Internal gear is a gear with its teeth cut in the internal surface of a cylinder and meshes with spur gears.
In its manufacturing, because of its shape, the usual hobbing machine found in spur gear creation can’t be used. Generally it is made out of gear shaper (or gear shaping machine) equipped with a pinion cutter. Recently, the efficiency of inner gear cutting offers been improved by a different process called skiving.
Due to the thinness of the band, it really is 
sometimes difficult to create high precision. On the other hand, gear grinding to boost precision grade is bound by the amount of manufacturers and their available grinding facilities resulting in very high cost.
Also, although it is normally too expensive to make helical teeth internal gears, Ever-Power has made it possible to create them by adopting equipment that can cut them simply by making pinion cutters rather than needing helical guides.
Normally, internal gears can be utilized with the same module spur gears, depending on the amount of teeth combinations, nonetheless it is necessary to note the possibilities to getting involute, trochoid or trimming interferences. For using Ever-Power standard stock gears, please seek advice from the PDF formatted “Specialized Info of Internal Gears” section for usable number of teeth combinations.
Internal gears are often used in applications involving planetary gear drives and gear couplings. There are three major types of planetary gear mechanisms: planetary, solar and celebrity types. Based on the type and the design of which shafts become input and result, many variations of rate tranny ratios and rotational directions are created.