Creep of Rail
Creep of Rail is defined as a longitudional movement of rail with respect to sleeper. Rail have the tendency to gradually move in the direction of dominant traffic. The creep of rail is common to all railway tracks and its value varies from almost nothing in some cases to about 130 mm/ month in creep.
There is wide variation in the amount of creep of rail. The principal causes which are responsible for the development of creep of rail are discussed below.
Causes of Creep of Rail
1. Acceleration or Starting of Train :
At the time of acceleration, Wheel gives lateral thrust which causes creep of rail.
2. Deaccelerating or Stopping of Train :
If sudden stopping of train takes place, braking effect tends to push the rail forward and thus causes creep in forward direction.
3. Wave action or Wave Theory :
As train is passing under the rolls the portion under the rolling wheels is compressed and depressed slightly due to wheel loads. As more the wheel moves this depression also moves and the portion which is under depression previously comes back to its original position.
4. Percussion Theory :
This type of creep of rail occurs due to impact of load. In this type, when the wheels of rail passes over the joint, the trailing rails gets depressed down and the wheels gives impact to the facing of rail.
In addition to this creep of rail may also be caused due to following reasons.
- Insufficient numbers of sleepers is laid.
- Uneven spacing of sleepers.
- Improper expansion joints.
- Use of improper and faulty sleepers.
- Rails too light for the traffic carried by them.
- Poor drainage work.
- Improper maintenance of track guage and joints.
Effect of creep.
- The results of creep are of very serious nature and hence great care should be taken to detect and repair the creep.
- The suspended joints starts becoming supported joints and rail ends get battered.
- The sleepers move out of their position and hence the rail gauge is disturbed and also the rail level. This result in bad running of train.
- Due to creep the position of point and crossing will be disturbed and it will be difficult to maintain a correct gauge and alignment.
- The interlocking mechanism of the signal are disturbed due to creep.
- Rail joints get opened out resulting in bolt holes getting elongated and premature fracture of fish plate and bolts.
Measurement of Creep.
Creep posts should be erected every kilometer on either side of the track and the position of joints should be marked on one-of the posts. The measurement of creep should be taken frequently at an interval of about 3 months in a prescribed register to he maintained by the P.W.T. Creep in excess of 150 mm (6 inches) should not be permitted on standard track and at one location not more than six consecutive rails should be found jammed in single rail track. In approaches of points & crossings, there should be no creep.
Adjustment of Creep :
When the creep becomes excessive (more than 6 inches) causing maintenance problems, the same should he adjusted by pulling back. The work is carried out under the protection of engineering signals after necessary caution order is given. A careful survey should be carried out of the expansion gaps and of the present position of rail joints.
The total creep proposed to be adjusted, the correct expansion gaps to be provided and the length of track to be tackled in one operation, should be 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 loosened removed. The rails are then pulled back one by one with the help of a rope attached to the hook.
Pulling back should be regulated in such a way that the rail joints remain central on the joint sleepers. Pulling back rail by rail is a slow process and can be done only for short isolated lengths. About 40 to 50 men per kilometer are normally required for adjusting the creep.
When creep is required to be adjusted for longer lengths, five rail lengths are tackled at a time. The procedure is almost similar except that instead of pulling by rope, a blow is given by a cut rail piece of about 5 meter length.
A creep adjuster is normally used when the work involved is of bigger nature. The creep adjuster is set at the center of the length to be tackled at a time, with the wide joints behind it and tight joints ahead of it. Expansion liners of correct size are then put in all the expansion gaps.
All the keys on this side of creep adjuster are removed and all fish bolts loosened. The creep adjuster is then made to operate so that it closes up the gaps to the required dimensions by pushing the rails forward and a gap of few inches between the rail ends opposite the adjuster is left behind.
The corrected rails are then keyed up. After that the rails on the other side of the adjuster are tackled. The keys are removed on that side and the fish bolts loosened. The rails are then pulled by the adjuster to the extent required, providing the necessary expansion gaps between the rail ends with the help of liners.
The operation leaves some of the expansion gaps to be too big which are tackled in the next position of the creep adjuster. The corrected rails are then keyed and the adjuster is shifted to the new position. The whole process is repeated again and again till such time the whole length is attended.
Remedial measure for the prevention of the Creep of Rail
- Pulling back the rail to original position.
- Pulling of anchors and anti creepers should be done .
- One should increase no of sleepers per rail length.
- One should provide sufficient ballast and packing with care.
- For good grip steel sleepers are used.
The creep anchor consist of cast pieces which are made to grip the rail. The creep anchor placed behind the sleeper for every third or fourth sleeper.
This arrangement prevents the movement of rail because the sleepers which are embedded in the bucket will also have to move if the creep has to take place.
The no of creep anchor are per rail length and it should be determined by the intensity of creep.
As per the Indian practices, 4 creep anchors per rail are provided for creep of 75 mm to 150 mm in the month and 6 creep anchor / rail are provided for the creep of 225 mm to 250 mm in month.
The additional creep anchors should be provided on level crossing or at places where heavy break applications are made near to station.