1. Field of the Invention
The present invention relates to a variable load type brake control system for a railway vehicle, and more particularly to a load responsive brake pressure control valve which prevents erroneous load sensor readings caused by normal railway freight car motions.
2. Description of the Prior Art
For at least the last 100 years, pneumatic braking has been utilized onboard railway freight cars in which a brake pipe runs the length of the freight train providing a source of pressurized air to each individual car of the train. Braking conditions of the train are controlled by changes in the brake pipe pressure through utilization of pneumatic valves. Traditionally, on board each freight car is a control valve which responds to the brake pipe condition in a multi-function role including: charging reservoirs onboard each individual freight car; instituting brake application; and controlling the release of the brakes on the train. Such systems generally utilized onboard pneumatic control valves such as ABD, ABDW, ABDX, or DB60 valves, with 26 type locomotive brake equipment or microprocessor with like EPIC sold by Westinghouse Air Brake Company. It was the general practice to use identical functioning pneumatic control valves in a related control sequencing on comparably equipped freight cars throughout the train, such that each car's braking sequencing would be similar. Generally, the pneumatic signal is initiated by the lead locomotives in the train. However, some systems have been prepared and used where brake pipe pressures can be controlled at the rear or at spaced intermediate positions within the train.
Freight cars have varying braking capabilities depending on (1) brake cylinder pressure in effect at the moment under consideration; (2) the brake cylinder size; (3) the design of the mechanical linkage (brake rigging) between the brake cylinder and the friction pair (usually a brake shoe and the tread of a steel wheel, with disc brake equipment acceptable but seldom employed); (4) the weight of the car plus its contents; (5) the speed of the car at the time brakes are applied; and (6) the friction force developed as a result of the normal (brake shoe) force by the particular friction pair. The preponderance of the North American freight car fleet uses composition brake shoes on wheel treads thus rendering factor (6) constant, while minimizing the effects of speed (5). Brake cylinder size (2) and mechanical linkage (3) are both constants, chosen at the time of car design and are thus of no importance as variables during train operation. The only variables of importance then during train operation are the brake cylinder pressure (1) and the weight of the car and its contents (4).
Brake cylinder pressure (1) is under the control of the driver as a result of his or her manipulation of brake pipe pressure (through an engineer's brake valve or an electronic control, for example). The weight of the car plus its lading are only determined when the actual load is placed in the car. For some cars (such as those that carry only coal or wheat) this variable has one or the other of two values. For other cars (which carry indeterminate amounts of cargo having indeterminate weight, such as general commodity box cars or intermodal flatcars) the weight of the car and its contents may take on a wide range of values.
In either case, the braking ability of the car will be reduced as its weight is increased. In the past, this has been dealt with by selecting the brake cylinder size and rigging at the time of car design such that the highest braking produced by the maximum brake cylinder pressure available to the driver would not slide the wheels of the light car, while the heaviest possible car with this same maximum brake cylinder pressure, would still provide an effective amount of braking. Standards set by the Association of American Railroads (AAR) permit a maximum braking ratio (the ratio of brake shoe force to actual weight of the car and contents) of about 35%, while a minimum of.+-.10% is permitted. Thus the contents of a car can weight no more than 2.5 times the weight of the empty car if the rule is to be followed.
In the past, freight trains usually had about as many empty cars as loaded ones, and the average braking ratio for the train as a whole was better than the minimum acceptable for an individual car as cited above. In recent times, however, two developments have made the use of a single value for maximum brake shoe force less acceptable than in former times. These are: (1) greater use of so called unit trains in which every car is carrying the same commodity and all cars in a train are fully loaded (the train average braking ratio in this case is no better than that of the individual cars); and (2) the fact that with today's engineering advances cars can be designed whose weight for the same carrying capacity is lower than older designs. The first condition leads to train braking performance being lower than might be desirable, and the second would lead to a condition where the lading weight was restricted to a value (2.5 times the car weight) significantly lower than equipment design would otherwise permit.
Empty cars cannot utilize the same level of braking as loaded cars because the highly loaded cars are more likely to have their wheels lock and skid at a braking level that would be acceptable on a fully loaded car. Special brake equipment is therefore necessary to increase the loaded car braking ratio without incurring the consequence of a wheel slide condition when braking an empty car. Such equipment automatically adjusts brake shoe force according to the load condition of the car. These special equipment arrangements fall into two categories: dual capacity empty/load braking and multi-capacity or continuously variable braking.
It is well known to those skilled in the art that overbraking and ensuing wheel lockup and wheel sliding on lightly loaded rapid and/or mass transit vehicles must be avoided since flat spots and damage to the wheels may occur during the braking of passenger trains. On heavily loaded railway vehicles there is the possibility that underbraking conditions may result in longer braking distances which may cause a railway train to over-run its normal stopping point at a station or a block section. In order to avoid an overbraking and underbraking condition, it is common practice to employ equipment which senses the load of the car and reduces the brake cylinder pressure on cars that are not fully loaded. Examples of these are disclosed in U.S. Pat. Nos. 5,106,168; 5,100,207; 5,005,915; 5,269,595; and 5,340,203.
In order to overcome these undesirable limitations, brake manufacturers have developed a number of devices for application to the cars which, in response to an engineer's command for maximum brake effort, provide higher brake shoe force on a loaded car than for an empty one, thus permitting light weight cars to carry greater loads, and improving the braking on unit trains of fully loaded cars.
These devices, generically referred to as empty-load brake controls, provide satisfactory service in the case of cars which are either empty or loaded, but they do not provide a complete solution to the problem. One example is a very light car (such as an intermodal spine car), in which case the car will frequently carry a load intermediate between its maximum design capacity and its empty weight. The problem here is that as car weight is increased by loading, the empty-load device produces no increase in maximum brake cylinder pressure until a certain point is reached, and above this point allows the maximum value (loaded car) brake cylinder pressure. Thus if this changeover point is at a high car weight, the brake cylinder pressure will remain at the low value chosen for the empty car and performance of the train will suffer, as the car with its lading could support a higher brake shoe force without danger of sliding wheels. If, on the other hand, the changeover point occurs at too low a total car weight, the maximum brake cylinder pressure for a full load could be made available to a car weighing only slightly more than an empty one, thus heightening the likelihood of sliding wheels.
In the ideal case, the maximum value of brake cylinder pressure permitted would be continuously variable with increased lading weight. This would eliminate both of the undesirable conditions mentioned above.
Of equal importance is the fact that where equipment is to be operated on trains segregated from the general interchange, such as integral trains, a continuous load limiting system can allow the use of much higher loaded braking ratios and hence provide much better control than can be achieved with the AAR standard values. For example, the loaded braking ratio could be made equal to that presently allowed for empty cars, i.e. .+-.30%. A solid train of such cars, when compared to a solid train of cars braked at 10% (a not unusual condition), would have three times the braking ability, and could safely descend a hill three times as steep or stop in one third of the distance required by the lower braked train. Alternatively, with a train of such cars, the speed could be increased by a factor of 1.732 (or the square root of 3) times the speed of the slower braked train (roughly a 73% of increase) while maintaining the ability to stop in the same distance. This performance improvement is available without demanding any more adhesion of the wheel and rail interface than present practice imposes every day.
In the variable load type equipment, braking pressure is proportioned to the actual load, generally throughout the full range of car loading. It will be appreciated, however, that the proportioned brake pressure is selected in accordance with the maximum brake pressure (emergency) capable of being developed from the maximum running pressure normally carried by a train (110 psi). Therefore, when making relatively light surface braking applications or when making a maximum brake application from a relatively low running pressure (70 psi), the proportioned brake pressure may be far less than that capable of being supported by the adhesion demand. Accordingly, less than optimum brake efficiency is realized under certain brake conditions with variable load type brake equipment, as well as single capacity brake equipment, in order to protect against wheel sliding on an empty car under maximum braking conditions.
In the variable load type equipment, it is common to mount a spring loaded proportioning valve between the railway vehicle car body and the wheel truck assembly. The load on the railway freight car is determined by the amount of deflection between the sprung car body and the unsprung wheel bearings and associated parts of its truck. Thus, for example, the maximum spacing between the bottom of the railway vehicle and the top of a freight type three piece wheel truck assembly is indicative of an empty vehicle, while a fully loaded vehicle is characterized by minimal spacing between the railway vehicle and these same parts. However, this separation distance does not remain constant for either empty or loaded railway vehicles. It is not uncommon for the vehicle and/or wheel assembly to undergo deflections, or rock and roll, as the wheels of the truck assembly traverse the rails during motion. Slight imperfections in the height of the rails, such as at the junction of rail segment ends or in rail high or low spots, can result in raising or lowering of the wheel truck with respect to the railway vehicle body. Additionally, during a curved section of track it is not uncommon for one side of the vehicle to be closer to the truck assembly than the other side, which typically occurs on the inside portion of a curved track section. These deflections can affect inputs to the load sensing valve assembly such that the variable load brake system may thereby be "sensing" a heavier or lighter vehicle than is actually the case.
What is needed then is a system to increase railway vehicle braking efficiency while minimizing errors occurring in load sensing braking systems caused by normal vehicle motion.
It is therefore an object of the present invention to provide a variable load railway braking system which allows the maximum value of braking cylinder pressure to be continuously variable with increased vehicle loading weight.
It is also an object of the present invention to provide a means for isolating the variable load sensing braking system from errors caused by normal railway vehicle deflections during typical train movement.
It is another object of the present invention to provide a variable load limiting valve continuously operable between a railway vehicle empty and fully loaded conditions.
It is a further object of the present invention to provide an easily adjustable maximum pressure output for use on different types of cars.
It is a still further object of the present invention to provide an easily adjustable ratio control of car height to maximum brake cylinder pressure.