We call a set of two or more gears meshed together a gear train. A gear train is, therefore, a kinematic chain in which motion is transmitted by gears. Some of the reasons to use gear trains to transmit motion are: (a) Space restrictions. When motion has to be transmitted between two axles that are far apart. In that case, two large wheels would be needed. A gear train with several small gears could be used, occupying much less space. (b) The transmission ratio is too high or low. To achieve such a transmission ratio with only two wheels it would be necessary to use one toothed wheel with a much larger pitch diameter than the other. This solution has some problems such as interference and more energy losses than a solution with more than two wheels. (c) Opposite output and input angular velocities are required. (d) Multiple outputs are needed for a single input.