are ones in which the teeth are at a helix angle with respect to the axis of the gear as shown in Figure 9-15a. Figure 9-16 shows a pair of opposite-hand helical gears in mesh. Their axes are parallel. Two crossed helical gears of the same hand can be meshed with their axes at an angle as shown in Figure 9-l7. The helix angles can be designed to accommodate any skew angle between the nonintersecting shafts.