Interference Problem In Spur Gears

In spur gear design, interference between mating gears can be a serious problem that prevents efficient work, even the spur gear system does not work.

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What Is Interference Between Mating Spur Gears?

Gear interference problem illustration(Image Source:

Interference between spur gear mates can de be described as; the interference between the tip of teeth of pinion gear which is a smaller gear in spur gear mates, and the root side of teeth of the bigger gear. In the design stage, the designer must know to alleviate this issue.

What Causes Interference Between Mating Spur Gears?

The most general reason for interference between gears is the size difference between the pinion and bigger gear. If the size difference is higher than the recommended values, interference could occur. In the design stage, the designer must consider the design recommendations of the mating of spur gears.

In pinion and rack pairs, interference is the most prominent issue compared with other gear mates. Because you can think of the pinion that has an infinite pitch.

How To Alleviate Gear Interference?

In the design or gear selection stage, you need to comply with the gear mating recommendations of the manufacturer or general recommendations about standard spur gears, to prevent interference problems.

For standards of pinion gears, there are the minimum number of teeth recommendations such as;

  • If the 25 degrees, involute form, and full-depth tooth form are used on pinion gears, it is supposed that the minimum number of teeth must not below 12.
  • If the pressure angle is 20 for the same tooth form, the recommended minimum number of teeth must be 18.
  • If the pressure angle is 14, the minimum number of teeth must be 32.

In general, 20 degrees of pressure angles are standard for spur gears. Also according to the number of teeth of pinion gears, some recommendations are available to prevent interference between spur gears, in terms of the number of maximum teeth used for bigger gears;

  • If the number of pinion teeth is 13, which is not encountered design procedure for 20 degrees of pressure angles, the maximum recommended number of bigger gear teeth is 16. This ratio is meaningless for most machinery applications. Because of that, the number of teeth for pinions are not used as this low.
  • If the number of pinion teeth is 14, the recommended number of bigger teeth is 26.
  • For 15 teeth for pinion, the maximum number of bigger gear teeth is 45.
  • For 16, this maximum number is 101.
  • And for 17 teeth for pinion, this recommended maximum number for bigger gear is 1309.

As you understand from the values above, the use of teeth numbers above 16 is common practice. Recommended maximum teeth for bigger gear increases exponentially with the increasing number of pinion teeth number.

If There Is Interference Problem In Existing Design Or Machinery?

Illustration of undercut(Image Source):

You can face an interference problem with the existing design or machinery. All the systems can be installed, but there is an interference problem. You can do the operation called ‘undercut’ on bigger gear teeth.

Undercut prevents gear interference in a very efficient way. With a proper machining process, you give round geometry on the root of teeth.

But you need to be aware that this operation decreases the strength of teeth.

There is a required strength to transmit power between gear mates. Undercut processes can reduce this strength.


These are the general information about the interference problem on gear mates, which is not considered in a good way by designers generally. A good designer must be aware of these kinds of facts.

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