The present invention is related to electro-pneumatic type brake systems for railroad freight cars and particularly to a car load compensating arrangement for preventing a wheel slide in the event of a fail-safe emergency brake application causing the electronic brake control to experience a power loss.
Electro-pneumatic brake systems that are suitable for railroad cars typically employ solenoid operated electro-pneumatic valves for directly pressurizing the car brake cylinder device under control of a microprocessor. Such systems have the potential for eliminating the need for a pneumatic back-up or emergency brake, since the solenoid valves can be arranged in a fail-safe configuration in which a source of compressed air is connected to the brake cylinder in a deenergized state. Such electro-pneumatic brake systems that employ a microprocessor have the further ability to perform the load control function electrically, thereby achieving more accurate, reliable, and economical brake control operation.
It is important to note, however, that without some means of retaining the load control function when a power loss occurs, the maximum pneumatic emergency brake cylinder pressure will be delivered in accordance with the fail-safe operation of the system solenoid valves. Accordingly, the potential exists for an empty or partially loaded car to slide its wheels when such an emergency occurs, which is undesirable from the standpoint of the high cost of wheel damage, not to mention the potential for derailment.
In order to realize the foregoing benefits attributed to an electro-pneumatic brake system, as above discussed, it is the object of the present invention to load limit brake cylinder pressure obtained in accordance with the fail-safe operation of the system solenoid valves in the event of a power loss that disables the electric load control.
It is an extension of the foregoing objective to proportion brake cylinder pressure in accordance with the car load condition to provide the aforementioned limit pressure.
Briefly, these objectives are carried out by means of a pressure limiting valve that is interposed in the delivery line leading from the application and release solenoid valves. An electric load sensor sets the maximum load limited brake cylinder pressure capable of being delivered by the limiting valve. The limiting valve arrangement is such that the load limited pressure setting is retained in the event a power loss occurs, causing the electric load sensor to be disabled.