On-track tampers are self-propelled vehicles which facilitate automatic tamping of sleepers through the controls provided in the operator's cabin. Heavy on-track tampers weigh 20 to 30 t and cannot be easily removed from the track. It is therefore vital that the work be done after putting up the necessary traffic blocks. However, the tamper can be put off the track using special equipment, provided that there are adequate bank extensions available. These tampers can automatically and simultaneously perform the tasks of lifting, aligning, levelling, and tamping. On American railroads, a sleeper is referred to as a tie. Therefore, since on-track tampers are used for tamping sleepers (or ties), they are also called tie tamping machines.
20.3.1 Principles of Working of On-track Tampers
The principles of the working of an on-track tamper (tie tamping machine) are described below.
Tamping is the most important application of tie tamping (TT) machines. Tamping consists of packing the ballast under the sleeper. This is achieved by vibrating the ballast, thereby making it fluid, and then compressing it by squeezing as shown in Fig. 20.2. Tamping is done with the help of either 16 or 32 tamping tools, depending upon whether single or double sleepers are to be packed at a time. Tamping is done by either the synchronous or the non-synchronous system of vibration. In the synchronous system, the movements of the two tamping tools on either side of a sleeper are similar and simultaneous, synchronizing with each other. In the case of non-synchronous systems, the two tools work independently.
Fig. 20.2 Squeezing of ballast
Aligning the track
The alignment of the track is corrected by the two-chord system in the case of machines manufactured by Messrs Plasser and Theurer. In this method, two chords of lengths 24 m and 12 m are taken and placed in the area between two rails, where they are stretched parallel to the track and kept a certain distance apart from each other.
BB'= (3/4) » CC' BQ = (1/3) x BB'
Fig. 20.3 Principle of alignment-two-chord system
It can be geometrically proved that the versine BQ of the short chord AC measured at a quarter point B' is one-third of the ordinate BB' measured for a long chord at the same quarter point (Fig. 20.3). The ordinate at the quarter point of the 24-m chord H and the versine at the centre of the 12-m chord h are measured using a measuring bogie. In a circular curve, the ratio of H:h is equal to 3:1 as indicated above. The two dimensions are measured at the same point by means of the measuring bogie and if the ratio is found to be equal to 3:1, it indicates that the curve is in order and that the alignment is correct. In the case of any defects, special rollers attached to the rail are used to slew the track at the location of the centre bogie till this ratio is achieved.
This procedure is used for correcting alignment defects to a value that is one-sixth the value of the original fault because of the relative positions of the measuring bogie and the central bogie as shown in Fig. 20.4. The latest TT machines can be used to correct alignment defects using the three-chord and four-chord methods.
Longitudinal levels are corrected on the basis of the principle of proportional levelling with the help of tampers along with an infrared transmitter, a shadow
board, and photocells. The distance between these three units is fixed and is so arranged that any error in the longitudinal level is reduced to one-fifth of its value, as is clear from Fig. 20.5.
Fig. 20.4 Correction of alignment defect