Internal Gears
Gears are used for the transmission of power. Internal Gears are gear teeth generated in the internal diameter of a cylinder while external gears have the gear teeth generated on the outside diameter of the component. While there are several different types of gears, only two can be produced as both internal and external gears, spur and helical gears. While internal gears are ideal for applications for high power transmission within small overall dimensions, there are design constraints that must be addressed in order to achieve the advantages that internal gears offer.
An internal gear can also be called a ring gear. Internal gears are used in planetary gear box applications, but can also be applied in other situations. Advantages of the use of a planetary gear train are low vibration, high speed reduction ratio, and the low cost for the entire train layout. Since the centers of the mating wheels can be closer than those of external gears, internal spur gears are preferred in applications where space is a constraint. Some of the common uses for planetary gear transmissions are robotic arms, hybrid vehicle power transmissions and turbine generators. Internal gears can also be cut in helical gears just as external gears can be cut in helical gears. These types of internal gears can be found in some internal gear drive, simple pairs, internal differential drives and internal planetary drives. Special housings have to be designed in order to accommodate the internal gear.
There are major differences between an internal and an external gear beside where the gear teeth are generated on the workpiece. For example, while meshing a pair of external gears, their rotation will go in opposite directions; but in the meshing on an external and internal gear their rotation will go in the same direction. Another major difference between the two types of gears is that a low ratio reduction is unsuitable in the case of an internal and external gear, but is completely acceptable in the meshing of an external and another external gear.
To secure an effective internal gear drive, much more effort must be put into its design than into that of external gears. Due to the nature of internal gears, a few checks are necessary during the design process. One major feature that must be checked during the meshing of external and internal gears is the interference in between the two gears. If the design features of a gear are chosen independently of one another then as a result the prototyping process can be longer due to the need for multiple trial and error processes in order to find the optimal design for the application.
A common design feature utilized to combat some of the limitations of an internal gear is a profile shift in the involute gear tooth profile. There are large amounts of profile shift that can be designed into internal gears more so than what can occur in external gears. Unlike what occurs in the profile shift between two external gears where when one gear’s profile shift is increased positive the other is decreased, if the external pinion has an increase in profile shift the internal gear’s profile shift must also be increased. The profile shift of internal gears and their pinions can be shifted more than one module. The advantage of these large profile shift values is that they can be sometimes be used to prevent tip interference and other problems.
Because of the geometry of the gear, gear hobbing cannot be used to produce this gear. Another type of gear generation can be used however, gear shaping is generally employed to produce the gear teeth. Shaping uses a gear-like tool that is reciprocated up and down to impart its tooth form on the gear blank. The cutter may be pinion shaped, a multi-tooth rack-shaped cutter, or a single-point cutting tool. The size of the cutter imposes restrictions on the tooth proportions of the internal gear. If the cutter is too large, the tips of two or more teeth in the internal gear will be trimmed as the cutter is fed to depth. To avoid this, smaller, special cutters are often employed. However, if the cutter is too small, imperfect tooth forms will be developed on the internal gear. The most practical design for generated gears will be one which avoids the need for special cutters. It is possible to wire edm an internal gear, but this should be avoided due to the finish left on gear teeth. This finish can cause excessive wear to the component and greatly reduce the life of the system.
