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"Easy" Gear Selection Screen

- Choose a gear by pitch, # of teeth, or bore size.
- 2D/3D Drawings Available
- PDF Catalog Available
- Strength Calculation Available

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Technical Information of Gear Rack


KHK stock racks are made for high precision linear motion applications. We offer a large selection of racks ranging from module 0.5 to 10 and lengths from 100 to 2000 mm. The following table lists the main features.

[NOTE 1] The catalog numbers in the above table with (F) suffix have both ends machined so that they can be butted against each other to make a ny desired length. The items with (D) have mounting screw holes for easier assembly.
[NOTE 2] Products with module less than 0.8 are thermal refined, without their gear teeth being induction hardened.
[NOTE 3] Precision grade standard of racks are set by KHK. Please see “Precision of Racks” in Selection Hints section for details.

For safe handling and to prevent damage such as deformation, KHK stock racks have round chamfering at the corners of the top land of the gear tooth. This rounded chamfered shape is patented by KHK. Because it is effective for reducing noise, all of KHK products, except for BSR and PR racks, have this chamfering treatment.
Black colored products are KHK stock gears that have an applied 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 notes before the final selection.

1. Caution in selecting the mating Gears

  1. With the exception of helical racks, KHK stock racks can mate with any spur gears of the same module. Products with different tooth width can also be mated as a pinion.
  2. There are limited choices for of mating gears for KRHG / KRHGF Ground Helical Racks and Helical Racks. There are limited choices for of mating gears for KRHG(F) Ground Helical Racks and SH Helical Racks. Be sure to check the helix hand (right or left) when selecting.

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

Mating Gear Selection Chart
Mating Gear Selection Chart
Caution in selecting the mating Gears

Calculation assumptions for Bending Strength of Gears

Calculation assumptions for 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/mm^2) were matched to the units needed in the equation.
[NOTE 2] The allowable bending stress at 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.
[NOTE 3] For SRG, or SRGF Ground Racks, with a module less than 0.8, the rack teeth are not induction hardened. Allowable bending stress and allowable hertz stress are referred to the value shown in the parentheses.
[NOTE 4] The values for DR m 1.5 racks were assumed by KHK. Usage conditions for SSDR (DR Rack Pinion) are the same for the SSCP Pinion, shown on page 227.

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 insufficient bending strength
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 allowabletangential 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

3. Selecting Racks By Precision

The precision standards of KHK stock racks are established by us. The table below indicates the tolerance ranges of our racks.

1. Pitch Errors of Racks (KHK R 001)

Our precision grades for pitch errors are established by referring to JIS Standards. The precision grades are set from 1 to 8, in accordance with the tolerance of a single pitch error (S.P.E.), adjacent tooth-to-tooth error (T.T.E.), and the total composite error (T.C.E.) for each module and length.

Precision Grades of Racks (KHK R 001)
Precision Grades of Racks (KHK R 001)
[NOTE] Since the pitch accuracy of racks may vary due to humidity, the precision grades are evaluated at the bottom surface of the product, at the temperature of 20 deg C . The dimensions of the KHK PR Plastic Racks may vary widely due to humidity. Therefore, the total composite error is assumed to be excluded from this accuracy standard. Please refer separate technical reference book to “Design of Plastic Gears” (Page 107) for change in dimensions.

Pitch inspection and a sample report using Karl Zeiss UMC-550 Coordinate Measuring Machine. (KHK R 001 Grade 1)
Karl Zeiss UMC 550
a sample report using Karl Zeiss UMC 550 Coordinate Measuring Machine

2. Precision of Rack Blanks

Tolerance on Face Width and Height

Tolerance on Face Width and Height
Tolerance on Face Width and Height
[CAUTION] The width and height tolerances of KHK R 001 grades 3 to 5 products are measured at 1mm inside from each corner. Dimensional tolerance for plastic racks is the value obtained when machining is performed, and the maximum tolerance value is +0.2 x Module (+0.40 for m2 products.), with consideration for aging.

Maximum Curvature Values (Flatness Tolerance L)

Maximum Curvature Values
Maximum Curvature Values
[CAUTION] The straightness tolerances of round racks are 0.15/500 mm and 0.2/1000 mm.

Tolerance on Overall Length

Tolerance on Overall Length
[CAUTION] For Type-F racks with machined ends, the dimensional tolerance is a calculated value according to assumed usage conditions, without consideration of pitch errors and aged deterioration.

3. Backlash of Rack Tooth

Backlash of Rack Tooth (Amount of Tooth Thinning)

Backlash of Rack Tooth
[NOTE] The values shown in the table are amount of tooth thinning. The theoretical backlash of assembled rack and pinion is given by:
Rack & pinion backlash = Amount of tooth thinning of the rack + Amount of tooth thinning of the pinion
Amount of tooth thinning of the rack : See above table.
Amount of tooth thinning of the pinion : Take 1/2 of backlash given in the product table.

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

1. Caution on Performing Secondary Operations

  1. Secondary operations can be performed on all KHK stock racks except for the racks with their gear teeth induction hardened. To avoid problems of gear precision, do not reduce the face width. The precision of ground racks and racks with mounting holes may drop if you do not exercise extreme caution during installation or while modifying.
  2. Pitch lines of racks are controlled by using the bottom surface as the reference datum and over-pin measurements on tooth thickness. If you machine the bottom surfaces, the precision of the racks may be affected.
  3. When connecting two racks, the machining of the mating ends requires careful consideration. The meshing will be poor if the pitch straddling the connection has a positive tolerance. We recommend a minus tolerance on pitch of at the connection. The below is an indication of pitch tolerance for each module.
    Caution on Performing Secondary Operations 1 Caution on Performing Secondary Operations 2
  4. To use dowel pins to secure racks, attach the racks to the base and drill both simultaneously.
  5. KHK stock racks made of S45C and SCM440 (except for ground racks) can be induction hardened. However, the precision of pitch is decreased.
  6. To be able to handle parts safely, all burrs and sharp corners should be removed after the secondary operations are done.
  7. If you are going to modify the gear by gripping the teeth, please exercise caution not to crush the teeth by applying too much pressure. Any scarring will cause noise during operation.

2. Points of Caution in Assembling

  1. KHK stock racks are designed to give the proper backlash when assembled using the mounting distance given by the formula below (mounting distance tolerance of H7 to H8 required). The backlash values are given in the table on Page 191. Make sure that the mounting distance stays constant for the length of the rack.

    Mounting distance a = Height of pitch line of rack + Pitch radius of pinion

    Points of Caution in Assembling
  2. KRG type of KHK stock ground racks have four surfaces ground parallel to within 10~15µm To maintain true angle, they should be mounted on high precision bases as shown below. It is even possible to correct for the angular errors of racks by compensating the mounting base. With recent increases in the requirement for zero backlash linear drives, such accurate assembly as shown is becoming more important.
    Points of Caution in Assembling
  3. If the racks are not secured properly to the base, they could shift during operation and cause unexpected problems. It is very important to insure firm mounting by the use of dowel pins or similar devices.
  4. Machined end type racks such as SRF and SRFD series have the pitch tolerance of -0.05 to -0.4mm at the end face. If you try to connect the racks without any space, the pitch at the connection will be too small and will cause problems. Please follow the following diagrams for assembly.

An example of Rack Joining, we recommend the following method.
An example of Rack Joining we recommend the following method01 An example of Rack Joining we recommend the following method02
An example of Rack Joining we recommend the following method03
[CAUTION] Joining gauge racks for helical racks must have the opposite hand from the racks. Please use Module 1~10 100 racks as a joining gauge rack, or alternatively the rack of the same specifications on hand.

How to mount racks on a mounting base (In case of SRF2-1000)

  1. Adjusting the pitches.
    Mount the SRF2-1000 Rack on the mounting base and connect with the SR2-100 Rack, then, fix with bolts temporally.
    Adjusting the pitches
    * SRF2-1000 Racks are designed so that they will have a clearance gap between 0.2mm and 0.6mm.
  2. Fixing the rack on the mountain base.
    Hit the rack with a plastic hammer to combine closely with the mountain base, then, retighten the bolts. (If a metal hammer is being used, be sure not to deform the gear teeth and use a pressure equalizing plate to protect them.)
    Fixing the rack on the mountain base
  3. Test and run the pinion on the rack to confirm the following.
    (1) Makes no large vibrations or abnormal noise.
    (2) Has appropriate backlash.
    (3) Has no uneven teeth-contact occurred.
    Test and run the pinion on the rack to confirm the following
  4. Secure the fixing to the mountain base.
    It is recommended to use knock pins to prevent slippage due to vibration etc.

    (1) Drill reamer bores simultaneously.
    Secure the fixing to the mountain base01

    (2) Hammer in the knock pins.
    Secure the fixing to the mountain base02
    *After installing the knock pins, retighten them. Making a mark with a pen beforehand will be of help to find looseness if it occurs.

Application Examples
Application Examples
Extrusion device with a round rack* (It can also be a lifting/lowering device by setting up vertically.)

Extrusion device with a round rack1
KRG Ground Rack and SSG Ground Spur Gear used as a work conveying device of the auto loader.

Extrusion device with a round rack2
SRO Round Rack used as a work storage device (fluctuating table) of the auto loader.

Related links :
Rack and Pinion

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