Electro-pneumatic freight brake control system

A railroad car brake control system comprising:

• a) a source of fluid under pressure;
• b) a brake cylinder device;
• c) application and release control valve means interposed between said brake cylinder device and respective ones of said source of fluid under pressure and atmosphere;
• d) electric communication means for conducting a brake command signal;
• e) processor means for establishing and interrupting electrical communication between a source of electric power and said application and release control valve means in accordance with said brake command signal, said application and release control valve means in a deenergized condition establishing communication between said source of fluid under pressure and said brake cylinder device;
• f) electric load sensing means for providing a car load signal corresponding to the load weight of said car;
• and
• g) pressure limiting valve means interposed between said application and release control valve means and said brake cylinder device, and subject to said car load weight signal for setting a maximum pressure limit above which the fluid pressure at said brake cylinder device is cut-off from said source of fluid under pressure, and for retaining the setting of said maximum pressure limit following a loss of said car load weight signal, wherein said limiting valve means includes:
• (i) an inlet connected to said application and release valve means;
• (ii) an outlet;
• (iii) a proportioning valve assembly including;
• (a) a differential area piston abutment;
• (b) a supply valve carried on said differential area piston abutment via which fluid under pressure effective at said input is connected to said outlet;
• (c) a load responsive piston connected to said differential area piston abutment and subject on one side to the fluid under pressure supplied to said inlet;
• and
• (iv) a non-rotatable spring seat spaced from the other side of said load responsive piston;
• (v) a proportioning spring in the space between said other side of said load responsive piston and said spring seat;
• (vi) electric motor means subject to said car load weight signal and having a screw-threaded output shaft threadedly engaged with said spring seat for adjusting the degree of compression of said spring in accordance with the degree of rotation of said output shaft, the degree of rotation of said output shaft being dependent on said car load weight signal, the fluid pressure effective at said outlet when said car load weight signal is absent in consequence of a loss of said source of electric power corresponding to said maximum pressure limit.