Chapters 7 is dedicated to the analysis of fixed-axis and planetary gear trains. They are used in many applications including clocks and watches, drills, automotive and helicopter power transmission, and power generation in wind turbines. In many cases, gear trains are used to allow us to choose how we would like to use our available power in a way that it suits the situation at hand. For example, an automotive transmission allows us to operate a car at low speed and high torque (at start) or at high speed and low torque (in a highway). In both fixed-axis and planetary gear trains, a major focus of the discussion is on methods that can be used for calculating the speed ratio (that is also related to mechanical advantage and torque ratio (see Chap. 11 )). We will see that speed ratio can be calculated using the rolling contact equations discussed in Chap. 3 .