Comparing Helical Gear Rating Standards: AGMA 2101-D04 and ISO 6336

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There are different gear rating standards used throughout industrial applications. Here’s what you need to know to compare AGMA 2101 and ISO 6336.
EngineersWith so many different gear rating standards used in the industry, it's challenging for end-users and OEMs to determine how much power can pass through a gear set—especially when comparing selections from different gear drive suppliers.
There are several different industry standards used to calculate the mechanical capacity of gear teeth, and what’s preferred by a gear designer or consultant can vary. The ratings include standard documentation nationally or internationally controlled by organizations such as the International Organization for Standardization (ISO) or the American Gear Manufacturers Association (AGMA).
To further complicate the issue, gear rating standards can also be written by application-specific organizations such as the American Petroleum Institute (API), which covers gearing for oil and gas applications. While making a clear side-by-side comparison is tough, we can look at the two primary standards for spur and helical gear teeth commonly used in the Americas and reach a strong consensus.
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Comparing Two Primary Gear Rating Standards

The two main comparisons used in the United States, Canada, Central, and South America, are ISO 6336 (Calculation of Load Capacity of Spur and Helical Gears) and AGMA 2101-D04 (Fundamental Rating Factors and Calculation Methods for Involute Spur and Helical Gear Teeth). These rating standards apply across gearing in general, and many application-specific guidelines will also refer to these standards as their basis.
Each of these gear rating standards has a corresponding bevel method as well (ISO 10300 and AGMA 2103, respectively). The bevel methods have a similar structure as the spur and helical calculations.
Both the AGMA and ISO methods calculate the risk of damage from fatigue-related macropitting along the gear tooth flanks and fatigue-related cracking in the gear tooth roots.

Understanding Gear Tooth Macropitting and Bending Fatigue

Since both standards calculate the risk of fatigue-related damage, it's essential to understand what the damage looks like. Macropitting occurs when fatigue cracks start at the surface or just below the surface of a gear tooth. The cracks typically travel parallel to the surface before eventually moving through and causing the material to separate.
The resulting macropit may have sharp, angular edges with cracks or beach marks at the bottom. Macropitting is often progressive and will eventually become severe damage on the flank of the gear tooth. The mechanical capacity to resist macropitting is commonly called the “durability” of the gear tooth.

Figure 1 – Initial macropitting (left) and progressive macropitting (right) on a gear tooth.
Pictures from AGMA 1010-F14, Appearance of Gear Teeth—Terminology of Wear and Failure

Cracking or breaking in the tooth roots happens when a gear tooth is loaded heavily enough to deflect and crack at the root fillet radius.
This type of gear tooth fatigue occurs in three stages:

  1. The first stage of gear tooth fatigue is when plastic deformation near the root of the tooth leads to microcracks.
  2. The second stage occurs as the cracks grow and travel perpendicular to the stresses on the tooth. Beach marks and ratchet marks are created across the crack zone as the tooth is cyclically loaded and unloaded in operation.
  3. The last stage of gear tooth fatigue and failure is when the tooth breaks. The mechanical capacity to resist tooth breakage is also known as the “bending strength” of the gear tooth.

Figure 2 – Top-down view of two adjacent helical gear teeth that have failed due to bending fatigue.
Picture from AGMA 1010-F14, Appearance of Gear Teeth—Terminology of Wear and Failure.

There are many other failure modes for gear teeth, some very similar to macropitting and bending fatigue failures. For example, micropitting appears as frosting or grey staining on a tooth surface but isn’t the same as macropitting. If you’re analyzing a damaged gear, it’s best to consult with a gear manufacturer or consultant. The gear rating standard AGMA 1010-F14 (Appearance of Gear Teeth – Terminology of Wear and Failure) is also a useful reference.
Gear rating standards AGMA 2101 and ISO 6336 both calculate the mechanical capacity of gear teeth by using roughly the same approach. Both methods use geometric calculations to account for the shape of the tooth flank and simulate the tooth as a loaded beam. The ratings both contain adjustments for the accuracy of the gear tooth machining and the modifications used to maintain tooth contact as the gearing deflects under load. The two documents also set requirements for material quality and recommendations for the stress each type of material can accept before failing.

The Differences Between Gear Rating Standards

Despite the similarities of the two commonly-used gear rating standards, the methods were developed in very different ways and result in different capacities.
AGMA 2101 was created as a design tool. The calculations determine the amount of torque that can be applied to a gear tooth before failure. Much of the AGMA 2101 development is based on empirical work intended for application in a broad range of gear designs. The gear rating standard AGMA 2101 evolved from the methods that AGMA initially published in 1946. Generally, the calculations are simplified and analytical; they are easily applied.
ISO 6336, first published in 1996, is a checking tool for an existing design. The calculation in ISO 6336 results in the safety factor between the allowable stress on the gear tooth and the applied stress of the application. ISO 6336 was developed from the German rating standard, DIN 3990. It’s complex and detailed methods were derived from laboratory testing in universities. There are multiple methods used to calculate influence factors, such as using 3D simulations, analytical methods, or graphical methods. The influence factors of ISO 6336 tend to take more into account than AGMA 2101. For example, it’s possible to account for the viscosity of the lubricant in the macropitting evaluation.

Despite the differences in the two gear rating standards, it’s not appropriate to say one method is more accurate than the other. They both present valid considerations for determining the mechanical capacity of gear teeth.

Comparing the Two Gear Rating Standards to Reach a Consensus

At Rexnord, we’ve noticed the following trends between AGMA and ISO gear tooth ratings:
  • ISO 6336 rates through-hardened gearing 5% to 35% lower than AGMA 2101.
  • The mechanical ratings for carburized gearing from ISO 6336 and AGMA 2101 fall within 10% of each other.
  • ISO 6336 only allows helix angles between 5° and 15° and penalizes designs falling outside of that range.
  • ISO 6336 overestimates the mechanical capacity of large, lower accuracy, slow speed gear sets.

So, how do you compare two different gear drive selections with different ratings?

When evaluating gear drive selections—one rated with AGMA 2101 and the other rated with ISO 6336—keep in mind both methods are calculations that assume ideal conditions for lubrication and operation. Remember that ISO 6336 allows the gear designer to take surface roughness and lubrication viscosity into consideration in the ratings. For accurate calculations and comparison, the gearing must be operated with the same load, speed, lubrication, and manufacturing accuracy as assumed in the original evaluation.

Lastly, it’s essential to remember gears may not control the rating of the gear drive. Gear drives are a system of components including gears, shafts, and bearings, but the gear drive mechanical rating is the lowest rating of these components. If the shafts or bearings determine the gear drive capacity, the method used to rate the gearing isn’t as important. It’s also possible for the gear drive to receive a thermal rating (the ability to dissipate heat during operation) much lower than the mechanical rating. Available cooling options then determine the resulting selection.

So, the answer to "which gear rating standard is better" isn't cut-and-dry. Gear rating standards are guidelines, but don't offer the full picture of a gear drive or the gear components within. If you need guidance, your Rexnord representative will assist you in determining the best solution for your unique application.

Robin Olson

Robin is the Director of Applications Engineering at Rexnord Industries, Gear Group. In 1995, Robin joined Falk, which was acquired by Rexnord in 2005, and has previously worked in the Engineering Technical Services, Warranty, Product Engineering, and Marine Product groups during her career.  She is active in the American Gear Manufacturers Association (AGMA), acting as a contributing member of the Helical Gear Rating Committee, Chairperson of the AGMA 925 (Gear Surface Distress) subcommittee, and is honored to act as US delegate to ISO Working Groups 6 (Gear calculations) and 15 (Micropitting).  Robin holds a Bachelor of Science in Physics from the University of Wisconsin -- LaCrosse and a Master of Science in Physics from the University of Wisconsin -- Madison.