Off-track tampers are portable and can be quickly taken off the track by just two people. These tampers work during the interval between the passage of trains and do not require any traffic blockage (Fig. 20.1). They consist of tools driven by compressed air, electricity, or petrol. There are generally two types of tampers, namely, self-contained tampers and those that are worked from a common power unit. Tampers may be vibratory or of the percussion type or a combination of both. In the vibratory type, tamping is achieved by vibration as well as by the weight of the tamper itself, while in the percussion type. tamping is achieved by imparting blows. The important off-track tampers tried on Indian Railways are Cobra tampers, Jackson tampers, Shibaura tampers, and Kango tampers.
1 Use of Off-track Tampers
Off-track tampers are placed diagonally under the rail and worked in pairs from the opposite sides of the sleepers in order to ascertain the maximum consolidation of the ballast. Using beaters, the ballast is first loosened around the rail seat in the crib for a length of 450 mm (18") on either side of the foot of the raft. The tamper is then inserted vertically and the tamping tool blades are kept about 75 to 100 mm away from the sleeper so that enough ballast is available between the two as shown in Fig. 20.1. During its working, the head of the tamper should be moved slightly backward in the shape of an arc of a circle so that the surrounding ballast is well compacted. The operator should not exert force on the tamper while tamping is being done using either the vibratory system or the percussion system.
The average progress achieved by one set of off-track tampers is about 3 km per month, after taking the repairs, overhauling , etc. into consideration.
2 Limitations of Off-track Tampers
Off-track tampers have not been much of a success on Indian Railways because of the following reasons.
(a) The maintenance of these tampers has been found to be extremely difficult because of the non-availability of spare parts, which are mostly imported.
(b) Transporting off-track tampers along with their power units to the site of work in the mid-section is quite problematic.
(c) Tamping with off-track tampers is very strenuous and a worker normally gets fatigued after 30-40 minutes. The quality of work done after this duration is likely to deteriorate.
(d) Intensive supervision is required to ensure the correct use of these tampers so that the work done is of the desired quality. This type of supervision becomes particularly difficult in the mid-section.
(e) The quality of tracks maintained using tampers is not very high compared to those maintained by manual methods.
(f) The use of off-track tampers following deep screening and relaying work has been found to be very unsuitable for the early restoration of normal
However, off-track tampers are yet to be tried under the following conditions on Indian Railways.
(a) For packing the points and crossings where normal packing cannot be done effectively due to limited space.
(b) For packing newly realigned curves, on which the track requires immediate consolidation so that its alignment remains undisturbed and normal speed can be restored along its length as early as possible.