Intermediate Station Time-Lock.
—We have another time-lock attached to the sending apparatus that has been already referred to in describing the “block system” used at intermediate stations; a time-lock to prevent carriers being inserted into the tube at intermediate stations while another carrier is passing that station. This time-lock is shown in Fig. 27 at W´, and is shown in detail by a sectional drawing, Fig. 31.
When a carrier closes an electric circuit in passing one of the boxes located in a manhole about three hundred feet from an intermediate station, it indicates its approach to the station by exciting the electro-magnet A, Fig. 31. This magnet pulls down its armature B and raises the small piston valve C, which admits compressed air to a small chamber, D. The air is supplied to this chamber from the main tube through the pipe E. In one end of this chamber is fitted a piston, F, held to one end of its stroke by a spring, G. When compressed air is admitted to the chamber D, this piston is moved to the left, and by such movement throws the controlling valve of the sending apparatus into its normal position (shown in Fig. 29) and holds it there. This forms a positive lock, and, no matter in what position the sending apparatus may be, it puts the tube B, Fig. 29, into line with the main tube so that the approaching carrier can pass through the apparatus. The piston-rod H, Fig. 31, is connected to the finger d, Fig. 29, and by rocking this finger moves the controlling valve, or prevents it being moved by the handle K.
Fig. 31.
INTERMEDIATE STATION TIME-LOCK.
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Returning now to Fig. 31, we have on the top of the chamber D, in addition to the electro-magnet A and its armature B, a differential cylinder and piston, K, L, whose function is to close the valve C when the chamber D is filled with air. The piston K is smaller than the piston L, and sustains a constant air-pressure, supplied through the small pipe M M, from the pipe E, which leads to the main tube. When the chamber D becomes filled from the pipe E through the valve C, the pressure in the chamber moves the piston L upward against the pressure on the piston K, because of the greater area of the piston L. This movement of the differential piston raises the lever I, which passes through a slot in the stem of the differential piston, and thus closes the valve C. The air in the chamber now gradually escapes to the atmosphere through a small orifice Q; in fact it has been escaping here all the time while the chamber was being filled, but the opening through the valve C is so many times larger than the orifice Q that the escape of air was not sufficient to prevent the chamber from filling. Now, however, that all supply to the chamber is shut off, the air in the chamber is gradually being discharged through the orifice. When nearly all the air has escaped, the piston F will return to its normal position, shown in the figure, and unlock the controlling valve. The time required for the air to escape from the chamber, D, is the time that the sending apparatus will be locked, and this time can be regulated by varying the size of the orifice Q. The opening of the orifice, or the time that the sending apparatus is locked, is indicated by an index and dial, P.
This locking mechanism is secured to a bracket on the side of the large cylinder H, Fig. 27, in a position where it can be easily inspected. The moving parts of the electro-magnetic valve—for such is the valve C, with the magnet A, Fig. 31—are made very light, in order that they may respond easily and quickly to the closing of the electric circuit.
It is a disadvantage to have stations too numerous upon the same line, especially if they do a large amount of business, for each station will delay the sending of carriers from the others more or less, and the interference will be greatest during the busiest hours of the day. This condition is inherent in any system of large tubes where carriers have to be run a certain minimum distance apart, and cannot be overcome by any mechanism. But the disadvantage is greatly overshadowed by the advantage of being able to connect several stations by one line, instead of having to run independent lines from each station to the central, especially when the business of the individual stations is not sufficient to occupy a separate tube all the time. It makes it possible to have stations where otherwise the business would not warrant the cost of installation and expense of operation. We recommend the establishing of not more than eight stations on a line, and usually a smaller number than this, depending, of course, upon the amount of business to be done at each station.