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Spur Gears Spur Gears - Technical Data

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To meet your requirements, KHK stock gears are made in a variety of types, materials, configurations, modules and numbers of teeth. We also offer products that allow for secondary operations to be performed on the bores, shafts, outside diameters, keyways and set screws. The following table lists the main features.

Secondary Operations:

Secondary Operations Image Key Features of spur gears
NOTE 1. Products with module less than 0.8 are thermal refined, without their gear teeth being hardened.
NOTE 2. KSA-shaped products with module less than 1 have no material thermal refinement treatment.
NOTE 3. The product accuracy class having a module less than 0.8 corresponds to ‘equivalent’ as shown in the table.

• By chamfering the corners of the top land, gear noise is reduced, and the chances of damage due to handling and transportation are decreased. All KHK gears larger than m1.5 have their teeth chamfered.

• Black products are KHK stock gears that have an applied black oxide coating for rust resistance; this is a product characteristic of KHK stock gears.

Caution in selecting the mating gears

1. Basically, all spur gears, internal gears and racks can be paired as long as the module and pressure angle match. Products with different materials, tooth widths, or methods of cutting the teeth can be mated.

2. When using a pinion with an internal gear with a small difference in the numbers of teeth, there are possibilities of involute interference, trochoid interference and trimming interference. See the internal gear interference portion of the technical section to avoid problems in assembling these items.

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. Also, KSUSF F-loc hub spur gears, KDSF F-loc hub spur gears and various F series that use the friction coupling method to fasten the gear shaft need addi-tional consideration for starting torque.

The table below contains the assumptions established for various products in order to compute gear strengths.

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)

Calculation of surface durability except where it is common with bending strength
NOTE 1. The gear strength formula is based on JGMA (Japanese Gear Manufacturers Association) specifications, "MC Nylon Technical Data" by Nippon Polypenco Limited and "Duracon Gear Data" by Polyplastic Co. 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 roσoFtli m 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.
NOTE 3. For KSSG Ground Spur Gears, with module 0.8 or less, thermal refining is applied. Allowable bending stress and allowable hertz stress values are shown in parentheses.
NOTE 4. For KSSS Spur Pinion Shafts, with module over 1.5, tooth induction hardening is not applied. Allowable bending stress and allowable hertz stress values are shown in parentheses.
NOTE 5. KSSS Spur Pinion Shafts with module 1 or less (KSA configuration) are set to cantilever support as they are single shaft types.

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 Handling

1. KHK products are packaged one by one to prevent scratches and dents, but if you find issues such as rust, scratches, or dents when the product is removed from the box after purchase, please contact the supplier.

2. Depending on the handling method, the product may become deformed or damaged. Resin gears and ring gears deform particularly easily, so please handle with care.

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.


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 Hardening location: Gear tooth surface or tooth surface and tooth root

    Hardness: Within the range of 45 to 60 HRC and 10 HRC width (Example: 48 to 58 HRC)
  • KHK’s Specifications for Heat Treatment Hardened location: Tooth surface, or Tooth surface and Tooth root Hardness: 50 to 60 HRC

* Hardness and Depth of Gear-teeth Induction Hardening The hardening method and the state of the hardened teeth area vary 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, 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.

Points of Caution during Assembly

1. KHK stock spur 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). For the backlash of each product, please refer to the dimension table.

Alert! - Important InformationBacklash may be adjusted by changing the center distance of mating gears.
For more information, please consult our technical section on gear backlash (Here).

Center Distance of Mating 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. Spur gears produce no thrust forces; however, be sure to fasten them firmly with stepped shafts, or collars, to prevent shifting toward the shaft.

Keyways are generally used in fastening gears to a shaft, and they should be secured by applying drilled holes for set screws, or applying flats to the shaft, in case of fastening only with set screws. There are also methods of secure settings using a Mecha-Lock, a POSI-LOCK, or a Spannring, which are parts for engaging the hole and the axis. 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 60hours of continuous operation.

4. 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.

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.

Surface Hardening Comes as Standard

Hardened Spur Gears and Gear Racks - Image
The surface durability is increased about 4 times with Hardened Plus. Ideal as a mating pinion for hardened racks (KH Series) and laser hardened racks (KHL Series). Use to improve the durability of gears.

Hardened Plus compatible products
  • KSS/KSSA/KSSCP Spur Gears
  • KKS/KKSSCP Thermal Refined Spur Gears

• Hardened Plus

Hardening is provided additionally to standard products when ordered. Products with H at the end of the Catalog No. support Hardened Plus.

  • Quick delivery, hardening completed in 4 working days Gear tooth induction hardening is completed in 4 working days excluding the day the order is placed.
  • The surface durability is increased about 4 times The surface durability is increased about 4 times.
  • Product unit price + hardening unit price The hardening unit price is added to the product unit price. For details, please see below:

    Catalog No. + H is the order method.

    Example: Catalog No.: KSS3-20,
    when hardening is added: KSS3-20H

• Industrial Hardening Specifications

    Area: Tooth surface hardening
    Hardness: HRC50 to 60
    Depth: 1 mm or more

  • Hardness and depth of gear-teeth induction hardening Hardened Plus compatible products The hardening method and the state of the hardened teeth area vary depending on the size of gears. The hardening depth is where the Vickers hardness from the tooth surface to the deep area is up to HV450 (from JIS G 0559: 2008).

    Note that hardening specifications of Hardening Plus above will be near the standard pitch diameter of the gear.

  • Note 1: The surface durability values shown in the table are calculated values according to the assumed usage conditions. Please calculate the actual surface durability in the Web Catalog.
  • Note 2: The gear precision decreases by about one grade after hardening. The bore dimension tolerance H7 will also be ungraded.
  • Note 3: Black oxide processing cannot be performed again after hardening.