Brake Tests

89 UC Equipment, PM Features ONLY. 191. Fig. 54 shows that when this equipment is arranged to give the equivalent of the operative features of the PM equipment only, the results obtained are substantially equivalent to those just described for Fig. 53. 192. This is further illustrated by the comparative stopping distances (Fig. 93), which are almost identical for the PM and the UC equipment having PM features only. UC Pneumatic Equipment. 193. Fig. 55 shows characteristic brake cylinder pressure cards obtained with this equipment, emergency application. The brake cylinder pressure rises almost instantly to its maximum value and is held without blow-down throughout the stop thus utilizing the air pressure available on each car to its fùllest extent and effect. The results to be expected from this quick rise of brake cylinder pres- sure offset to a certain extent, however, by the relatively slow rate of transmission of serial quick action which resulted in the stop being somewhat longer and not as smooth as would have been the case other- wise. 194. The brake cylinder indicator cards (Fig. 55), show that the time of transmission of serial quick action was slightly longer than with the PM equipment. This was due to operation of the valve mechan- ism and it was found possible to quicken the pneumatic serial quick action feature without any material change in the design of the parts. When so modified the time of transmission of quick action with the UC pneumatic equipment is practically the same as that with the PM equipment. It was not thought of sufficient importance to try out any of these valves so modified in the series of tests under discus- sion. But the universal valves being supplied for cars going into ser- vice at the present time are improved in this particular and the cyl- inder cards shown in Figs. 55A illustrate the results obtained with this improved valve in rack tests. Considering the improvement in the time of transmission of quick action indicated by these cards the stops would be improved correspondingly. 195. The quick rise of cylinder pressure and the high emergency pressure obtained with the UC pneumatic equipment is effective in pro- ducing a materially shorter stop, namely, from 200 to 250 feet shorter than with the PM equipment. (Fig. 93.) For a discussion of the effect of the complete pneumatic equipment on the smoothness of low speed (10 m.p.h.) stops, see Par. 426.