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Helical Gears: Technical Information

Characteristics∧ top

KHK stock helical gears are quiet, compact and economical. They are suitable wherever you require high-speed rotation including in machine tools, speed reducers and other industrial machinery. The following table lists the main features.

Catalog No. KKHG KSH
Module 1~3 2~3
Material SCM440 S45C
Heat Treatment Thermal refined, Gear teeth induction hardened
Tooth Surface Finish Ground Cut
Precision JIS B 1702-1:1998 N6 N8
Secondary Operations Possible except for tooth Possible
Features Have excellent strength and wear resistance which allow your designs to be more compact. Having larger contact ratios compared to the SS spur gears, effective in reducing noise and vibration.

Advanced Grinding Equipment Allows for Efficient Production
The use of electro deposition grinding wheel produces consistent precision with shorter grinding usage, making products affordable.

Gear Grinding Machine

Selection Hints∧ top

It is important to thoroughly understand the contents of the product tables as well as "CAUTION" notes before making the selection. You must specify the right or left hand by including the letter R or L in the catalog number when ordering.

1. Caution in Selecting the Mating Gears.
We have two different types of KKHK helical gear products, one is a KKHG gear type, and the other is a KSH gear type. Each type of gear has different module systems, pressure angle designations and helix angles. Since the KKHG Gears are of the transverse module style, and the KSH gears are of normal module style, KKHG and KSH gears are not interchangeable. Please keep this in mind when making your selection. Also, right hand and left hand helical mating gears are packaged as a set. See the photos below for reference and for help in making a proper selection. The table shows the possible combinations.

Mating Helical Gear Selection Chart (O = Allowable, X = Not Allowable)
Catalog No. & Helix Hand KKHG KSH KKRHG, KKRHGF KSRH

Helix Direction
Helix Direction

Transverse Module and Normal Module
The difference between transverse module and normal module is defined as the difference of basic tooth form. As shown on the right, the module of tooth datum orthogonal to the center axis of gear is called transverse module. The module of tooth datum orthogonal to the thread helix is called normal module. The characteristics of each are shown as below.

Characteristics of Transverse Module and Normal Module
Style Advantages Disadvantages
Transverse module (KKHG) Replaces spur gears having the same module, number of teeth, and center distance. Special gear cutting or grinding machines are required for processing each helix angle.
Normal module (KSH) Modifications of spur gears are made by gear cutting or grinding machines, even if they have different helix angles. Have a center distance value different from that of a spur gear, although they have the same module size and the same number of gear teeth. The center distance value is rarely an integral number.

Normal and Transverse Module (CAUTION) Above is for illustration purpose only and not a representation of the true tooth forms. For detailed technical information, please refer to the section of "4.3 Helical Gears"
2. Caution in Selecting Gears Based on Gear Strength
Allowable bending strength and surface durability values shown in product tables were computed by assuming a certain application environment. They should be used as reference only. We recommend that each user computes his own values by applying the actual usage conditions. To find more information on gear strength calculations, please refer to the technical reference, in the section "Bending Strength of Spur and Helical Gears" or "Surface Durability of Spur and Helical Gears".
Catalog No. Item KKHG KSH
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 Same number of teeth
Rotation 600rpm 100rpm
Durability Over 107 cycles
Impact from motor Uniform load
Impact from load Uniform load
Direction of load Bidirectional
Allowable bending stress at root σFlim(kgf/mm²) NOTE2 30 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 bending strength (JGMA402-01)
Kinematic viscosity of lubricant 100cSt (50°C)
Gear support Symmetric support by bearings
Allowable Hertz stress σHlim(kgf/mm²) 116 49
Safety factor SH 1.15
NOTE 1: The formula for gear strength is based on JGMA Standard. The units for the rotational speed (rpm) and the load (kgf/mm²) were matched to the units needed in the equation. 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.
Bending Damage Definition of bending strength by JGMA 401-01(1974)
The allowable bending strength of a gear is defined as the allowable tangential force at the pitch circle based on the mutually allowable root stress of two meshing gears under load.

Surface Damage Definition of surface durability by JGMA 402-01(1974)
The surface durability of a gear is defined as the allowable tangential force at the pitch circle, which permits the force to be transmitted safely without incurring surface failure.

Application Hints∧ top

In order to use KHK stock gears safely, carefully read the Application Hints before proceeding. If there are questions or if you require clarifications, please contact our technical department.

Heat Treatment
If you apply induction hardening to the gear teeth of S45C products, you need to designate the hardness and where to apply the heat treatment. Below is an example of common specifications and KHK's specifications for hardening:

• Common Specifications for Heat Treatment
Area: Tooth surface, or, Tooth surface and Tooth root
Hardness: Within 10 HRC in the range from 45 to 60 HRC. (e.g. 48 - 58 HRC)

• KHK 's Specifications for Heat Treatment
Area: Tooth surface, or, Tooth surface and Tooth root
Hardness: From 50 to 60 HRC.

*Hardness and Depth of Gear-teeth Induction Hardening. The hardening method and the state of hardened teeth area are varied depending on the size of gears. Since different hardening treatment is applied in accordance with the module and number of teeth, the hardness level you designate is referred to as the hardness of the reference diameter. For some of our products, there may be a case that the hardness at tooth tip / root may not be equal to the hardness you designated. As to the effective case depth for S45C, it is specified by JIS, as "The distance from the surface of the case to the area with hardness HV450". The case depth differs from area to area of a tooth.
1. Caution on Performing Secondary Operations
  1. If you are reboring, it is important to pay special attention to locating the center in order to avoid runout.
  2. The reference datum for gear cutting is the bore. Therefore, use the bore for locating the center. If it is too difficult to do for small bores, the alternative is to use one spot on the bore and the runout of the side surface.
  3. If the rework requires using scroll chucks, we recommend the use of new or rebored jaws for improved precision. If chucking by the teeth, please apply the pressure carefully to avoid crushing the teeth which will lead to noisy gears. Lathe Operation
  4. The maximum bore size is dictated by the requirement that the strength of the hub is to be higher than that of the gear teeth. The maximum bore size should be 60% to 70% of the hub diameter (or teeth root diameter), and 50% to 60% for keyway applied modifications.
  5. In order to avoid stress concentration, leave radii on the keyway corners. Keyway Tapping and Slotting
  6. To avoid problems of reduced gear precision and other manufacturing difficulties, do not attempt to machine the gears to reduce face widths.
  7. KKHG Ground Helical Gears are already stress relieved. But if you subject them to a heavy turning operation such as removing the hubs, the residual stress may cause deformation.
  8. When heat-treating KSH Helical Gears, it is possible to get thermal stress cracks. It is best to subject them to penetrant inspection afterwards. If the tooth strength is not sufficient, it can be increased approximately four times by heat-treating. On the other hand, the precision of the gear will drop about one grade.
2. Points of Caution in Assembling
  1. KHK stock helical gears are designed to give the proper backlash when assembled using the center distance given by the formula on the right (center distance tolerance of H7 ~ H8). The amount of backlash is given in the product table for each gear.
    Center Distance
  2. Please refer to overall length tolerance for Helical Gears on Spur Gears Technical Information Page
  3. Because of the helix of the gear teeth, helical gears in mesh produce thrust forces in the axial directions. The axial thrust bearings must be able to resist these forces. The direction of the thrust forces depend on the helix hand and the direction of rotation as shown below. For details, please refer to the technical reference, section of "Gear Forces".
    Direction of Rotation and Thrust Force
3. Application Examples

Application Example