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Internal Gears: Technical Information

Characteristics∧ top

KHK 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. KSI KSIR
Module 0.5 ~ 3 2 ~ 3
Material S45C S45C
Heat Treatment
Tooth Surface Finish Cut Cut
Precision JIS B 1702-1:1998 N8 Note 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∧ top

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
KHK 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.

Allowable Mating Pinions and Number of Teeth
No. of teeth of
Internal Gear
No. of teeth of Allowable Mating Pinions
Lower limit No. of teeth by Involute interference Upper limit No. of teeth by Trochoid interference Upper limit No. of teeth by Trimming interference
50 22 41 33
60 21 51 43
80 20 72 64
100 19 92 84
120 19 112 104
160 19 152 144
200 18 192 184

Established equipment and technology. Custom Gears are also available.
Diameter Ø700mm maximum, Module 6.5 maximum, Cutting Stroke 170 mm

CNC Gear Shaper

2. Caution in Selecting Gears Based on Gear Strength
The gear strength values shown in the product pages were computed by assuming a certain application environment. Therefore, they should be used as reference only. We recommend that each user computes his own values by applying the actual usage conditions. The table below contains the assumptions established for these products in order to compute gear strengths.

Catalog No. Item KSI KSIR
Calculation Assumptions for Bending Strength of Gears
Formula NOTE 1 Formula of spur and helical gears on bending strength (JGMA401-01)
No. of teeth of mating gears 30
Rotation 100rpm
Durability Over 107cycles
Impact from motor Uniform load
Impact from load Uniform load
Direction of load Bidirectional
Allowable beam stress at root σFlim (kgf/mm²) NOTE 2 19
Safety factor SF 1.2
Calculation assumptions for Surface Durability (Except where it is common with bending strength)
Formula NOTE 1 Formula of spur and helical gears on surface durability (JGMA402-01)
Kinematic viscosity of lubricant 100cSt (50°C)
Gear support Symmetric support by bearings
Allowable Hertz stress σHlim (kgf/mm²) 49
Safety factor SH 1.15
Note 1: The gear strength formula is based on JGMA (Japanese Gear Manufacturers‘ Association) The units for the rotational speed (rpm) and the stress (kgf/mm2) are adjusted to the units needed in the formula. Note 2: The allowable bending stress at the root σFlim is calculated from JGMA401-01, and set to 2/3 of the value in the consideration of the use of planetary-, idler-, or other gear systems, loaded in both directions.

Application Hints∧ top

In order to use KHK stock internal gears safely, read the Application Hints carefully before proceeding. Also "1. Caution on Performing Secondary Operations>", "3. Notes on Starting Operations" and "4. Other Points to Consider in Applications" in the Spur Gear KHK Technical Section should be consulted.

1. Point of Caution in Assembling
  1. a KHK stock internal gears are designed to give the proper backlash when assembled using the center distance given by the formula below. The amount of backlash is given in the product table for each gear.
    a = d2 - d1 Where
    a : Center Distance
    d1 : Pitch Diameter of Pinion
    d2 : Pitch diameter of internal gear
  2. Note that the direction of rotation of the internal gear is different from that of two spur gears in mesh.
    Gear Ratio and Direction of Rotation
  3. To use as a planetary gear drive, the following conditions must be satisfied.
    Conditions formulas
Example of Combinations
No. of teeth of internal gear No. of planet gears No. of teeth of sun gear No. of teeth of planet gears Reduction ratio of planetary type Reduction ratio of solar type Reduction ratio of star type
60 3 18 21 4.333 1.3 – 3.333
80 3 16 32 6 1.2 – 5
80 3 40 20 3 1.5 – 2
100 3 20 40 6 1.2 – 5
100 3 50 25 3 1.5 – 2

Gear Reduction Mechanism