Integrated Railmotive Systems Inc.

Excellence in Engineering


    WABTEC international sales had an opportunity to provide brake equipment for passenger equipment to a major Indian railway but were stymied because an empty-load equipment was required and all WABTEC empty-load systems were built for use with direct release type AAR controls.  This type empty-load would not function properly with the Indian system, which was built to permit graduated release of brakes as the train slowed. The Indian railway felt that reduced braking in this situation was desired both to smooth the stop and avoid wheel slide. The non compatibility between the existing equipment and the customer’s desire had to be addressed and Railmotive was asked to advise a practical solution.
    The basic problem was that when brake cylinder pressure was decreased but not fully released, (which is impossible with AAR type freight brakes) the brake cylinder output from the empty-load control would not reduce in step with the engineer’s command. Cars would then be braked at too high a rate during the final portion of a stop, and rough handling would result. Our solution was to modify the AAR standard
P-1 proportioning valve by the addition of a controlled vent valve as shown in exhibit
17-1 so as to provide the empty car with a brake cylinder pressure which would both rise and fall in step with the Engineer’s command.

This solution worked.  The small added vent valve did not increase the manufacturing cost significantly. The cars were equipped and the revised valve (including a more refined vent valve than that shown here) became a new addition to the WABTEC product line.

    Referring to Exhibit 17-2 below:  in operation with an empty car, the feedback chamber above the diaphragm is connected to the brake cylinder through the load sensor. When brakes are applied, the feedback pressure on the piston’s top is opposed by the brake cylinder control pressure beneath this piston.  The control pressure is acting on a smaller area, however, so the piston will move down to the position shown, contact the check and seal off further flow to the brake cylinder. This occurs when cylinder pressure builds up to a fraction of the control pressure determined by the designed ratio of the two piston areas, for example 60%. Thus Brake cylinder pressure on an empty car will always be 60% of what it would be on a loaded car, for the same control action by the engineer. The connections to the cylinder, load sensor and control valve are shown in the figure.

With an un-modified valve, when the engineer lowers the control pressure, this change will have no effect on the brake cylinder because the supply check valve is closed and will not permit air from he cylinder to flow upstream. If a direct release is made, as with AAR equipment, this is no problem because, when the control pressure falls below 60% of its initial value, (which is the amount trapped in the cylinder) the check valve will open and cylinder pressure will fall to zero along with control pressure.    
    With a graduated release though, when control pressure is reduced, an immediate reduction in cylinder pressure is required.  The Added vent valve provides this by opening when he piston moves down in response to the dropping control pressure beneath it. This reduces brake cylinder pressure in step with but at 60% of the value of, the now reduced control pressure.  If control pressure ceases to fall, the venting of pressure above the piston will cause it to rise and close the vent. The new lower braking value will then be maintained in accord with the engineer’s wishes.
    Thus whether the engineer is increasing or decreasing control pressure, the brake cylinder pressure will always be the desired value. in this example, 60% of control pressure. Patents on the modification were applied for and assigned by Railmotive to WABTEC.