Gear Failure

To under stand Gear failure, finding the root cause of damage is very vital. A lot of work needs to be done for this. It includes the need for reviewing the service history, conduct interviews with those who were present, or employ technical tools like vibration and oil analysis. But unless the condition of the teeth is completely checked, the exact cause of failure cannot be ascertained. To avoid the recurrence of Gear Failures, a comprehensive understanding of the failure modes is essential. This can be done by knowing the condition of the teeth, and understanding the operating conditions and maintenance requirements.

Why Gears Fail
The principle causes for gear failure are given here - a) An error of design, b) an application error, c) it is likely that there is a manufacturing error. Design errors may be due to causes like improper gear geometry, use of wrong materials, quality, lubrication and other specifications. Application errors can be due to problems like vibration, mounting and installation, cooling and maintenance. While manufacturing errors can be in the form of mistakes in machining or problems in heat treating. Before we embark on the process to define the typical reasons of gear failure it would be better to have a look at the different types of terminology in a gear tooth profile.



The American Gear Manufacturers Association or AGMA has recognized the four basic types of gear failure, with a fifth to comprehensively cover everything.

Wear Failure
Wear, is the first in the category of failure mode. It takes place with the metal gradually wearing away from the contact points of the gear teeth. It takes place in an uniform manner. Some wear of course is quite normal. Wear as such is of several degrees and of many types. Some of the types of wear are discussed here:

• Moderate Wear
  Moderate wear comes up as a type of contact pattern where a metal gets removed from both the addendum and dedendum tooth surfaces. While operating pitch line stay put as a continuous line. The cause may be lubricant contamination. It cannot be avoided sometimes, due to certain limitations, like gear speed, lubricant viscosity, and temperature.
  
  Solutions can be a total replacement of a splash-fed lubrication system by filtered positive-spray system. Further ways to solve this problem can be reducing of gear load. As also changing of the gear geometry, materials, or even hardness.
  
  
• Abrasive Wear
  Abrasive wear is characterised by lapped surfaces that has problems on the surface where the tooth contacts. This can show up in the form of radial scratches or grooves. This can take place shortly after commencement of a new installation or even on an open gearing. The cause is generally particles in the lubricating system.
  
  Solutions can range from a prior cleaning of gearbox and lubrication system before use. Other solutions are high viscosity oils, changing filters or using a more fine replacement filter along with periodic oil changes.
  
  
• Corrosive Wear
  Corrosive Wear is marked by a visible surface deterioration. The main reason for Corrosive Wear is the chemical reaction of active ingredients found in the lubricants. These may be acid, foreign body particulates, moisture, extreme-pressure additives etc.
  
  Solutions range from checking oils periodically for breakdown and changing at regular gap. Other solution is sealing of the gears that are exposed to liquids, chemicals etc. This steps can minimize corrosive wear.
  
  
• Scoring
  Scoring can be attributed to the failure of the lubricant film. Which can be caused by overheating in the mesh area. Other reasons are deflection, misalignment and non uniform loads or temperatures. Scoring can be moderate, localized or sometimes can take a destructive form.
  
  Solutions can be downing of temperature in the mesh area. Other solutions are using a lubricant that has extreme-pressure additives, honing etc.
  
  

Surface Fatigue Failure
Surface fatigues comes to notice as the gradual removal of metal and cavity formations. Surface fatigues can be big, small. They may grow or remain as it is. It typically takes place as repeated stresses make the gear material to fail. As there is a limit to the endurance of metals to take the repeated stresses. Here in are few of the types of Surface fatigue failure.

• Pitting
  Pitting failures are attributed to the surface contact stress along with the number of stress cycles. Initial pitting takes place with small pit areas in localized sections of gear teeth. These range from a diameter of 0.015 - 0.030 inch in diameter.
  
  In destructive pitting, pits are bigger and more visible than initial pitting. This takes place in the dedendum area of a gear teeth. These large pits are result of severe overload conditions which is not that cannot all eviated by initial pitting.
  
  
• Spalling
  Spalling is something similar to destructive pitting. But in case of Spalling the pits may be bigger, shallow and non uniformly shaped. Typical symptoms are quick breaking away of the edges of pits, formation of large and irregular interconnected voids. Cause of Spalling is exceedingly high contact stress levels.
  
  Solutions are a reduction in the contact stress on the gear surface. Another way out is hardening the material, that will the surface strength and it can fight Spalling.
  
  
• Micropitting
  Micropitting is also a kind of contact fatigue. It comes up as a staining, under a very thin film conditions. Appearance wise it is frosting or gray. The surface gradually gets an etch-like finish. The first signs of Micropitting shows up on the dedendum area of driving gear. This can begin on the addendum section also. Causes of Micropitting are high surface loads along with heat generation. This consequently thins the lubrication film and eventually leading to marginal lubrication.
  
  Solutions can start from improving the surface finish. This can be done in manufacturing techniques like hard honing and grinding or through a break-in cycle.
  
  
• Case Crushing
  Case crushing is a typical surface fatigue failure in a heavily loaded case hardened gear. It can take place in gears that has already been nitrided, carburized or induction hardened. Actually Case crushing is a subsurface fatigue. The following image shows the problem of case crushing.
  
  Solutions range from increasing the depth of a case hardening. Another viable solution is to increase a bit the hardness of the core material.

Plastic Flow Failure
Plastic flow is a type of deformation in surface. It can happen with a vicious combination of high contact stresses with sliding and rolling action of a meshing gear teeth. It is a cause for cold working of the tooth surfaces. It can affect soft as well as those gears that are heavily loaded. Plastic Flow Failures take these forms:

• Rippling
  Rippling is a regular occurrence on hardened gear surfaces. The effects of Rippling can be dangerous only when it has reached an advanced stage.
  
  
  
  
  
  
• Ridging
  Ridging occurs by the combined action of compressive stress that has high contact and a low sliding velocity. It results in plastic flow failure in both surface and subsurface material. Frequent problems occur in heavily loaded worm gear drives. Also found in hypoid and pinion gear drives.
  
  

Breakage Failure
Breakage Failure is the result of a fracture of a tooth either wholly or in a substantial part of a tooth. Reasons are overload as well as cyclic stressing beyond endurance of the gear tooth material. It can be of three types. They are:

• Bending fatigue breakage
• Overload breakage
• Random fracture

Associated Gear Failures
There are three types of Associated gear failures. Associated gear failures can happen as a result of:
- Improper processing
- Environmental conditions
- Even accidents

• Quenching Cracks
  
  
  
• Grinding Cracks
  
  
• Rim and web failures
  
  
• Electric current damage
  Electric current damage are tiny pits that happens in a well-synchronised pattern. It is seen to be distributed along the gear surfaces in an uniform fashion.
  
  
  
  
  

How to Minimize Gear Failures

• Install good house keeping procedure
  As gear systems get contaminated during the field assembly it is crucial to install good house keeping procedure.
  
  
• Handle gears with attention
  Store it properly during the rebuild steps procedure. Gears can be easily chipped. As shown in the image.
  
  
• Follow the User's manual
  To change the drive train oils at regular intervals please go through the User's manual.
  
  
• Quick Replacement
  Replace immediately dirty breathers.
  
  
• Proper Maintenance
  Maintain a proper and accurate history file for all the gears.
  
  
• Accurate Records
  Record accurately the number of builds as well as the hours on all the gears.
  
  
• Proper Handling Process
  Avoid handling mismanagement as the image shows.