Railway Signalling & Interlocking

6.4 Point Machines 161 The generalised block diagram is shown in figure 6.14: – In an electro-mechanical point machine, electric power is transformed into mechanical by means of an AC or DC electric motor M. The motor rotation is spread on to the reduction gear R meant to strengthen the angular momentum and to reduce the rotary speed of the motor. The motor is connected with reduction gear via branch sleeve which allows an insignificant radial displacement of shafts while retaining a parallel position of their axes. To protect the motor from overloads, e. g. if the blades do not reach their end position due to an obstacle, and to ensure the braking of the revolving parts of the EPM after the end of switching the points, a friction gear is inserted into the gear. The rotating movement is transferred into the progressive motion of the throw bar TB in the last cascade of the reduction gear. – In an electro-hydraulic point machine, the electric motor M rotates the pump of the hydraulic gear R. This pump pumps oil from one cylinder into another and causes a relative movement between the cylinder and a piston. Either the cylinder or the piston is mechanically connected with the throw bar TB, the other is fixed. In both forms, the throw bar impacts upon the blades of the points through the point drive rod. The detection contacts DC provide checking of point positions and commutate the electric controlling circuits. Obtaining the checking signal about of point end position is only possible if the position of the detection bars DB conforms to that of the throw bar. An important factor for exchangeability of EPM's of different manufacturers is compatibility of two kinds: – Electrical compatibility of the EPM in the operation and supervision circuitry. An example for a standard is the German four-wire point circuitry (chapter – Mechanical compatibility at the interface between EPM and point drawbar, regarding mechanical connections, switching length and others. Often compatibility is provided in one country, but not internationally. For degraded mode operation and for maintenance, EPM shall enable the possibility of switching the points by the hand crank. During hand cranking, electrical movement must be prevented for safety. The EPM influences directly the safety of train movements, since is checks the actual position of a set of points. The idea of supervision of point position is to verifiy the conformity between the detection bars and the throw bar. In order to check point position, one checking drawbar is attached to each blade. These drawbars are connected with the detection bars which move inside the EPM. 6.4.3 Supervision of Point Position on the Example of SP-6 To illustrate the principle of proving the point position, let us examine the checking block of the Russian EPM SP-6 (Reznikov 1985, Sapožnikov et al. 2008). According to the classification given in chapters 6.4.1 and 6.4.2, this EPM is electro-mechanical with possibility of using Figure 6.14: EPM block diagram (R is a mechanical gear in electro-mechanical and a hydraulic gear in electro-hydraulic point machine)