As the name itself suggest, an internal gear is the polar opposite of an external gear not only its' definition but also in its' application and usage. In the internal gear, the teeth branch inwards as opposed to the branching outward as happens in the case of external gears. The position of the addendum and dedendum is reversed completely in the internal gears from their traditional positions in external gears. Ami Gears, a custom internal Gears Company has kept up with that trend.
Most of the internal gears have a non-intersection set of shafts that could transmit torque with significant reduction in speeds. Although the center distance in external gears is a lot higher than in the internal gears, the ratio stays the same as long as the internal gear is used with a standard spur pinion. Certain applications demand that the two non-intersecting shaft move in the same direction rather than opposing each other, in those applications, an internal gear renders an idler gear obsolete since it is itself capable of doing that without employing any other body or object.
The usability of internal gears depends as much on the installation as it does on the manufacture. Out of numerous benefits that internal gears carry, one of them is a reduction in sliding motion. Unlike the external gears that have different lengths when it comes to the teeth on the pinion and external gears, internal gears have similar if not the same length of teeth as the pinions being used, which gives a 1:1 tooth ratio.
As the gears function, the teeth of the internal gear slide over those on the pinions, this creates a sliding action, which causes heat and friction. Both of these factors contribute to the wear and tear in the tooth, which makes internal gears preferable over the external ones as they greatly reduce the sliding action that can only custom internal Gears Company can provide.