Gear trains and speed reducers are mechanical components often used for obtaining a desired angular velocity of an output shaft, while the input shaft rotates at a different angular velocity. The angular-velocity ratio between input and output members must usually remain constant. The value of this ratio can be adjusted in some arrangements (usually with a friction or hydraulic drive and/or clutch arrangement), while in others the ratio is not adjustable. In order, therefore, to design or select a gear train or speed reducer, one of the first tasks Is the determination of the angular velocities. This will now be considered.