Machinery consists of complex machines. Such complex machines may be defined as two or more simple machines connected together. There are six simple machines: the lever, the wheel and axle, the pulley, the inclined plane, the wedge, and the screw. These simple machines are all used to help men do different kinds of work—to lift, carry, turn, pulí, and cut. A combination of any of the simple machines is a complex machine. Most complex machines are designed to do special kinds of work.One of the complex machines easiest to understand is the gear. A gear is made up of two simple wheel and axle machines placed next to one another. When one turns, it causes the other to turn. Usually the wheels are of different sizes and have teeth cut into their edges to prevent slipping. The smaller gear turns faster, with less turning force, trian the larger gear, which then has more turning force. Every time the small gear turns all the way around, the large gear turns only part way. The large wheel and its axle are turning more slowly but can lift something heavier or turn something too difficult for the axle of the small wheel. By cutting the edge of the gears at an angle, the axes of rotation of the two gears can be at an angle with each other rather than being along the same direction. In this way, a force in a vertical direction can be changed to one in a horizontal direction. Gears are used in watches, automobiles, outboard motors, trains, and many other complex machines.
