U.S. patent number 3,813,052 [Application Number 05/351,098] was granted by the patent office on 1974-05-28 for web tension control system.
This patent grant is currently assigned to Arcata Graphics Corporation. Invention is credited to Gerald W. Siminski, Clifford Swann.
United States Patent |
3,813,052 |
Swann , et al. |
May 28, 1974 |
WEB TENSION CONTROL SYSTEM
Abstract
A system for use in controlling the substitution of a
replacement roll of paper for an expiring roll of paper, while
maintaining essentially uniform tension conditions in the paper web
forwarded to a web printing machine. The system utilizes dual web
tension responsive controls for controlling web tension during the
roll replacement or transition stage, and provides an emergency
stop control for arresting rotation of both of the replacement and
expiring rolls.
Inventors: |
Swann; Clifford (Fort Erie,
Ontario, CA), Siminski; Gerald W. (Buffalo, NY) |
Assignee: |
Arcata Graphics Corporation
(Depew, NY)
|
Family
ID: |
23379558 |
Appl.
No.: |
05/351,098 |
Filed: |
April 13, 1973 |
Current U.S.
Class: |
242/420.3;
242/421.5; 242/421.6; 242/554.5; 242/555.7 |
Current CPC
Class: |
B65H
23/085 (20130101); B65H 19/1821 (20130101); B65H
19/1868 (20130101) |
Current International
Class: |
B65H
19/18 (20060101); B65H 23/08 (20060101); B65H
23/06 (20060101); B65h 019/00 (); B65h
021/00 () |
Field of
Search: |
;242/58.1,58.2,58.3,58.4,75.41,75.42,75.43
;156/502,504,505,509 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Watkins; Donald E.
Assistant Examiner: Jillions; John M.
Attorney, Agent or Firm: Bean & Bean
Claims
We claim:
1. In a system for controlling operation of a device for feeding a
web to a printing machine, wherein said device includes a rotatable
reel for supporting an expiring roll from which said web is
withdrawn for passage to said printing machine when said expiring
roll is in "run-off" position and at least one replacement roll
movable upon rotation of said reel successively through a "pasting"
position and into said "run-off" position to replace said expiring
roll when said web is substantially expired therefrom; a web
splicing and severing mechanism for splicing said web passing from
said expiring roll to a leading end of a web carried by said
replacement roll upon movement of the latter into said "pasting"
position and thereafter severing said web between a resultant
splice and said expiring roll; a web tensioning mechanism arranged
for surface engagement with said web when withdrawn from said
expiring roll; a transition brake associated with said replacement
roll for applying transition tension to said web withdrawn
therefrom after severing of said web as aforesaid and until said
replacement roll is disposed in said "run-off" position; web
tension sensing means movable in response to changes in web
tension; and a pneumatic control circuit means responsive to
movement of said web tension sensing means for adjustably
controlling the pressure of air supplied to said web tensioning
mechanism and said transition brake whereby to vary web tension,
characterized in that the pressure of air required by said
transition brake to maintain a given web tension is substantially
less than that required by said web tensioning mechanism to
maintain said given web tension, the improvement wherein said
control circuit means comprises:
first conduit means for connecting said web tensioning mechanism in
flow communication with a source of air under a given pressure and
incorporating a valve responsive to movements of said web tension
sensing means for adjustably controlling pressure of air applied to
said web tensioning mechanism;
second conduit means for separately connecting said transition
brake to said source and incorporating means for reducing pressure
of air available for supply to said transition brake to a given
value substantially below said given pressure; and
a branch conduit connected into said second conduit means
intermediate said pressure reducing means and said transition
brake, said branch conduit incorporating a dump valve controlling
communication of said branch conduit with the atmosphere for
reducing pressure applied to said transition brake to a value below
said given value, said dump valve being responsive to said web
tension sensing means.
2. In a system according to claim 1, wherein an other branch
conduit is connected into said second conduit means intermediate
said branch conduit and said transition brake, said other branch
conduit incorporating a valve for slowly bleeding air from said
second conduit means to the atmosphere.
3. A system for controlling operation of a device for feeding a web
to a printing machine, said device comprising in combination:
a rotatable reel for supporting an expiring roll from which said
web is withdrawn for passage to said printing machine when said
expiring roll is in "run-off" position and at least one replacement
roll movable upon rotation of said reel successively through a
"pasting" position and into said "run-off" position to replace said
expiring roll when said web is substantially expired therefrom;
web tension sensing means movable in response to changes in web
tension;
a web splicing and severing mechanism for splicing said web passing
from said expiring roll to a leading end of a web carried by said
replacement roll upon movement of the latter into said "pasting"
position and thereafter severing said web between a resultant
splice and said expiring roll;
a web tensioning mechanism arranged for surface engagement with
said web when withdrawn from said expiring roll;
first circuit means for connecting said web tensioning mechanism to
a source of air under a given pressure, said first circuit means
including flow control valve means responsive to movements of said
web tension sensing means for adjustably varying operation of said
web tensioning means;
a transition brake associated with said replacement roll for
applying transition tension to said web withdrawn therefrom
incident to the severing of said web as aforesaid and during
transitional movement of said replacement roll into said "run-off"
position;
a second circuit means for connecting said transition brake
directly to said source incident to severing of said web and during
transitional movement of said replacement roll as aforesaid, said
second circuit means including means for reducing pressure of air
available to said transition brake to a value below said given
pressure and second valve means connected into a portion of said
second circuit means intermediate said pressure reducing means and
said transition brake, said second valve means being responsive to
web tension increase sensing movements of said web tension sensing
means for connecting said portion of said second circuit means to
the atmosphere whereby to further reduce pressure of air supplied
to said transition brake;
an expiring roll brake;
a third circuit means including third valve means operable for
connecting said expiring roll brake to said source after the
severing of said web and removal of said expiring roll from said
"run-off" position; and
fourth circuit means connected into said source and operable in
response to a breaking of said web to supply braking air to said
portion of said second circuit means and to said third circuit
means intermediate said third valve means and said expiring roll
brake, whereby rotation of said expiring roll is arrested when said
web breaks while said expiring roll is in said "run-off" position
and rotation of said replacement roll is arrested when said web
breaks during said transitional movement, said fourth circuit means
incorporating check valve means for normally blocking communication
of said second and third circuit means with said fourth circuit
means and for blocking communication between said second and third
circuit means.
Description
BACKGROUND OF THE INVENTION
Feeding devices for use in feeding a web of paper to a web printing
machine normally comprise a roll stand or reel having a pair of
rotatable spiders whose arms carry chucks for releasably engaging
opposite ends of an expiring and one or more replacement rolls; a
tension control mechanism including a plurality of driven tension
belts for maintaining tension in a web withdrawn by the printing
machine from the expiring roll; an auxiliary roll drive for
bringing the peripheral speed of a replacement roll up to the speed
of the web; and a mechanism for pasting or splicing an adhesively
coated leading end of the web on the replacement roll to the web
passing from the expiring roll and immediately thereafter severing
the web of the latter.
In operation of these prior feeding devices, when the expiring roll
reaches some given diameter, an operator depresses a cycle start
button, which serves to rotate the reel to position the replacement
roll in a pasting position closely adjacent the running web; to
operate the auxiliary drive in order to accelerate the replacement
roll up to web speed; and to position a pasting and severing
mechanism adjacent its operating position. When the expiring roll
has become substantially expired, the operator depresses a splice
button, which initially causes the pasting and severing mechanism
to press the web against the replacement roll in order to effect a
splice and immediately thereafter to sever the web between the
splice and the expiring roll. After the web is severed, the pasting
and splicing mechanism and the auxiliary drive are removed from
their operating positions and the reel is further rotated to
position the replacement roll in the normal web "run-off" position
previously occupied by the expiring roll. A suitable pneumatically
operated transition brake is normally associated with the
replacement roll in order to supply transition tension to the web
simultaneously with the act of severing the web; this transition
brake being maintained "ON" until the replacement roll has reached
the "run-off" position and the web is under the sole control of the
tension belts. When the replacement roll is in "run-off" position,
the expiring roll or "butt" is removed from the reel and another
replacement roll is loaded in its place.
With present commercial systems constructed in accordance with the
general teachings of U.S. Pat. No. 2,596,189, a single valve
controlled by a "floating" roller is employed to control supply of
compressed air to both tension cylinders, which control tension in
the tension belts, and to the transition brake associated with the
replacement roll. A problem is encountered with present systems,
due in part to the fact that it has been found that the air
pressure required by the transition brake is often less than that
required by the tension cylinders to maintain the same web tension.
Thus, when air is supplied to the transition brake and the
transition brake goes "ON," there tends to be a rapid rise in web
tension to which the "floating" roller responds for the purpose of
lowering the applied air pressure to a value required by the
transition brake to maintain a desired web tension. As a result,
the web is subjected to a rapidly changing tension condition termed
a "tension spike," which may under certain conditions produce a
tearing or severing of the web. Further, it will be understood
that, since the same reduced air pressure is also supplied to the
tension cylinders, the belt tension is correspondingly reduced to a
value substantially below that required to maintain web tension
when the belts are the sole controlling factor.
Also, it will be understood that, as the replacement roll is moved
towards the "run-off" position, the web is stretched and is thus
subject to additional tension, which is sensed by the "floating"
roller. As a result there tends to be an additional decrease in air
pressure supplied to the tension cylinders, as well as the
transition brake. Thus, when the transition brake is turned "OFF,"
as the replacement roll approaches the "run-off" position, the belt
tension is substantially below that required to maintain a normal
web tension condition and the web tends to run away. The immediate
resultant "slack" or run-away condition of the web is sensed by the
"floating" roller and full air pressure is immediately supplied to
the tension cylinders; this resulting in the production of a large
and extremely rapid increase in web tension. As time progresses,
the web tension is of course reduced to its normal value, but
ofttimes the "tension spike" resulting from the sharp increase in
belt tension is sufficient to break the web. It will be understood
that this "tension spike" is normally substantially in excess of
the "tension spike" produced when the web is severed and the
transition brake of the replacement roll goes "ON."
SUMMARY OF THE INVENTION
In accordance with the present invention the drawbacks of the above
described commercial system are overcome by connecting the
transition brake into the air supply independently of the
"floating" roller controlled valve and placing a cam controlled
dump valve immediately upstream of the transition brake. The dump
valve is mechanically coupled to the "floating" roller such that
when the "floating" roller senses an increase in web tension air is
dumped from the transition brake circuit. A manually adjustable
bleed valve is arranged between the dump valve and the transition
brake in order to bleed a small percentage of air from the latter
whereby to reduce shock occasioned by introduction of air to the
brake, and thus cushion shocks to which the web is exposed incident
to the braking operation. The use of this bleed valve further
reduces "hunting" of this system.
With the present arrangement, that portion of the second "tension
spike" occasioned by turning "ON" the transition brake is
substantially reduced, since the air pressure initially applied to
the transition brake may be adjusted independently of the
"floating" roller controlled valve in order to provide for proper
web tension conditions. The small spiking effect or tension curve
discontinuity noted at this point, is believed to be primarily
occasioned by the web severing operation.
As the reel is rotated to position the replacement roll in
"run-off" position, the resultant slight increase in web tension
occasioned by stretching of the web is sensed by the "floating"
roller and the dump valve operated to reduce pressure on the
transition brake in order to maintain essentially constant web
tension conditions.
Since the transition brake and the "floating" roller controlled
valve supplying the tension cylinders are furnished with separate
air supplies, the belt tension tends to remain more or less
constant during the transition stage, and thus when the transition
brake is turned "OFF," the tension belts are immediately effective
to resume substantially normal tension control of the web. A slight
tension curve discontinuity noted as normally occurring at this
point in the operational cycle is far below that which will cause
the web to tear or rip.
DRAWINGS
The nature and mode of operation of the present invention will now
be more fully described in the following detailed description taken
with the accompanying drawings wherein:
FIG. 1 is a view of a web feeding device with which the control
system of the present invention possesses utility;
FIG. 2 is a diagrammatic view illustrating a control system
previously employed with web feeding devices of the type
illustrated in FIG. 1;
FIG. 3 is a diagrammatic view illustrating the control system of
the present invention; and
FIG. 4 is a graphic comparison of web tension conditions
encountered when utilizing the control systems of FIGS. 2 and
3.
DETAILED DESCRIPTION
Reference is now made particularly to FIG. 1, wherein a web feeding
device is generally designated as 10 and shown as including a reel
or roll stand 12 arranged in association with a web tension control
mechanism 14, a web pasting and severing mechanism 16, and a
replacement roll or auxiliary drive mechanism 17.
Reel 12 is formed with a pair of spiders 18, which are supported on
and spaced apart axially of a common shaft or axis 20. Although
only one of the spiders is illustrated, it will be appreciated that
each includes a plurality of arms 22, which carry chucks, not
shown, for rotatably supporting opposite ends of a plurality of
paper rolls. For purposes of reference, these rolls are designated
as being an expiring roll 24, a replacement or new roll 26, and a
second replacement roll 28; and reel 12 is shown in full line as
being in its rotatable position at which expiring roll 24 is
disposed in a normal web "run-off" position. When expiring roll 24
is disposed in "run-off" position, a web of paper 30 is withdrawn
therefrom by web forwarding mechanism associated with a printing
machine, not shown; web 30 while in transit to the printing machine
passing over a plurality of guide rolls 32 and a "floating" web
tension sensing roller 34 and being maintained under desired
tension by mechanism 14.
Web tension control mechanism 14 includes a plurality of driven,
transversely spaced endless belts 36, which are arranged in face to
face frictional engagement with the surface of web 30, as it is
withdrawn from expiring roll 24. The tension of belts 36 and thus
the resultant tension in web 30, is controlled by one or more
tension cylinders 38, which are in turn under the control of a flow
control valve 40 mechanically associated with "floating" roller 34.
Roller 34 is supported for vertically directed swinging movements
by a mounting bar 42 journaled on a pin shaft 44; a free end of bar
42 carrying a force transmitting roller 46 engageable with a
movable part 40a of valve 40.
Valve 40, which is of conventional construction, generally includes
a spring biased diaphragm carrying an exhaust valve seat, and a
double headed exhaust valve stem spring biased to normally close a
stationary air inlet valve seat arranged in alignment with the
movable exhaust valve seat. The arrangement is such that when the
"floating" roller moves to the left, as viewed in FIG. 1, as a
result of a decrease in web tension, roller 46 exerts pressure on
part 40a and thus on the top of the pressure regulating spring and
diaphragm. As the diaphragm moves downwardly, the exhaust valve
seat pushes the exhaust valve stem off its inlet valve seat and air
from an inlet conduit 48 flows through valve 40 and out to
cylinders 38 through conduit 50, whereby the cylinders are extended
to effect an increase in the tension in belts 36.
When the air pressure being delivered to tension cylinders 38
overcomes the mechanical force being exerted on the top of the
diaphragm, the diaphragm lifts and the valve stem closes against
the inlet valve seat, while the exhaust valve seat remains in a
closed condition, this cutting off further supply of air pressure
to the tension cylinders while preventing any escape of air
pressure through the exhaust conduit 52. This is the condition of
valve 40 when "floating" roller 34 senses a desired web tension
condition.
When "floating" roller 34 moves to the right in response to an
undesired increase in web tension, there results a reduction of the
mechanical force on the top of the diaphragm, and air pressure
below the diaphragm overcomes the mechanical force on top of it and
the diaphragm raises. When this occurs, the inlet valve remains
closed, but the exhaust valve seat is moved sufficiently far to
permit same to open in order to exhaust air pressure from the
tension cylinders through conduit 52 until such time as the air
pressure below the diaphram again balances the mechanical force on
top of it.
Mechanism 16 is of conventional construction and generally includes
separate "brush" and "knife" devices, not shown. The "brush" device
is operable to force web 30 into web splicing engagement with an
adhesive surface provided on the leading end of a web carried by
replacement roll 26, when reel 12 is rotated to move the
replacement roll into the position illustrated in phantom line in
FIG. 1; and the "knife" device is subsequently operable to extend
between belts 36 for the purpose of severing web 30 intermediate
the splice and expiring roll 24.
Mechanism 17, which is of conventional construction, generally
includes a power driven belt or belts selectively movable into
engagement with the surface of replacement roll 26 for the purpose
of bringing same up to the running speed of web 30 prior to
operation of mechanism 16 to effect splicing of web 30.
Reference is now made particularly to FIG. 2, which illustrates a
portion of a conventional control system used in conjunction with a
prior web feeding device of the type thus far described. In
addition to the previously described tension cylinders 38 and valve
40, this system includes an air commutator 60 arranged in
association with reel 12 to control feeding of air to brake
devices, generally designated at 24a, 26a and 28a, which are
employed for selectively braking or retarding rotation of rolls 24,
26 and 28, respectively when reel 12 is in different rotatable
positions thereof. Commutator 60 is selectively supplied with three
separate air supplies, namely, a replacement roll transition brake
or first air supply established by a conduit 62 connected in flow
communication with tension cylinder control conduit 50, which is
under the control of a solenoid operated transition control valve
64; an expiring roll brake or second air supply established by a
conduit 66 connected in flow communication with a common system air
source 68, which is under the control of a pressure regulator 70
and a "butt" brake solenoid operated valve 72; and a second
replacement roll or third air supply established by a conduit 74
connected in communication with conduit 66 intermediate regulator
70 and valve 72, which is under the control of a manually operated
lock valve 76.
Operation of the above described prior system will now be discussed
with reference to FIGS. 1, 2 and 4; it being assumed that reel 12
is initially in the full line position illustrated in FIG. 1 with
web 30 being withdrawn from expiring roll 24. At this point in
time, the tension in web 30 is controlled by operation of valve 40
in response to web induced movements of "floating" roller 34. When
expiring roll 24 has reached some given diameter, an operator will
depress a cycle start button, not shown, which serves to rotate
reel 12 into the phantom line position illustrated in FIG. 1,
wherein replacement roll 26 is disposed in a "pasting" position
closely adjacent web 30. Simultaneously therewith, mechanism 17 is
operable to accelerate replacement roll 26 up to a speed
essentially matching the running speed of web 30 and mechanism 16
is moved into an operating position closely adjacent the running
web. When expiring roll 26 has become substantially expired or
depleted, the operator presses a splice button, not shown, which
serves to initially cause the "brush" device of mechanism 16 to
press web 30 against the surface of replacement roll 26 in order to
effect a splice; such splicing operation effecting a slight
stretching of and/or braking action on web 30, resulting in the
production of a small "tension spike," which is designated as "A"
in the web tension-time curve illustrated in FIG. 4. Immediately
thereafter, the "knife" device of mechanism 16 is fired to sever
web 30 between the splice and expiring roll 24. Simultaneously
therewith, valve 64 is opened to place conduit 62 in flow
communication with conduit 50 with the result that the transition
brake 26a is turned "ON." The web severing operation and the
turning "ON" of transition brake 26a result in the production of an
enlarged spike designated as "B" in the web tension-time curve,
which under certain conditions is sufficiently large to effect
breaking or severing of the web 30 now being withdrawn from
replacement roll 26.
After the web between the splice and expiring roll 24 is severed,
mechanism 16 and 17 become inoperative and reel 12 is further
rotated to position replacement roll 26 in the "run-off" position
initially occupied by expiring roll 24. As replacement roll 26
approaches the "run-off" position, transition brake 26a is turned
"OFF," i.e., valve 64 operated to vent the transition brake to the
atmosphere, and the tension in web 30 is thereafter under the sole
control of tension belts 36. At this point in time there is
experienced an excessively large "tension spike" designated as "C"
in FIG. 4, which quite often effects breaking or severing of the
web even under otherwise ideal operating conditions.
After the web severing operation described above, valve 72 is
automatically operated in order to arrest rotation of the now
expired or "butt" roll 24, during the period of time reel 12 is
rotated to move roll 26 from its "pasting" into the "run-off"
position. After reel rotation is completed, the now expired roll
occupies the position previously occupied by roll 28; roll 28
having been moved into the original position of roll 26 to await
the next splicing and severing operation. The expired roll may now
be removed and replaced by another replacement roll; this operation
being facilitated by manually operating valve 76 in order to
momentarily lock the expired roll and/or new replacement roll
against rotation.
After extensive investigation, it has been found that the
pronounced "tension spikes" "B" and "C" are caused in part by the
fact that the value of the air pressure required by brake 26a,
associated with replacement roll 26, is substantially less than
that required by tension cylinders 38 to maintain the same web
tension. Thus, when air is supplied to brake 26a and the brake goes
"ON," there tends to be a rapid rise in web tension, i.e., "tension
spike B" to which "floating" roller 34 responds for the purpose of
lowering the pressure of air in conduit 50 to a value required by
brake 26a to maintain desired web tension. The substantial
reduction in air pressure within conduit 50 causes a corresponding
reduction in the tension applied to belts 36 by cylinders 38.
Accordingly, when replacement roll 26 is moved into "run-off"
position and transition brake 26a goes "OFF," belts 36 are
initially ineffective for controlling web tension and the web tends
to become "slack" and "run-away." Of course, this reduction in web
tension is immediately sensed by "floating" roller 34 and high
pressure applied through conduit 50 to cylinders 38. However, the
resultant large and rapid increase in belt tension, produces a
correspondingly large and rapid increase in web tension, which is
in the form of "tension spike C."
In accordance with the present invention the previously described
system is modified in the manner illustrated in FIG. 3, by
connecting conduit 62 into common supply conduit 68 and
incorporating a pressure regulator 78 in conduit 62 upstream of
valve 64, in order to reduce available air pressure to a value
providing for proper operation of transition brake 26a. Further, a
branch conduit 80, which incorporates a dump valve 82 is connected
into conduit 62 downstream of valve 64; and a branch conduit 84,
which incorporates a manually adjustable air bleed valve 86, is
connected into conduit 62 intermediate conduit 80 and commutator
60.
Again referring to FIG. 1, it will be understood that the condition
of dump valve 82 is controlled by a cam operator 88, which is fixed
for rotation with mounting bar 42 about the axis of pin shaft
44.
The overall mode of operation of the system thus far described in
connection with FIG. 3 is similar to that discussed above in
connection with FIG. 2, except that when transition brake 26a goes
"ON" the air pressure initially applied thereto is of a value
required by such brake to maintain a proper web tension condition;
bleed valve 86 serving to "cushion" the shock of operation of the
brake. As a result, the "tension spike" associated with this part
of the operational cycle is due essentially to the operation of the
"knife"device and is of an acceptable value, as indicated at "B" in
FIG. 4.
During further rotation of reel 12 to position roll 26 in the
"run-off" position, the tendency for the tension in web 30 to
increase, due to slight stretching of the web, is sensed by the
"floating" roller, with the result that dump valve 82 is
progressively opened by cam operator 88 as required to reduce the
pressure of air supplied to brake 26a and thus maintain web tension
essentially constant. It will of course be understood that slight
movements of the "floating" roller necessary to control operation
of dump valve 82 produce only slight adjustments of valve 40 and
thus the pressure of air supplied to cylinders 38 is also
maintained essentially constant and at essentially its proper value
even though the pressure of air supplied to brake 26a is reduced.
As a result, when brake 26a is turned "OFF," belts 36 are
substantially immediately effective to properly tension the web.
While system factors tend to produce a slight web tension curve
discontinuity at this point in time, which is designated as "C" in
FIG. 4, such discontinuity is well within acceptable limits.
The web tension curve illustrated in FIG. 4 is not meant to
represent any actual curve obtained from tests of the prior and
present systems. Rather, it is meant to illustrate the number and
relative magnitude of "tension spikes" occuring during the roll
change operation with each system, and to indicate that a
substantial difference in magnitude has been noted over the testing
period between the values of tension spikes "B" and "B'" and "C"
and "C'."
Again referring to FIG. 3, it will be appreciated that the present
system additionally features a novel system modification permitting
emergency braking of roll 24 during the normal web feeding
operation; and of rolls 24 and 26, during the roll change operation
in the event the web breaks or its motion is otherwise arrested
within the printing apparatus. This modification includes the
provision of a conduit 90 including a pressure regulator 92 and an
emergency responsive, solenoid operated stop valve 94, wherein
conduit 90 is connected into conduit 62 via a branch conduit 96
incorporating a check valve 98 and directly into conduit 66.
Further, a check valve 100 is provided in conduit 90 intermediate
branch conduit 96 and conduit 66; and a branch conduit 102 is
connected into conduit 66 downstream of butt brake solenoid valve
72 and incorporates an air bleed valve 104. While valve 94 could of
course be manually operated, it is preferably under the automatic
control of web condition sensing devices incorporated within the
printing apparatus.
If a web break is detected, valve 94 is opened and high pressure
air is immediately supplied to conduits 62 and 66. If rolls 24 and
26 are within their normal range of movements incident to a roll
change operation, brakes 24a and 26a are immediately turned "ON" to
arrest roll rotation. If roll 24 is in "run-off" position, only
brake 24a is turned "ON." Check valves 98 and 100 serve to prevent
coupling or flow communication between conduits 62 and 66, and air
bleed valve 104 permits bleed off of air from conduit 66 downstream
of valve 72 when valve 94 is rendered inoperable.
* * * * *