6 - Gears

Abstract

This chapter introduces various types of gears and details the design, specification, and selection of spur gears in particular. Gears are toothed cylindrical wheels used for transmitting mechanical power from one rotating shaft to another. This chapter introduces a wide range of gears and calculations enabling simple gear trains to be analyzed. A preliminary selection procedure for spur gears based on the Lewis formula for bending stresses has been presented. This serves as the starting point in proposing the details for a gear train, but more sophisticated analysis is necessary if a robust gearbox is required. A speed change can be achieved by running discs of different diameter together, or alternatively cones for turning corners as well. All gear mechanisms and gear trains demand continuous lubrication, which must be pressure fed for high-speed gears to counteract centrifugal effects on the oil. The layout and geometry for a pair of meshing spur gears can be determined by the procedure, which assumes access to a computer aided design (CAD) drawing package. It should be noted that gears are commonly available as standard items from specialist manufacturers and suppliers and need not necessarily be designed from scratch. A gear train is one or more pairs of gears operating together to transmit power. The torque speed characteristic of a petrol internal combustion engine delivers low torque at starting and at low speeds.