Sleepers in Railways
Sleepers are members generally laid transverse to the rails. On sleepers, the rails are supported and fixed, to transfer the loads. The load transfers from rails to sleeper, sleeper to ballast, ballast to subgrade below.
Functions of sleepers
- hold the rails to correct gauge.
- hold the rails in proper level or transverse tilt.
- act an elastic medium in between the ballast and rails to absorb vibrations.
- distribute the load from rails to ballast (or girders in case of bridges).
- support the rails at proper level in straight tracks and at proper super elevation on curves.
- also add to the longitudinal and lateral stability of the permanent track on the whole.
- also provide means to rectify track geometry during service life.
Requirements of sleeper
- Sleeper to be used should be economical i.e. they should have minimum possible initial and maintenance cost.
- Fittings of the sleeper should be such that they can be easily adjusted during maintenance operations.
- Weight of sleeper should not be too heavy or excessive light i.e. ease for handling.
- Designs of sleeper should be such that the gauge, alignment of track and levels of the rails can be easily adjusted and maintained.
- Bearing area of sleeper below the rails seat and over the ballast should be enough to resist the crushing due to rail seat; and crushing of the ballast underneath the sleeper.
- Sleeper design and spacing should be such as to facilitate easy removal and replacement of ballast.
- Design of the sleeper should be such that they are not pushed out easily due to moving trains especially with steel sleeper with rounded ends.
- Insulation of rails should be possible for track circuiting, if required, through sleepers.
Classification of sleepers
Sleepers can be classified according to the materials used in their construction, in the following categories –
- Wooden sleeper
- Metal sleeper (i) Cast iron (ii) Steel
- Concrete sleeper (i) Reinforcement concrete (ii) Prestressed concrete
Different types of sleepers are manufactured, are described in the following steps –
1. Wooden or Timber sleepers
Wooden sleepers are regarded to be best. These are ideal sleepers. The life of timber sleepers depends upon their ability to resist
- Attack by vermin i.e. white ants
- Quality of timber used
- Timber is easily available.
- Fittings for wooden sleeper are few and simple in design.
- Wooden sleeper is easy to lay, relay, pack, lift and maintain.
- Wooden sleeper is suitable for all types of ballast.
- Overall more economical.
- The sleepers are subjected to wear, decay, attack by white ants, spike killing, wrapping, cracking, end spitting, rail cutting etc.
- It is difficult to maintain the gauge in case of wooden sleepers.
- Track is easily disturbed i.e. alignment maintenance is difficult.
- Service life is minimum (12 to 15 years) aa compared to other types of sleepers.
- Maintenance cost is highest.
Types of Timber for sleepers
- Hard wood such as sal and teak
- Soft wood such as chir and deodar
2. Metal sleepers
Due to growing scarcity of wooden sleeper, their high maintenance cost and short life; metal sleepers are now being adopted in India.
Metal sleeper are either of steel or cast iron. Cast iron is largely used because it is less prone to corrosion.
- Metal sleepers are uniform in strength and durability.
- In metal sleeper, the performance of fittings is better and hence lesser creep occurs.
- Metal sleepers are economical, as life is longer and maintenance is easier.
- Gauge can be easily adjusted.
- For metal sleeper, frequent renewal is not required.
- They have good scrap value and not susceptible to fire hazards.
- More ballast is required.
- Greater number of fittings are required, which is difficult to maintain and inspect.
- These are liable to rusting/corrosion.
- Metal being good conductor of electricity interferes with tracking circuit.
- These are unsuitable for bridges, level crossings and also points and crossings.
- Therefore these are suitable only for stone ballast.
Types of cast iron sleeper
- Pot or bowl sleeper
- Plate sleeper
- Box sleeper
- C.S.T.-9 sleeper (combination of plate and box type)
- Rail free duplex sleeper
Types of steel sleeper
- Lugs or Jaws pressed out of metal
- With loose lugs or Jaws
- Clip bolt type
- Saddle or spring type
3. Concrete sleepers
These sleeper were ended due to chronic shortage of good wooden sleeper and need for better design and economy of sleeper on sustainable basis.
- These sleeper are free from natural decay and attacks by vermin, insects etc.
- Maximum life time i.e. about 40 to 60 years.
- Not affected by moisture, chemical action of ballast, cinder and sub-soil salt.
- There is no difficulty in the track circuiting also, required for electrifying the track.
- Greater stability of track and thus better resistance against temperature variation.
- The sleeper have higher elastic modulus and hence withstand stresses induced by fast and heavy traffic.
- Concrete sleeper in the elastic fastenings offer an ideal track in every respect such as gauge, cross-level, alignment.
- Weight of sleeper is as high as 2 to 3 times of wooden sleeper, requiring mechanical appliances for handling.
- These sleeper require pads and plugs for spikes.
- They damage the bottom edge during the packing.
- Scrap value is almost nil.
- Damages to the concrete sleeper is very heavy in case of derailment.
On the other hand, main disadvantages of Prestressed concrete sleeper are –
- Heavily damaged in case of derailments.
- The bed of the ballast is specially prepared.
- These are uneconomical.
- More rigid in nature.
- Design is complicated.
Types of concrete sleepers
These sleeper are mainly of two types –
- Reinforced concrete sleeper
- Prestressed concrete sleeper
Further concrete sleeper is of two types –
- Through type
- Composite or block and tie type
Comparison of all types of sleepers
Sleeper density or spacing of sleeper
The space between two adjacent sleepers determines the effective span of the rail over the sleepers. The spacing of sleepers, therefore, in a track depends on axle load and lateral thrust. Axle load which the track is expected to carry and thus lateral thrust of locomotives to which it is subjected.
Therefore, Number of sleepers in a track is indicated by the number per rail length. However the number of sleepers can’t be increased indefinitely as certain minimum space between sleepers is required for packing of ballast.
In India, this minimum packing distance is kept 30.5 to 35.5 cm except at joints. The number of sleepers per rail varies from M+4 to M+7 for main tracks.
In America, this distance is reduced to 25 cm. The number of sleepers per rail varies from M+9 to M+11 for tracks.
Where, M = length of rail in meters
In Britain, N+4 sleepers are used.
Where, N = length of rail in yards
Therefore, sleeper density is the number of sleepers per rail length and it is specified as M+x or N+x.
Factors governing the sleeper density
- Axle load and speed
- Type and section of the rails
- Type of ballast and ballast cushion
- Strength and type of sleeper i.e. bearing area of particular sleeper on the ballast
- Nature of foundation
Spacing of sleeper is always not uniform. Thus three or four sleepers on either side of the joints are kept down close together. The spacing between joint sleeper is 30 to 45 cm, whereas intermediate it is 75 to 90 cm or as per specifications.