Mind the Doors, Please!

MIND the DOORS, PLEASE!

The Guard’s “Magic” Switchboard

From The Modern Boy magazine, #342, Vol. 14; 25 August 1934.

Digitized by Doug Frizzle. Stillwoods.Blogspot.Com

EVERY fellow who has travelled on London’s Underground Railways knows the sliding doors which open—somewhat mysteriously—by themselves, and when everyone has got into or out of a coach, shut again. Nobody touches them, or so it seems!

Next time you get the chance, get into the last coach on the train, where the guard always travels. He’s the man in charge of the doors. He’s in charge of the whole train really, as we shall see presently.

When the train gets to a station, watch what he does. He’ll step up to a switchboard, and, as soon as the train stops, will push a button on the board. Watch those doors, and you’ll see them slowly open and stay open. You may hear a slight hiss of escaping air, too.

When the train is ready to go off again, the guard presses another button, and the doors shut. As soon as they’re shut, he presses a third button—the middle one on the switch-board—and that gives the signal to the driver that all’s well and he can go ahead—which he docs.

How’s it done? To each sliding door there is an engine, and it is fixed underneath and behind the seat cushions nearest to the door it has to work. It consists of a cylinder, with a piston working inside it. The piston rod has a bar fixed to it with cog teeth cut on its end. These teeth engage with a cogwheel, called a “pinion.” If the piston rod moves, it turns the pinion.

On the pinion is fixed a long lever with a roller bearing at its end, which runs in a groove fixed to a framework extension of the door itself. So you see—look at the picture—that if you turn the arm, you pull open the door. That’s all that happens.

When the guard presses the button, he closes a switch which allows an electric current to work a valve. This valve, in its turn, allows air under pressure to go into the door-engine cylinder, and this, acting on the piston, pushes it one way or the other. As the piston moves, so it turns the pinion, which moves the arm, which moves the door!

The air that does the work is the same air that works the brakes, only the pressure is let down a bit first.

As it is important to secure the doors in the shut position, air is kept in the cylinders of the engines all the time. When the doors are to be opened, this air has to be released first, and that hissing noise you may hear will be caused in that way.

When each door is truly shut, an electric tell-tale sends its signal to the guard, and until all these signals have come in it, isn’t a bit of use his sending the wire to the driver, because the driver won’t get it!

On one of each pair of sliding doors there is a spring arm, so that if anyone did get trapped between the doors the spring would come into action. The passenger could easily get away, and then the door would close properly—but until it was fairly closed the train couldn’t start!