Although a rail joint has always been an integral part of the railway track, it is looked upon as a necessary evil because of the various problems that it presents. Earlier, rails were rolled in short lengths due to difficulties in rolling and the problem of transportation. With increase in temperature, rails expand and this expansion needs to be considered at the joints. It was, therefore, felt that the longer the rail, the larger the required expansion gap, and this too limited the length of the rail. A rail joint is thus an inevitable feature of railway tracks, even though it presents a lot of problems in the maintenance of the permanent way. This chapter discusses the various types of rail joints and their suitability on a railway track.
ILL EFFECTS OF A RAIL JOINT
A rail joint is the weakest link in the track. At a joint, there is a break in the continuity of the rail in both the horizontal and the vertical planes because of the presence of the expansion gap and imperfection in the levels of rail heads. A severe jolt is also experienced at the rail joint when the wheels of vehicles negotiate the expansion gap. This jolt loosens the ballast under the sleeper bed, making the maintenance of the joint difficult. The fittings at the joint also become loose, causing heavy wear and tear of the track material. Some of the problems associated with the rail joint are as follows.
Due to the impact of moving loads on the joint, the packing under the sleeper loosens and the geometry of the track gets distorted very quickly because of which the joint requires frequent attention. It is generally seen that about 30 per cent extra labour is required for maintenance of a joint.
Bonded main line 6-bolt rail joint on a segment of 76.9 kg/m rail. Note how bolts are oppositely oriented to prevent complete separation of the joint in the event of being struck by a wheel during a derailment.
The life of rails, sleepers, and fastenings gets adversely affected due to the extra stresses created by the impact of moving loads on the rail joint. The rail ends particularly get battered and hogged and chances of rail fracture at joints are considerably high due to fatigue stresses in the rail ends.
A lot of noise pollution is created due to rail joints, making rail travel uncomfortable.
Wherever there is a rail joint, there is a potential danger of the removal of fish plates and rails by miscreants and greater susceptibility to sabotage.
Impact on quality
The quality of the track suffers because of excessive wear and tear of track components and rolling stock caused by rail joints.
The presence of rail joints results in increased fuel consumption because of the extra effort required by the locomotive to haul the train over these joints.
REQUIREMENTS OF AN IDEAL RAIL JOINT
An ideal rail joint provides the same strength and stiffness as the parent rail. The characteristics of an ideal rail joint are briefly summarized here.
Holding the rail ends:
An ideal rail joint should hold both the rail ends in their precise location in the horizontal as well as the vertical planes to provide as much continuity in the track as possible. This helps in avoiding wheel jumping or the deviation of the wheel from its normal path of movement.
An ideal rail joint should have the same strength and stiffness as the parent rails it joins.
The joint should provide an adequate expansion gap for the free expansion and contraction of rails caused by changes in temperature Flexibility It should provide flexibility for the easy replacement of rails, whenever required.
Provision for wear:
It should provide for the wear of the rail ends, which is likely to occur under normal operating conditions.
It should provide adequate elasticity as well as resistance to longitudinal forces so as to ensure a trouble-free track.
The initial as well as maintenance costs of an ideal rail joint should be minimal.
TYPES OF RAIL JOINTS
The nomenclature of rail joints depends upon the position of the sleepers or the joints.
Classification According to Position of Sleepers
Three types of rail joints come under this category.
In this type of joint, the ends of the rails are supported directly on the sleeper. It was expected that supporting the joint would reduce the wear and tear of the rails, as there would be no cantilever action. In practice, however, the support tends to slightly raise the height of the rail ends. As such, the run on a supported joint is normally hard. There is also wear and tear of the sleeper supporting the joint and its maintenance presents quite a problem. The duplex sleeper is an example of a supported joint (Fig. below).
In this type of joint, the ends of the rails are suspended between two sleepers and some portion of the rail is cantilevered at the joint. As a result of cantilever action, the packing under the sleepers of the joint becomes loose particularly due to the hammering action of the moving train loads. Suspended joints are the most common type of joints adopted by railway systems worldwide, including India (Fig.)
The bridge joint is similar to the suspended joint except that the two sleepers on either side of a bridge joint are connected by means of a metal flat [Fig. (a)] or a corrugated plate known as a bridge plate [Fig. (b)]. This type of joint is generally not used on Indian Railways.
|(a) Bridge joint with metal flat Joint
Classification Based on the Position of the Joint
Two types of rail joints fall in this category.
In this case, the joints in one rail are exactly opposite to the joints in the other rail. This type of joint is most common on Indian Railways (Fig. below).
In this case, the joints in one rail are somewhat staggered and are not opposite the joints in the other rail. Staggered joints are normally preferred on curved tracks because they hinder the centrifugal force that pushes the track outward (Fig. below).
WELDING A RAIL JOINT
The purpose of welding is to join rail ends together by the application of heat and thus eliminate the evil effects of rail joints.
There are four welding methods used in railways.
a) Gas pressure welding
b) Electric arc or metal arc welding
c) Flash butt welding