Creep is defined as the longitudinal movement of rails with respect to sleepers in a track. Creep is common to all railway tracks, but varies in magnitude considerably, the rail, in some places, moves by several centimetres in a month while in other locations the movement of rails may be negligible. It is observed that the rails have tendency to move gradually in the direction of dominant traffic.
Indications of Creep
Occurrence of creep can be noticed from the following observations :
- Closing of successive expansion spaces at rail joints in the direction of creep and opening out of joints at the point from where the creep starts.
- Marks on flanges and webs of rails made by spike heads, by scraping or scratching as the rails slide.
Theories of Creep
Various theories are propounded to explain the different causes of creep but none of them gives the true picture for real cause (or root cause) of creep in rails. The various theories propounded for explaining the probable causes of creep in rails are described in following paragraphs :
1. Wave Action or Wave Theory
Wave motion is set up by moving loads of wheels. The vertical reverse curve ABC (as shown in Fig. 8.2) is formed in the rails ahead of the wheels, resulting from the rail deflection under the load, is the chief cause of creep. The wheels push the wave with a tendency to force the rail in the direction of traffic. On a particular rail, the joint action by several wheels causes creep. As the wheels move, the lift in front of the moving load is thus carried forward by the wheels and causes creep, whereas the lift at the rear of the wheel gets back to its normal position.
The pitch and depth of wave depend upon the following :
- Track modulus
- Stiffness of track
- Stability of formation.
The wave action can be reduced (i.e. creep is reduced) by adopting following measures :
- Angular and heavy ballast-which develops good interlock
- Increased stiffness of track
- Lesser sleeper spacing
- Bigger, section of the rail
2. Percussion Theory
This theory states that the creep is due to impact of wheels at the rail end ahead at joints. The horizontal component ‘P’ of ‘R’ tends to cause creep while the vertical component tends to bend down the rail end vertically, i.e., to make a battered rail end. Hence as and when the wheels leave the trailing rail and strike the facing rail end at each joint, it pushes the rail forward resulting in creep. Though the creep is very small in single impact but cumulative effect of number of wheels in quick succession results in sufficient creep.
The creep by this theory will increase due to :
- weak and loose fish bolts
- worn out fish plates
- loose packing at joints
- wide expansion gap
- heavy axle loads moving at high speed.
3. Drag or Dragging Theory
It states that backward thrust on driving wheels of the locomotive of train has got a tendency to push the rail off the track backward while the other wheels of the locomotive and the vehicles (i.e.wheels of coaches and wagons) push the rail in the direction of travel as explained in Wave Action Theory and they have greater effect (as compared to drag effect). This results in creep of rails in the direction of movement of trains.
4. Starting, Accelerating, Slowing down or Stopping of a Train
When a train is starting or accelerating, the backward thrust of the engine driving wheels tends to push the rails backward. When it is slowing down (i.e. decelerating) or coming to a stop, the braking effect tends to push the rails forward.
5. Expansion or Contraction of Rails due to Temperature
Creep also occurs due to variation in temperature. The creep in this case is influenced by the range in temperature variation, location of track, whether exposed or shady surroundings, etc.
6. Unbalanced Traffic
(a) In a single line system if heavy equal traffic (both in number and load) runs in both directions, the creep is almost balanced. Otherwise, heavy traffic in one direction will cause creep, which is partly balanced by light traffic in opposite direction.
(b) In the double line system, trains on a particular line being unidirectional, creep occurs in both the lines. The above mentioned causes are main but by no means the only causes of creep.
Following factors also govern the magnitude and direction of creep:
(i) Alignment of Track
Creep is observed greater on curves than on tangent railway track
(ii) Grade of Track
Creep is more with steep gradient particularly if the trains move downward with heavy loads. Though in the opposite direction, it is not impossible for creep to develop.
(iii) Type of Rails
The old rails have more creep than new rails.
(iv) Direction of Heaviest Traffic
If the loaded trains run in one direction (such as from production centres to the markets) and empty trains in opposite direction, creep will usually be found in the direction of loaded trains.
(v) Poor Maintenance of Track Components
Poor Maintenance of Track Components and ill-design of super-elevation, curves, joints, etc. will also increase the creep. It is observed that creep is not constant over any given time nor does it vary at uniform rate, nor does it continue in one direction, nor do both the rails creep by an equal amount. In fact, the direction and extent of creep cannot be predicted. Such variations may occur daily or may be seasonal.
Effects of Creep
The effects of creep are several and serious too, the most serious effect being buckling of the track. If a buckled track is not attended at proper time, it may easily derail a train and result in serious losses. The common effects of creep, are the following :
- Sleepers move out of square and out of position. This affects the gauge and alignment of track. As sleepers move, naturally the surface is also disturbed and finally results in an uncomfortable riding.
- Rail-joints are opened out of their limit in some cases and stresses are set up in fish-plates and bolts due to which the bolts sometimes break. The rails are also battered at ends due to excessive gaps at joints. While at other places, joints are jammed and prevent required expansion due to temperature variation.
- Points and crossings get distorted and it becomes very difficult to keep them to correct gauge and alignment. The movement of switches is made difficult (i.e., difficult to operate the switches) and interlocking is thrown out of gear.
- If any rail is removed from the track for any purpose, it becomes difficult to fix it again at proper position because by the time gap becomes too short or too long due to creep.
- Besides these effects, smashing of fish-plates and fish-bolts, bending of bars, kinks at joints of rails and forging of ballast ahead, are common effects of creep.
Measurement of Creep
“Creep Indicator” is used to measure the creep of rails. A chisel mark is made at the side of bottom flange of rail and two rail posts are fixed to the formation with their top levels flushing with top of sleepers. A fishing string is stretched below the rails on the marks on the top of posts and creep is thus measured. The distance between chisel mark to the string is the amount of creep.
As per Indian Railways practice, the creep should be measured frequently at an interval of about 3-months. A creep in excess of 150 mm (15 cm) should not be permitted on standard track and at any location more than 6 consecutive rails should not be found jammed in a single railway track. Moreover, no creep should be permitted on approaches of points and crossings.
Track Portions Susceptible to Creep
The following portions of track are found to be more susceptible to should be frequently attended :
- Steel Sleeper Track (CST – 9) joining/melting wooden sleeper track.
- Dips in stretches with long gradients.
- Approaches to major structures.
- Approaches to level crossings and points crossings.
Remedies / Prevention of Creep
Prevention is always better than cure. If creep is not prevented in time, it will result in derailment. Following are the common methods adopted to prevent creep.
1. Pulling Back the Rails
If creep is distinctly visible, the remedy is to pull back the rails to their original position. For doing this, first inspect the track, note the extent of pulling back distance and determine the point from which to begin. Now start pulling the rails back to their original positions by means of crow bars and hooks provided through the fish bolt holes of rail. In pulling back, the positions of joints relative to sleepers must be maintained, and both the rail joints must be in their relative positions.
Pulling back the rails is a very slow and tedious process and is only possible when a small length is to be dealt. It has, moreover, been noticed that rails start creeping immediately after pulling back.
2. Provision of Anchors or Anticreepers
The creep of the track can be prevented by use of Anchors and sufficient crib ballast. For creep of 7.5 cm to 15 cm, in a month 4-anchors per rail and for creep of 22.5 cm to 25 cm. 6-anchors per rail are used in the Indian practice.
Anchors are fastened to the foot of rail and kept in perfect contact with the side of the sleeper being the side opposite to the direction of creep. If creep occurs in both directions, anticreepers are provided on both the sides of sleepers, starting from the centre of the rail and should never be fixed near the joints.
Anchors are fixed to rails either (i) by wedging action, (ii) by clamping (iii) by a spring grip. There are several varieties of anchors based on these three types.
The following points should be considered for efficient maintenance of Anchors:
- The creep anchors should be strong enough to resist the movement of rails due to creep.
- The anchors should butt against the sleepers otherwise they do not function effectively.
- The anchors should be uniformly distributed over the entire rail length.
- Defective anchors must be renewed to prevent accumulation of creep.
- Anchors should be fixed to good sound sleepers only.
- Anchors, which depend on spring effect for their grip, must not be driven along a rail as this will destroy the spring.
- As far as possible, creep anchors should not be provided on the railway bridges.
- It is a good practice to use number of anchors at approaches to the yards, in yards, at level crossings or at places where heavy brake applications are made, since the creep adversely affects the alignment, gauge and position of points and crossings at these locations.
3. Use of Steel Sleepers
Sleepers should be of such a type and with such fittings that they effectively prevent the rail from creeping on them. Secondly, the sleepers must have a good grip with the ballast to resist the movement of the sleepers in the ballast. Steel trough sleepers are the best for this purpose. Increase in the number of sleepers will, therefore, also help in the prevention of creep.
The longitudinal movement of rails with respect to sleepers, called creep of rails, results in bucking which may lead to derailment of trains and hence serious losses of life and/or property. There are several theories put forward to explain the reasons of occurrence of creep, but none of them is able to give the perfect reasoning. Creep does not vary at uniform rate, nor does it continue in one direction, nor both the rails creep by an equal amount. The direction and magnitude of creep cannot be predicted. However, creep must be prevented by pulling back, or provision of anti-creepers, or use of steel trough sleepers.