Note: Descriptions are shown in the official language in which they were submitted.
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Descrlption
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Draw Works Transmission Control
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Technical Field
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This invention relates to controls for
hydraulically operated winches, and more specifically,
hydromechanical boom or hook type la~er draw works
transmission controls.
Background Art
10 Prior art of possible relevance includes
United States Letters Patent 4,048,799, issued Sep~
tember 20, 1977 to Golan et al., and assigned to
the assignee of the present invention.
Winches, or as more generally known, draw
works, are used in a large variety of operations
and, as a consequence, there are draw works construc-
tions available with widely varying degrees of sophis-
ticated control and drive equipmen~. Some of the
more sophisticated draw works constructions are
hydraulically operated and include a hydraulic motor
for driving a draw works drum. Typically, there
is provided a hydraulically disengaged brake which
brakes the drum to prevent unduly rapid lowering
of a load to be hoisted and also mul~iple-speed,
hydraulically controlled transmissions interconnect
the drum and the drive motor therefor.
There is also provided means for regulating
the amount o~ control fluid applied to the hydraul-
ically disengaged brake to control the deyree oE
disengagement of stlch brake and thereby control the
rate oE descent oE an elevated load.
In the usual case, two types of load-
lowering are desirable. The first is the so-called
"drop and catch" lowering wherein the brake is fully
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disengaged and the load is allowed to fall at a maximum
rate. When the load has been lowered the desired
amounti the brake is once again engaged and further
lowering is arrested. The second type of lowering is
the so-called "controlled" lowering in which the brake
is only partially disengaged, thereby allowing a load
to be lowered at some controlled rate less than the
maximum speed.
During drop and catch lowering, initial
engagement of the brake to catch the load will cause
"grabbing" of the brake rather than smooth engagement.
This, in turn, communicates a considerable shock to the
internal components of the brake and draw works com~
ponents connected to the output of the brake. This
shock reduces the useful life of such components by
causing premature wear and failure thereof.
It is therefore highly desirable to prevent
such shock loading by providing for gradual application
of the brake when arrest of drop and catch lowerin~ is
desired.
During controlled lowering, however, such a
feature is not desirable since modulation of brake
application will result in additional travel of the
load after the operator applies the brake thereby
making fine control of load position difficult~ More-
over, such modulation is not necessary since changes in
torque levels are not so drastic as to cause the
detrimental shock noted above.
Disclosure of the Invention
In one aspect of the present invention, there
is provided a control for a hydraulically operated draw
works or the like having a hydraulica:lly disengaged
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brake, a transmission, a brake control valve including
an inlet and an outlet, the outlet connected to said
brake, and a source of fluid under pressure connected
to said inlet, the improvement comprising; means con-
nected between the outlet of the brake control valveand the brake for causing modulated application o~ said
brake when the brake is applied after having been fully
released, and for causing unmodulated application of
said brake when the brake is applied after having been
only partially released.
Brief Description of the Drawings
Fig. 1 is an schematic of a hydraulic control
system and illustrates mechanical details of manual
actuators therefor;
Fig. 2 is a sec-tional view of a control valve
and schematically illustrates peripheral components
utilized in the system;
Fig. 3 is a sectional view taken approximately
along the line III-III of Fig. 2; and
Fig. 4 is a sectional view o the accumulator ~`
and schematically illustrates peripheral components.
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Best Mode for Carrying Out the Invention
An exemplary embodiment of a control system
for a hydraulically operated draw works or the like is
illustrated in the drawings. With reference to Fig. 1,
a multi-speed, hydraulically controlled transmission
includes a high speed section shown schematically at 10
and a low speed section schematically illustrated at
12. As will be seen, the
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transmission including the components 10 and 12
will be of the type that when fluid under pressure
is directed to the high speed section 10, the output
speed of the transmission will be in high gear, and
when fluid under pressure is directed to the low
speed section 12, the output speed of the trans-
mission will be in the low range.
The usual draw works assemblage will
include a spring-engaged, hydraulically disengaged
brake which may be of a conventional construction
and which is illustrated schematically at 14. The
system will also include a metering pump 16 which
will be suitably coupled to the draw works drum
(not shown) through a one-way clutch (not shown)
so as to be driven thereby when the load is lowered
at a speed proportional to the rate of rotation of
the draw works drum. The purpose of the metering
pump 16 is to act as a governor and limit the rota-
tional velocity of the drum.
When the draw works system is used in a
vehicle as, for example, a pipe layer, there will
be a number of additional hydraulically controlled
components associated with the vehicle, which com-
ponents are schematically illustrated at 18 and
may include steering clutches, power transmission
and vehicle brake elements. Fluid under pressure is
provided to the system by a hydraulic pump 20, typ-
ically driven by the prime mover of the vehicle.
The pump 20 receives oil from a reservoir 22 and
directs the same, under pressure, to a junction 24.
one side of the junction 24 extends to a priority
valve 26 which, in turn, permits the Elow of fluid
to the vehicle components 1~. The other side of
the junction 24 extends to the control system of
the present invention.
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- The priority valve 26 is of conventional con-
struction and is operative to ensure delivery of fluid
to the~ control system of the present invention at a
pressure equal to or exceeding a predetermined minimum
pressure. Frequently, hydraulic fluid flow require-
ments of the vehicle components 18 will cause the
pressure to drop to a relatively low value which is
insufficient to ~aintain engagement of the components
of the transmission. The priority valve 26 prevents
such from occurring.
The control system includes a control valve,
generally designated 28, which comprises two valves in
a common housing. Manual actuators, generally desig-
nated 30, are provided for the valve 28 in, for
example, an operator area. The manual actuators 30
include, for example, a handle 32 which ~ay be grasped
by the operator to perform a variety of functions to be
described. A console within the operator area is pro
vided with a slot 34 in which the handle 32 may be
moved.
A first mechanical link, shown schematica]ly
at 36, is attached to the handle 32 and extends to the
control valve 28 to convey thereto mechanical motion of
the handle 32 directing the selection of a particular
transmission output speed. A similar linkage, shown
schematically at 38, extends to a brake control section
of the valve 28 to convey mechanical movement of the
handle 32 to the valve 28 to direct the flow of hydrau-
lic fluid under pressure to the brake 14 to control its
degree of disengagement.
~ third linkaqe, shown schematically at ~0,
extends to a motor speed and direction control system
(not shown) which is operative to control the speed oE
the hydraulic drive motor Eor the draw works as well as
its ~irectional output.
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The linkages 36, 38 and 40 may be conven-
tional in nature and, for example, in the form of con-
trol cables or linkages. It is only necessary that
the linkage 36 be responsive to movement of the
handle 32 in the right-left direction, as viewed
in Fig. 1, and nonresponsive to other directions
of movement thereof. The linkages 38 and 40 are sim-
ilar, but are responsive only to up-down movements
of the handle 32, as viewed in Fig. 1, and nonres-
ponsive to left-right movement.
The slot 34 defines a shift pattern for
the handle 32. It includes a horizontally elongated
slot 42. When, as viewed in Fig. 1, the handle 3
is disposed in the le~t-hand end of the slot 42,
the control valve 28 will direct the transmission
to select its high speed output. When the handle 32
is in the right-hand extremity of the slot 42, it
will direct the control valve 28 to select the low
spee~ range of the transmission.
At each end of the slot 42, there are pro-
vided downwardly extending slots 44 and 46~ When the
handle 32 is aligned with either of the slots 44
and 46, and depressed therein, the linkage 40 will
direct the motor speed and direction control system
to drive the drum of the draw works to elevate the
load. The degree of depression of the handle 32
in either of the slots 44 or 46 will control the
speed of the drive motor for the winch.
Also included is an upwardly extending
slot 48 intermediate the ends of the slot 42. When
the handle 32 is alicJned with -the slot 48, a direction
by the valve 28 to the transmission will cause the
latter to assume a neutral condition. As the handle
32 is elevated in the slot 48, the brake 1~ will
be allowed to progressively disengage and increase
in speed as the handle 32 moves further up the slot.
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At the same time the linkage 40 may be directed
after the backlash has been taken up to drive the -
drum motor in a direction to lower the load at a `
particular speed, but only i the load is not of
sufficient magnitude for gravity to overcome the
~riction of the draw works assemblage.
A short, downwardly extending slot 50
intersects the slot 42 intermediate its ends. When
the handle 32 is directed downwardly into the slot
50, there ~ill be a direction to the motor speed
and direction control system to energize the drive
motor for the draw works.
Whenever the handle 32 is aligned with eith-
er of the slots 48 or 50 or in between the two, the
transmission will be directed, by the val~e 28, to
remain in neutral. Thus, the use of the slot 50
enables the energization of the drive motor for the
winch while the transmission is in neutral to enable
warmup of the components without changing the position
of the load carried by the draw works. This feature
of the invention, when used, ensures excellent res--
ponse of the system in cold environments.
Returning to the junction 24, hydraulic
fluid under pressure is directed along a line 52
to the transmission control side of the valve 28 in
a manner to be described in greater detail herein-
a~ter. It is also directed -to a check valve system
5~. The check valve system 54 includes a first check
valve 56 which precludes backflow from any down-
stream component to the junction 24. Just down-
stream of the check valve 56 there i5 located a
junction 58. Connected to the junction 58 is a
check valve 60 which extends to the metering pump
16. The check valve 60 precludes discharcJe o~ an
accumulator 62 e~cept through the valve 28.
Turniny now to Fig~. 2 and 3, the construc-
tion of the control valve 28 will be described in
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greater detail. The valve 28 includes a housing
100 formed of a center housing 102, a right end hou-
sing 104 and a left end housing 106. The le~t end
hous-ng 106 receives, in a conventional fashion,
cable ends 108 and 110 of the linka~es 36 and 38,
respectively. The center housing 100 includes a
transmission control bore 112 and a brake control
bore 114~ The housing 104 includes cavities 116
which are aligned with the bores 112 and 114 and
house bi-directional spring centering assemblies
118 which are operative to center respective ones
of a transmission control spool 120 in the bore
112 and a brake control spool 122 in the bore 114
to the positions illustrated in Fi~. 2 regardless
of whéther the spools 120 and 122 are shifted to
the right or to the left.
The spools 120 and 122 have leftward ex-
tensions which extend into the housing 106 for con-
nection to the cable ends 108 and 110 whereby the
spools 120 and 122 may be shifted to the right or
to the left in their bores by manipulation of the
handle 32, as mentioned previously.
The transmission control bore 112 includes
a first outlet port 124 which may be connected to
the high section 10 of the transmission to be con-
trolled and a second outlet port 126 which may be :
connected to the low section 12 of the transmission.
Intermediate the outlet ports 124 and 126 is an
inlet port 128 which is connected to the junction
2~ (Fig. 1). On the sides of the outlet ports 12~and 126 opposite from the inlet port 128, the bore
112 :is provided with drain ports 130 and 132, res-
pectively, which dr~in ports are also common to the
brake control bore 11~ and which are connected to
the reser~oir 22.
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The spool 120 includes spaced lands 134 and
136. Dependent upon the position of the spool 120
within the bore 112, the land 134 will either preclude
fluid communication between the ports 124 and 128 or
the ports 126 and 128. The land 136 will either pre-
clude fluid communication between the ports 126 and 128
or the ports 126 and 132. In the position of the valve
illustrated in Fig~ 2, which corresponds to a position
directing the transmission to be in neutral, the lands
134 and 136 block the flow of pressurized fluid into
either of the transmission sections 10 and 12, while at
the same time allow fluid flow fro~ those sections to
the reservoir 22 through the drain ports 130 and 132
respectively.
To command the transmission to operate in its
low range, the handle 32 i~ moved to the right in the
slot 42, as mentioned previously. This will cause a
commensurate shift of the spool 120 to the right within
the bore 112. This, in turn~ will establish fluid com-
munication between the inlet 128 and the outlet 126.
Flow to drain through the port 132 is blocked by the
right-hand side of the land 136 in such a case, while
flow to drain through the port 130 continues because
the land 140 does not move far enough to block com-
munication between the outlet port 124 and the drain
port 130. Flow from the inlet 128 to the outlet 124
continues to be blocked but now by land 222 instead of
land 134.
Conversely, when the handle 32 is shifted to
the left, as viewed in FigO 1, to direct the trans-
mission to operate in its high range, the spool 120
will shift to the left within the bore 112 from t.he
position shown. At this time, the land 222 will shift
to the le~t to preclude fluid communication between the
port 124 and the drain port 130
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while the land 134 enables fluid flow from the inlet
port 128 to the port 124. The rather long axial length
of the land 136 will continue to block -the flow of
fluid to the outlet port 126.
Turning now to the brake control section of
the valve 28, the brake control bore 114 includes a
cavity 150 which is connected to junction 58 for
receipt of flu d under pressure. Just to the right of
the cavity 150 as seen in Fig. 2, is an outlet port 152
which is adapted to be connected to both a brake accum-
ulator 174 and to the metering pump 16, as shown in
Fig. 4. The outlet port 152 is disposed between the
cavity 150 and the outlet port 132 which extends to the
reservoir 22.
The spool 122 includes a land 158 having a
relatively long axial length which is normally opera-
tive to preclude the flow of fluid from the cavity 150
to the outlet port 152 while allowing flow of fluid
from the outlet port 152 to drain through the drain
port 132 or to interrupt fluid communication between
the drain port 132 and the outlet port 152 and allow
fluid to flow from the cavity 150 to the outlet port
152 under circumstances to be described in greater
detail hereinafter.
As seen in Figs. 2 and 3, the land 158
includes oppositely disposed, axially extending grooves
160, 162 and 164 in its periphery. Each of the grooves
160, 162 and 164 opens to the inlet side of the land
158 and, as can be best seen in Fig. 2, the groove 160
has a relatively long axial length, the groove 162 has
an intermedia~e axial length, while the groove 164 has
a relakively short axial length. As seen in Fig. 3,
the grooves 162 and 164 have re~atively large cross
sections, while the groove 160 has a relatively small
cross section.
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Moreover, all three grooves have a progressively
decreasing cross section from left to right~
In the case of a brake in a draw works,
it is desired that there be an infinite number
of degrees of disengagement so that the speed of
descent of the load can be regulated. The grooves
160, 162 and 164 serve as metering grooves to assist
in attaining such a degree of brake disengage~ent
control. Specifically, the further the spool 122
is moved to the right, as viewed in Fig. 2, the
greater the fluid flow from the cavit~ 150 to the
outlet 152 through the groove 160. The greater the
fluid flow, the greater the degree of disengagement
of the brake 14 which, it will be recalled, is of
the hydraulically disengaged type. For greater
rightward shifts of the spool 122 within the bore
114, fluid communication between the cavity 150 and
the port 152 will be established through the larger
groove 162 so that fluid flow will be less restricted.
For even greater rightward shifts of the spool 122,
: fluid communication between the cavity 150 and port
152 will be established through all three grooves
160, 162 and 164. Fluid flow will then be at its
maximum allowing the load to be lowered at its
maximum rate.
To enable the operator to feel when the
condition of maximum lowering rate is being approached,
a spring 166 has been included to provide positive
feedback to the operator. As the leading edge of
the groove 164 approaches the leftmost surface of
cavity 168 which leads to port 152, the washer 167
contacts a shoulder 170 of the bore 114. An~ further
rightward movement of spool 122 compresses the spring
166 and provides for a positive operator feel w.hen
the maximum speed groove 16~ has been entered~
As best seen in Pigs. 1 and 4, ~luid ~low-
ing from the port 152 w~ll flow to a junction 172
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and simultaneously to the metering pump 16 and a brake
accumulator, generally shown at 174.
As explained above, the brake is of the spring
engaged-hydraulically disengaged type and therefore the
brake 14 may be disengaged, and the load lowered, only
by supplying brake 14 with hydraulic Eluid from the
port 152. In such a brake, the greater the fluid pres-
sure the greater the degree of disengagement and the
greater the quantity of fluid required to cause pro-
gressive disengagement. As shown in ~igs. 1 and 4, the
brake 14 can only be supplied with hydraulic fluid
through the brake accumulator 174.
~ydraulic fluid metered through the grooves
160, 162 and 164 is supplied to a brake accumulator ~ -
port 176 in a housing 178 of the accumulator 174.
Located within the housing 178 is a bore 180 which con-
tains a piston 182. The piston 182 has a relatively
thin wall 184 which is perforated by a number of
passageways 186. As viewed in Fig. 4, the piston is
urged to the left by springs 188 and 190.
When fluid is supplied to the port 176, it
will flow through the piston passageways 186 and into a
spring pocket 192. The spring pocket 192 is ultimately
connected to the brake 14 by a passageway 194, a check
valve 196, an accumulator chamber 198 and an accumu-
lator port 200.
Located within the brake 14 is a brake release
piston 202, a brake return spring 204 and a brake
cylinder 2060 The brake 14 is otherwise conventional
in construction and need not be further explained. It
is sufficient to note that when the piston ~02 is in
the leftmost position, as viewed in Fig. 4, the brake
14 will be fully applied and as the piston 202 is
forced an increasing distance to the right by pres
surized Eluid in the cylinder 206, the brake 14 will be
increasingly released.
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The piston return spring 204 creates a force
opposing fluid pressure in the cylinder 206. There-
fore, if fluid pressure in chamber 206 is insufficient
to overcome the force created by the sprin~ 204, the
piston 202 will be urqed to the left and the brake 14
will be applied.
As stated above, the pressure of the hydraulic
fluid supplied from the outlet port 152 to the accumu-
lator port 176, and consequently the brake 14, may be
varied by operator actuation o~ the brake spool 122.
Depending on the pressure o~ fluid supplied to the
brake 14, brake release will be accomplished by one of
two methods. If fluid supply pressure is high, the
force on the piston 202 will be sufficient to com-
pletely overcome the force created by the spring 204.~n this case, the brake piston 202 will be forced fully
to the right and the brake 14 will he completely
released. As a consequence of this large piston 202
movement, a large volume o~ hydraulic fluid will enter
the brake cylinder 206.
A relatively low hydraulic ~luid pressure will
be insufficient to completely overcome the force
created by the return spring 20~ and the brake piston
202 will be forced only a shoft distance to the right~
This piston movement will be insufficient to completely
release the brake 14 but will cause slippage. In this
situation, the volume of fluid entering the piston
cylinder 206 will be relatively small.
As will be seen in Fig. 2, when the brake
spool 122 is returned to the neutral position, the out-
let port 152 will be connected to the relieE port 132
and conseq~lently to the tank 22. This will cause the
pressure of the fluid supplied to the accumulator port
176, and consequently the spring chamber 192, to drop
to a very low level. The pressurized fluicl
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remaining in the accumulator chamber 198 and the brake
cylinder 206 will then cause the check valve 196 to
seat, precluding reverse fluid flow through the spring
chamber 192. All flu.id from the brake 14 will thus be
forced through the chamber 198 and towards the left end
of the piston 182.
Located on the piston 182 are piston surfaces
208 and 210 upon which the fluid located in the accumu-
lator chamber 198 will act. Located within a piston
wall 212 is a metering orifice 214 which establishes
fluid communcation between the accumulator chamber 198
and the accumulator port 176 through the piston pass- :
ageways 186.
When the pressure of fluid supplied to the
15 brake 14 is relieved, reverse flow to the tank 22 ~-
through the ports 152 and 132 will be accomplished by
one of two modes depending upon the volume of fluid
contained in the piston cylinder 206.
If the pressure, and therefore the volume, of
fluid located in the brake cylinder 206 is low, as is
the situation during controlled lowering when the brake
14 is only partially released, there will be insuffi-
cient fluid in the accumulator chamber 198 and brake
cylinder 206 to fully compress the spring 188 and 190.
Therefore, while the accumulator piston 182 will travel
a slight distance to the right, this travel will not
compress the springs 188 and 190 far enough to create a
fluid backpressure within the accumulator chamber 198
and brake cylinder 206 sufficient to cause slippa~e of
the brake 14. Any fluid located in the accumulator
chamber 198 and the brake cylinder 206 will simply flow
through the metering orifice 21~ as the piston 182
slowly moves to khe left and will be ultimately
returned to the tank 22 through the control valve 28
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If the pressure, and therefore the volume, of
fluid supplied to and entering the brake 14 is large,
as when the brake 14 is fully releasedF reverse flow to
the control valve 28 will be somewhat different~ In
this situation, fluid pressure in the accumulator
chamber 198 and the brake cylinder 206 will be suffi-
cient, when acting on the accumulator piston surfaces
208 and 210, to fully compress the springs 188 and
190~ As the piston surface 208 moves beyond a port
edge 216, ~he accumulator piston 182 will act as a
relief valve and allow a large volume of fluid to flow
to the accumulator port 176. As the residual pressure
in the brake 14 and the accumulator chamber 198
decreases, the spring 188 and 190 will once again force
the piston 182 to the left. The springs 188 and 190
are chosen such that the force on the piston 182 will
cause a backpressure in the accumulator chamber 198 and
the brake cylinder 206 sufficient to main~ain slippage
. in the brake 14.
Once the piston 182 has been urged a short
distance to the left~ the accumulator chamber 198 will
no longer be in direct fluid communication with the
port 176 and any remaining fluid in the accumulator
chamber 198 and the brake cylinder 206 must flow
through the metering orifice 214 and the piston pass-
ageways 186. The metering orifice 214 will limit the
flow of fluid from the piston cylinder 206 and thus
cause gradual application of the brake 14.
Industrial Applicability
In operation, there will be two distinct modes
of load lowering available to the operator~ The first
is "drop and catch" lowering where it is desired to
lower the load for a distance at the
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maximum rate available and then arxest lowering
at a particular point. The second mode is "con-
trolled" lowering where it is desired to lower
a load at a speed somewhat less than the maximum
5 available. As described above, when lowering at
the maximum rate is desired, the brake spool 122
will be in the rightmost position as viewed in
Fig. 2. High pressure fluid will be allowed to
flow through the grooves 160, 162 and 164 from
1~ the cavity 15~ to the outlet port 152. As shown
in Fig. 4, fluid flow from the outlet port 152
will proceed to the junction 172 and then to the
accumulator port 176 and the metering pump 16.
Upon reaching the accumulator port 176, the fluid
15 flows through the piston passages 186, the spring
pocket 192, the passageway 194, the check valve
196 and ~inally through the accumulator port 200
to the brake cylinder 206. The accumulator chamber
198 will become completely filled during this
20 process.
Since fluid pressure and flow is at
its maximum, the brake piston 202 will be forced
completely to the right and the brake 14 will be ?
~ully released. The load will therefore drop at
25 its maximum rate.
When~it i5 desired to stop descent of
the load, the brake spool 122 is shifted to the
neutral position as shown in Fig. 2. As indicated
above, the outlet port 152 will then be in fluid
30 communication with the drain port 132, and conse-
quently, the tank 22. The accumulator spring pock~
et 192 and the passageway 194 will thus be drained
by the drain port 132. Fluid pressure in the accu-
rnulator chamber 198 and the piston cylinder 206
35 will then cause the check valve 196 to be completely
seated.
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Since the check valve 196 is closed, all
hydraulic fluid to be discharged from the brake 1
must move the accumulator piston 182 against the
springs 188 and 190. Since the fluid pressure in
the accumulator chamber 198 and the piston cylinder
206 is high, the fluid acting upon the piston sur-
faces 208 and 210 will create a force sufficient
to fully compress the springs 188 and 190.
~s the piston 182 moves to the right,
the piston surface 208 will clear the edge 216 of
the port 176 and the piston 182 will act as a
relief valve. As the pressure of fluid in the ac-
cumulator chamber 198 and the piston cylinder 206
decreases, the springs 188 and 190 will force the
piston 182 to once again move towards the left,
closing off accumulator port 176.
Since the springs:~.l88 and 190 are greatly
: compressed, they will cause the piston I82 to act
upon fluid contained in the accumulator chamber
198 with a force sufficient to cause a fluid back-
pressure in the accumulator chamber 198 and the
piston cylinder 206 such that the brake 14 will
continue to slip.
With the accumulator chamber 198 no longer
in direct fluid communication with accumulator port
176, any further fluid flow from the accumulator
chamber 198 and the brake cylinder 206 must be
through the metering orifice 214. The limited flow
through the metering orifice 21~ will cause the
gradual re}ief of fluid pressure in the brake cyl-
inder 206 and thus provide modulated brake appli-
cation and gradual arrest of the load.
During controlled lowering, :Eluid flow
from the cavit~ 150 to the outlet 152 w:ill be
throu~h less than all of the groo~es 160, 162 and
16~. I'his will generate a fluid pressure in the
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brake cylinder 206 which will be less than that
required to completely release the brake 14. However,
fluid pressure in the cylinder 206 will be of a magni-
tude that will cause slippage and limited ro~ation of
the brake 14. Since the brake piston 202 will move
~nly a slight distance to the righ~, a relatively small
volume of fluid will be contained in the brake cylinder
206.
When the brake valve 122 is returned to the
neutral position, fluid flow from the accumulator
chamber 198 and the piston cylinder 206 will once again
be limited to the left end of the piston 182 by action
of the check valve 196.
Since during controlled lowering the pressure
and volume of fluid contained in brake cylinder 206 is
low, the volume of fluid acting on the piston surfaces
208 and 210 will not fully compress springs 188 and
190. Therefore, the springs 188 and 190 will not be
compressed a distance suf~icient to create the back-
pressure required to cause an indicated slippage of thebrake 14. Brake application will be immediate.
It will thus be seen that the addition of the
accumulator 174 to a draw works control system will
provide the dual advantages of modulated braking during
high-speed lowering and unmodulated rapid brake appli-
cation during controlled lowering.
The system also includes means wherehy the
accumulator 62 may be discharged when the pump 20 or
engine is inoperative. The fluid from the accumulator
62 is discharged through ori~ice 224 to either drain
130 to sump 22, or port 128 back through pump 20 ~o
sump 22, depending upon the position o~ spools 122 and
140.
Other aspects, objects and advantages o~ this
invention may be obtained from a study o~ the drawingsrthe disclosure and the appended claims~
,~, i,,~, . .
