class411
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Post by class411 on Apr 24, 2013 10:58:36 GMT
A couple of things I've often wondered about:
Why is that it's very easy to stop a door from closing but virtually impossible to force it any further open.
It's not something I make a habit of so I don't know if it's still the case but it's something I've noticed over the years.
Also:
Many years ago (Late seventies IIRC) there was an incident where someone got their arm stuck in the doors and the train went of with said arm outside the carriage. I don't think he suffered anything more serious than a broken arm - I knew the victim by sight as he drank in the pub that doubled as our office canteen and he just had his arm in a sling for a few weeks.
To get to the point, at the time it was reported in the papers (so it must be true) that the common understanding that underground trains would not start if the doors were not properly closed was incorrect. From the comments in the 'pilot light' thread it seems that the common undertsnding was, indeed, correct and (shock, horror gasp, steps back in amazement) the papers were wrong!.
Are there any circumstances (apart from faulty sensors or where the driver has overridden the system) where a train (this would haver been the new(!) C Stock) will start with a door open sufficiently for an arm to protrude?
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Chris M
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Post by Chris M on Apr 24, 2013 11:42:30 GMT
The door sensors do have a tolerance, and you can sometimes see them come open about a centimetre or so without the train stopping. However this shouldn't be anywhere near enough to accommodate an adult's arm.
I think there might have been discussion about this (the general case, I don't recall hearing about the specific incident before) as part of a thread about the 09 stock's sensitive door edges, but I can't immediately find it. I'll keep looking though.
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Post by railtechnician on Apr 24, 2013 17:51:32 GMT
Simple answers one feels! The doors were opened and closed pneumatically thus it was easy enough to hold a door open against the operating air pressure. It really didn't take much resistance at all to counter the action of the air motor. However, once the doors were closed mechanical 'locks' prevented them being forced open more than two or three inches. The pilot on the older stocks could indeed go out as trains accelerated in tunnel and sudden air pressure forced the doors against the locks but that would not impede traction, I suspect there was a time element to the detection but I am guessing.
Things may be different today on new and refurbished stocks gut it was quite possible to travel between two stations on 73 stock with an arm outside the double doors, on one occasion I witnessed such a train entering the eastbound platform at Kings Cross with a man's arm trapped outside the doors but otherwise unharmed when the doors opened. In the rush hour it was not uncommon to witness trapped coats outside the cars when trains were packed like sardines.
I have no doubt that the technology in use on modern stocks is different in terms of equipment and tolerance although it may be generally of the same type.
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class411
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Post by class411 on Apr 24, 2013 18:55:01 GMT
Simple answers one feels! The doors were opened and closed pneumatically thus it was easy enough to hold a door open against the operating air pressure. It really didn't take much resistance at all to counter the action of the air motor. However, once the doors were closed mechanical 'locks' prevented them being forced open more than two or three inches. That I could understand but it seemed that no matter where you 'intercepted' the doors, even if they had only closed a few inches, it was impossible to push them further back. If you were holding one open and let it close a couple of inches you could never regain the lost distance. It was as if they were operating on a ratchet, but if the were, it was completely silent. |
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Deleted
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Post by Deleted on Apr 24, 2013 21:24:41 GMT
I have a theory, when the door is being held open the air pressure is still building up in the ram and the average person isn't strong enough to push the air back into the holding tank.
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Post by railtechnician on Apr 25, 2013 9:13:08 GMT
I have a theory, when the door is being held open the air pressure is still building up in the ram and the average person isn't strong enough to push the air back into the holding tank. Of course the available air pressure depends upon the pressure of the compressed air in the compressor reservoirs. In the old stocks one could see that pressure as the gauges were visible beneath the seats about 8" off the floor. One could observe the pressure increase as the compressors cut in and decrease as brakes and doors were operated. I have no idea if the pressure gauges are visible to passengers in modern stocks. |
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Deleted
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Post by Deleted on Apr 25, 2013 9:17:23 GMT
I've seen them on the 09/S stock but they just show the break pressure.
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Post by Dstock7080 on Apr 25, 2013 12:15:31 GMT
Of course the available air pressure depends upon the pressure of the compressed air in the compressor reservoirs. In the old stocks one could see that pressure as the gauges were visible beneath the seats about 8" off the floor. One could observe the pressure increase as the compressors cut in and decrease as brakes and doors were operated. I have no idea if the pressure gauges are visible to passengers in modern stocks. On older Stock the pressure gauges visable inside cars would have been either for brake cylinder pressure or train-line air, door engines operated from main-line air. |
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neilw
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Post by neilw on Apr 25, 2013 14:19:03 GMT
It's to do with the geometry of the door engine arrangement, there is an arm which moves through 180 deg and takes the door with it in a slot. One of each pair of double doors also has a spring which allows an inch or two of movement on the end of the arm. You can see the effect in the speed of the door movement, starts off slowly, speeds up, then slows down again to close. This applies to traditional "inside" doors designs , not externally hung ones.
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