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Internal gear is definitely a gear using its teeth cut in the internal surface area of a cylinder and meshes with spur gears.

In its manufacturing, because of its shape, the usual hobbing machine found in spur gear production cannot be used. Generally it is made out of gear shaper (or gear shaping machine) equipped with a pinion cutter. More recently, the efficiency of inner gear cutting has been improved by a different procedure called skiving.

Because of the thinness of the ring, it is sometimes difficult to create high precision. However, gear grinding to boost precision grade is limited by the amount of producers and their available grinding facilities resulting in very high cost.

Also, although it is generally too expensive to make helical teeth internal gears, Ever-Power has made it possible to create them simply 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 amount of teeth combinations, nonetheless 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 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 main types of planetary equipment mechanisms: planetary, solar and celebrity types. With respect to the type and the pattern of which shafts act as input and result, many variations of rate tranny ratios and rotational directions are created.