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.
