Internal Gears

Internal gear is a gear with its teeth cut in the inner surface of a cylinder and meshes with spur gears. Internal gears are primarily used for planetary gear drives.



Internal gears, also known as ring gears, have the same standards as an external gear except that the teeth are cut into the inside diameter while the outside is smooth. Internal gears can provide a compact solution that delivers significant speed reductions and reduced sliding wear action, meaning you’ll get increased longevity. When it’s necessary to have two parallel shafts rotate in the same direction, internal gears eliminate the need for an idler gear. Internal gears can be applied in a wide range of equipment and applications.

A special feature of spur and helical gears is their capability of being made in an internal form, in which Internal Gear mates with ordinary external gear. This offers considerable versatility in the design of planetary gear trains and miscellaneous instrument packages. Advantages of an internal gear are the following:

1) Well suited to compact design since the center distance is less than for external gears.
2) A high contact ratio is possible.
3) Good surface endurance due to a convex profile surface working against a concave surface.

Internal Gears: Technical Information

HZPT stock internal gears are offered in modules 0.5 to 3 in 50 to 200 teeth. They can be used in many applications including planetary gear drives.

Catalog No. SI SIR
Module 0.5 ~ 3 2 ~ 3
Material S45C S45C
Heat Treatment
Tooth Surface Finish Cut Cut
Precision JIS B 1702-1:1998 N8Note 1 N9
Secondary Operations Possible Possible
Features A popular type of internal gear; low cost and suitable for many applications. Ring gear large in size / number of tooth. It can be cut to make segment gears and corner racks.

Note 1: The Product accuracy class having a module less than 0.8 corresponds to ‘equivalent’ as shown in the table.

Selection Hints

Please select the most suitable products by carefully considering the characteristics of items and contents of the product tables. It is also important to read all applicable notes before the final selection.

1. Caution in Selecting Mating Gears
HZPT stock internal gears can mate with any spur gears of the same module, however, there are cases of involute, trochoid and trimming interference occurrences, depending on the number of teeth of the mating gear. Various types of interference and their symptoms and causes are tabulated below, also shown, the number of teeth of allowable mating pinions.

Interferences and the Symptoms

Involute interference The tip of the internal gear digs into the root of the pinion. Too few teeth on the pinion.
Trochoid interference The exiting pinion tooth contacts the internal gear tooth. Too little difference in number of teeth of the two gears.
Trimming interference Pinion can slide in or out axially but cannot move radially. Too little difference in number of teeth of the two gears.


How are internal gears manufactured?

Internal gears are created using a pinion cutter and a shaping procedure. External gears are shaped mainly by hobbing, milling, and shaping with a rack cutter. However, other gear cutting techniques such as punching, etching, and laser engraving may be used in some cases.

The first step in the manufacturing process is to select a material for the gear teeth. These materials include steel, aluminum, plastic, wood, and others. Each has advantages and disadvantages that make one material or another better suited for particular applications.

Next, the geometry of the gear teeth is cut into the selected material using a gear cutting machine. The depth of the cut determines the number of teeth on the gear wheel. Each time the gear cutting machine cuts into the material, it leaves a layer of waste material called “dross” that must be removed from the tooth’s surface before it will provide adequate protection against wear and tear.

After the teeth have been cut, they need to be polished to remove any remaining trash and give them a smooth finish. This is usually done by hand using a slurry of oil and water or alone as a lubricant. The gears are rotated against a stone plate with varying pressure to achieve the desired finish.

Applications of Internal Gears

  • Rollers
  • Electric Screwdrivers
  • Pumps
  • Positioning Equipment
  • Automotives
  • Bicycle components
  • Planetary gear drives