Fluid-pressure and analogous brake systems – Load control – Empty and load type
Reexamination Certificate
2000-01-26
2001-04-17
Butler, Douglas C. (Department: 3613)
Fluid-pressure and analogous brake systems
Load control
Empty and load type
C188S195000, C303S022700
Reexamination Certificate
active
06217130
ABSTRACT:
BACKGROUND OF THE INVENTION
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 to
Buchanan Ingersoll P.C.
Butler Douglas C.
Westinghouse Air Brake Technologies Corporation
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