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What is Creep of Rail?
It is common to all railway tracks but varies considerably in magnitude. In some places, its value is negligible but at some places, its value may be as high as 15.5 cm per month.
Indications of Creep
- Closing of successive expansion spaces at rail joints in the direction of creep and opening out of joints at the point from where creep starts.
- Marks on rail flanges and webs made by spike due to scratching as the rails slide.
Causes of Creep
Following are the main causes of the development of creep:
Development of wave motion in the rails by a moving train – Due to the load of the wheel, the portion of the rail under load is depressed slightly. As the wheels move, the depression also moves with them and previous depressed portion regains their original level. Thus under the wheel of a train, wave motion is developed. This wave motion tends the rail to move forward.
Force acting at the time of starting, accelerating, slowing down or stopping the train causes creep. as shown in the figure during the starting operation, the wheel pushes the rail backward, while during the stopping operation the rails are pushed forward.
Creep may also develop due to unequal expansion and contraction of rails due to variations in temperature.
Percussion Theory– According to this theory creep is developed due to the impact of wheels at the rail end head of joints. The horizontal component p of reaction R tends to creep and the vertical component tends to bend the rail end vertically i.e. to better the rail end. Thus as and when wheels leave the trailing rail and strike the facing rail end at each joint, it pushes the rail forward resulting in creep.
In addition to the above main reasons for the development of creep, following minor causes also help in the development of creep in rails:
- Good quality sleepers are not used.
- Rail not tightly or properly fixed with sleepers.
- Inadequate consolidation of formation of the track.
- Insufficient drainage arrangement.
- Track is not maintained properly.
- Gauge maintained either too tight or too slack.
- Rail joints are not properly maintained.
- The uneven spacing of sleepers.
- Defective packing.
- Insufficient ballast etc.
Magnitude and Direction of Creep
The following factor governs the magnitude and direction of creep
Alignment of the track – Creep is found greater on curves than straight portions of the track.
Gradient of the track – Generally creep develops in the direction of down gradient but it changes direction frequently.
The direction of motion of trains – In the direction in which more numbers of train move, develops greater creep. Also, the load of the vehicle influences the development of creep.
Effects of Creep
Following are the main effects of the development of creep:
- Development of disturbance in gauge and alignment.
- Widening of gaps.
- Disturbance at points and crossing
- The interlocking mechanism of signals is disturbed and thrown out of gear due to creep.
- If a crept rail is removed from the track, it becomes difficult to refix it. either it is found too short or too long due to creep.
- Bucking of the track takes place in extreme cases.
- Rail end gets battered and suspended joints start becoming supported joints.
Measurement of Creep
To measure the creep of rails, creep indicators are used. A chisel mark is made at the side of the bottom flange of the rail and two rail posts are fixed to the formation with their top levels at the top of sleeper. A fishing string is stretched below the rails on the marks on the top of posts. The distance between the chisel mark and the string will be the amount of creep.
Method of Correcting Creep
The following method can be used to correct the creep:
- Pulling back of rails
- Use of creep anchors
- Use of steel sleepers
- Increase in sleeper density
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