Turnout:Deficiencies of existing turnouts

Turnout:-

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loops have been replaced by the curved switches laid on concrete sleepers.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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