gears

ISO 6336; Standard for calculating the load-carrying capacity of gears

December 8, 2025
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Gears are crucial components of power transmission systems – from automotive transmissions and turbines to industrial plants and power plant components. Understanding their load-carrying capacity and behavior under mechanical stress is essential for precise and reliable gear design. ISO     6336
is among the most reliable international standards in this field and contains numerous provisions for calculating the load-carrying capacity of spur and helical gears. This standard is an indispensable tool for design engineers and mechanical engineers, enabling them to predict gear life, ensure safety, and prevent premature failure.

Introduction to ISO 6336

ISO 6336,     “Calculation of the load-carrying capacity of spur and helical gears,”     is a series of standards consisting of several parts and addressing various aspects of the mechanical strength of gears. The most important parts include:

  • Part One:     Basic principles, general concepts and common influencing factors

  • Part Two:     Calculation of surface resistance (corrosion and surface pores)

  • Part Three:     Calculating the bending force of the tooth root

  • Part 5:     Assessment of material quality and its influence on mechanical properties

  • Section 6:     Calculation of service life under variable load

Together, these chapters form a comprehensive evaluation system for assessing equipment performance under various operating conditions. The main purpose of this standard is to provide a standardized basis for calculating load capacities and comparing different designs.

The importance of applying ISO 6336 in gear design

Gears are mainly     subjected to two types of stress:

  1. Hertzian contact stress     : This arises from the contact between two tooth surfaces and, if it increases significantly, can lead to a reduction in the tooth surface area.

  2. Pressure on the tooth root can     lead to tooth loss.

These two types of load are influenced by numerous factors, including rotational speed, temperature, lubrication, surface finish, design, and load conditions.
ISO 6336 takes all these factors into account and provides a systematic method for the precise calculation of load-carrying capacity. This helps designers ensure the safety, long service life, and optimal performance of gearboxes.

Basic steps for calculating load-bearing capacity according to ISO 6336

1. Determination of process parameters and operating parameters

First, the geometric parameters of the gear must be determined, such as module, compression angle, number of teeth, helix angle, tooth width, gear ratio, and circumferential speed.
Additionally, the operating conditions, including torque, rotational speed, ambient temperature, lubricant type, and load type, must be taken into account.

2. Calculation of principal pressures

The next step is to identify the two most important pressure factors:

  • Surface contact stress (σH):     is used to test for the possibility of pitting corrosion.

  • Root bending stress (σF):     is used to assess the mechanical failure of teeth.

Sections 2 and 3 of the standard contain calculation formulas for each type of expense.

3. Application of correction factors

ISO 6336 identifies several factors that must be considered when assessing environmental, design, and performance impacts. The most important of these include:

  • KA:     Application or load factor (impact effects and load variability)

  • KV:     dynamic coefficient or velocity

  • KHβ and KHα:     Load distribution coefficients across the width and circumference of the tooth

  • YF and YS:     Geometric parameters and bending stresses

  • ZNT and YNT:     Transactions related to material quality and heat treatment

These transactions make it possible to extend calculations beyond purely theoretical cases and to approach reality more closely.

4. Calculation of the safety factor

After determining the actual stress and comparing it to the material’s permissible stress, a safety factor is calculated for each failure mode. If the resulting value exceeds the  minimum value recommended in the standard      , the design is considered acceptable. Otherwise, parameters such as tooth width, modulus of elasticity, or surface hardness must be adjusted.

Update to version 2019

The new version of the ISO 6336-1:2019 standard contains a number of important changes, including:

  • Consideration of the correlation coefficient between surface quality and lubrication.

  • Improving the relationship between dynamic parameters and construction quality

  • Updated material and hardness tables to take into account new heat treatment processes.

  • The limitations of this method are explained in detail, for example the    pressure angle     of 15 to 25 degrees and the maximum spiral angle of 30 degrees.

These changes led to higher computational accuracy and a better agreement between theoretical results and experimental data.

Industrial application of ISO 6336

This standard is widely used in many industries, including the following areas:

  • Industrial and mining gearboxes

  • Automobile and motorcycle manufacturing

  • Turbines and generators for power plants

  • Shipping and aviation industries

  • Power transmission systems and precision machines

The use of ISO 6336 in gearbox design can lead to increased reliability, reduced system weight and lower manufacturing and maintenance costs.

Advantages of using this standard

  • Accurately estimate the permissible load capacity and service life of the gears.

  • To create a common ground between designers, manufacturers and consumers.

  • Reduced risk of unexpected breakdowns and maintenance costs

  • To make it easier to compare different designs

  • Ability to create clear design documentation and quality control reports.

Problems and limitations

Although ISO 6336 is very precise,   the following points should be observed when applying it:

  1. This method is only applicable to cylindrical gears with straight or helical teeth.

  2. Some parameters are determined based on    empirical   data and must be verified through practical experience.

  3. In fact, factors such as insufficient lubrication, high temperatures, or shock loads can influence the calculation results.

  4. At extremely high speeds or under abnormal conditions, numerical methods such as finite element analysis (FEA) should be used.

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Tips for engineers and designers

  • Please read and use the latest version of ISO 6336.

  • When selecting materials, special attention should be paid to the surface hardness, the depth of the hard layer, and the thermal properties.

  • The calculations must  take into account the actual operating conditions, including vibrations, temperature and lubrication.

  • The calculation results were compared with experimental data from similar gearboxes.

  • Document the project report and the details of the transactions and assumptions to ensure transparency in the quality control process.

  • To perform more accurate calculations according to the ISO 6336 standard, use professional passport analysis software (such as KISSsoft or Romax).

Sensitivity of the response to project parameters

In conclusion

ISO      6336   is one of the most important and comprehensive international standards for calculating the load-carrying capacity of spur gears. This standard provides a precise tool for assessing the integrity and service life of gears, taking into account materials, manufacturing quality, lubrication, and operating conditions. Understanding the concepts and calculation methods of this standard is essential for every     gear system designer
. Applying ISO 6336 enables the development of more reliable, lighter, and more cost-effective designs and prevents expensive failures.