Abstract:
A brake cylinder limiting valve having a first portion that determines actual brake cylinder pressure and a second portion that determines intended brake cylinder pressure based on brake pipe pressure reduction. The two portions are combined so that brake cylinder pressure will be vented if the actual brake cylinder pressure exceeds intended brake cylinder pressure by a predetermined threshold amount, which is preferably two and one-half times the brake pipe pressure reduction. An exhaust cut-off valve may be used to prevent venting of the brake cylinder pressure if it falls below a predetermined value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     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 
     Control valves used in freight car brake systems, such as the DB-60 control valve manufactured by New York Air Brake Corporation of Watertown, N.Y., or the AB-type control valves manufactured by Wabtec Corporation of Wilmerding, Pa., 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. 
     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. 
     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 
     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. 
     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 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) 
       The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic of a brake cylinder limiting valve according to the present invention; 
         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; 
         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; 
         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 
         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 
     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 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. 
     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 closes 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. 
     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  32 . 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 . 
     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 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. 
     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. 
     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 . 
     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 . 
     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 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. 
     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. 
     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  10  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. 
     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 .