Q&A about Gears – Section for studying gear technology –

In this “Q&A About Gears” section, we have supplemented the textbook with such things as gear related vocabulary. There are many elements that must be learned to correctly use gears such as strength and size calculations, material knowledge, drawing conventions, etc. To assist you, we will continue to improve the contents of this section based on the actual technical questions we receive and their answers.

Related links :
Q&A关于齿轮 – 中文页

Manufacturing and Sales of KHK Stock Gears

Where can I buy KHK Stock Gears?
Is it possible to request secondary operations of KHK Stock Gears, or request custom-made gears? And, what is the price?
Can I request design of custom-made gears or gearboxes?
Do you produce gears which have smaller bores than KHK Stock Gears? And, do you manufacture gears that have no bores?
Do you manufacture plastic gears made with materials other than MC901 (MC Nylon)?
Can I request positioning of gear-teeth and keyways for KHK Stock Gears?
Are Raydent or plating treatments available?
If I send you a broken, used gear, do you think you can produce the same exact gear?
Can I consult you about gears that I had have produced for other gear manufactures?
Can I request measurement of other manufacturer’s gear products?
Are the products with model numbers that cannot be found in the catalog, no longer manufactured?

Gear Technologies

What is “Module”?
What is DP (Diametral Pitch)?
What is “Reference Diameter”?
What is a “Pressure angle”?
How do you distinguish right-hand teeth (R) from left-hand teeth (L)?
Direction of spiral (or hand of helix), and direction of thrust.
What is “Center distance”?
What is “Crowning”?
What does “Gear tooth modifications” mean?
What is “Semi-topping”?
What is the “Shaft angle”?
What is Seizure?
What is the solution to make gears rotate smoothly?
What is “Tooth thickness”?
What is “Contact Ratio”?
Does the reference diameter of a profile-shifted gear vary too?
How do I distinguish between a Transverse system and a Normal system?
What is “Black oxide coating”?
Is the black oxide coating removable?
How often do I apply a lubricant?
In case of fastening a gear to the shaft with a set screw, how much torque can be transmitted?
What methods are available to fasten gears to shafts?

Backlash

What is “Backlash”?
How much backlash should be set up?
Is it a problem to use gears with no backlash?
I want to remove backlash completely, what am I supposed to do?
For gears which have the center distance separated (or close), what is the calculation method for the backlash?

Gear Strength

What is the difference between Bending strength and Surface durability?
When selecting gears, which factors should be considered, bending strength or surface durability?
How can I manage to use a gear where the surface durability is very low compared with the bending strength?
What is the surface durability of plastic gears?
How much can you raise the strength by quenching?
What materials or products available are stronger than S45C-made gears with gear teeth induction hardened?
What is the lifediv of a gear?
Is it possible to calculate the surface durability for gears made of Free-Cutting Brass?

Specific usages

Is it possible to use gears in a high-temperature environment?
Is it possible to use KHK Stock Gears in a clean room?
What gear is suitable for use in food machinery?
Which types of gears are resistant to rusting?
What types of gears are suitable for high speed rotation?
Which types of gears are quiet?
Are there any gears that can be used without lubrication?
Is grease lubrication applicable?

Calculations and Specifications for Gears

Can I request gear calculations (recalculation) ?
Do you offer information on ISO or DIN Standards?
Is it possible to calculate rotation for more than 3600 rpm?
Is CAD data for tooth profiles available?
What is gear efficiency?
What are the numbers (0 to 60) listed on the left in the Involute Function Tables?
What is the calculation method for thrust force produced in a radial direction?

Spur Gears

How much allowable center distance is usually set for spur gears?
What is the difference between MSGA and MSGB Ground Spur Gears in KHK Stock Gear series?
Is it possible to use gears which have undercutting?
Why do gears with a small number of teeth have less strength?
Is it possible to mate spur gears which have different accuracy, face width, or are made of different material?
How do I fasten DS Spur Gears to the shaft?
How do I decide tolerances of div measurements for k teeth?

Helical Gears

What is the difference between Spur gears and Helical gears?
What is the difference between Transverse module KHG Helical Gears and Normal module SH Helical Gears?
How much thrust force is produced?
Do you manufacture Double Helical Gears?

Internal Gears

What is interference in Internal Gears?
Interference occurs when I use an internal gear. What is the solution?
How do I make a planetary gear system?
Do you accept requests for gear-tooth grinding of internal gears?

Racks

What is the difference between m (Module) and CP (Circular pitch)?
When a pinion rotates one revolution, how many millimeters has the mated rack traveled?
I want to use several racks in joint. Is it possible to do that?
How do I apply lubrication to racks?
Interference occurs when a rack and the pinion mesh on and off. What is the solution?
What gears can be used to move a 100kg object horizontally?
Is it possible to mesh DR Molded Flexible Racks, similar to spur gears?

Bevel Gears

What is Miter Gears?
Is it possible to mate miter gears, where one has 20 teeth and the other has 25 teeth?
What is the “Mounting distance” (Locating distance)?
In the dimensional tolerance of the mounting distance, how well do you decide the exact dimension?
Are spiral bevel gears able to make both positive rotation and negative rotation?
What is “Zerol Bevel Gear”?
Do you carry bevel gears that have a shaft angle other than 90°?
Do you accept requests for manufacturing high-ratio hypoid gears, other than products introduced in your catalog?
Does self-locking occur in high-ratio hypoid gears?
How can I confirm proper installment of bevel gears?
Do you carry sets of spiral bevel gears which have opposite helix hands?
How do I fasten DM Injection Molded Miter Gears or DB Injection Molded Bevel Gears to a shaft?

Screw Gears

How do I have a screw gear shifted?
I am having screw gear wear problems. What is the solution?

Worm Gears

What is the difference between “single thread” and “double threads” in worms?
Is it possible to use duplex worm gear pairs at zero backlash?
Are duplex worm gear pairs able to make both positive rotation and negative rotation?
How do I make self-locking worm gear pairs?
How much thrust force is produced?
Is it possible to mesh worms or worm wheels with racks?
What is a JIS3 Worm tooth profile?
Is it possible to calculate the bending strength of worm gears?
How do I enhance the efficiency of worm gears?
How can I confirm proper installment of worm gear pairs?
I am using worm gears in a device to lift heavy objects, but doesn’t work smoothly in the reverse direction. Any ideas for solving this problem?
Please provide a tip in understanding the principle of self-locking worm gears.
For shaft-angle errors or assembly errors of worm gears, please advice on the allowable values?

Other Products

Is lubrication necessary for gear couplings?
Can I request the repair of One-Cycle Clutches? Or do you have spare products/parts available for them?

Plastic Gears

Do you manufacture plastic gears made with materials other than MC901 (MC Nylon)?
What is the surface durability of plastic gears?
What is a suitable backlash for Resin (Plastic) Gears?
Breakage occurs with resin-made (plastic) gear teeth near the pitch point, are there any preventive solutions?
Why is it better to mate resign (plastic) gears with metal gears?

Manufacturing and Sales of KHK Stock Gears

Where can I buy KHK Stock Gears?
Please purchase KHK Stock Gears from a nearby distributor in your country. KHK Stock Gears are not available for direct purchase from us. To find KHK international distributors, please see here.
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Is it possible to request secondary operations of KHK Stock Gears, or request custom-made gears? And, what is the price?
We do accept orders for secondary operations of KHK Stock Gears and orders for custom-made gears, however, not directly. So, prepare drawings and specifications then ask a nearby distributor for a quotation. The distributor will respond to you and will let you know if production is possible, in accordance with your requests and lead-time.
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Can I request design of custom-made gears or gearboxes?
We do not offer design services for either custom-made gears or gearboxes. For the design of gears, please ask a professional designer. However, we do accept orders for manufacturing custom-made gears, but we do not manufacture gearboxes.
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Do you produce gears which have smaller bores than KHK Stock Gears? And, do you manufacture gears that have no bores?
We do not manufacture gears which have smaller bores than KHK Stock Gears, due to the limited capacity in our manufacturing facilities. We do produce gears with no bores as custom-made gears, however, it will cost more than regular products and require a longer lead -time, and is handled as a shaft gear and tooth-cutting procedure.
For a quote, please contact a nearby distributor in your country.
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Do you manufacture plastic gears made with materials other than MC901 (MC Nylon)?
It depends on the shape of gears and the material used, and we accept orders for custom gears made with commercially available materials. For plastic gears, commonly used materials are; POM (also called Polyacetal, DURACON or Delrin), MC602ST (MC Nylon) having higher strength than MC901, and PEEK, which has an excellent anti-chemical durability. It is not possible to manufacture plastic gears made of fiber reinforced resin such as CFRP (Carbon Fiber) or GFRP (Glass Fiber).
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Can I request positioning of gear-teeth and keyways for KHK Stock Gears?
In any series of KHK Stock Gears, we do not have products where teeth and keyways are positioned. For custom-made gears, we accept requests to modify keyways so to make the positioning in accordance with the top land and the bottom land of the teeth, however, the positioning accuracy will be somewhere around  ±0.1mm, as the positioning is done by scribing.
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Are Raydent or plating treatments available?
We offer several types of plating treatments; Raydent, Electro-galvanizing (Trivalent Chromate etc.), and Electroless nickel plating. However, for areas like bores, with an uneven thickness of plating film, it might negatively influence the smooth insertion of the shaft. Since the plating treatment creates a thin film of only a few μm. Do not use gears without lubrication.
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If I send you a broken, used gear, do you think you can produce the same exact gear?
It is not possible to produce the same gear by analysis of a used gear. Since it is too difficult to clarify specifications of used gears, especially when the gears have wear and breakage, we do not produce gears by analysis of actual products.
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Can I consult you about gears that I had have produced for other gear manufactures?
We do not accept requests for inspection, investigation or trouble shooting of other manufacturer’s gear products. In the case you are planning to replace other manufacture’s gears with KHK Stock Gears, please contact us for consultation.
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Can I request measurement of other manufacturer’s gear products?
We do not accept requests for measurement of other manufacturer’s gear products. You can ask the manufacturer who produced the gears, or a gear-measurement instrument manufacturer.
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Are the products with model numbers that cannot be found in the catalog, no longer manufactured?
Please see here to find Discontinued KHK Stock Gear Product Lists. For these discontinued products, except for the One-Cycle Clutches, we handle requests by offering equivalent products or custom-made gears.
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Gear Technologies

What is “Module”?
Module is the unit of size that indicates how big or small a gear is.
The module is the length of the pitch diameter per tooth. To represent the tooth size. CP and DP are also used. (CP stands for circular pitch, and DP stands for diametral pitch.)
*Reference*   “Pitch”  is the distance between corresponding points on adjacent teeth.
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What is DP (Diametral Pitch)?
DP (Diametral Pitch) is the unit to denote the size of the gear tooth. Transformation of DP to module is accomplished by the following equation.
If DP is 8
1 inch (25.4㎜) ÷ 8 = Module 3.175
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What is “Reference Diameter”?
The reference diameter is the diameter of the pitch circle of a gear It is also known as “Pitch diameter”. The two essential factors in determining the size of a gear are the number of teeth and the module. Then the reference diameter is:
Number of teeth x Transverse module
JIS gear terms for “reference diameter” before the amendment was “pitch diameter”.
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What is a “Pressure angle”?
Pressure angle is the angle generated with a radial line and a line of the tooth profile, which intersect at a pitch point. In simple terms, it is described as an angle of the gear tooth leaning against the normal reference line, and the gear force is transmitted toward this angle in the same direction. Generally, pressure angle means a pressure angle on base circle. The most frequently used pressure angle is 20°, however, other angles, such as 14.5°, 15°, 17.5° and 22.5° are also used.
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How do you distinguish right-hand teeth (R) from left-hand teeth (L)?
When the gear is placed on a flat surface, the teeth of a left-hand gear lean to the left and the teeth of a right-hand lean to the right.
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Direction of spiral (or hand of helix), and direction of thrust.
The meshed gears develop, under operating conditions, and thrust force. The only exception is spur gears, where the teeth are straight and parallel to the shaft axis. For relations between the direction of spiral and the direction of thrust on each type of gear, please see the following:
* For helical gears and helical racks* Click here
* For spiral miter gears* Click here
* For spiral bevel gears* Click here
((For spiral bevel gears with gear ratio under 1.5, the direction is the same as for spiral miter gears.) In the case of straight bevel gears, the thrust is always backward toward the hub.
* For screw gears* Click here
* For worm gear pair* Click here
For the magnitude of the gear forces** Click here
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What is “Center distance”?
The center distance is the distance between the center of the shaft of one gear to the center of the shaft of the other gear. The backlash is inseparably related to the center distance; a positive permissible deviation of the center distance means a larger backlash, and a negative permissible deviation of the center distance means a smaller backlash.
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What is “Crowning”?
Crowning is removal of a slight amount of the tooth from the center on out to the reach edge, making the tooth surface slightly convex.
This method allows the gear to maintain contact in the central region of the tooth and avoids edge contact. The larger the amount of crowning, the less the amount of tooth surface contact.
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What does “Gear tooth modifications” mean?
It means the intentional deviations from the proper tooth profile to avoid excessive tooth load deflection interference, by chamfering the tooth surface. It is similar to crowning except that it is a simple process and only an approximation to crowning.
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What is “Semi-topping”?
Semi-topping is the chamfering of the tooth’s top corner which is accomplished, simultaneously, by tooth generation.
The advantages of semi-topping are:
(1) Prevents burrs at the tooth tip when applying gear cutting.
(2) Prevents dents at the tooth tip that tend to occur when shipping.
[NOTE] Tooth tip: the tangential line of the tooth face and the top land.
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What is the “Shaft angle”?
Intersecting axis gears (bevel gears) and nonparallel & nonintersecting axis gears (screw gears and worm gear pair) connect from one shaft to another. The angle of these two shafts is called the shaft angle. The angle is normally 90° . Setting up the shaft angle accurately, as per specification, is very important as it affects the tooth contact and the backlash.
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What is Seizure?
Seizure is the adhesion occurring with the mating gear’s tooth surface, which is caused by a shearing lubricant film that makes direct contact with each steel tooth surface. Seizure is also called Scoring or Scuffing. It occurs automatically when no lubricant is applied and in non-lubricant operations, and usually involves the risk of rapid abrasion in the adhered area with the film partially ripped off.
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What is the solution to make gears rotate smoothly?
(1) Check and make sure if a proper backlash is maintained.
Gears are unable to rotate smoothly without backlash (the state of being meshed)
(2) Check and make sure if proper lubrication is applied
Gears are unable to rotate smoothly without lubricant. Also, too much lubricant increases resistance to stirring.
(3) Adjust tooth contact properly

If gears are set with improper tooth contact, the shaft might be inclined. Excessive uneven contact causes abrasion. Especially for bevel gears and worm gears, proper tooth contact is crucial, so it must be adjusted to position near the center. Gears with better accuracy can rotate more smoothly. Gears having tooth grinding applied after heat treatment are able to rotate more smoothly than gears where the accuracy is decreased after applying heat treatment.If gears are set with improper tooth contact, the shaft might be inclined. Excessive uneven contact causes abrasion. Especially for bevel gears and worm gears, proper tooth contact is crucial, so it must be adjusted to position near the center. Gears with better accuracy can rotate more smoothly. Gears having tooth grinding applied after heat treatment are able to rotate more smoothly than gears where the accuracy is decreased after applying heat treatment.
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What is “Tooth thickness”?
Tooth thickness is the thickness of each gear-tooth. Tooth thickness is not the face width. It is usually produced, smaller than the basic dimension to apply backlash. Methods for measuring tooth thickness are: div measurement for spur or spiral gears.Over pin measurement for rack and worm gears.
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What is “Contact Ratio”?
There are two types of Contact Ratio, one is Transverse contact ratio and the other is Overlap ratio.
Transverse contact ratio is a value expressed by this calculation:
Length of path of contact divided by Base pitch. For example, if a transverse contact ratio is 1.5, therefore it should rotate and keep tooth contact with a single tooth and with two teeth, alternately. If transverse contact ratio is 2, it should rotate and always keep tooth contact with two teeth.
Overlap ratio is a value involving gears which have helix hands like helical gears, and is expressed by this calculation: Face width divided by Pitch in the direction of the face width.
Total contact ratio is the sum of Transverse contact ratio and Overlap ratio. It is said that less noise is produced if total contact ratio becomes an integer, such as 2 or 3. As for strength, higher contact ratio leads to more strength as the load per tooth is dispersed.
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Does the reference diameter of a profile-shifted gear vary too?
Even if it is a profile-shifted gear, the reference diameter will not vary. The change in center distance is due to the change in the working pitch diameter.
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How do I distinguish between a Transverse system and a Normal system?
There is no certain method of distinguishing between a Transverse system and a Normal system, even by viewing actual products.
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What is “Black oxide coating”?
Black oxide coating is the surface treatment for rust prevention by creating a film of ferrosoferric oxide. This is a simple rust-prevention treatment by forming a black rust film on a surface.
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Is the black oxide coating removable?
Black oxide films can be removed by acid pickling, however, please be aware that the surface will get rusty easily after removal.
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How often do I apply a lubricant?
In regards to the frequency of lubrication, there are no specific guidelines, therefore application varies depending on the torque or condition of the machine operation. Please determine the situation by performing tests or use your own experience and knowledge.
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In case of fastening a gear to the shaft with a set screw, how much torque can be transmitted?
Fastening with a set screw is a simple method. For secure fastening, please use keys or dowel pins in combination.
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What methods are available to fasten gears to shafts?
The common fastening methods are the use of keyways. There are also methods that use frictional fasteners, such as the Power-Lock (TSUBAKIMOTO CHAIN CO.), the Lamp-Lock (NABEYA BI-TECH KAISHA), or the Mechalock (ISEL CO., LTD.). In the case that these frictional fasteners cannot be used, due to small diameters, dowel pins, set screws and press fitting are used as simplified fastening methods.
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Backlash

What is “Backlash”?
Backlash is the clearance space between the teeth of mating gears Backlash is necessary for smooth operation of gears.
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How much backlash should be set up?
Data of backlash for KHK Stock Gears, complied as theoretical values, are available on the KHK Web Catalog. Since backlash is the data obtained from knowledge and experiences of each machine manufacturer, there’s no specific uniform value defined. Backlash on JIS B 1703 for Spur Gears and Spiral Gears are discontinued, but we introduce the data only as reference.
Backlash for Spur Gears and Helical Gears
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Is it a problem to use gears with no backlash?
Backlash is required so that gears are able to rotate smoothly. Less backlash causes lubrication problem, leading to abrasion. Also, without backlash, excessive noise is produced as gears should contact with both tooth surfaces. Higher precision gears produce fewer problems when backlash is set at a small value, however, no-backlash must be avoided.
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I want to remove backlash completely, what am I supposed to do?
There’s no product in KHK Stock Gears that allows zero backlash. There are gears that allow to adjust backlash to a minimum, but, it is not recommended to set backlash to zero. The products that allow backlash adjustment are Tapered racks & pinions and Duplex worm gear pairs.
For details please see our reference book (PDF)
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For gears which have the center distance separated (or close), what is the calculation method for the backlash?
Please see our reference book (PDF) or KHK Catalog to find this information.
The methods introduced there, are basic calculations and the calculation results might involve some errors. The error might become larger, in case of calculation for profile shifted gears.
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Gear Strength

What is the difference between Bending strength and Surface durability?
Strength of gears is generally considered from bending strength and surface durability. Bending strength denotes the strength against breakage at tooth root, when force is produced as a gear transmits power. Surface durability is surface-endurance properties against wear on tooth surface, produced when mating teeth contact each other repeatedly.
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When selecting gears, which factors should be considered, Bending strength or Surface durability?
In general, both the bending strength and the surface durability should be considered. However, there are cases where only bending strength is considered for selecting gears, which have less-frequency in use, or gears meshed at low speed (slow as rotating by hand) In any case, gears should be selected in accordance with usage conditions.
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How can I manage to use a gear where the surface durability is very low compared with the bending strength?
Gears without heat treatment have low surface durability. Applying heat treatment encourages surface durability of gears from about 3 to 5 times larger, however, there is noise increases as the heat from the treatment deteriorates the precision of gear. Grinding tooth surface after heat treatment, for modifying roughness and the accuracy, will enable you to produce stronger gears for high precision. Grinding tooth surface encourages surface durability as the roughness level becomes lower.
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What is the surface durability of plastic gears?
MS Nylon is a material excellent in wear resistance. Since gears made of MC Nylon have surface durability exceeding the bending strength, it is not necessary to consider the surface durability. As for gears made of DURACON, without lubrication, the surface durability should also be considered.
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How much can you raise the strength by quenching?
Quenching enhances the surface durability about 3 to 5 times, however, the precision of the gear is deteriorated by heat and it decreases allowable bending strength. To cope with this condition, grinding is usually applied after quenching, enhancing precision.
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What materials or products available are stronger than S45C-made gears with gear teeth induction hardened?
In general, the following materials are available. (In descending order, according to strength)
(1) SCM415, Carburized (MSGA(B) Series, MMS Series, etc.)
(2) SCM440, Induction hardened after thermal refining (KHG Series, KWG Series, etc.)
(3) S45C Induction hardened after thermal refining (SSGS Series, etc.)
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What is the lifediv of a gear?
The lifediv of gears cannot be figured out by calculation only. Generally, if the gear is used properly in accordance with usage conditions, breakage or wear caused by lifediv problems do not usually occur.
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Is it possible to calculate the surface durability for gears made of Free-Cutting Brass?
No, it is not possible. In regards to the material properties of free-cutting brass, the allowable hertz stress is not specified in the formulas of JGMA. (Japan Gear Manufacturers Association’s Standards).
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Specific usages

Is it possible to use gears in a high-temperature environment?
KHK Stock Gears should be used at normal temperature (approx. 40℃). Usage in a high-temperature environment may cause problems such as distortion or decrease of precision due to heat and functional deterioration in the lubricant. As for heat-treated gears, the hardness may be reverted and unable to maintain inherent strength. Metal gears can be used as long as temperatures are set under 150℃, where the hardness will not decrease. To use plastic gears, please consider the following temperature guide; less than 120℃ for MC Nylon Gears and less than 95℃ for DURACON Gears.
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Is it possible to use KHK Stock Gears in a clean room?
There are no KHK Stock Gears that can be used in a clean room. To use KHK Stock Gears in a clean room, an enclosed device should be applied such as a gearbox.
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What gear is suitable for use in food machinery?
Since rust and oil contaminations are undesirable for food machinery, stainless steel gears and plastic gears are suitable. To meet these requirements we provide various gears made of stainless steel and MC Nylon. MC Nylon gears can be used without lubrication, and they can be compliant with food sanitation laws if boiling sterilization is applied. However, care should be taken as dimensions might vary through water absorption.
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Which types of gears are resistant to rusting?
Stainless steel and plastic gears are good against rusting. Using plastic against plastic is not desirable since they tend to heat up and expand, resulting in decrease of backlash. Mating a plastic gear with a metal gear is desirable.
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What types of gears are suitable for high speed rotation?
Since gear strength decreases more at higher peripheral speed, gears desirable to use are gears that are strong enough under required usage condition and have high precision. Therefore, spiral gears or high precision gears finished with tooth grinding after heat treatment, are the most suitable.
(1) High precision ground tooth gears
Ground tooth gears are better in their accuracy than cut or hobbled gears, and therefore, they are stronger and quieter. KHK carries stock ground tooth gears for the following types of gears: spur gear, rack, helical gear, helical rack, spiral miter gear and spiral bevel gear.
(2) Gears with spiral teeth
Gears with spiral teeth have better contact ratio than straight teeth gears, and therefore, they are stronger and quieter. It is recommended that you choose helical gears in preference to spur gears, and spiral bevel gears to straight bevel gears.
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Which types of gears are quiet?
High precision gears with a grounded finish are quiet.  The use of helical gears in preference to spur gears is more effective in reducing noise level, since the teeth of helical gears are cut at an angle to the hole (axis), which increases the contact ratio. Plastic gears are also quiet, however, they are less strong.
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Are there any gears that can be used without lubrication?
Metal gears mated with metal gears cannot be used without lubrication. Mesh of metal and metal gears causes wear, galling and seizure. Plastic gears can be used without lubrication, as long as it involves only light loads, however, it is recommended to apply lubrication to plastic gears too. For the use of plastic gears, it is also recommended you mate a plastic gear with a metal gear, as plastic gears tend to heat up and expand resulting in decrease of backlash.
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Is grease lubrication applicable?
The lubrication applied to a gear system is usually oil bath lubrication, but grease lubrication is often applied for a gear system as such compound with racks & pinions, that is open and runs at low speed / light loading. However, care should be taken so that proper grease is used and frequent maintenance is done to prevent grease decay. Grease lubrication is not suitable for worm or screw gears as they are easily get hot, and grease lubrication has a low cooling effect.
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Calculations and Specifications for Gears

Can I request gear calculations (recalculation) ?
We don’t offer calculation (and recalculation) services for gear specifications and strength for custom made gears. The gear strength for KHK Stock Gears can be calculated at our Web Catalog. For calculation of gears other than Stock Gears, please try out GCSW, the automatic gear calculation software (Japanese version only).
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Do you offer information on ISO or DIN Standards?
We are not offering services for answering questions on these standards. To find information on these standards, please contact and make an inquiry to the Japanese Standards Association (JSA), who may provide information on international standards.
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Is it possible to calculate rotation for more than 3600 rpm?
In accordance with the strength calculation method defined by the Japan Gear Manufacturers Association (JGMA) that we are using, the strength can only be calculated under the condition of; rotation is lower than 3600 rpm and peripheral speed is lower than 25m/s.
Even if the strength cannot be calculated, it doesn’t mean the gear cannot be used. However, since it does not allow consideration of strength from the calculation, users are supposed to use the gear in accordance with data obtained, only, from user’s own experiences or knowledge.
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Is CAD data for tooth profiles available?
CAD data for our products includes no data on tooth profiles. Data for tooth profile is not available.
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What is gear efficiency?
The data on general efficiency is available on our reference book (PDF). For worm gears, an equation for calculating efficiency is available, however, accurate efficiency cannot practically be calculated due to loss by bearing or resistance to stirring.
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What are the numbers (0 to 60) listed on the left in the Involute Function Tables?
The numbers on the left in the Involute Function Tables denote “Minute” in DMS: Degrees/Minutes/Seconds. For example, 21°20′ is of 0.018217, the value at the intersection of a line in section “21°” and the line “20” in the vertical section on the left.
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What is the calculation method for thrust force produced in a radial direction?
Calculations for gear forces are introduced in our product catalog and
gear technology reference (PDF).
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Spur Gears

How much allowable center distance is usually set for spur gears?
We recommend plus tolerance at H7 to H8 for center distance. Japan Gear Manufacturers Association (JGMA) recommends ±tolerance targeting zero of reference value. The standard related to this value is JGMA 1101-01(2000).
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What is the difference between MSGA and MSGB Ground Spur Gears in KHK Stock Gear series?
These ground spur gears, divided type A and B, have the exact same specifications other than their bore sizes.
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Is it possible to use gears which have undercutting?
Undercutting occurs on all of our stock spur gears, which are not profile shifted and have a pressure angle at 20°, and the number of teeth are 17 or less. However, undercutting also might occur on the stock spur gears with 18 teeth, if the amount of tooth thinning is large. It should be no problem to use those gears as long as they have enough strength and have a contact ratio at 1, or over. For details, see here.
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Why do gears with a small number of teeth have less strength?
Gears with a small number of teeth have weakness in profile at the tooth root, and become weak if undercutting occurs. For details, please see our guide book “The ABCs of Gears – The intermediate course”. Also, since a lesser number of teeth simply have smaller diameters, the allowable torque declines.
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Is it possible to mate spur gears which have different accuracy, face width, or are made of different material?
Gears with different face width, material and accuracy can be mated as long as the module and the pressure angle are matched.
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How do I fasten DS Spur Gears to the shaft?
Since DS Spur Gears are produced with a minus tolerance, use a shaft (H7) with a force fit. For secure fastening, use tapping screws or dowel pins.
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How do I decide tolerances of div measurements for k teeth?
Since tolerances for tooth thickness are determined by each gear manufacturer, in accordance with their own experiences and empirical data, there is no uniform values specified.
Decreases (negative tolerance) in div measurement over k teeth affects the backlash value.
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Helical Gears

What is the difference between Spur gears and Helical gears?
Under the same conditions (similar accuracy, material, etc.), helical gears have more strength than spur gears, and helical gears are much quieter. However, helical gears produce thrust force so care should be taken for those involving bearings or in a gearbox.
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What is the difference between Transverse module KHG Helical Gears and Normal module SH Helical Gears?
They have different basic profiles and each of them has advantages and disadvantages as listed below: Transverse module KHG Advantage: allows easy design as the values of the reference diameter and the center distance of a KHG correspond to that of a spur gear.
Disadvantage: (from the viewpoint of manufacturer) The special hob or cutter is necessary. Normal module SH Advantage: (from the viewpoint of manufacture) The hobs or cutters for a spur gear can be used for a SH helical gear. No special hob or cutter is necessary, which results in reasonable costs.
Disadvantage: It will be necessary to make a new gearbox as the center distances of a SH and a spur gear are different.
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How much thrust force is produced?
It varies depending on the torque and other specifications, please calculate in accordance with the reference below.
See here for details
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Do you manufacture Double Helical Gears?
We are not manufacturing double helical gears. It is not practical to use double helical gears as they are assembled with right-handed and left-handed teeth that are the same in number, but are mismatched in pitch and are uneven in tooth thickness.
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Internal Gears

What is interference in Internal Gears?
Internal gears produce interference depending on mated spur gears.
Involute Interference : This occurs between the dedendum of the external gear and the addendum of the internal gear. It is prevalent when the number of teeth of the external gear is small.
Trochoid Interference : This refers to an interference occurring at the addendum of the external gear and the dedendum of the internal gear. It tends to happen when the difference between the numbers of teeth of the two gears is small.
Trimming Interference : This occurs when the internal gear is shifted in the radial direction. It tends to happen when the difference between the number of teeth of the internal and external gears is small. Trimming causes problems when applying gear cutting with a pinion cutting machine or when assembling, however, there is no problem in ordinary meshing.
For details see here.
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Interference occurs when I use an internal gear. What is the solution?
Since all interference that happen in gear meshing with internal gears occur at the tooth tip of the internal gear, the interference is avoidable if you design the internal gear with short -addendum teeth (with large tip diameter). However, care must be taken in the design so that the contact ratio is never smaller than 1, while the gears should have stub teeth. Interference is also avoidable by applying a positive correction on the internal gear.
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How do I make a planetary gear system?
To create a planetary gear system, it is essential to fulfill the following six conditions:
Involute interference, Trochoid interference, Trimming interference, Matching of center distance, Even-positioning, and Interference between planetary gears.
Planetary gear systems offer features such as compact size and high reduction ratio, but these systems are for advanced users, as it is difficult to fulfill all necessary interferences and conditions.
Interference in Internal Gears
Planetary Gear System
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Do you accept requests for gear-tooth grinding of internal gears?
No, we don’t. We are not able to handle gear-tooth grinding for internal gears as we do not have the required machining equipment.
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Racks

What is the difference between m (Module) and CP (Circular pitch) ?
Both are a way to express the size of gear teeth.
CP (Circular pitch) denotes the reference pitch in millimeters; this is also the units for m (module).
Module is obtained by dividing CP by Pi. The relational expression for m and CP is :
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When a pinion rotates one revolution, how many millimeters has the mated rack traveled?
Traveling distance made by one revolution of a pinion is obtained from the calculation: Module x No. of teeth x Pi.
(e.g. a pinion, which has module 3 and, no. of teeth is 20, rotates one revolution:3 x 20 x pi = 188.5)
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I want to use several racks in joint. Is it possible to do that?
To string together racks for use, it is necessary to adjust pitch to fit at joint parts. One of the ways to adjust pitches at a joint is to engage short racks as “gauge racks” when assembling.
For details, see here.
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How do I apply lubrication to racks?
It is recommended to apply an oil bath or forced lubrication for gear operations, but, as for racks, grease lubrication is most commonly applied while it is difficult to apply an oil bath.
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Interference occurs when a rack and the pinion mesh on and off. What is the solution?
In case the pinion meshes on and off in the traveling direction, interference may occur between the pinion and the rack at their tooth tips, when they mesh again. So far, there’s no method for avoiding this interference, therefore, it is not recommended to use racks and pinions in such a way.
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What gears can be used to move a 100kg object horizontally?
Gear selection cannot be made under such unclear conditions. Even for a heavy weight, the inertia becomes small if addition-subtraction speed is low. Also, if a guide having less resistance, like a linear guide, is used, the load produced when traveling becomes small. Selection cannot be made without data on the expected load produced on racks & pinions.
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Is it possible to mesh DR Molded Flexible Racks, similar to spur gears?
The DR Molded Flexible Rack has a straight-line tooth profile. Gears with a straight-line tooth profile cannot be used together as they do not mesh properly.
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Bevel Gears

What is Miter Gears?
Spiral gears where gear ratio is 1:1 and the shaft angle is at 90°, are called Miter Gears.
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Is it possible to mate miter gears, where one has 20 teeth and the other has 25 teeth?
Since the number of teeth of a mating gear is determined when manufactured, it is not possible to mate miter gears with gears of a different number of teeth.
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What is the “Mounting distance” (Locating distance)?
Mounting distance (or Locating distance) denotes the distance between a corn tip and positioning surface (reference surface for mounting) of bevel gears. Mounting distance is a very important dimension that concerns tooth contact and backlash.
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In the dimensional tolerance of the mounting distance, how well do you decide the exact dimension?
The best way is to aim at near-zero for the proper backlash and tooth contact. It is recommended that you aim at the specified dimensional standard (zero) from JS7 to JS9.
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Are spiral bevel gears able to make both positive rotation and negative rotation?
Bevel gears can be used for both right and left rotation. There’s no specific restriction. However, the direction of the thrust force should be carefully considered. It might produce thrust force in the direction of the mated gear, and that thrust force is in a negative direction. This thrust force must be securely received by bearings.
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What is “Zerol Bevel Gear”?
Zerol bevel gear is a spiral bevel gear having a helix angle less than 10°, similar to a straight bevel gear.
Advantages:
(1) The forces acting upon teeth are the same as for a straight bevel gear.
(2) Has higher strength, and is quieter compared to straight bevel gears.
(3) The teeth are grindable, which means it is possible to produce high accuracy products.
* Tip: The spiral angle of spiral bevel gear is generally at 35°.
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Do you carry bevel gears that have a shaft angle other than 90°?
Our product, SAM Angular Miter Gears, are available with shaft angle at 45, 60 and 120°. We also accept orders for custom-made gears with a shaft angle between 45 to 120°, however, there might be cases where we can’t manufacture as requested, due to limited capability in our machining equipment.
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Do you accept requests for manufacturing high-ratio hypoid gears, other than products introduced in your catalog?
Since high-ratio hypoid gears are special products, we do not manufacture them, other than products introduced in our product catalog.
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Does self-locking occur in high-ratio hypoid gears?
Self-locking occurs in high-ratio hypoid gears which have a gear ratio 1/45 or over. However, since the state of self-locking is influenced by machining accuracy or lubrication, another brake mechanism should also be provided if complete prevention of reverse is required.
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How can I confirm proper installment of bevel gears?
To confirm if a bevel gear is mounted properly, you need to check the tooth contact and the backlash. If the bevel gear has an improper tooth contact or too small backlash, it causes wear and noise. It is recommended to have a preconditioning interim operation before start using.
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Do you carry sets of spiral bevel gears which have opposite helix hands?
In any series of KHK Stock Gears, we do not have gears which have opposite helix hands. We do produce these gears as custom-made gears. For a quote, please contact a local distributor in your country.
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How do I fasten DM Injection Molded Miter Gears or DB Injection Molded Bevel Gears to a shaft?
Since DM Injection Molded Miter Gears and DB Injection Molded Bevel Gears are produced with a minus tolerance, you can use them by putting in a shaft (H7) with a force fit. For secure fastening, use tapping screws or dowel pins.
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Screw Gears

How do I have a screw gear shifted?
Shifting a screw gear in accordance with the mounting distance, involves change in the shaft angle. In addition, the reference diameter is varied if you change the helix angle to adjust the shaft angle, and results will show that the mounting distance is also varied. Therefore, calculation for transposition of screw gears is very difficult and it is not recommended.
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I am having screw gear wear problems. What is the solution?
Although wear hardly occurs as long as the gear does not receive excessive load against the allowable torque (surface durability), wear occurs when lubricant is depleted, as the screw gear transmits power by slippage on tooth surface. Mating a screw gear with a screw gear made of the same material tends to involve seizure, so, wear is avoidable by mating gears made of different material. Mating a gear with a gear made of material that has better slippage qualities, such as copper alloy, will have less wear than mating screw gears made of steel.
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Worm Gears

What is the difference between “single thread” and “double threads” in worms?
The thread in this instance is a continuous helical rib on a worm. The single thread means that a worm has one thread. The double threads means that a worm has two threads. The thread is the equivalent of the gear tooth. The larger the number of thread, the wider the lead angle.
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Is it possible to use duplex worm gear pairs at zero backlash?
Basically, it is not possible. There are possibilities that the gear pair does not turn well. It is possible that backlash change at a certain point during a turn due to runout error, which causes a jam. Backlash is necessary for smooth operation of the gears. Too small amount of backlash will create these problems:
(1) Abnormal wear of the gear due to a lack of space for lubrication.
(2) Deformation of the gear due to excessive heat and a consequent jam of the gear due to zero-backlash condition.
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Are duplex worm gear pairs able to make both positive rotation and negative rotation?
As with regular worm gear pairs, duplex worm gear pairs can also be used for both positive rotation and negative rotation. The arrow included on our worm gear products is to indicate the mounting direction of the worm and the worm wheel. The arrow is not a mark to designate the rotational direction.
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How do I make self-locking worm gear pairs?
Self-locking is a characteristic of worm meshes, where a worm cannot be driven by the worm wheel. It says that self-locking occurs when the lead angle is 4° or less, but it is not easy to establish precise self-locking conditions as it is instable due to friction. Also, self-locking might occur even if the lead angle is more than 4°, according to conditions.
For details, see here.
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How much thrust force is produced?
It varies depending on the size of load and gear specifications. Please refer to our reference book, Gear
Forces, and calculate the force that acts on the gear.
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Is it possible to mesh worms or worm wheels with racks?
Since worms have a lead angle, they cannot be meshed in parallel with regular racks. Worm wheels cannot be meshed with racks as they have a different tooth profile. Please use, in combination, a worm gear pair and a pair of rack & pinion.
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What is a JIS3 Worm Tooth Profile?
It is a form of worm tooth profile. The form is created by a grinding or milling machine with a trapezoidal-shaped tool. In JIS Standards, four kinds of tooth profiles are defined; Type A (Type 1), Type N (Type 2), Type K (Type 3) and Type I (Type 4), where the machining method and the form are different for each. Type 3 is most commonly used and KHK worm products also have the type-3 tooth profile.
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Is it possible to calculate the bending strength of worm gears?
For worm wheels, we are able to calculate only surface durability. This is because we use a calculation formula from Japan Gear Manufacturers Association (JGMA), that allows calculating only surface durability where no data on calculation for bending strength is available. In a few references like “Haguruma Binran”, calculation formulas for bending strength are also introduced, but we do not use them. As worm gears transmit power by slippage, it is necessary to design gears in accordance with the surface durability. If the surface durability is not satisfied, it might cause tooth brakeage due to tooth thinning by friction.
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How do I enhance the efficiency of worm gears?
Worm wheels made of phosphor bronze have more efficiency than those made of aluminum bronze, or cast iron. If the material is the same, the worm wheel with a larger lead angle has more efficiency. However, having a larger lead angle leads to lesser self-blocking effects. Also, higher rotation speed brings better efficiency as a higher sliding speed brings a decrease in the coefficient of friction.
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How can I confirm proper installment of worm gear pairs?
To confirm if a worm gear pair is mounted properly, you need to check the tooth contact and the backlash. If the worm gear pair has an improper tooth contact or too small backlash, it causes wear and noise. It is recommended to have a preconditioned interim operation before start using.
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I am using worm gears in a device to lift heavy objects, but doesn’t work smoothly in the reverse direction. Any ideas for solving this problem?
It is hard to find exactly what causes this problem, but chances are that the lubrication is working properly when rotating for lift, and does not work in reverse; as there might be uneven tooth contact occurring when assembled. In this case, check tooth contact to see if there’s no offset in assembling. It is also notable that worm wheels produce thrust forces and the tooth contact may deteriorate if it is not fastened firmly in the shaft direction, leading to the worm wheel moved by the load. Also, improper lubrication will cause the same problem.
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Please provide a tip in understanding the principle of self-locking worm gears.
Self-locking is a phenomenon that can be explained by a similar phenomenon occurring between a bolt and a nut; the bolt does not rotate when the nut (or wheel) is pulled upward.
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For shaft-angle errors or assembly errors of worm gears, please advice on the allowable values?
Please see this page to read about points of caution in worm gear assembling.
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Other Products

Is lubrication necessary for gear couplings?
Gear couplings is a gear product where grease lubrication is necessary. If you use gear couplings without a lubricant, there is a strong possibility that abrasion may occur.
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Can I request the repair of One-Cycle Clutches? Or do you have spare products/parts available for them?
We stopped the manufacturing of One-Cycle Clutches in 1980. The parts for these products are no longer in stock, therefore we no longer service One-Cycle Clutches.
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Plastic Gears

What is a suitable backlash for Resin (Plastic) Gears?
In the case of combining a plastic gear with a metal gear, adequate backlash is 6% to 10% of the module. However, it is necessary to perform calculations if the gears have a large number of teeth or usage is at high operating temperatures.
[ Calculating formula ]
j t=0.06×m+(K×d×tana)
j t : Circumferential backlash mm
m : Module mm
K : Temperature Correction Factors ((Diagram below)
d : Reference diameter mm
a : Pressure angle °
QA_poripenco
Reference : Technical information for MC Nylongs provided by POLYPENCO JAPAN LTD.
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Breakage occurs with resin-made (plastic) gear teeth near the pitch point, are there any preventive solutions?
Breakage might occur when using resin gears as a drive-gear, or as an idle-gear, due to the space generated near the pitch point. It is recommended to use resin gears as a driven-gear.
Reference : Design of plastic gears for power transmission, published by Gihodo Books.
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Why is it better to mate resign (plastic) gears with metal gears?
Mating of a plastic-gear with another plastic-gear reduces heat generation, compared to the combination of a metal-gear and another metal-gear. Since the strength of plastic gears decline due to heat generation, it is recommended to mate plastic gears with metal gears, in usage. Compared to plastic gears, metal gears produce less dimensional changes, and reducing the effects of water absorption or by heat.
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