Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
BRAKE CYLINDER LIMITING VALVE
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
[0001] The present invention relates to rail or freight car brake systems
and, more
particularly, to a brake cylinder limiting valves for an AAR-type freight car
brake that prevents
over-pressurization of the brake cylinder.
2. DESCRIPTION OF THE RELATED ART
[0002] Control valves used in freight car brake systems, such as the DB-
60 control valve
manufactured by New York Air Brake Corporation of Watertown, New York, or the
AB-type
control valves manufactured by Wabtec Corporation of Wilmerding, Pennsylvania,
typically
supply air pressure to the brake cylinder of a freight car. If the brake
cylinder or the plumbing
between the control valve and the car has a leak, however, the brake cylinder
will not maintain
the original set pressure. In addition to brake cylinder leakage, resulting in
low brake cylinder
pressure, the brake system can leak into the brake cylinder, resulting in high
brake cylinder
pressure.
[0003] Brake control systems on rail or freight cars that comply with AAR
standards are
referred to as displacement type system and the brake cylinder pressure is
proportional to the size
of the auxiliary reservoir and brake cylinder volumes, which are proscribed by
AAR regulations
and controlled by means of the brake control valve. Control of the brake
cylinder pressure is in
response to modulation of the brake pipe pressure by the train driver.
Although these systems
are very reliable, they operate in an open loop mode with the brake cylinder
pressure being the
result of the relationship between auxiliary reservoir and brake pipe
pressures. As a result, there
is no feedback of brake cylinder pressure for the purpose of closed loop
control. Leakage into or
out of the brake cylinder may therefore result in brake cylinder pressures
that are higher or lower
than desired without any recognition by the system that the pressures are
abnormal. While
recently improvement to AAR brake systems include the addition of brake
cylinder maintaining
valves that compensate for brake cylinder leakage, the issue of brake cylinder
over-
pressurization is still a problem and may occur as the result of leakage in
the quick service
limiting valve, in the auxiliary reservoir, in the emergency reservoir, in the
auxiliary reservoir, or
in the brake pipe pressure into the brake cylinder while the brakes are
applied.
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[0004] A brake failure that results in over-pressurization of the brakes
on a car in train is
very hazardous and may result in "hot wheels," which damages the wheels and
raises the
potential for a subsequent wheel failure and even train derailment. The train
driver is usually
unaware that a car has over-pressurized brakes due, in part, to the length of
the train and the
number of cars in the train. The only existing method of addressing this
problem is to install a
network of hot wheel detectors along a predetermined location in the
continental rail system that
can detect a hot wheel on a car using a thermal sensor, identify the car ID
using an RFID tag, and
then send an alarm to a dispatch center so that a dispatcher can contact the
train driver. Such
systems are costly, require significant modifications to the existing
infrastructure, and are limited
in geographic scope. As a result, rail car mounted system that can prevent
over-pressurization of
the brake cylinder and avoid the resulting hot wheel problem would be a
significant safety
improvement.
BRIEF SUMMARY OF THE INVENTION
[0005] The present invention comprises a brake cylinder limiting valve
having a first
portion that determined actual brake cylinder pressure and a second portion
that determined
intended brake cylinder pressure, and then allows for venting of the brake
cylinder pressure if the
actual brake cylinder pressure exceeds intended brake pipe pressure by a
predetermined
threshold. The intended brake pipe pressure is determined based on a reduction
in brake pipe
pressure relative to emergency reservoir pressure and the preferred threshold
for venting is a
brake cylinder pressure that is more than two and one-half times the brake
pipe pressure
reduction, plus a nominal amount for tolerance.
[0006] In one embodiment, the first portion comprises a first chamber in
communication
with a source of brake cylinder pressure, a second chamber in communication
with atmospheric
pressure, and a first diaphragm separating the first and second chambers and
having a first wetted
area, with the diaphragm configured to open a brake cylinder pressure exhaust
port against the
bias of a spring. The second portion comprises a third chamber in
communication with a source
of brake pressure, a fourth chamber in communication with a source of
emergency reservoir
pressure, and a second diaphragm separating the third and fourth chambers and
having a second
wetted area that is greater than the first wetted area by a threshold ratio,
where the diaphragm is
moveable to impart a second force via a floating pin that also biases the seat
into the closed
position. Thus, the brake cylinder pressure in the first chamber will be
exhausted when it
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overcomes the bias force of the spring and any bias force being applied by the
second
diaphragm. The wetted area ratio of the second diaphragm to the first
diaphragm is preferably
2.5 to 1, thereby providing for the same ratio of brake pipe pressure
reduction to brake cylinder
pressure increase required in an AAR compliant braking system. The brake
cylinder limiting
valve may be interconnected to the existing 4-port testing interface of a pipe
bracket, or
integrated into any number of locations in a conventional brake control valve.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)
[0007] The present invention will be more fully understood and
appreciated by reading
the following Detailed Description in conjunction with the accompanying
drawings, in which:
[0008] Fig. 1 is a schematic of a brake cylinder limiting valve according
to the present
invention;
[0009] Fig. 2 is a perspective view of an AAR control valve retrofitted
in a first
configuration with a brake cylinder limiting valve according to the present
invention;
[0010] Fig. 3 is a perspective view of an AAR control valve retrofitted
in a second
configuration with a brake cylinder limiting valve according to the present
invention;
[0011] Fig. 4 is a perspective view of a brake cylinder limiting valve
adaptor according
to the present invention for interconnecting to an AAR control valve; and
[0012] Fig. 5 is a schematic of a brake control valve showing three
alternative locations
for the installation of a brake cylinder limiting valve according to the
present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0013] Referring now to the drawings, wherein like reference numerals
refer to like parts
throughout, there is seen in Fig. 1 a brake cylinder limiting valve 10 for
preventing over-
pressurization of a brake cylinder. Valve 10 is a 2.5:1 differential pressure
limiting valve which
has a first portion that pneumatically determines the intended brake cylinder
pressure and a
second portion that compares the intended brake cylinder pressure to the
actual brake cylinder
pressure. The 2.5:1 differential pressure is selected to account for the ratio
of brake pipe
pressure to brake cylinder pressure required to be in an AAR compliance
system. More
specifically, because of the volumetric relationship between the auxiliary
reservoir and the brake
cylinder in an AAR braking system, a reduction in the brake pipe pressure will
cause an increase
in brake cylinder pressure which is 2.5 times the brake pipe reduction. For
example, when an
operator makes a 10 psi brake pipe reduction to actuate the brakes, the brake
cylinder pressure is
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increased by 25 psi. Thus, it should be recognized that the present invention
could be configured
for a different differential pressure as desired or required by a non-AAR
compliant system or
system having different requirements.
[0014] As seen in Fig. 1, valve 10 comprises a first port 12 in fluid
communication with a
source of brake cylinder pressure BC, a second port 14 in fluid communication
with an exhaust
EX (atmospheric pressure), a third port 16 in fluid communication with a
source of brake pipe
pressure BP, and a fourth port 18 in communication with a source of emergency
reservoir
pressure ER. Valve 10 further comprises a first diaphragm 20 separating a
first chamber 22 that
is in communication with first port 12 from a second chamber 24 that is in
communication with
second port 14 and exhaust EX. A spring 26 biases diaphragm 20 to move a seat
28 positioned
thereon to selectively opens and close communication between first port 12 and
an exhaust port
30. Spring 26 is configured to provide the equivalent biasing force of between
5 and 10 psi.
[0015] A second diaphragm 32 is positioned in valve 10 to separate a
third chamber 34 in
communication with third port 16 and brake pipe pressure BP from a fourth
chamber 36 in
communication with fourth port 18 and emergency reservoir pressure ER.
Movement of second
diaphragm 32 is communicated to first diaphragm 20 via a floating pin 38,
thereby allowing a
decrease in brake pipe pressure BP to adjust the amount of force necessary to
open seat 28. The
wetted area of second diaphragm 32 separating the emergency reservoir pressure
ER chamber 36
and brake pipe pressure BP chamber 34 is selected to be about 2.5 times the
wetted area of first
diaphragm 22. As a result, valve 10 will not open seat 28 and vent brake
cylinder pressure BC to
exhaust port 30 unless brake cylinder pressure BC in chamber 22 exceeds both
the bias force of
spring 26 and 2.5 times any force applied to diaphragm 20 by pin 38 and
diaphragm 32, which is
the amount of reduction of brake pipe pressure BP in chamber 34. Thus, the
first portion of
valve 10 comprises an actual brake cylinder pressure feedback that is compared
against the
intended brake pipe pressure as determined by brake pipe pressure. As a
result, valve 10 can
determine whether the actual brake cylinder pressure exceeds the intended
brake cylinder
pressure and exhaust the brake cylinder if it is over pressurized by an amount
equal to the bias
force of spring 26.
[0016] Exhaust port 30 is preferably connected to the inlet 40 of an
exhaust valve 42
having a pilot 44 in communication with brake cylinder pressure BC that acts
against a valve
spring 46 to selectively connected exhaust port 30 with an exhaust EX. Valve
spring 46 is
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configured to provide a biasing force equal to about 20 psi and thus will
close exhaust valve 42 if
brake cylinder pressure BC falls below about 20 psi. Conventional AAR brake
systems include a
retainer valve that, when manually activated, will bottle up the brake
cylinder pressure by sealing
the brake cylinder exhaust. This allows the train driver to bottle up the
brakes on the cars, and
then make a release and recharge of the brake pipe and all of the control
valves on the train while
the retainer bottles brake cylinder pressure. Retainers are typically used
while descending long
grades. By AAR standard, the retainer will bottle 20 psi in the high pressure
setting. Exhaust
cut-off valve 42 thus disables the brake cylinder limiting valve in retainer
operations to comply
with AAR standards.
[0017] In release and recharge, both the emergency and auxiliary
reservoirs are
pressurized to the brake pipe pressure, usually 90 psi. During a service brake
application, the
emergency reservoir pressure is unchanged from the original charge state. The
brake cylinder
limiting valve thus uses the difference between the emergency reservoir
pressure and the brake
pipe pressure to determine the brake pipe reduction, which is the brake
command signal. The
brake reduction is thus compared to the actual brake cylinder feedback
pressure.
[0018] As explained above, during a normal brake application the brake
cylinder pressure
BC will be about 2.5 times the brake pipe reduction. Brake cylinder limiting
valve 10 will
therefore be in force balance and exhaust port 30 will be held closed by valve
spring 26, which
has a nominal preload of between about 5 and 10 psi. This preload prevents
undesired leakage
from the brake cylinder limiting valve 10 in the balanced state, and
accommodates tolerance
variations of the brake system. If brake cylinder pressure BC increases as a
result of any
undesired leakage into the brake cylinder, such as from the brake pipe, the
auxiliary reservoir, or
the emergency reservoir, and does so in an amount equal to or greater than the
value of spring
26, first diaphragm 20 will move downwardly, as seen in Fig. 1, thereby
opening seat 28 and
allowing brake cylinder pressure in chamber 22 to escape out of exhaust port
30.
[0019] In an emergency brake application, brake pipe pressure is vented
to zero psi and
the emergency and auxiliary reservoirs and brake cylinder pressures are at
equilibrium. Due to
the ratios of the wetted areas in brake cylinder limiting valve 10, exhaust
port 30 is held firmly
closed by seat 28.
[0020] While Fig. 1 shows a brake cylinder limiting valve 10 having
flexible diaphragms
20 and 32, as well as floating pin 38 to provide force communication, the
function of brake
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cylinder limiting valve 10 could be implemented using other comparable valve
structures, such
as a combination of pistons and seals that provide the requisite 2.5 to 1 area
ratio between the
actual brake cylinder feedback portion and the intended brake cylinder
pressure determining
portion.
[0021] As seen in Fig. 2, valve 10 may be provided in a module 50 adapted
for
interconnection to a single-sided pipe bracket 52 via the existing 4-port
interface 54 that is
provided for periodic connection to a single car testing device. 4-port
interface 54 includes
conduits that provide for fluid communication to brake pipe pressure BP,
auxiliary reservoir
pressure AR, emergency reservoir pressure ER, and brake cylinder pressure BC
and can thus
provide all needed inputs for valve 10. In Fig. 2, module 50 is connected
directly to 4-port
interface 54 of pipe bracket 52. As a result, module 53 would have to be
removed so that a
single car testing device could be connected to 4-port interface 54 for
periodic testing of the
braking system.
[0022] As seen in Figs. 3 and 4, valve 10 may be incorporated into an
module 60 that is
attached directly along a first side 66 to 4-port interface 54 and that
contains a series of conduits
62 formed therein to provide fluid communication to valve 10 as well as to a
corresponding set
of ports 64 on a second side 68 that allow a conventional testing device to be
attached to module
60 for periodic testing purposes. Although module 60 is shown in Fig. 3 to be
attached to 4-port
interface 54 with valve 10 above pipe bracket, module 60 could be configured
to position valve
below pipe bracket 52. As further seen in Fig. 3, a test adaptor 70 may be
bolted over adaptor
60 to allow for connection to a single car testing device.
[0023] It should be recognized by those of skill in the art that valve 10
may be configured
into any portion of a braking system control valve, such as by redesigning the
packaging of the
control valve, as a module that interfaces to the release valve interface, or
as a module fitted
between either the service portion and the pipe bracket or the emergency
portion and the pipe
bracket (or by including valve 10 in any other location that has pneumatic
access to brake pipe,
emergency reservoir, and brake cylinder pressures). As seen in Fig. 5, valve
10 may be
integrated into one of at least three different locations, Alt 1, Alt 2, and
Alt 3, respectively, of a
control valve 72.
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