Copper is one of the most widely used non-ferrous metals in industry. It is not found in pure state from under the earth. It occurs in some minerals such as copper glance (Cu2S), copper pyrites (CuFeS2)’ malachite (CuCO3.CuO2H2) and azurite (3CuCO3.CuO2). In India, copper ores are found at Ghatsila in Bihar and Khetri in Rajasthan.
Properties and Uses
It is a soft, malleable and ductile metal with a reddish-brown appearance. Its specific gravity is 8.9 and melting point is 1083°C. It is a good conductor of electricity. It is largely used in making electric cables and wires, for electric machinery and appliances. It is used for making coins and household utensils. It may be cast, forged, rolled and drawn into wires. It is non-corrosive under ordinary conditions and resists weather very effectively. Copper in the form of tubes is used widely in mechanical engineering. It is also used for making ammunitions. It is used for making useful alloys with Tin, Zinc, Nickel and Aluminium.
Copper may be alloyed with a wide range of other elements to produce many different alloy groups of industrial importance. The most important are:
- Copper aluminium (the aluminium bronzes).
- Copper tin antimony (Babbitt metal).
- Copper tin (the tin bronzes).
- Copper tin phosphorus (the phosphorus bronzes).
- Copper zinc (the brasses).
- Copper nickel.
Aluminium Bronze. Copper alloys with Aluminium and forms aluminium bronze which contains 6% and 10% of Aluminium and Copper respectively. Aluminium gives the alloy lightness, while the addition of Copper to pure Aluminium increases its strength.
Uses. Aluminium bronzes are widely used for making gears, propellers, condenser bolts, pump components, tubes, air pumps, slide valves and bushings etc. Cams and rollers are also made from this alloy. The 6% aluminium alloy has a fine gold colour which is used for imitation jewellery and decorative purposes.
Lead is the heaviest of the common metals. It is extracted from the lead ore called galena, which is a sulphide of lead. India is having very little lead ores. Lead has a bluish grey colour and a dull metallic lusture, but this is lost on exposure to the air, the surface becoming dull grey.
Properties and Uses. Lead is very soft, malleable and ductile metal and can be rolled easily. It is resistant to corrosion and many acids have no chemical action on it. Because of this, it is used for water pipes, roof covering, sheathing the electric cables and for construction material of chemical plants. The melting point of lead is 327°C.
Lead Base Alloys
The lead base alloys are employed where a cheap and corrosion resistant material is required. Alloy containing 83% Lead, 15% Antimony, 1.5% Tin and 0.5% Copper is used for large bearings subjected to low load service.
Tin is obtained from tinstone and oxide by a refining process carried out in a reverberatory furnace. In India, its sources are quite negligible. It is a brilliant white metal with a yellowish tinge. The melting point of tin is 232°C.
Properties and Uses. Soft, malleable and ductile, it can be rolled into very thin sheets. Tin does not corrode in wet and dry climates, making it useful as a protective coating for iron and steel. It is also used for tinning copper wire before the latter is made into cables.
Bearing metals may be classified into following groups:
- Copper-base bearing metals containing Copper, Tin and Lead.
- Tin-base bearing metals containing Tin, Antimony and Copper.
- Lead-base bearing metals containing Lead, Tin and Antimony.
- Cadmium-base bearing metals containing Cadmium and Nickel.
Those alloys in which Tin, Lead and Cadmium are predominating elements are designated as white-metal bearing alloys. They have the properties of high plasticity combined with the low hardness and comparatively low melting point, which facilitates the formation of bearing by casting the metal directly and usually require no machining. (Refer Fig 12.6)
The Copper-base alloys are harder and stronger than the white metals and are used for bearings, which require resisting heavier pressures. Tin-base white metals are used where bearing are subjected to high pressure and load, whereas for light loads and pressure, lead-base alloys are used. Of the lead-base alloys, the alloys, which contain 80 percent, Lead and 20 percent Antimony is generally used. Cadmium-base bearing metals have more favourable properties especially at elevated temperatures, than the tin-base alloys. The compressive strength of the cadmium-base bearing alloys is greater than those of the tin-base alloys.
To give an efficient bearing combination, the following conditions are necessary:
- That the shaft and the bearing should be dissimilar in nature and the bearing should be softer than the shaft.
- Bearing with a softer body embedded with smaller pieces of a comparatively hard metal will give more efficiency.
- The bearing metals should have sufficient compressive strength to carry the bearing pressure.
- The bearing should develop a minimum of friction when they actually come in contact, as for example, when a shaft is starting or stopping.
- The bearing should wear to smooth surfaces as they rub together.
Tin Base Alloys (Babbitt Metal)
A tin base alloy containing 88% Tin, 8% Antimony and 4% Copper is called Babbitt metal. It is a soft material with a low coefficient of friction and has little strength. It is the most common bearing metal used with cast iron boxes where the bearings are subjected to high pressure and load.