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Helical Gears Helical Gears - Technical Data

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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.

Features of helical gears - data table

Selection hints

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.

Caution in selecting the mating gears

We have two different types of helical gear products: one is the KKHG gear type, and the other is theK SH 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 the 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 pack-aged as a set.

See the photos below for reference and for help in making a proper selection.

• Direction of helix

Direction of helix in mating helical gears - image
Direction of helix in mating helical gear racks - image

• Mating Helical Gear Selection Chart

O = Allowable • X = Not allowable
Mating Helical Gear Selection Chart

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 their own values by applying the actual usage conditions.

Alert! - Important Information To learn more about helical gears, please refer to our separate technical reference section, here.

• Calculation of Bending Strength of Gears

Calculation of bending strength of gears - table

Calculation of surface durability (except where it is common with bending strength)

Calculations of surface durability
NOTE 1. The gear strength formula is based on JGMA (Japanese Gear Manufacturers Association) specifications.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.

When selecting standard gears, glance over the Cautions on Product Characteristics and Cautions on Performing Secondary Operations in the respective dimension tables.

1. Products not listed in this catalog or materials, modules, number of teeth and the like not listed in the dimensional tables can be manufactured as custom items.

2. The color and shape of the product images listed on the dimension table page of each product may differ from the actual product. Be sure to confirm the shape in the dimension table before selection.

3. The details (specifications, dimensions, prices, etc.) listed in the catalog may be changed without prior notice.

Alert! Important InformationThe most important factor in selecting gears is the gear strength.

Determine the actual load torque applied to the gear and the gear type suitable for the purpose.

Definition of bending strength of gears

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.

Example of failure due to insufficient
bending strength

Definition of bending strength of gears - image

Definition of surface durability of gears

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. The allowable gear tooth load of a gear is defined as the allowable tangential force at the pitch circle based on the mutual gear tooth strength of two meshing gears under load.

Example of wear due to insufficient
surface durability

Definition of surface durability of gears - image
Select provisionally from the allowable torque table of the Master Catalog based on the load torque.
Provisional selection from the master catalog - image

Alert! - Important InformationWe recommend that each user computes their own values by applying the actual usage conditions to determine the suitability of the gear strength.

Calculate the strength formally using the various gear strength formulas (Get them here).

Application Hints
In order to use KHK stock gears safely, carefully read the Application Hints before proceeding. If there are questions or you require clarifications, please contact our technical department or your nearest distributor. TEL: (646) 396-GEAR FAX: (516) 437-6700 E-mail: qtcsupport@qtcgears.com

Caution on Performing Secondary Operations

1. If 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 reworking using scroll chucks, we recommend the use of new or rebored jaws for improved precision. Please exercise caution not to crush the teeth by applying too much pressure. Any scarring will cause noise during operation.

Lathe Operations - Image
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 tooth root diameter), and 50% to 60% for keyway applied modifications.

5. In order to avoid stress concentration, round the keyway corners.
Tapping & Keyway 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. When induction-hardening S45C products, thermal stress cracks may appear. Also, note that the precision grade of the product declines by 1 or 2 grades, as deformation on material may occur. If you require tolerance for bore or other parts, machining is necessary after heat treatment.

Points of Caution during Assembly

1. KHK stock helical gears are designed to give the proper normal direction 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. For the center distance of KSH, refer to the dimensional table page.
Center Distance of Mating Gears for Helical Gears - Image
2. The table below indicates the tolerance on the total length of KHK stock spur gears. Please refer to this data when designing gear boxes or other components.

Total Length Tolerance for Spur and Helical Gears
Total Length Tolerance for Spur and Helical Gears.
Note. The following products are excluded from this table: Spur pinion shafts, Injection molded spur gears, F-loc hub spur gears, and MC nylon products.

3. Verify that the two shafts are parallel. Incorrect assembly will lead to uneven teeth contact which will cause noise and wear. (Check the assembly by painting a thin layer of red lead primer or the like on the gear teeth, meshing them together and rotating them.)

Test example:
Abrasion occurred on KSSG3-30 due to poor edge contact (only 30% with proper contact).
Poor edge contact with abrasion - image
Poor tooth contact and pitting
Gear oil (equivalent to JIS gear oil category 2 No. 3)
The design conditions were load torque at 278 rpm,
42.5 kg/m (12 kW), 1.5 times the allowable bending strength,
and 3 times the allowable surface durability torque.
The pitting occurred on the poor tooth contact area after
60 hours of continuous operation.

4. 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 direction and the direction of rotation as shown below.
Direction of Rotation and Thrust Force
Alert! - Important InformationFor more details, please refer to our separate technical reference for, "Helical Gears".

Cautions on Starting

1. Check the following items before starting:
  • Are the gears installed securely?
  • Is there uneven tooth contact?
  • Is there adequate backlash?
    Be sure to avoid zero-backlash.
  • Has proper lubrication been supplied?

2. If gears are exposed, be sure to attach a safety cover to ensure safety. Also, be careful not to touch rotating gears.

3. Gears can be lubricated with the "grease lubrication method", "splash lubrication method (oil bath method)", or "forced lubrication method (circulation lubrication method)". For initial operation, the lubricant may deteriorate markedly, so check the condition of the lubricant after starting.

Alert! - Important InformationFor more technical information, please see the section "Gear Lubrication" of our technical reference book.
4. If there is any abnormality such as noise or vibration during startup, check the gears and assembly condition. "High gear accuracy", "smooth gear teeth surface" and "correct tooth contact" are some of the measures against gear noise.
Alert! - Important InformationFor more technical information, please see the section "Gear Noise" (Here).
KHK considers safety a priority in the use of our products. When handling, adding secondary operations, assembling, and operating KHK products, please be aware of the following issues in order to prevent accidents.

Warning: Precautions for preventing physical and property damagePrecautions for preventing physical and property damage

1. When using KHK products, follow relevant safety regulations (Occupational Safety and Health Regulations, etc.).

2. Pay attention to the following items when installing, removing, or performing maintenance and inspection of the product.
  • Turn off the power switch.
  • Do not reach or crawl under the product.
  • Wear appropriate clothing and protective equipment for the work.

Caution in preventing accidentsCautions in preventing accidents

1. Before using a KHK product, read the precautions in the catalog carefully in order to use it correctly.

2. Avoid use in environments that may adversely affect the product.

3. Our products are manufactured under a superior quality control system based on the ISO9000 quality management system; if you notice any malfunctions upon purchasing a product, please contact the supplier.