Design of Mechanical Elements

11Gears

11.1 Introduction

A gear is a rotating mechanical element with “teeth” that engage or mesh with the “teeth” of another gear to transmit torque and rotary motion between two mechanical elements, such as two shafts. Meshing gears of different sizes enable tuning the rotational speed and torque of the shafts to which the gears are mounted. We refer to several meshing gears working in sequence as a gear train or a transmission. Figure 11.1 compares the rotation direction of (a) a belt/pulley and (b) a gear transmission. A finite amount of slip exists between the belt and pulley, which does not occur in gear transmissions. Furthermore, in a belt/pulley transmission (with noncrossed belt), the two pulleys rotate in the same direction, whereas the two meshing gears rotate in opposite direction.

11.1.1 Types of Gears

Figure 11.2 schematically illustrates the four basic categories of gears, showing (a) spur gears or “straight‐cut” gears, (b) helical gears, (c) bevel gears, and (d) worm gears. We briefly describe the different gear categories in the following paragraphs.

Spur gears (Figure 11.2a) project teeth radially outward and the teeth are parallel to the axis of rotation. Thus, they transmit power and rotary motion between parallel shafts. No axial thrust loads are created between meshing spur gears because all force components act radially and tangentially to the gear. Spur gears are the simplest type of gear and, therefore, are used abundantly. They perform well at moderate ...