In many cases, failed parts and inspection data don’t yield enough information to determine the cause of failure. When this happens, gear design calculations and laboratory tests are usually needed to develop and confirm a hypothesis for the probable cause.
Gear design calculations. The gear geometry data collected earlier aids in estimating tooth contact stress, bending stress, lubricant film thickness, and gear tooth contact temperature based on transmitted loads for each gear. These values are calculated according to American Gear Manufacturers Association standards such as ANSI/AGMA 2001-B88 for spur and helical gears. Comparing these calculated values with AGMA allowable values helps to determine the risk of macropitting, bending fatigue, and scuffing.
Laboratory examination and tests. A microscopic examination may confirm the failure mode or find the origin of a fatigue crack. Both light microscopes and scanning electron microscopes (SEM) are useful for this purpose. An SEM with an energy dispersive X-ray is especially useful for identifying corrosion, contamination, or inclusions.
If the primary failure mode is likely to be influenced by gear geometry, check for any geometric or metallurgical defects that may have contributed to the failure. For example, if tooth contact patterns indicate misalignment or interference, inspect the gear for accuracy on gear inspection machines. Conversely, where contact patterns indicate good alignment and the calculated loads are within rated gear capacity, check the teeth for metallurgical defects.
Conduct nondestructive tests before any destructive tests. These nondestructive tests, which aid in detecting material or manufacturing defects and provide rating information, include:
• Surface hardness and roughness.
• Magnetic particle inspection.
• Acid etch inspection.
• Gear tooth accuracy inspection.
Then conduct destructive tests to evaluate material and heat treatment.These tests include:
• Microhardness survey.
• Microstructural determination using various acid etches.
• Determination of grain size.
• Determination of nonmetallic inclusions.
• SEM microscopy to study fracture surfaces.
