This Article Covers Creep Of Rail Their Causes, Effect, Adjustment And Measurements One By One.
Creep Of Rail
Creep defined as the longitudinal movement of the rail with respect to the sleepers. Rails have a tendency to gradually move in the direction of dominant traffic.
Creep is common to all railway tracks, but its magnitude varies considerably from place to place; the rail may move by several centimetres in a month at few places, while at other locations the movement may be almost negligible.
THEORIES FOR THE DEVELOPMENT OF CREEP
Various theories have put forward to explain the phenomenon of creep and its causes, but
none of them have proved to be satisfactory.
The important theories briefly discussed in the following sections.
Wave Motion Theory
According to wave motion theory, wave motion is set up in the resilient track because of moving loads, causing a deflection in the rail under the load.
The portion of the rail immediately under the wheel gets slightly depressed due to the wheel load. Therefore, the rails generally have a wavy formation.
According to percussion theory, creep developed due to the impact of wheels at the rail end ahead of a joint.
As the wheels of the moving train leave the trailing rail at the joint, the rail gets pushed forward causing it to move longitudinally in the direction of traffic, and that’s how creep develops.
Though the impact of a single wheel may be nominal, the continuous movement of several wheels passing over the joint pushes the facing or landing rail forward, thereby causing creep.
According to drag theory, the backward thrust of the driving wheels of a locomotive has the tendency to push the rail backwards, while the thrust of the other wheels of the locomotive and trailing wagons pushes the rail in the direction in which the locomotive is moving.
This results in the longitudinal movement of the rail in the direction of traffic, thereby causing creep.
Causes Of Creep
The main factors responsible for the development of creep are as follows.
Ironing effect of the wheel
The ironing effect of moving wheels on the waves formed in the rail tends to cause the rail to move in the direction of traffic, resulting in creep.
Starting and stopping operations
When a train starts or accelerates, the backward thrust of its wheels tends to push the rail backwards.
Similarly, when the train slows down or comes to a halt, the effect of the applied brakes tends to push the rail forward.
This in turn causes creep in one direction or the other.
Changes in temperature
Creep can also develop due to variations in temperature resulting in the expansion and contraction of the rail.
Creep occurs frequently during hot weather conditions.
In a double-line section, trains move only in one direction, i.e., each track is unidirectional.
Creep, therefore, develops in the direction of traffic. In a single-line section, even though traffic moves in both directions, the volume of traffic in each direction is normally variable.
Creep, therefore, develops in the direction of predominant traffic.
Poor maintenance of track
Some minor factors, mostly relating to poor maintenance of the track, also contribute to the development of creep.
These are as follows:
- Improper securing of rails to sleepers
- Limited quantities of ballast resulting in inadequate ballast resistance to the movement of sleepers
- Improper expansion gaps
- Badly maintained rail joints
- Rail seat wear in metal sleeper track
- Rails too light for the traffic carried on them
- Yielding formations that result in uneven cross levels
- Other miscellaneous factors such as lack of drainage, and loose packing, uneven spacing of sleepers
Effects Of Creep
The following are the common effects of creep.
Sleepers out of square
The sleepers move out of their position as a result of creep and become out of square.
This in turn affects the gauge and alignment of the track, which finally results in unpleasant rides.
Expansion in gaps get disturbed
Due to creep, the expansion gaps widen at some places and close at others.
This results in the joints getting jammed.
Undue stresses created in the fish plates and bolts, which affect the smooth working of the switch expansion joints in the case of long welded rails.
Distortion of points and crossings
Due to excessive creep, it becomes difficult to maintain the correct gauge and alignment of the rails at points and crossings.
Difficulty in changing rails
If, due to operational reasons, it required that the rail Can changed, the same becomes difficult as the new rail found to either too short or too long because of creep.
Effect on interlocking
The interlocking mechanism of the points and crossings pets disturbed bv creep.
Possible buckling of track
If the creep excessive and there is negligence in the maintenance of the track, the possibility of buckling of the track cannot ruled out.
There are other miscellaneous effects of creep such as breaking of bolts and kinks in the alignment, which occur in various situations.
Measurements Of Creep
Creep can measured with the help of a device called creep indicator.
It consists of two creep posts, which generally rail pieces that driven at 1 km intervals on either side of the track. For the purpose of easy measurement, their top level is generally at the same level as the rail.
Using a chisel, a mark made at the side of the bottom flange of the rail on either side of the track. A fishing string is then stretched between the two creep posts and the distance between the chisel mark and the string is taken as the amount of creep.
According to the prescribed stipulations, creep should measured at intervals of about three months and noted in a prescribed register, which maintained by the permanent way inspector (PWI).
Creep in excess of 150 mm (6 in.) should not permitted on any track and not more than six consecutive rails should found jammed in a single-rail track at one location.
There should be no creep in approaches to points and crossings.
Adjustment Of Creep
When creep in excess of 150 mm resulting in maintenance problems, the same should adjusted by pulling the rails back.
This work carried out after the required engineering signals have put up and the necessary caution orders given.
The various steps involved in the adjustment of creep are as follows:
- A careful survey of the expansion gaps and of the current position of rail joints is carried out.
- The total creep that has been proposed to be adjusted and the correct expansion gap that is to be kept are decided in advance.
- The fish plates at one end are loosened and those at the other end are removed. Sleeper fittings, i.e., spikes or keys, are also loosened or removed.
- The rails are then pulled back one by one with the help of a rope attached to a hook. The pulling back should be regulated in such a way that the rail joints remain central and suspended on the joint sleepers.
The pulling back of rails is a slow process since only one rail dealt with at a time and can done only for short isolated lengths of a track. Normally, about 40-50 men required per kilometre for adjusting creep.
When creep required to adjusted for longer lengths, five rail lengths tackled at a time. The procedure almost same as the preceding steps except that instead of pulling the rails with i rope, a blow given to them using a cut rail piece of a length of about 5 m.
A creep adjuster normally used when extensive work involved
The creep adjuster set at the centre of the length of the track, to tackled, with the wide joints behind it and the jammed joints ahead of it.
The following steps are adopted while using a creep adjuster:
- Expansion liners of the correct size are put in all the expansion gaps,
- All the keys on the side (with wide joints) of the creep adjuster are removed and all fish bolts loosened,
- The creep adjuster is then used to close up the gaps to the required extent by pushing the rails forward. A gap of a few inches is left between the rail ends opposite the adjuster,
- The corrected rails then fastened with keys. After that, the rails on the other side of the adjuster tackled,
- And The operation leaves some of the expansion gaps too wide which tackled by the creep adjuster when it is set in the next position,
- The corrected rails are then fastened and the adjuster shifted to the new position,
- The whole process repeated again and again till the requisite attention has paid to the entire length of the rail. In the end it may be necessary to use a rail with the correct size of closure (bigger or smaller) to complete the work.
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