Helical gears are critical components in machinery, but they are prone to various failure modes that can lead to costly downtime and repairs.
From bending fatigue to tooth breakage, these failures are often caused by factors like overloading, misalignment, and inadequate lubrication.
In this blog post, we’ll explore the common failure modes of helical gears.
Failure Modes of Helical Gears
Bending Fatigue
Bending fatigue is one of the most common failure modes in helical gears. It occurs due to repeated stress on the gear teeth, causing cracks to initiate at the tooth root area. As the gear continues to operate under cyclic loading, these cracks propagate until eventual tooth breakage occurs.
Contact Fatigue
Contact fatigue manifests as pitting or spalling on the contacting surfaces of helical gear teeth. It is caused by repeated high contact stresses that exceed the surface durability limit of the gear material.
Scuffing
Scuffing is a severe form of adhesive wear that occurs when the lubricant film breaks down, allowing metal-to-metal contact between the mating gear teeth surfaces. It is characterized by localized welding and tearing away of material, resulting in a rough, torn, and matte-gray appearance.
Wear and Abrasion
Abrasive wear occurs when harder asperities or particles plow into and remove material from the softer gear tooth surface. It can be accelerated by the presence of contaminants like dust, sand, or internally generated wear debris.
Adhesive wear happens when microwelding and tearing take place between contacting asperities under high local pressures and temperatures.
Plastic Deformation
Plastic deformation happens when the contact stresses exceed the yield strength of the gear material, causing permanent tooth deformation. It is more prevalent in gears made from softer materials, those operating under high loads, and at elevated temperatures.
Plastic flow can also occur due to high contact stresses and frictional heating in localized areas. It alters the tooth profile, affects gear mesh, and intensifies dynamic loads, leading to other failure modes.
Micropitting
Micropitting appears as a matted or frosted gray surface on gear teeth flanks caused by numerous tiny micropits typically 10-20 microns in diameter. It is a surface fatigue phenomenon resulting from repeated stress cycling and plastic deformation of surface asperities.
Tooth Pitting and Fatigue
Pitting is a surface fatigue failure characterized by small cavities on the gear tooth surface formed by the propagation of subsurface cracks under repeated contact stresses. Pits grow and coalesce into larger cavities, leading to progressive loss of tooth profile.
Gear Tooth Breakage
Tooth breakage is a catastrophic failure mode where a substantial portion of the gear tooth breaks away from the rim. It can occur due to overload, impact, or as the final outcome of other progressive failure modes like bending fatigue or pitting.
Causes of Helical Gear Failures
- Overloading: Subjecting gears to loads beyond their designed capacity leads to various failure modes like tooth breakage, pitting, scuffing and plastic deformation.
- Misalignment: Poor alignment between the mating gears causes uneven load distribution across the face width, concentrating stresses and accelerating failure.
- Inadequate Lubrication: Ineffective lubrication due to incorrect lubricant selection, insufficient supply, or poor maintenance can result in adhesive wear, scuffing, micropitting and contact fatigue.
- Contamination: Contamination of gear lubricants with solid particles, water or liquid chemicals can disrupt lubricant films and promote abrasive wear, corrosion and cavitation.
- Material Defects: The presence of inclusions, pores, segregation, or improper microstructure in the gear material can act as weak points for crack initiation and propagation.
- Thermal Overload: Excessive heat generation due to high sliding, poor cooling, or elevated ambient temperatures can break down lubricants and compromise gear material properties.
- Corrosion: Exposure to corrosive agents from the environment or lubricant degradation products can lead to pitting corrosion, etching, and uniform corrosion of gear surfaces.
- Dynamic Load Fluctuations: Operating conditions involving shock loads, severe vibrations, or resonance can induce dynamic overloads and cause impact fractures or fatigue failures.
- Overhung Loads: The presence of excessive overhung loads on gear shafts generates additional bending moments and misalignment, increasing the risk of fatigue and wear-related failures.
