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| Re: Real Long Read On NYCPropulsion/Braking (334050) | |||
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Re: Real Long Read On NYCPropulsion/Braking |
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Posted by Stephen Bauman on Sun Nov 5 07:18:41 2006, in response to Re: Real Long Read On NYCPropulsion/Braking, posted by RonInBayside on Sun Nov 5 00:24:33 2006. So for emergency braking a constant brake pressure is good!The primary objective for emergency braking is to stop a train before a collision. The ability to do this is sometimes expressed as a maximum stopping distance but more often as the average deceleration rate (regardless of the actual deceleration profile). There are preferences among actual deceleration profiles that have the same average deceleration rate. Namely, the PCC discovered that constant acceleration/deceleration rates are safer for passengers. Consequently, their streetcar was designed to reduce jerk (rate of change of acceleration/deceleration). The PCC service braking rate is significantly greater than the emergency braking rate for NYCT rolling stock without any increase in passenger injuries. Mr. JeffH stated: NYCT emergency brake cylinders maintain constant pressure and NYCT brakes have an an inverse relation between speed and coefficient of friction. One would infer from these statements that the NYCT emergency braking profile was one whose deceleration rate increased with decreasing speed. One would also infer that the reason others chose to bleed cylinder pressure during the application of emergency braking was an attempt to maintain a more nearly constant deceleration rate. Given the research of the PCC regarding constant braking rates, one would conclude that NYCT emergency brake applications should result in more passenger injuries than those of other passenger cars whose emergency brakes bled during application. Perhaps, this was the root cause of those passenger injuries that prompted NYCT to reduce emergency braking rates, without adequately considering the safety implications of such an action. |
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