pressure level of the pilot valve. Metering section 82
also includes a shoulder portion 86 extending into pressure
end 26a of bore 24a in which one or more metering slots 87
are formed. A reduced diameter portion 88 of metering
section 82 spaces shoulder portion 86 from a head portion
90 of metering section 82 defining therebetween a bleed
flow chamber 28a and having slots 87 in communication
therewith and in which one end of bleed flow passage 46a
terminates. Bleed flow chamber 28a is in communication
with pressure passage lOa through a bleed flow orifice
18a formed in head portion 90.
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Spring end section 83 is held in abutting relation-
ship with metering section 82 by a spring member 34a and
is slightly smaller in diameter than shoulder portion 86
and head portion 90 of metering section 82. The di~fer-
ential area formed by the slight differences in diametersis acted on by load pressure in pressure end 22a generating
a force tending to open the pilot valve against the force
exerted by spring member 34a.
Spring member 34a is positioned in pilot bore 24a
between spring end section 83 and an adjustment member 94
in threaded engagement with body 70. Adjustment member 94
provides a means for varying the amount of compression of
spring 34a thereby providing for adjusting the cracking
pressure level of the pilot valve.
A main piston 50a is movably mounted in main bore 36a
to yieldingly shut off fluid flow between pressure passage
lOa and return passage 12a. Main piston 50a includes a
large area or rear end 58a and a spool section 96 extending
from a tapered portion of piston 50a into return passage 12a
and~terminates at a head end 51a. Tapered portion 52a is
adapted for seated engagement with a valve seat 54a for~ed
on body 70 at the juncture of pressure passage lOa and
main bore 36a. Spool section 96 is formed with a plurality
of radial notches 98 terminating adjacent tapered portion
52a. Tapered portion 52a is proportioned for low leakage
when in seated engagement with valve seat 54a. When fulLy
closed, poppet action at tapered portion 52a on the valve
seat 54a provides a virtually leak proof seal. As tapered
portion 52a moves away from valve seat 54a, main piston
50a behaves as a sliding or spool type valve and fluid
flow is metered through radial notches 98. By selective
dimensioning of the width of radial notches 98, it is
possible to control flow gain through the valve as con-
trasted to a very high flow gain that would result if
the main piston was a pure poppet type valve with only
poppet action between the main piston and the valve seat.
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Fluid pressure in pressure passage lOa acts on main
piston 50a on a first differential area formed by tapered
portion 52a contacting valve seat 54a at a diameter less
than that of rear end 58a. Fluid pressure in return
passage 12a acts on a second differential area of main
piston 50a formed by the radial surfaces of spool section
96 exposed to the fluid in the return passage.
Main piston 50a further includes a counterbore 60a
formed in rear end 58a and a metering passage 62a extending
from the bottom of counterbore 60a through spool section 96
into return passage 12a. Counterbore 60a, main bore 36a,
and bearing member 72 define therebetween a bleed chamber
42a in main bore 36a which is connected to return passage
12a through metering passage 62a and to bleed flow chamber
28a in pilot bore 24a through bleed flow passage 46a.
A servo valve 64a having a stem portion 100 supported
for sliding movement in bearing member 72 extends from
servo bore 76 through bearing member 72 and terminates in
coné shaped end 65a. Cone shaped end 65a is yieldingly
urged into engagement with metering passage 62a to shut
off fluid flow therethrough by a spring member 77 arranged
in bleed chamber 42a between bearing member 72 and a spring
seat 104 positioned on stem 100 adjacent cone end 65a.
Movement of cone end 65a into or out of engagement with
metering passage 12a generates a variable orifice therebetween
for metering fluid flow between bleed chamber 42a and return
passage 12a.
A servo piston 66a associated with poppet valve 64a is
mounted for movement relative thereto in servo bore 76.
Servo piston 66a includes a counterbore 106 which together
~ with bearing member 72 and the walls of servo bore 76
; define a control pressure chamber 44a in communication with
control pressure passage 48a. A relatively light spring
member 107 positioned between bearing member 72 and the
bottom of counterbore 106 keeps servo piston 66a in contact
with a shoulder portion 108 of servo valve 64a and prevents
servo piston 66a from blocking passage 48a. Under the
urging of control pressure in chamber 44a, servo piston 66a
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acts to unseat cone end 65a through shoulder portion 108
for controlling the variable orifice metering action of
the servo valve 64a.
Note that in this embodiment, spring member 77 com-
bines the functions of springs 56 and 68 shown anddescribed above in relation to Fig. 1. As previously
mentioned, the resulting higher spring force limits the
use of this embodiment as an anti-cavitation check valve.
One of the features of the control valve is low
leakage in the shut off position. The seats between main
,piston 50a and body 70 and poppet valve 64a and metering
orifice 62a can be considered as positive seals with zero
leakage. With reasonable tolerances very low leakage
rates can be maintained.
It will be apparent to those skilled in the art that
many changes may be made to the above described invention
without departing from the spirit of the invention and the
scope of the appended claims.
one such change, by way of example, is shown in Fig. 3
wherein like elements have the same reference numerals as
in Fig. 1 with the suffix b added.
Fig. 3 shows a spool type servo valve 108 in place of
the servo piston 66 and poppet type servo valve 64 of Fig.
1. Servo valve 108 includes a piston end 110 positioned in
control chamber 44b, a spool member 112 extending from
piston end 110 into and through a bore 114 formed in main
piston 50b, and having a metering passage 62b formed
therein in communication with return chamber 38b through
the end of spool member 112 positioned in bore 114. Servo
valve 108 is mounted for movement in bore 114 and is yield-
ingly urged by spring member 68b to shut off fluid flow
between bleed chamber 42b and return chamber 38b through
metering passage 62b. Servo valve 108 is operable by
control pressure applied to piston end 110 for generating
a variable orifice as passage 62b is exposed to bleed
chamber 42b thereby metering fluid flow through metering
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passage 62b between bleed chamber 42b and return chamber
38b.
Servo valve 108 has the advantages that system pressure
forces have less effect on the spool type valve force
balance than on the poppet type valve force balance and
movement of servo valve 108 is not required when the cir-
cuit is functioning as a relief valve. A disadvantage of
the spool type valve, as previously mentioned, is that it
is susceptible to leakage through the clearances between
bore 114 and spool member 112.
