Calculation of load capacity of spur and helical gears — Part 1: Basic principles, introduction and general influence factors

SCOPE:

This document presents the basic principles of, an introduction to, and the general influence factors for the calculation of the load capacity of spur and helical gears. Together with the other documents in the ISO 6336 series, it provides a method by which different gear designs can be compared. It is not intended to assure the performance of assembled drive gear systems. It is not intended for use by the general engineering public. Instead, it is intended for use by the experienced gear designer who is capable of selecting reasonable values for the factors in these formulae based on the knowledge of similar designs and the awareness of the effects of the items discussed.

The formulae in the ISO 6336 series are intended to establish a uniformly acceptable method for calculating the load capacity of cylindrical gears with straight or helical involute teeth.

The ISO 6336 series includes procedures based on testing and theoretical studies as referenced by each method. The methods are validated for:

• normal working pressure angle from 15° to 25°;
• reference helix angle up to 30°;
• transverse contact ratio from 1,0 to 2,5.

If this scope is exceeded, the calculated results will need to be confirmed by experience.

The formulae in the ISO 6336 series are not applicable when any of the following conditions exist:

• gears with transverse contact ratios less than 1,0;
• interference between tooth tips and root fillets;
• teeth are pointed;
• backlash is zero.

The rating formulae in the ISO 6336 series are not applicable to other types of gear tooth deterioration such as plastic deformation, case crushing and wear, and are not applicable under vibratory conditions where there can be an unpredictable profile breakdown. The ISO 6336 series does not apply to teeth finished by forging or sintering. It is not applicable to gears which have a poor contact pattern.

The influence factors presented in these methods form a method to predict the risk of damage that aligns with industry and experimental experience. It is possible that they are not entirely scientifically exact. Therefore, the calculation methods from one part of the ISO 6336 series is not applicable in another part of the ISO 6336 series unless specifically referenced.

The procedures in the ISO 6336 series provide rating formulae for the calculation of load capacity with regard to different failure modes such as pitting, tooth root breakage, tooth flank fracture, scuffing and micropitting. At pitch line velocities below 1 m/s the gear load capacity is often limited by abrasive wear (see other literature such as References [23] and [22] for further information on such calculation).

FOREWORD:

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The procedures used to develop this document and those intended for its further maintenance are described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of ISO documents should be noted. This document was drafted in accordance with the editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).

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This document was prepared by Technical Committee ISO/TC 60, Gears, Subcommittee SC 2, Gear capacity calculation.

This third edition cancels and replaces the second edition (ISO 6336-1:2006), which has been technically revised. It also incorporates the Technical Corrigendum ISO 6336-1:2006/Cor.1:2008.

The main changes compared to the previous edition are as follows:

• incorporation of ISO/TS 6336-4, ISO/TS 6336-20, ISO/TS 6336-21 and ISO/TS 6336-22 into Clause 4 (failure mode);
• update of application factors in Clause 5;
• integration of Clause 10 "Parameters of Hertzian contact";
• integration of Clause 11 "Lubricant parameters at given temperature."

A list of all parts in the ISO 6336 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A complete listing of these bodies can be found at www .iso .org/ members .html.

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