Pin-Brazing History and Overview
When first introduced in 1950, Pin brazing was described as 'A revolutionary solution to an old problem' wherein the attachment of rail bonds could be accomplished in just one second by means of a revolutionary new automatic brazing method.
The electrical current for railway signalling installations is fed through the rails. This is the case both for ordinary passage signals and for block signals. On electrified railways the power current for driving the train also passes through the rails.
Irrespective of weather the rails are used for conductors for signalling or power currents and it is a disadvantage that the resistance at the rail joints is relatively high and variable. The high resistance is due to the fact that the current has to pass from the rails to the fishplates and back to the rails again. Since both the rails and the surfaces of the fish plates are more or less rusty and abut against one another with varying pressures, it follows that the resistance will also vary appreciably.
Numerous attempts have been made to bridge over the rail joints by a more stable current path in the form of so-called rail bonds attached in different ways. These consist of a flexible copper cable provided at its ends with lugs. In the initial stages different forms of mechanical attachment were tried out, but connections of this kind tended to work loose in the course of time. It was natural therefore, when seeking for a more stable form of connection, to employ welding and brazing. For this purpose the copper cable was welded or brazed to the lugs, which in turn were fixed to the rails in the same manner.
The welded or brazed contact bonds had a low stable electrical resistance. However the process of attaching them had been found to give rise to difficult problems however, in the solution of which many different methods had been tried. Owing to the high thermal capacity of the rails it was necessary to heat them for relatively long periods in order to bring the material to the melting or brazing temperature, with the result that the copper cable also becomes very hot and is exposed too long to the detrimental effect of the heat. The heating up of the copper acts on the latter in such a way as to cause it's strength to deteriorate. The copper becomes brittle, in part owing to the precipitation of cuprous oxide at the grain boundaries and partly on account of the abnormal grain growth due to intense heating for relatively long periods. Oxide precipitation begins at 800 ºC and grain growth at 600 ºC.
If a rail bond is examined when a train is passing over it, it will be observed that the whole joint is pressed downward and that the end of the rail is raised in relation to the other. The rail end over which a wheel is moving, is pressed down further than the adjacent end. The copper cable of the contact is thus subjected to repeated bending each time a wheel passes over a joint. The cable will then bend along its whole length but owing to the softness of the material, this will not damage it. At the point of transition between the lug and the copper cable, however, this repeated bending will have a more dangerous effect if the copper in this region has become brittle owing to the heat set up in welding or brazing, so that it may eventually break after having been in service for some time. It is therefore important that the copper cable should be exposed as little as possible to heating, which could cause deterioration of its structure.
A solution was found in a new method, which was termed Pin Brazing. This utilised a silver brazing material, which had a low melting point but was however very expensive. It was, therefore, necessary to select a method in which dosage was just sufficient to render brazing possible. With this in mind a specially designed Brazing pin was produced. Consisting mainly of brass but having a flux and silver solder alloy tip. Today the pin brazing method is virtually the same process. The pin is inserted into a chuck, which is connected to one pole of a source of electric current. The other pole of this supply source is connected to the work-piece on which brazing is to be carried out by the pin. The pin is then lowered into contact with the work-piece whereupon the circuit is closed. The current which passes through an electromagnet on its path to the chuck sets up a magnetic force at the moment of short-circuit which raises the chuck together with the pin fixed in it. An arc is struck which heats up the work piece and sprays the brazing material at the point of the pin down onto the work-piece. An annular porcelain Ferrule surrounding the point of the brazing pin protects the brazing position during the time the arc is burning. A time control device connected in the circuit interrupts after an accurately permitted time. When the current ceases to flow through the electromagnet, the magnetic force disappears and a spring pushes the metal pin down into the melted bath, after which the braze solidifies and the pin is rigidly brazed to the underlying base.
How the Pin Brazing method is employed for brazing rail bonds to the rails
The rail bond which is specially designed for the pin brazing method consists of a copper cable with lugs fixed to its two ends. The lugs are constructed in the form of tubes, the entire length of which is threaded over a copper cable. The outer end of the lug together with the conductors which pass through it are pressed together so that a flat end piece is obtained. A hole is driven both through the flattened tube walls and the conductor wires.
The rail bond is brazed to two rail-ends adjoining one another in such a way that the flat side of the cable lug is laid against the side of the railhead. The brazing pin is placed in the chuck of a so-called brazing gun inserted in the hole on the rail bonds. Current is passed through the brazing pin for a short time and during the period the arc is maintained, the flux and brazing material within the pin are melted and flow between the contact surfaces of the lug and the rail, the walls of the hole and the pin and also between the conductors, which are pressed together inside the lug. On the interruption of current the metal pin is forced down into the hole and brazed tightly to the rail and the rail bond. The shaft of the pin is then broken off level with the kerf so that only that the part below the kerf remains in the hole on the rail bond. By this means the cable ends, the surrounding parts of the lug, the end of the metal pin and the brazing material are caused to form a single compact unit with the adjoining parts of the rail.
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