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Helical Gears

 Helical Gears • Technical Information

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

Item Catalog Number
KHG SH
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 Excellent strength and wear resistance, which allows your designs to be more compact. Having larger contact ratios compared to the SS spur gears
Makes them effective in reducing noise and vibration.

Advanced Grinding Equipment Allows for Efficient Production
The use of an electrodeposition grinding wheel allows for consistent precision with shorter production time, making products affordable.
Electro Deposition Grinding Wheel and Gleason Cylindrical Gear Grinding MachineGleason Cylindrical Gear Grinding Machine (TAG400)
Characteristics Selection Hints:
It is important to thoroughly understand the contents of the products 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 KHK helical gear products, one is a KHK gear type, and the other is an SH gear type. Each type of gear has different module systems, pressure angle designations, and helix angles. Since the KHG Gears are of the transverse module style, and the SH gears are of normal module style, KHG and SH 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 photos below for reference and for help in making a proper selection. The table shows the possible combinations.

Mating Helical Gear Selection Chart
Catalog Number &
Helix Hand
KHG SH KRHG
KRHGF
SRH
RH LH RH LH RH LH RH LH
KHG RH Not
Allowable
Allowable Not
Allowable
Not
Allowable
Not
Allowable
Allowable Not
Allowable
Not
Allowable
LH Allowable Not
Allowable
Not
Allowable
Not
Allowable
Allowable Not
Allowable
Not
Allowable
Not
Allowable
SH RH Not
Allowable
Not
Allowable
Not
Allowable
Allowable Not
Allowable
Not
Allowable
Not
Allowable
Allowable
LH Not
Allowable
Not
Allowable
Allowable Not
Allowable
Not
Allowable
Not
Allowable
Allowable Not
Allowable


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 in the illustration below, 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:

Transverse Module
(Caution) The above is for illustration purpose
only and not a representation of the true tooth
forms.


Characteristics of Transverse Module and Normal Module
Style Advantages Disadvantages
Transverse Module
(KHG)
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
(SH)
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.

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 their own values by applying the actual usage conditions.


Calculation Assumptions for Bending Strengths of Gears
Item Catalog Number
KHG SH
Formula (Note 1) Formula of spur gears on bending strength (JGMA401-01)
No. of teeth of mating gears Same number of teeth
Rotation 600 rpm 100 rpm
Durability Over 10 7 cycles
Impact from motor Uniform load
Impact from load Uniform load
Direction of load Bidirectional
Allowable bending stress at root
σFlim (kgf/mm 2) (Note 2)
30 19
Safety Factor
SF

1.2

Calculation Assumptions for Surface Durability
(Except where it is common with Bending Strength):

Item Catalog Number
KHG SH
Formula (Note 1) Formula of spur and helical gears on suface durability (JGMA402-01)
Kinematic viscosity of lubricant 100cSt (50°C)
Gear support Symmetric support by bearings
Allowable Hertz Stress
σHlim (kgf/mm 2)
116 49
Safety Factor
SH
1.15

(Note 1) The formula of gear strength is based JGMA (Japanese Manufacturers' Association). The units for rotational speed (rpm) and the load (kgf/mm 2) were matched to the units needed in the equation.

(Note 2) Since the load is bidirectional, the allowable bending stress at root σFlim is set to 2/3 of the value.

Definition of Bending Strength by
JGMA 401-01 (1974)

Bending Strength
Example of the failure due to
insufficient bending strength

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.
Definition of Surface Durability by
JGMA 402-01 (1975)

Bending Strength
Example of the defacement
due to insufficient surface durability.

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.



Characteristics Application Hints:
In order to use KHK stock gears safely, carefully read these Application Hints before proceeding. If there are questions or if you require clarifications, please contact us at qtcsupport@qtcgears.com or call 516-437-6700.

1. Caution on Performing Secondary Operations
If you are reboring, it is important to pay special attention to locating the center in order to avoid runout.

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.

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 Operations
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 within 60% to 70% of the hub diameter (or tooth root diameter), and 50% to 60% for keyway applied modifications.

In order to avoid stress concentration, leave radii on the keyway corners.
Tapping and Keyway Slotting
To avoid problems of reduced gear precision and other manufacturing difficulties, do not attempt to machine the gears to reduce face widths.

KHG 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.

When heat-treating SH 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.

• 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 45 to 55 HRC.

Hardness and Depth case 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 pitch line. For some of our products, 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.

2. Points of Caution in Assembling
KHK stock helical gears are designed to give the proper backlash when assembled using the center distance given by the formula below (center distance tolerance of H7 – H8). The amount of backlash is given in the product table for each gear.
Helical Gear Center Distance Formula

Overall Length Tolerance For Spur & Helical Gears:
Overall Length Tolerance For Spur Gears and Helical Gears

(Note) Following products are excluded from this table: Spur Pinion Shafts, Injection Molded Spur Gears, Fairloc® Hub Spur Gears, and MC Nylon products.

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.
Direction of Rotation and Thrust Force

Application Examples Application Examples:

Helical Coupling Application Examples