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:
- The first stage of gear tooth fatigue is when plastic deformation near the root of the tooth leads to microcracks.
- 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.
- 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.
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.