Gear Failure & Analysis

1. TOPIC 1
INTRODUCTION
Gear is most essential element of power transmission prefer for short distance. It is very
economical and very effective way of power transmission. It is used almost all engineering
purpose for power transmission. A gear is a machine element designed to transmit force and
motion from one mechanical unit to another. The design and function of gears are usually closely
associated, since gears are designed for a specific function. Various types of gears have been
developed to perform different functions, the most common of these being spur gears, helical
gears, straight and spiral bevel gears, and hypoid gears.
The characteristics of various gear types are discussed in most mechanical design texts like all
mechanical components, gears can and do fail in service for a variety of reasons. In most cases,
except for an increase in noise level and vibration, total gear failure is often the first and only
indication of a problem. Many modes of gear failure have been identified, for example fatigue,
impact, wear or plastic deformation. Of these, one of the most common causes of gear failure is
tooth bending fatigue.
Fatigue is the most common failure in gearing. Tooth bending fatigue and surface contact
fatigue are two of the most common modes of fatigue failure in gears. Several causes of fatigue
failure have been identified. These include poor design of the gear set, incorrect assembly or
misalignment of the gears, overloads, inadvertent stress raisers or subsurface defects in critical
areas, and the use of incorrect materials and heat treatments [1]. A special emphasis is given gear
failure due to misalignment of gear teeth while meshing with each other while other techniques
also covered this paper consists of different overview by the different researcher by using various
methodologies to calculate various aspects of gear failure and its conclusion to reduce the gear
failure to some aspect. Gear failure can occur in various modes. In this chapter details of failure
are given. If care is taken during the design stage it to prevent each of these failures a sound gear
design can be evolved. The gear failure is explained by means of flow diagram in Fig. 1. [1]

2. When an important gear failure occurs, someone becomes responsible for analyzing the
failure, determining its cause, and recommending a solution. A company can select its own
engineer, an outside consultant, or both. If a consultant is called in, this should be done as early
in the process as possible. Though similar procedures apply to any failure analysis, the specific
approach can vary depending on when and where the inspection is made, the nature of the
failure, and time constraints.[2]
 When and where:-
Ideally, the engineer conducting the analysis should inspect the failed
components as soon after failure as possible. If an early inspection is not possible,
someone at the site must preserve the evidence based on instructions from the analyst. If
a suitable facility for disassembling and inspecting the gearbox is not available on-site, it
may be necessary to find an alternate location or bring the necessary equipment to the
site.
 Nature of failure:-
The failure conditions can determine when and how to conduct an analysis. For
example, if the gears are damaged but still able to function, the company may decide to
continue their operation and monitor the rate at which damage progresses. In this case,
samples of the lubricant should be collected for analysis, the reservoir drained and
flushed, and the lubricant replaced. If gearbox reliability is crucial to the application, the
gears should be examined by magnetic particle inspection to ensure that they have no
cracks. The monitoring phase will consist of periodically checking the gears for damage
by visual inspection and by measuring sound and vibration.
 Time constraints:-
In some situations, the high cost of shutting down equipment limits the time
available for inspection. Such cases call for careful planning. For example, dividing tasks
between two or more analysts reduces the time required.

3. TOPIC 2
DIFFERENT MODES OF GEAR FAILURE
Gear failure can occur in various modes. In this chapter details of failure are given. If
care is taken during the design stage itself to prevent each of these failure a sound gear design
can be evolved. The different modes of gear failure is given as below:[3]
2.1 SCORING
Scoring is due to combination of two distinct activities: First, lubrication failure in the
contact region and second, establishment of metal to metal contact. Later on, welding and tearing
action resulting from metallic contact removes the metal rapidly and continuously so far the load,
speed and oil temperature remain at the same level. The scoring is classified into initial,
moderate and destructive.
Fig. 2.1 Different Modes of Gear Failure

4. 2.2 WEAR
As per gear engineer’s point of view, the wear is a kind of tooth damage where in layers
of metal are removed more or less uniformly from the surface. It is nothing but progressive
removal of metal from the surface. Consequently tooth thins down and gets weakened. Three
most common causes of gear tooth wear are metal-to-metal contact due to lack of oil film,
ingress of abrasive particles in the oil and chemical wear due to the composition of oil and its
additives.
2.3 PITTING OF GEARS
Pitting is a surface fatigue failure of the gear tooth. It occurs due to repeated loading of
tooth surface and the contact stress exceeding the surface fatigue strength of the material.
Material in the fatigue region gets removed and a pit is formed. The pit itself will cause stress
concentration and soon the pitting spreads to adjacent region till the whole surface is covered.
Subsequently, higher impact load resulting from pitting may cause fracture of already weakened
tooth. However, the failure process takes place over millions of cycles of running. There are two
types of pitting, initial and progressive.
2.4 PLASTIC FLOW – COLD FLOW
Plastic flow of tooth surface results when it is subjected to high contact stress under
rolling cum sliding action. Surface deformation takes place due to yielding of surface or
subsurface material. Normally it occurs in softer gear materials. But it can occur even in heavily
loaded case hardened gears.
2.5 TOOTH FRACTURE
Tooth fracture is the most dangerous kind of gear failure and leads to disablement of the
drive and frequently to damage of other components (shafts, bearings, etc.) by pieces of the broken
teeth. Tooth breakage may be the result of high overloads of either impact or static in nature,
repeated overloads causing low-cycle fatigue, or multiple repeated loads leading to high cycle
fatigue of the material.

5. 2.6 GEAR NOISE
The gear noise arises due to several reasons. At the contact point due to error in the gear profile,
surface roughness, impact of tooth and sliding and rolling friction; bearings, churning of the
lubricant, and windage.
The principal methods of combating noise are: improving the tooth finishing operations, changing
over to helical gearing, modifying the profile by flanking, increasing the contact ratio, equalizing
the load along the face width of the tooth rim, using crowned gears, and improving the design of
the covers and housings.

6. TOPIC 3
GEAR INSPEACTION
3.1 Preparing for inspection
Before visiting the failure site, interview a contact person located at the site and explain
what you need to inspect the gearbox including personnel, equipment, and working conditions.
Request a skilled technician to disassemble the equipment under your direction. But,
make sure that no work is done on the gearbox until you arrive. This means no disassembly or
cleaning. Otherwise, a well meaning technician could inadvertently destroy evidence.
Verify that the gearbox drawings, disassembly tools, and adequate inspection facilities
are available. Ask for as much background information as possible, including manufacturer’s
part numbers gear and bearing runtime (hr), service history, and lubricant type.
Now, it’s time to assemble your inspection equipment, including items such as a
magnifying glass, measuring tools, felt tip markers, lubricant sampling equipment, and
photographic equipment. A well-designed set of inspection forms for the gearbox, gears, and
bearings should be at the top of your priority list.
3.2 FAILURE INSPECTION
Before starting the inspection, review the background information and service history
with the contact person. Then interview those involved in the design, installation, operation,
maintenance, and failure of the gearbox. Encourage them to tell everything they know about the
gearbox even if they feel it is not important. After completing the interviews, explain your
objectives to the technician who will be working with you. Review the gearbox assembly
drawings with the technician, checking for potential disassembly problems.
3.3 VISUAL INSPECTION
Before disassembling the gearbox, thoroughly inspect its exterior. Use an inspection form
as a guide to ensure that you record important data that would otherwise be lost once
disassembly begins. For example, the condition of seals and keyways must be recorded before
disassembly. Otherwise, it will be impossible to determine when any damage may have occurred
to these parts.

7. After the initial inspection, wash the components with solvents and re-examine them.
This examination should be as thorough as possible because it is often the most important phase
of the investigation and may yield valuable clues. A low power magnifying glass and pocket
microscope are helpful tools for this examination. It is important to inspect the bearings because
they often provide clues as to the cause of gear failure.
3.4 OBSERAVATION OF GEAR GEOMETRY
The load capacity of the gear set will need to be calculated later. For this purpose, obtain
the following geometry data, either from the gears and gear housing or their drawings:
• Number of teeth.
• Outside diameter.
• Face width.
• Gear housing center distance for each gear set.
• Whole depth of teeth.
• Tooth thickness (both span and top land measurement).
3.5 Specimens for laboratory tests
During the inspection, you will begin to formulate hypotheses regarding the cause of
failure. With these hypotheses in mind, select specimens for laboratory testing. Take broken
parts for laboratory evaluation or, if this is not possible, ensure that they will be preserved for
later analysis. Oil samples can be very helpful. But, an effective lubricant analysis depends on
how well the sample represents the operating lubricant. To take samples from a gearbox drain
valve, first discard stagnant oil from the valve. Then take a sample at the start, middle, and end
of a drain to avoid stratification. To sample from the reservoir, draw samples from the top,
middle, and near the bottom. Examine the oil filter and magnetic plug for wear debris and
contaminants.

8. TOPIC 4
DETERMINATION OF GEAR FAILURE
Now it’s time to examine all of the information and determine how the gear (or gears)
failed. Several failure modes may be present and you need to identify which is the primary mode,
and which are secondary modes that may have contributed to failure.
4.1 BENDING FATIGUE
This common type of failure is a slow, progressive failure caused by repeated loading. It
occurs in three stages:
• Crack initiation :- Plastic deformation occurs in areas of stress concentration or
discontinuities, such as notches or inclusions, leading to microscopic
cracks.
• Crack propagation :-A smooth crack grows perpendicular to the maximum tensile stress.
• Fracture :- When the crack grows large enough, it causes sudden fracture.
As a fatigue crack propagates, it leaves a series of “beach marks” — visible to the naked
eye - that correspond to positions where the crack stopped, Figure 2. The origin of the crack is
usually surrounded by several concentric curved beach marks.
Most gear tooth fatigue failures occur in the tooth root fillet, Figure 3, where cyclic stress
is less than the yield strength of the material and the number of cycles is more than 10,000. This
condition is called high-cycle fatigue. A large part of the fatigue life is spent initiating cracks,
whereas a shorter time is required for the cracks to propagate.
Stress concentrations in the fillet often cause multiple crack origins, each producing
separate cracks. In such cases, cracks propagate on different planes and may join to form a step,
called a ratchet mark, Figure 2.

9. 4.2 CONTACT FATIGUE
In another failure mode, called contact or Hertzian fatigue, repeated stresses cause
surface cracks and detachment of metal fragments from the tooth contact surface, Figure 4. The
most common types of surface fatigue are macro-pitting (visible to the naked eye) and micro-
pitting. Macro-pitting occurs when fatigue cracks start either at or below the surface. As the
cracks grow, they cause a piece of surface material to break out, forming a pit with sharp edges.
Based on the type of damage, macro-pitting is categorized as non-progressive,
progressive, spall, or flake. The non-progressive type consists of pits less than 1 mm dia. in
localized areas. These pits distribute load more evenly by removing high points on the surface,
after which pitting stops. Progressive macro-pitting consists of pits larger than 1 mm dia. that
cover a significant portion of the tooth surface.
In one type, called spalling, the pits coalesce and form irregular craters over a large area.
In flake macro-pitting, thin flakes of material break out and form triangular pits that are
relatively shallow, but large in area. Micro-pitting has a frosted, matte, or gray stained
appearance. Under magnification, the surface is shown to be covered by very fine pits (less than
20 mm deep). Metallurgical sections through these pits show fatigue cracks that may extend
deeper than the pits.
4.3 WAER
Gear tooth surface wear involves removal or displacement of material due to mechanical,
chemical, or electrical action. The three major types of wear are adhesion, abrasion, and
polishing.
Adhesion is the transfer of material from the surface of one tooth to that of another due to
welding and tearing, Figure 5. It is confined to oxide layers on the tooth surface. Adhesion is
categorized as mild or moderate, whereas severe adhesion is termed scuffing (described later).
Typically, mild adhesion occurs during gear set run-in and subsides after it wears local
imperfections from the surface. To the unaided eye, the surface appears undamaged and
machining marks are still visible. Moderate adhesion removes some or all of the machining
marks from the contact surface. Under certain conditions, it can lead to excessive wear.

10. 4.4 SCUFFING
Severe adhesion or scuffing transfers metal from the surface of one tooth to that of
another, Figure 8. Typically, it occurs in the addendum or dedendum in bands along the direction
of sliding, though load concentrations can cause localized scuffing. Surfaces have a rough or
matte texture that, under magnification, appear to be torn and plastically deformed. Scuffing
ranges from mild to severe. Mild scuffing occurs on small areas of a tooth and is confined to
surface peaks. Generally, it is non-progressive.
Moderate scuffing occurs in patches that cover significant portions of the teeth. If
operating conditions do not change, it can be progressive.
Severe scuffing occurs on significant portions of a gear tooth (for example, the entire
addendum or dedendum). In some cases, surface material is plastically deformed and displaced
over the tooth tip or into the tooth root. Unless corrected, it is usually progressive.

11. TOPIC 5
CONCLUSION
In this seminar report, here a brief review of gear failure analysis different conventional
and recent techniques were discussed for particularly helical and spiral bevel gear through
fatigue failure in gear while operation at various region. And after the review of this paper
following points were calculated.
The misalignment in gear teeth while meshing is the one of main causes of gear teeth
fatigue failure. Due to this crack is also initiated in the vicinity of gear teeth. A proper alignment
in gear wheel and pinion is necessary to reduce this failure.
Crack failure in gear possibly due to the presence of the number of inclusion cluster
consisting of Al2O2 Complex inclusion in the crack origins zones is mainly responsible for
cracking of the gears. To reduce the thing a attention need give while carburizing quenching
process where due presence inclusion cluster the crack generates.
The failure zones were examined with help of scanning electron Microscope equipped
with EDX facility. For further investigation an analysis through SEM was carried out close to the
crack initiation, it was found that the damage in the bevel gear were by fatigue fracture mode.
The SEM analysis shows that the gear teeth were under serves contact stress during the
operation.
A fatigue analysis has been performed following the FITNET FFS procedure it has been
concluded that no fatigue problem should have occurred in failure section and also that this
section should not have been the most stressed one the hypothesis. The conclusion inspired to
further research to reduce the fatigue failure in gears to incorporate other parameters and
symptoms with fatigue features develop more robust expert systems for fatigue failure in gears.

12. REFERENCES:-
1. Arvind Yadav, “International Journal of Science, Engineering & Technology Research”,
Vol. 1, Issue 5, November 2012, Page no. 86
2. Robert L. Errichello, Jane Muller, “Geartech, Power Transmission Design”, March 1994,
Page no. 35-40
3. Prof. K. Gopinath, Prof. M. M. Mayuram,”Machine Design II”, Indian Institute of
Madras.