Internal gear is certainly a gear using its teeth cut in the inner surface area of a cylinder and meshes with spur gears.
In its manufacturing, because of its shape, the most common hobbing machine found in spur gear creation can’t be used. Generally it is made with gear shaper (or gear shaping machine) built with a pinion cutter. More recently, the efficiency of inner gear cutting offers been improved by a different procedure called skiving.
Because of the thinness of the band, it really is sometimes difficult to produce high precision. However, gear grinding to boost precision grade is limited by the amount of manufacturers and their obtainable grinding facilities resulting in very high cost.
Also, although it is normally too expensive to create helical teeth internal gears, Ever-Power has made it possible to produce them simply by adopting equipment that can cut them by simply making pinion cutters and not needing helical guides.
Normally, internal gears can be utilized with the same module spur gears, based on the amount of teeth combinations, but it is necessary to note the possibilities of getting involute, trochoid or trimming interferences. For using Ever-Power standard stock gears, please consult the PDF formatted “Technical Info of Internal Gears” section for usable amount of teeth combinations.
Internal gears are often used in applications involving planetary gear drives and gear couplings. There are three main types of planetary equipment mechanisms: planetary, solar and celebrity types. 
With respect to the type and the design of which shafts act as input and result, many variations of quickness tranny ratios and rotational directions are created.