INTERLOCKING SIGNALS.
A signalman cannot lower or restore his signals to their normal positions in any order he likes. He is compelled to lower them as follows:—Starting and home; then distant. And restore them—distant; then starting and home. If a signalman were quite independent, he might, after the passage of a train, restore the home or starting, but forget all about the distant, so that the next train, which he wants to stop, would dash past the distant without warning and have to pull up suddenly when the home came in sight. But by a mechanical arrangement he is prevented from restoring the home or starting until the distant is at danger; and, vice versâ, he cannot lower the last until the other two are off. This mechanism is called locking gear.
LOOKING GEAR.
There are many different types of locking gear in use. It is impossible to describe them all, or even to give particulars of an elaborate locking-frame of any one type. But if we confine ourselves to the simplest combination of a stud-locking apparatus, such as is used in small boxes on the Great Western Railway, the reader will get an insight into the general principles of these safety devices, as the same principles underlie them all.
Fig. 93.—A signal lever and its connections. To move the lever, C is pressed towards B raising the catch-rod from its nick in the rack, G G G, guides; R R, anti-friction rollers; S, sockets for catch-rod to work in.
The levers in the particular type of locking gear which we are considering have each a tailpiece or "tappet arm" attached to it, which moves backwards and forwards with the lever (Fig. 93). Running at right angles to this tappet, and close to it, either under or above, are the lock bars, or stud bars. Refer now to Fig. 94, which shows the ends of the three tappet arms, D, H, and S, crossed by a bar, B, from which project these studs. The levers are all forward and the signals all "on." If the signalman tried to pull the lever attached to D down the page, as it were, he would fail to move it on account of the stud a, which engages with a notch in D. Before this stud can be got free of the notch the tappets H and S must be pulled over, so as to bring their notches in line with studs b and c (Fig. 95). The signalman can now move D, since the notch easily pushes the stud a to the left (Fig. 96). The signals must be restored to danger. As H and S are back-locked by D—that is, prevented by D from being put back into their normal positions—D must be moved first. The interlocking of the three signals described is merely repeated in the interlocking of a large number of signals.
Fig. 94.
