Modern Research Improves the Breed
Page 44
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Developments in Nickel-bronze Alloys, Copper Materials, Bimetal Strip for Thermostats, Nickel-chromium Stainless Steel, Design of Engine Valves, Die Casting and Tungsten Plating
ARAPID survey of the efforts being put forward at present by chemists and metallurgists all over the world would discover many items of intense interest to the commercial-motor industry. In Germany, for example, illuminating gas as fuel for vehicles is more than ari experiment; it is an established -fact.
Several German cities have established filling stations for providing gaseous fuel. Berlin buses are supplied with illuminating gas, as fuel, in this way. A compressor maintains in three gas flasks, with a combined capacity of 750 litres, a pressure of 350 atmospheres. In the gas holders in the buses, a pressure of 200 atmospheres is desired. This is furnished by a pressure-reducing valve.
Automatic Compression.
. Whenever the pressure in the fillingstation flasks has decreased to 280 atmospheres, the compressor starts up automatically and runs until the flask pressure is again 350 atmospheres. Thus, the filling time for a bus has been reduced to six minutes, as against 35 minutes before the introduction of the intermediate flasks.
In the United States, nickel is being added to bronze for the purpose of providing new alloys of special value for particular parts and purposes. For example, babbitt-lined bearing backs, bushings, etc., are being made from an alloy containing 88 per cent, copper, 5 per cent. nickel, $ per cent. tin and 2 per cent, zinc.
The nickel raises the fluidity of the metal, increases the tortgliness, gives a better grain structure and, besides improving the elasticity of the alloy, increases its resistance to corrosion.
Similarly, for automobile bushings, an alloy having 85 per cent, copper, 2i per cent. nickel, 2i per cent. tin, 9 per cent, lead and 1 per cent, zinc is being adopted. This alloy has a tensile strength of 12 tons per square in., and an elongation of 12 per cent. in 2 ins.
Alloy for Gear Cones.
Another alloy of commercial-motor interest contains 87.75 per cent, copper, 3 per cent, nickel, 8 per Cent. tin and 1.25 per cent. lead. This is employed for the cones of synchronizer gears in transmission, and has a tensile strength of 15 tons per square in., with an elongation of 12 per cent. in 2 ins., and a hardness of about 90 Brinell.
A new British industry is the production of bimetal strip, which is being introduced on an increasing scale. This material consists of two different metals closely united at the surfaces of contact by brazing or rolling at high temperature, so that both metals are firmly joined together.
a38 The commercial-vehicle and automobile industries are rapidly finding novel uses for this strip—for instance, in the automatic control of the water-circulating system. In this, when the engine is running cold, the radiator is short-circuited and cooling • water is allowed to reach the engine only when the temperature of the water in the engine increases. The valve that enables the radiator to be cut out is • worked by an element composed of bimetallic strip, which is plunged in the water stream itself.
An automatic carburetter has also employed bimetallic strip to advantage. Here, the quality of the mixture is controlled by thermostatic means, in accordance with the temperature of the engine, so that when it is cold, the mixture is rich, and vice versa.
Again, bimetallic strip has been used for the deflection of exhaust gases from vehicle-heating apparatus when the, temperature of the inside of the body rises to a predetermined figure, and vice versa. The feature of the strip that makes these results possible is that the differing rates of thermal expansion in the two united metals cause the strip to warp or distort under heat,
Stainless-steel Bodywork.
In the Near East, one sees nickelchromium stainless steels being employed in the construction of coach semi-trailers of the lightweight streamlined type, for service between Baghdad and Damascus. These vehicles are drawn by 150 b.h.p. tractors and have cut 20 hours off the scheduled time for the existing heavy coaches.
Some ingenious methods of making valves for large buses have been introduced. In one instance, a hollow valve is used. The shank is bored out by means of a twist drill, and the hollow thus formed is filled in with solid sodium, lithium chloride, or some similar substance possessing a relatively low melting point and fairly good heat conductivity.
When the valve is operating in normal use, the material in the stem melts and splashes up and down, transferring the heat from the head down into the shank, whence it is dispersed through the valve guide into the side walls of the engine case and thence to the atmosphere or the water. Valves of this type have shanks measuring from in, to I in., which is rather thicker than that of the ordinary valve. The valve steel used is generally a high chromium-nickel austenitic steel, hut some makers restrict this material to the head, which is welded on to a shank or stem of a steel possessing a slightly lower nickel and chromium content. A third procedure adopted, in some instances, is to cast the valve as a whole.
Zinc-alloy die castings are being increasingly developed for the automobile industry. Carburetters and the like are being made from these alloys, as are automobile radiator grilles.
Zinc-alloy Castings.
A new dip treatment has greatly extended the possibilities of zinc-alloy castings. A film is deposited by the new process and this prevents the casting from corroding on the surface. It is specially advantageous when white salts, formed by entrapped moisture, severe external exposure, or contact with saline atmospheres or materials, are likely to be met with.
In the process, the zinc-alloy castings a re dipped for a second or two in a simple chemical bath of patented design, cleansed and dried in an air blast. Thus, a thin, close film of transparent golden-brown is formed on the surface.
Magnesium die castings are slowly gaining in reputation. A typical alloy contains 10 per cent. aluminium, 0.1 per cent. manganese, 0.5 per cent. silicon, the balance being magnesium. Parts of large and complex design can quite satisfactorily be cast in magnesium alloy, with excellent definition and surface finish, so long as corners are ra.diused, and larger fillets are ad opted .
Beryllium for Hardening.
Surface hardening, or cementation, with beryllium has been another discovery of recent date, and has been given particular attention in France. Cementation consists of forming a surface alloy on a metal part by heating it when embedded in the powder of another metal—in this case, beryllium. At present, beryllium cementation is in the experimental stage, although some practical results are already being claimed.
Finally, one may mention the new tungsten plating which is being developed as a substitute for chromium plating. Tungsten plating has a bright silvery sheen, but is not, as chromium is, attacked by the commoner mineral acids, and can be dissolved only by a mixture of nitric and hydrofluoric acids. The plating solution is said -to be easy to prepare, but it is not perfectly stable, and will gradually deteriorate if kept continually in use.