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Technical Information of Spur Gear


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 secondary operations to be performed on the bores, shafts, outside diameters, keyways and set screws. The following table lists the main features.


NOTE 1 / Products with module less than 0.8 are thermal refined, without gear teeth hardened.
NOTE 2 / SA-shaped products with module less than 1 have no material thermal refinement treatment.
NOTE 3 / For products which are smaller than module 0.8, the accuracy grade is equivalent to the value shown.

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 colored products are KHK stock gears that have black oxide coating for rust resistance; this ‘blackness’ is a product characteristic of KHK stock gears.

Selection Hints

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 “CAUTION” notes before the final selection. Use of catalog numbers when ordering will simplify and expedite the processing of your order.

1. Caution in selecting the mating Gears

  1. Basically, all spur gears, internal gears and racks can be paired as long as the module matches. 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 for involute interference, trochoid interference and trimming interference. See the internal gear interference portion of the technical section to avoid problems in assembling these items. (Page 182)

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. Also, SUSL Fairloc hub spur gears, DSL Fairloc hub spur gears and SSAY/K spur gears with built-in clamps need additional considerations of the 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 Surface Durability (Except where it is common with Bending Strength)


NOTE 1 /
JGMA (Japanese Manufacturers’ Association), “MC Nylon Technical Data” of Nippon Polypenco Limited and “Duracon Gear” of Polyplastic Co. The units for 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 􀄱Flim 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 SSG Ground Spur Gears, with module 0.8 or lesser, thermal refining is applied. Allowable bending stress and allowable hertz stress are referred to as the value shown in the parentheses.
NOTE 4 /
For SSS Spur Pinion Shafts, with module over 1.5, tooth induction hardening is not applied. Allowable bending stress and allowable hertz stress are referred to the value shown in the parentheses.
NOTE 5 /
SSS Spur Pinion Shafts with module 1.0 or lesser (SA configuration) are set to cantilever support as it is a single shaft type.

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.
Example of the failure due to
Example of the failure due to insufficient bending strength.

Definition of surface durability by JGMA 402-01(1975)

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

Application Hints

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 or your nearest distributor.
PHONE: 81-48-254-1744 FAX: 81-48-254-1765
E-mail /

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 60% to 70% of the hub diameter (or teeth 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.
If you apply induction hardening on gear teeth, please be aware of potential thermal stress cracks. 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.

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.

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.

2. Points of Caution in Assembling

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).
Backlash may be adjusted by changing the center distance of mating gears. For more information, please consult the technical section on gear backlash (page 56) in separate technical reference book.
tolerance of center distance
a : Center distance
m : Module
Z1 : Number of teeth of pinion
Z2 : Number of teeth of gear
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.
Overall Length Tolerance for Spur and Helical Gears
Overall Length Tolerance for Spur 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.
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 MACHALOCK, a Posi-Lock, or a Shupanring, which are parts for the engaging the hole and the axis.
Verify that the two shafts are parallel. Incorrect assembly will lead to uneven teeth contact which will cause noise and wear. (After assembly, the gear mesh can be checked by applying a contact pattern compound and rotating the gears.)
Test example: Abrasion occurred on SSG3-30 due to poor edge contact (only 30% with proper contact).
In this example, the gear oil used is equivalent to the 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.
Poor tooth contact and pitting
Poor tooth contact and pitting

Application Examples

application of spur gear 1
Full-automatic Forming Machine by Jey Machine Co.
SSA and SS Spur Gears are used for stirring devices.

application of spur gears 2
Takashima High-Speed Wire Straightening & Cutting Machines by Takashima Sangyo Co., Ltd.
SS Spur Gears are used at the feeder.

application of spur gears 3
Automatic Packing Machine by New Max
SS Spur Gears, segment shaped by secondary operation, are used at the crimping device.

application of spur gears 4
Electric Wire Winder by Sakuma Tekko KK.
SS Spur Gears are used at the stopper of handgrip.

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