Internal gear is usually a gear using its teeth cut in the inner surface area of a cylinder and meshes with spur gears.
In its manufacturing, due to its shape, the most common hobbing machine used in spur gear production can’t be used. Generally it is made with gear shaper (or equipment shaping machine) built with a pinion cutter. More recently, the efficiency of internal gear cutting has been improved by a different process called skiving.
Due to the thinness of the band, it is sometimes difficult to create high precision. On the other hand, gear grinding to boost precision grade is limited by the number of manufacturers and their obtainable grinding facilities leading to very high cost.
Also, although it is normally too expensive to create helical teeth internal gears, Ever-Power has managed to get possible to produce them simply by adopting equipment that may 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 notice the possibilities to 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 quantity of teeth combinations.
Internal gears tend to be 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 pattern of which 
shafts become input and output, many variations of quickness tranny ratios and rotational directions are created.