Internal gear is certainly a gear with its teeth cut in 
the inner surface area of a cylinder and meshes with spur gears.
In its manufacturing, due to its shape, the usual hobbing machine used in spur gear production can’t be used. Generally it is made with gear shaper (or gear shaping machine) built with a pinion cutter. Recently, the efficiency of inner gear cutting offers been improved by a different process called skiving.
Because of the thinness of the ring, it really is sometimes difficult to create high precision. On the other hand, gear grinding to boost precision grade is limited by the amount of producers and their offered grinding facilities resulting in very high cost.
Also, although it is generally too expensive to create helical teeth internal gears, Ever-Power has made it possible to produce them by adopting equipment that can cut them by simply making pinion cutters and not needing helical guides.
Normally, internal gears can be used with the same module spur gears, based on the number of teeth combinations, but it is necessary to notice the possibilities of getting involute, trochoid or trimming interferences. For using Ever-Power standard stock gears, please consult the PDF formatted “Specialized Information of Internal Gears” section for usable quantity of teeth combinations.
Internal gears tend to be used in applications involving planetary gear drives and gear couplings. There are three main types of planetary gear mechanisms: planetary, solar and superstar types. With respect to the type and the pattern of which shafts become input and output, many variations of speed tranny ratios and rotational directions are produced.