Drug delivery devices and related methods of assembly are disclosed. The drug delivery device may include a main housing having an interior surface defining an enclosed space, and an exterior surface releasably attachable to a patient. A container may be disposed in the enclosed space and include a reservoir containing a drug and a stopper. A drive assembly may also be disposed in the enclosed space and configured to move the stopper through the reservoir to expel the drug from the reservoir. The drive assembly may include a rotational power source and a gear module. The gear module may include a mounting plate and a plurality of gears rotatably connected to the mounting plate. Furthermore, the mounting plate may be separate from the main housing.