Engineering Wonders of the World
Volume I

232 ENGINEERING WONDERS OF THE WORLD. Fig. 13.—AN ELECTRIC POINT-SHIFTING MOTOR. Yet another advantage is the means of making sure that the points lie for the direc- tion that corresponds with the signal. This is done effectively in. power signalling, and the signalman is also told that the lever has done its work, and that the points or signals have responded to the lever. Where mechan- ical signalling is employed, the man can often tell by the “ feel ” of the lever that nothing has become disconnected ; but in a power frame there is no “ feel.” Consequently a “ return indication ” is given after the work has been done. In most systems the electrical circuits are the valves opened, by the initial movement of the lever, which cannot be given its full stroke, because, when it is about half way over, a lock stops further progress, and has to be withdrawn by the up, or Return Indications. Automatic Stroke Completion. “ return indication ” coming from the point or signal, and certifying that the work has been done. The lever can then be moved the remainder of the distance, and will then release any sympathetic levers that have to follow. The low-pressure pneumatic method of automatically completing a stroke that has been begun by the signalman is very in- teresting, and demands notice. In this system the locking frame has slides instead of levers, and these have to be pulled out to alter a signal or a pair of points, and pushed back to change the position again. Signals and points are operated by air compressed at 15 lbs. pressure, and admitted through a main pipe into the signal and point motors, the admission being controlled» through valves moved by air conveyed in independent pipes at a pressure of 7 lbs. to the square inch. The general arrangement of an installation of this kind is shown in Fig. 14. There is a reservoir, X, from which a main pipe, Y, leads 15-lb. air to a point motor, C, through the valves R4 and R5. These determine on which side of the piston of C the air shall act. Branches from Y take air also to valves under the slide L2 and to valves R2 and R3, by which it is admitted to cylinders I and I2. The slide has two parts—L and L2. When the handle is pulled forward into position 2 (half stroke), pipe a is put into communica- tion with X, and the slot in L forces down the piston of I to the same level as that of I2. The bottom corners of this slot being square, the slide is arrested at half stroke by the pin on the top of the right-hand piston rod. Meanwhile air travels through pipe a to R6, which opens to allow air from, the main pipe, Y, to reach the motor cylinder, C, and press plate, M, towards the right. The left-hand slot in M The Low- Pressure Pneumatic System.