Welded Joints. Several forms of welded joints are in use. All these welded joints fasten the ends of the rails together so that the rail is practically continuous—just as if there were no joints—so far as the running surface of the rail is concerned. It was thought at one time that a continuous rail would be an impossibility because of the contraction and expansion of the rail under heat and cold, which, it was thought, would tend to pull the rails apart in cold weather and to cause them to bend and buckle out of line in hot weather. Experience has conclusively shown, however, that contraction and expansion are not to be feared when the track is laid in a street where it is covered with paving material or dirt. The paving tends to hold the track in line, and to protect it from extremes of heat and cold. The reason that contraction and expansion do not work havoc on track with welded joints, is probably that the rails have enough elasticity to provide for the contraction and expansion without breaking.
It is found that the best results are secured by welding rail joints during cool weather, so that the effect of contraction in the coldest weather will be minimum. In this case, of course, there will be considerable expansion of the track in the hottest weather, but this does not cause serious bending of the rails; whereas occasionally, if the track is welded in very hot weather, the contraction in winter will cause the joint to break.
PORTABLE CUPOLA FOR CAST-WELDING JOINTS OF STREET CAR RAILS.
Cast-Welded Joints. The process of cast-welding joints consists in pouring very hot cast iron into a mould placed around the ends of the rails. These moulds are of iron; and to prevent their sticking to the joint when it is cast, they are painted inside with a mixture of linseed oil and graphite. Iron is usually poured so hot that, before it cools, the base of the rail in the center of the molten joint becomes partially melted, thus causing a true union of the steel rail and cast-iron joint. This makes the joint solid mechanically and a good electrical conductor. To supply melted cast iron during the process of cast-welding joints on the street, a small portable cupola on wheels is employed. [Fig. 79] gives an idea of the process of making cast-welded joints.
Fig. 79. Process of Cast-Welding Joint.
Electrically Welded Joints. An electrically welded joint is made by welding steel blocks to the rail ends. A steel block is placed on each side of the joint, and current of very large volume is passed through from one block to the other. This current is so large that the electrical resistance between the rail and steel block causes that point to become molten. Current is then shut off, and the joint allowed to cool. There is in this case a true weld between the steel blocks and the rails and joint. An electric welding outfit being expensive to maintain and operate, this process is used only where a large amount of welding can be done at once. Current is taken from the trolley wire. A rotary converter set takes 500-volt direct current from the trolley wire, and converts it into alternating current. This alternating current is taken to a static transformer which reduces the voltage and gives a current of great quantity at low voltage, the latter current being passed through the blocks and rails in the welding process. A massive pair of clamps is used to hold the blocks against the rails, and to conduct the current to and from the joint while it is being welded. These clamps are water-cooled by having water circulated through them so that they will not become overheated at the point of contact with the steel blocks.
Thermit Welding. A process of welding rail joints which was developed after the cast-welding and electric-welding processes, is known as the Goldschmidt process, which makes use of a material called “thermit” for supplying heat to make the weld. A mould is placed around the joint and the thermit is put in this mould and ignited. The heat produced by the thermit is so intense as to reduce the iron in the thermit mixture and make a welded joint. The thermit consists of a mixture of finely powdered aluminum and iron oxide. When this is ignited, the aluminum oxidizes, that is, absorbs oxygen so rapidly that an intense heat is the result. In the process of oxidation, the aluminum takes the oxygen from the oxide of iron, leaving molten metallic iron, which metallic iron makes the weld by union with the molten rail ends. This process has the advantage over other welding processes, of not requiring an elaborate apparatus and a large crew of men to operate it; and consequently it can be used where but a few joints are to be welded.
