Internal gear is a gear with its teeth cut in the internal surface of a cylinder and meshes with spur gears.
In its manufacturing, due to its shape, the usual hobbing machine found in spur gear production cannot be used. Generally it is made out of gear shaper (or equipment shaping machine) equipped with a pinion cutter. More recently, the efficiency of internal gear cutting provides been improved by a different procedure called skiving.
Due to the thinness of the band, it really is sometimes difficult to produce high precision. However, gear grinding to boost precision grade is bound by the amount of producers and their offered grinding facilities leading to very high cost.
Also, although it is generally 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 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 amount of teeth combinations, but it is necessary to notice the possibilities of getting involute, trochoid or trimming interferences. For using Ever-Power standard share gears, please seek advice from the PDF formatted “Technical Information 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 star types. According to the type and the design of 
which shafts become input and output, many variations of velocity tranny ratios and rotational directions are created.