Turnout:Deficiencies of existing turnouts ~ MECHTECH GURU

Turnout:Deficiencies of existing turnouts


Turnout is a track structure which permits movement of train from one track to another. It is the most complicated component of track. Because of its complicated design and certain inherent deficiencies, speed on turnout while negotiating towards turnout side in Indian Railways till recently was limited to 15 kmph.  To improve traffic potential, speed potential of turnout is being increased. Increase of speed on turnout requires better design of turnout, better maintenance practices and perfect layout arrangement. Over the last 20 years, all most all the turnouts of main line and running loops have been replaced by the curved switches laid on concrete sleepers. In goods yards, a small percentage of turnouts are yet to be converted to turnouts on concrete sleeper. New design of turnout laid on concrete sleepers is sturdier than the earlier designs and permits higher speed on turnout side, at the same time it requires less maintenance. In the last decade, Indian Railway have gone for increase in axle load and by now all major routes have been converted to CC+8+2 routes.  So practically axle load has gone up by 12%. Axle load has been further raised to 25T on few select routes. With the increase in axle load, the stress transferred to various components of turnout increases drastically.  On turn out, the wheels move from one rail to another on switch as well as over crossing, hence wear and tear on account of increased axle load is have more adverse effect on  turn out than other components of track.  Hence better maintenance systems for turn out are required.  If a turnout is laid properly at the time of initial laying, maintenance requirement is significantly lesser than the turnout which has been laid with deficiencies or improper layout.

Deficiencies of existing turnouts:-

  There are few inherent geometrical deficiencies in the design of turnout, which come in the way of smooth riding over them. Engineers in the field should be aware of such issues, although it is difficult to overcome them in the field. However, good maintenance practices will reduce its ill effects to minimum. The main geometrical deficiencies of turnouts are: Weakness of tongue rail due to severe reduction of section by heavy machining.  Tongue rails are further weakened by heavy wear and tear by traffic. So these are to be replaced frequently. With increase in thickness at toe of switch or switch entry angle obstruction level increases. Hence a jerk is experienced by vehicles on account of these obstructions. Twist in the track over a short distance on tongue rails machined from rail of same cross section, due to overriding nature of the tongue rail.  Tongue rail are machined lower than stock rail near ATS. Height of tongue rail is increased progressively.   At certain location, top of tongue rail matches with top of stock rail, there after it becomes 6 mm higher than stock rail at JOH. Such rising of tongue rail above stock rail creates twist. However, no such twist is there in thick web switch, Structural weakness of the entire switch assembly to withstand lateral forces at higher speeds. As the turnout assembly is not provided with 1 in 20 cant, the wheel loads are concentrated over a small strip near gauge face. Hence wear and metal flow is high on turnout. Crossings are made straight; hence when turnout is laid on curve, a small straight patch is introduced on curved main line. Because of the existence of these inherent geometrical deficiencies, running of train cannot be as smooth as on normal track. In case turnout is not well maintained, roughness of train running increases.

Throw of switch:-

  Minimum throw of switch for BG for new work should be 115mm. If proper stretcher bars are used, such a throw is possible from track structure point of view.  The actual amount of throw is provided by point machine. Point machines are designed with maximum stroke of 143mm, to enable throw of 115mm. If less throw of switch is provided, it may pose troubles such as improper bearing of tongue rail against stock rail on one side. On opposite side clearance between the back face of open tongue rail and stock rail gauge face will be inadequate for passage of wheel, hence the wheel may rub against non gauge face of tongue rail which may set tongue rail and the stretcher bars in oscillatory motion. Hence, achieving proper throw of switch is important. For existing work, minimum throw of switch is 95mm.

Spherical washers:- 

Spherical washers are required to be provided for the bolt joining 2 planes which are not parallel to each other. At many locations on turnout, such as heel of switch, distance blocks behind heel and in the crossing portion, non parallel planes (rails) are connected with each other with the help of bolts with spherical washer. At such locations, holes are drilled perpendicular to one of the rail and on the other rail, where direction of bolt is not perpendicular to the rail, spherical washers are provided. In case, spherical washer is not provided while joining two non parallel surfaces, one edge of bolt will press against the web of rail causing bending/breakage of bolt.

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