Arc welding

Shielded metal-arc welding accounts for the largest total volume of welding today. In this process an electric arc is struck between the metallic electrode and the workpiece. Tiny globules of molten metal are transferred from the metal electrode to the weld joint. Arc welding can be done with either alternating or direct current. A holder or clamping device with an insulated handle is used to conduct the welding current to the electrode. A return circuit to the power source is made by means of a clamp to the workpiece.

Gas-shielded arc welding, in which the arc is shielded from the air by an inert gas such as argon or helium, has become increasingly important because it can deposit more material at a higher efficiency and can be readily automated. The tungsten electrode version finds its ma­jor applications in highly alloyed sheet materials. Either direct or alternating current is used, and filler metal is added either hot or cold into the arc. Consumable elec­trode gas-metal arc welding with a carbon dioxide shield­ing gas is widely used for steel welding. Metal transfer is rapid, and the gas protection ensures a tough weld.

Submerged arc welding is similar to the above except that the gas shield is replaced with a granulated mineral material as a flux.

Wettability of metals

Carbon and low-alloy steels are the most widely used materials in w^'ded construction. Carbon content largely

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determines the weldability of carbon steels. Low-alloy steels are generally regarded as those having a total al­loying content of less than 6 percent. There are many grades of steel available, and their relative weldability varies.

Aluminum and its alloys are also generally weldable. A very thin oxide film on aluminum tends to prevent good metal flow, however, and suitable fluxes are used for gas welding. Fusion welding is more effective with alternat­ing current when using the gas-tungsten arc process to enable the oxide to be removed by the arc action.

Copper and its alloys are weldable, but the high ther­mal conductivity of copper makes welding difficult. Me­tals such as zirconium, niobium, molybdenum, tantalum, and tungsten are usually welded by the gas-tungsten arc process. Nickel is the most compatible material for join­ing, is weldable to itself, and is extensively used in dis­similar metal welding of steels, stainless steels and cop­per alloys.

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Gear

Gear is a toothed wheel or cylinder used to transmit rotary or reciprocating motion from one part of a ma­chine to another. Two or more gears, transmitting mo­tion from one shaft to another, constitute a gear train. At one time various mechanisms were collectively called gearing. Now, however, gearing is used only to describe systems of wheels or cylinders with meshing (постоянное зацепление) teeth. Gearing is chiefly used to transmit rotating motion, but can, with suitably designed gears and flat-toothed sectors, be employed to transform re­ciprocating motion into rotating motion, and vice versa.