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Read the text. The principal function of the Permanent Way (PW) is accepting the imposed dynamic and static loads as rolling stock runs over the track






The principal function of the Permanent Way (PW) is accepting the imposed dynamic and static loads as rolling stock runs over the track. Structural design of the Permanent Way depends on the track class determined by traffic density and maximum speed of freight and passenger trains on the definite track section. According to traffic density all tracks are divided into 6 groups (A, B, C, etc.) and according to maximum speed – into 7 categories.

e.g. Track 2A4 means: 2 - class

A - group

4 - category

Permanent Way is the structure consisting of the rails, sleepers ( bridge timbers, etc.), ballast, fastenings, as well as turnouts and crossings.

The rail can be seen as the most important component of the track structure and has the following functions:

- it guides the wheels of the rolling stock;

- it accommodates the wheel loads and distributes these loads over the sleepers or supports;

- it provides a smooth running surface and distributes accelerating and braking forces by means of adhesion;

- it acts as an electrical conductor on electrified lines and conducts signal currents.

The parts of the rail (Figure 4.2) are known as the head, the web and the foot.

 
Figure 4.2

 

For reliable service the rails must be strong, durable and resistant to wear.

According to weight and profile the rails are classified into three main types (Figure 4.3):

Type Weight, kg/m
R 50 51, 67
R 65 64, 72
R 75 74, 71
Figure 4.3

Sleepers are a very important part of the track structure. They are used:

- to support the rails and keep them at the correct gauge;

- to transfer the loads from the rails to the ballast.

The basic requirements for sleepers are: minimum initial and maintenance cost, strength, elasticity, possibilities for track circuiting.

Three types of sleepers are used in railways: wooden, concrete and steel. The advantages of wooden sleepers are the following: easy to manufacture and handle, suitable for track circuiting, good dampening effect and correction to alignment is easy. At the same time the wooden sleepers have some disadvantages: short service life (15-17 years) due to decay wear, decreasing availability of timber.

Concrete sleepers are preferred for use nowadays. They are progressively replacing wooden sleepers because of long service life (40-50 years), stability of the track (due to greater weight and size), smooth running of trains, low maintenance cost. On the other hand concrete sleepers have some disadvantages: difficult handling (due to great weight), susceptibility to shock and impact, high rigidity and current conductivity.

Steel sleepers are comparatively light, don’t decay and are commonly used in tropical countries. Principal technical difficulties are the unsuitability of steel sleepers on electrified lines because of current leakage. Some more disadvantages are corrodibility and formation of cracks.

Ballast has to provide the following functions:

- to transmit the sleeper pressure to the subsoil as evenly as possible;

- to have high resistance to longitudinal and lateral sleeper displacement;

- to ensure the track elasticity in order to minimize dynamic forces;

- to provide effective drainage;

- to enable maintainability.

The most commonly used type of ballast is crushed rock. But gravel, crushed gravel, rock wool, crushed shells and sand are also in use.

 

 

 







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