Gear Teeth Surface Wear: Causes, Detection & Prevention

Date: June 05, 2024

Gear failure is often the result of undetected and ongoing improper wear to gear teeth. When a gear set grinds to a halt, it’s usually preceded by a series of signs requiring keen attention to detail and a solid knowledge set to diagnose. Worn teeth on a helical gear

Hopefully, these signs are discovered early and any resulting failure occurs during gear design or redesign stages. As long as an engineer knows what to look out for and understands possible remedies to these wear issues, significant resources can be spared by addressing them before a gear set is integrated into a system. 

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Gear wear is a mechanical or chemical phenomenon where contacting surfaces result in the removal or distortion of layers of the gear’s teeth surface material. Typical methods of reducing gear surface wear include:

  • Lubrication
  • Gear realignment
  • Gear load management
  • Gear speed adjustments
  • Temperature management
  • Adjusting warmup & cool off periods
  • Changing gear material (swapping in harder gears)

Understand the Causes of Surface Wear

Understanding the causes of premature gear wear is essential to measuring and correcting the issue before gear failure occurs. Without a firm grasp on these aspects of gear wear, an engineer will be reactive to issues with gear sets that are already in use, creating huge headaches and additional costs for the equipment’s end user. In contrast, an engineer knowledgeable in the signs, conditions and causes leading to gear surface stress, wear and failure can address these issues early and prevent costly downtime and even costlier equipment repair. 

Determining the Extent of Gear Wear: Qualitative & Quantitative Approaches

To understand whether gear wear is excessive or just part of the run-in period, it’s helpful to consider the issue in terms of the type of wear (qualitative) and the amount in which it occurs over a period of time (quantitative). Some wear is inevitable for gear sets in motion, but the amount (measured against a specific timeframe) and the type will help indicate whether it's an issue or just part of the natural aging of gears in use. 

  • Qualitative Gear Wear Analysis: determining the type of wear that is occuring
  • Quantitative Gear Wear Analysis: determining the intensity of the wear - normal, moderate or excessive over a set period of time
    • Normal Wear: typical wear over the lifespan of a gear or a break-in period
    • Moderate Wear: occurs over a long period of time, often a result of inadequate lubrication
    • Excessive Wear: the end result of normal wear progression gone unchecked, excessive wear causes rough performance, which can lead to failure

This basic observational approach is only a first step, but is crucial -  any gear stress analysis should include these carefully considered elements to determine the next steps in curing the issue. From there, a number of advanced techniques can be implemented to properly identify, test and resolve the issues discovered in these first steps; these may include boroscopic or endoscopic inspections, vibration analysis, oil analysis and much more. This data collection must lead to careful analysis and calculations to ensure proper operation of gears and gear sets.

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Operating Conditions Contributing to Gear Teeth Wear

Surface deterioration is complex and often the result of several conditions at play. In any given application, more than one of these factors can contribute to surface wear in gears:

  • General operating conditions - exposure to external contaminants, excess vibration, usage patterns, etc. 
  • Type of load - consistent vs. inconsistent load application, tangential/axial/radial forces involved, input & output torque, etc.
  • Relevant speeds of surfaces in contact - rotational speeds, torque increases & reduction of gears involved
  • Lubrication - viscosity & purity of lubricants applied & their additives, mineral based vs. synthetic, etc. 
  • Temperature - air velocity & temperature, humidity,  friction coefficients, operating temperatures, run up & cool off periods
  • Gear surface - the teeth surface hardness (typically measured via Brinell Scale or Rockwell Hardness Test) & roughness (via stylus profilometry or optical interferometry) 
  • Material - the characteristics of a gear’s material makeup and its compatibility with other components/elements of the application

Equations for Calculating Surface Wear in Gears

  1. Hertzian Contact Stress Equation: factoring in load, radius of curvature and elasticity of gear surfaces, this equation is used to calculate the contact stress between two surfaces in contact. 
  2. Archard's Wear Equation: calculates the volume of material worn away due to sliding contact between two surfaces. Accounts for factors such as the load, sliding distance and hardness of the surfaces.
  3. ISO 6336 Gear Rating Equation: calculates the gear rating based on factors such as the load, speed, and material properties. Accounts for factors like tooth strength, bending stress and pitting resistance.
  4. AGMA 925-A03 Surface Durability Equation: calculates gear tooth life span based on factors such as the surface hardness, load and lubrication. Accounts for factors including maximum tooth stress, contact stress and durability factor.

 

Signs of Critical Teeth Wear engineer testing a gear during design

The best defense against premature gear teeth wear is vigilance, proper operational usage and good inspection techniques. Regardless, gears will wear out and sudden unforeseen issues can arise in complex machinery. Sudden qualitative changes in performance are often a good indicator of a problem. If a system develops certain characteristics, it’s often a sign of excessive surface fatigue, which will lead to critical wear and eventual failure. 

Systemic characteristic changes to look out for include:

  • New increase in vibration and/or noise
  • Noticeable increase in heat generated near the gear
  • Increase in gear backlash
  • Excess lubricant smearing

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Forms of Surface Wear in Gears

Surface wear in gear teeth is often the result of mechanical processes like abrasion, adhesion and corrosion. These specific wear patterns often result in tooth surface fatigue, which is brought about by the repeated application of a load on the tooth’s surface or when the load exceeds the material’s endurance.

Gear Teeth Abrasion 

Abrasion occurs in two forms: two-body and three-body wear patterns. Two-body wear patterns occur between two gears, where pressure and friction are the main factors affecting gear wear. This type of abrasive wear often cuts material from one surface, which may either be shed harmlessly or may get incorporated into the gear mesh, which leads to three-body wear. 

Three-body wear occurs when small particles or debris are introduced in a gear mesh and come into contact with the gear surface, causing a grinding type of damage. These microparticles could be a byproduct of the gear’s machining process; this is normal and their abrasion action should subside after an initial run-in time. However, external contaminant exposure can cause added abrasion via the sliding of metals or materials together in the meshing process. This three-body exposure is most problematic in industrial settings where harder components may shed microparticles which find their way into softer teeth of a gear set, causing accelerated wear. 

Where to Find Gear Teeth Abrasion

Most wear resulting from abrasion will be found early on in gear usage and will be most apparent where teeth sliding speeds are highest: at the tooth’s tip circle and/or at the dedendum circle (at the bottom of the gap between teeth).

What to Look forGear scuffing from lubrication failure

Identifiable characteristics of gear teeth abrasion within the wear zone include: 

  • Smoothing Machining Marks - facets on the surface from hobbing, cutting streaks from machining etc. are visually diminished by wear
  • Luster & Coloration Changes (metal only) - teeth appear more lustrous above the pitch, duller below
  • Scoring & Streaks - fine grooves in the areas of highest sliding speed: most often the initial mesh contact points

Types of Abrasive Wear found in Gears

Typical wear patterns resulting from abrasive wear include: 

  • Pitting - the result of contact stresses compressing the tooth surface, wear is commonly focused near the pitch circle and identifiable by divots in the gear tooth surface
  • Scuffing - (also indicative of adhesive wear) occurs near the points of highest sliding speed (generally to tooth tops), resulting from excessive running temperatures which generates adhesion of material from one tooth surface to a mated tooth 
  • Indentations - defined impressions on the tooth’s surface typically found in thermoplastic materials, often as a result of an introduction of a foreign particle into the gear mesh 
  • Rippling - caused by high loading on a gear’s tooth, characterized by the appearance of wavy ridges on the tooth’s flank
  • Spalling - larger & shallower versions of pitting, typically caused by high-contact stresses

Variables in Quantitatively-Measurable Wear

Factors influencing the amount of abrasive wear in a gear surface include: 

  • Lubricant quality & thickness
  • Surface hardness
  • Surface roughness
  • Speed of operation
  • Gear load

If an engineer finds excessive signs of gear fatigue, stress and wear they should assess these factors while working toward a solution. 

How to Prevent General Abrasive Wear Lubrication being added to a worm gear

An ounce of prevention is worth a pound of cure when it comes to preventing abrasion wear. Some methods include: 

  1. Draining & flushing lubricant from gear boxes before initial startup
  2. Installing new filters or breathers where appropriate
  3. Using high-viscosity lubricants where possible
  4. Using gears with surface hardened teeth

Gear Teeth Surface Adhesion

Generally, this type of wear occurs when two surfaces stick together and then break apart, causing material to tear away from one surface and stick to the other. Adhesion is achieved when the pressure between two surfaces during gear mesh is great enough to cause micro-weldings, material polishing, or plastic deformations due to friction-related overheating. Inefficient or failing lubrication is the primary cause, but gear misalignment can also contribute to surface adhesion wear. 

Where to Find Gear Teeth Adhesion

Though gear surface adhesion can be discovered anywhere between gear surfaces at the meshing point, the rate of wear is typically very low. Gear surface peaks are often the first area to get worn and polished down, but the characteristics of adhesive wear will cause discrete and incremental damage which can be hard to detect. 

Scoring

The phenomenon of gear surface scoring is often the result of lubricant failure at the point of gear mesh which allows direct contact between the gear teeth. This creates a tearing of material from one gear followed by its welding onto another, which removes material from the teeth surfaces. Gear misalignment has also been attributed to this wear pattern.

Polishing 

Mating components are distorted and eventually laminated, giving the appearance of smooth, polished surfaces. Polishing generally occurs in metal teeth at low speeds with underperforming lubricants or the presence of abrasive particulates, often sheared from a mated gear. Though not always a serious issue, polishing can lead to other damage if not discovered and corrected.

Micro-Welding Adhesion 

This is a highly-localized occurrance in the high-slippage zones of two teeth flanks. Particles pulled off of one flank leaves a void in the surface it was pulled from, often to be found adhering to an adjoining tooth of the other gear. Superficial scratches and streaks are detectable from the sliding of the newly micro-welded material, beginning near the created void. This damage can also be caused by welding of foreign bodies under the same pressure. The irregularities here will diminish the effectiveness of lubrication.

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How to Prevent Adhesive Wear 

Most adhesion wear is the result of poor lubrication performance caused by excessive heat in the gear mesh. Some prevention methods include: 

  • Reducing the transmitted load 
  • Lowering speed of gear rotation
  • Switching to a high-pressure lubricant
  • Lowering oil input temperature

Gear Teeth Surface Corrosion

Oxidation is a simple indicator of chemical corrosion, but not all corrosion is as obvious to diagnose as identifying brown or red colored rust on a gear’s teeth. Electric corrosion, for example, is identifiable by micropitting on the gear surface, but also requires knowledge of the gear’s operating environment and the presence of electrical discharge.

Causes of Corrosion 

Corrosion can be caused by environmental factors like humidity and moisture, as well as the acids in the lubricating oil breaking down and attacking the gear’s surface. Occasionally, gear manufacturing processes involve chemical processes that can lead to corrosion later on. 

How to Prevent Corrosive Wear

Consistent observation of lubricants with anti-wear and anti-scoring additives will help prevent their breakdown and consequent corrosion of the gear surfaces. Changing the lube oil at regular intervals is essential as well. Any instances where environmental factors are determined to be the cause should include a sealed gearbox as a part of the solution. 

Other Origins of Gear Teeth Wear

Running-in Wear

This happens whenever gears get used for the first time, resulting in a smoothing of the tooth flank texture creating a highly polished surface visible to the naked eye.

Surface Fatigue

When a gear is subjected to variable and repeated loading and unloading it results in the development of small cracks, eventually leading to failure.

Overloading

When the gear is subjected to excessive loads it causes the surface to wear more quickly than it should.

Improper Lubrication

When lubrication types or amounts are improperly implemented, or when the substance begins to break down, this causes increased friction and wear between the surfaces.

Misalignment

When the gears are improperly aligned it causes excess stress on the teeth which leads to failure over time.

Issues with Gear Tooth Wear? Consult WM Berg

WM Berg has over 50 years of experience designing and manufacturing precision gears. Our engineers will help you diagnose gear wear issues, aid in choosing the right gear materials and provide guidance on proper lubrication and maintenance. We also offer custom gear design and manufacturing services to meet your specific needs if excessive wear is found and a full or partial system re-engineer is required. 

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