6.5 Point Locking Mechanisms 163 the beak-shaped end of the connecting lever is dropped into in the superimposed notches of the detection bars 1 and 2. Figure 6.15a shows the state of details of an EPM checking mechanism when the blades are located in their end position, with the right blade fitted to the stock rail and the left blade free. Herewith the operational contacts 3 (controlling a control circuit of an EPM) are connected with the connecting lever 4, while the checking contacts 10 – with the connecting lever 11. When the points are switched to the opposite position, the main drive shaft of the reduction gear rotates in a clockwise direction. Firstly, the roller of the switching lever 9 rolls onto the surface of the collar 8. That results in the following successive movements: the connecting lever 11 moves aside, disconnecting the checking contacts 10, and the operational contacts 12 become connected. Henceforth the throw bar begins moving and the blades of the points do the same together with it. This, consequently, provokes the movements of the detection bars 1 and 2. At the moment of the final movement of the throw bar, the notch of the collar of the main drive shaft becomes positioned under the roller of the switching lever 5, which causes it to move to the right under the influence of the spring 7. That results in the connecting lever 4 disconnecting the work contacts 3. If all elements of the EPM and the points are in working order, and therefore the blades and detection bars have moved to their end positions, the beakshaped end of the connecting lever 4 drops into the superimposed notches of the detection bars. Owning to that, the checking contacts 6 (figure 6.15b) become connected. They will not be connected, however, if at least one of the detection bars does not move for the specified distance, e.g. as a result of a breakage. In this case its notch will not be positioned under the beak-shaped end of the connecting lever and it will be propped against the surface of the detection bar (figure 6.15c). Neither the contacts 4 nor 6 will be connected. In case of trailing of the points, the collar 8 and the main drive shaft do not revolve, but the detection bars do move. The beak-shaped end of the connecting lever is pushed to the surface of the detection bar by the splayed edge of the notch of this bar. In that case the connecting lever occupies an intermediate position disconnecting the checking contacts. At that time, the connecting and switching levers of the other blade do not change their position, and the operational contacts remains to be connected. 6.5 Point Locking Mechanisms Under dynamic impact from passing rail vehicles, blades should be locked. As was already pointed out earlier, their locking can be external or internal. Besides, locking of the blades in the end position can be either form fitted (not trailable) or force fitted (trailable). In the following, some examples for locking mechanisms are described. 6.5.1 External Locking Mechanism: Clamp Lock In these countries which use external locking mechanisms, there is a large variety of locks. However, the by far most widely used solution is the clamp lock. The clamp lock (figure 6.16) is trailable. Besides the clamp lock, in recent years modern optimised external locking mechanisms have been developed by different manufacturers, which are optimised for low friction and are therefore used for points which shall be switched very frequently. On high speed lines, a problem of trailable points can be the danger of unintended switching by dynamic impact. Therefore, special locks are often applied. The drive rod is fixed to the point machine via the throwbar, but not to the blades. Instead, the blades are mounted to special lock arms. When the blade is unlocked (right blade in figure
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