Failure investigation of Planetary Gear Train due to Pitting

Yadav S. H.1, Mirza M.M.2 & Lokhande M.S.3

1&2
Department of Mechanical Engineering, P.G. Student, R.I.T., Sakharale, India
3
Kavitsu Trans. And Automation Pvt. Ltd., Satara, India
E-mail : Sunny.in11@gmail.com1, Manzoorahamad.mirza@ritindia.edu2, Maxima.mahesh@gmail.com3

Abstract - Planetary gear train is a gear system consisting Planetary gears have been in motion for many
of one or more planet gears, revolving about a sun gear. years. The advantages are the higher torque capacity,
And it is wildly used in industry, such as paper industries, smaller size, lower weight and improved efficiency
wind mills, and sugar industries. An epicyclical gearing characteristics of a planetary design. The small size and
system is particularly well suited for achieving a high-
modular construction of planetary gearboxes also means
reduction ratio in a relatively small, power dense package.
It is widely recognized that the load sharing is not equal that they can be assembled in several stages, providing
among the planetary gear meshes. Similarly, the stress high reduction capability from a highly compact
distribution at each mesh point contains variability. Pitting package. And it can be wildly used in industry, such as
is a surface fatigue failure of the gear tooth. It occurs due printing lathe, automation assembly and sugar industry.
to –misalignment; wrong viscosity selection of the
lubricant used, and Contact stress exceeding the surface Because a planetary gear box is smaller and lighter,
fatigue strength of the material up to half the size and 60% lighter than conventional
heavy engineered gearboxes, it is tempting to suppose
As we know gear is one of the most critical component in a that it is not as strong. Companies are using planetary
mechanical power transmission system, failure of one gear gears not just for the weight saving, the reduction
will affect the whole transmission system. Therefore it is capability and the compact size, but also, in some cases
necessary to find the root cause which results into failure because the units are weight balanced, i.e. in instances
of gear and try to eliminate these causes. Failure analysis is where a conventional gearbox is used the shaft is not in
an engineering approach to determining how and why line with the bulk of the gearbox and it's casing, hence
equipment or a component has failed. The goal of a failure there is an overhang and unbalanced weight to deal
analysis is to understand the root cause of the failure so as with. Because planetary gearboxes operate around a
to prevent similar failures in the future. central shaft, they can be used in-line with turbines,
pumps and wheel drives.
Keywords - Planetary gear train, Pitting.
Wear is nothing but progressive removal of metal
I. INTRODUCTION from the surface. Consequently tooth thins down and
gets weakened. Pitting is a surface fatigue failure of the
Gears are used extensively for transmission of
gear tooth. It occurs due to misalignment; wrong
power. Gearing is one of the most critical components in
viscosity selection of the lubricant used, and contact
a mechanical power transmission system, and in most
stress exceeding the surface fatigue strength of the
industrial rotating machinery. It is possible that gears
material. Material in the fatigue region gets removed
will predominate as the most effective means of
transmitting power in future machines due to their high and a pit is formed. The pit itself will cause stress
concentration and soon the pitting spreads to adjacent
degree of reliability and compactness. Planetary gear
region till the whole surface is covered. Subsequently,
trains are one of the main subdivisions of the simple
higher impact load resulting from pitting may cause
epicyclic gear train family. The epicyclic gear train
fracture of already weakened tooth.
family in general has a central “sun” gear which meshes
with and is surrounded by planet gears. The outermost Gears analysis in the past was performed using
gear, the ring gear, meshes with each of the planet gears. analytical methods, which required a number of
The planet gears are held to a cage or carrier that fixes assumptions and simplifications. In general, gear
the planets in orbit relative to each other. analyses are multidisciplinary, including calculations

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related to the tooth stresses and to tribological failures KAVITSU TransmissionPvt. Ltd., Satara is
such as like wear or scoring. Designing highly loaded manufacturing Industrial GearBoxes, Electric Motors &
spur gears for power transmission systems that are both Slew Bearing. Company is facing pitting failure
strong and quiet requires analysis methods that can problem in the 4KT-18HX model (Gear Box).As we
easily be implemented and also provide information on know gear is one of the most critical components in a
contact and bending stresses, along with transmission mechanical power transmission system, failure of gear
errors. The finite element method is capable of will affect the whole transmission system. Therefore it is
providing this information. The finite element method is necessary to find the root cause of pitting failure in
very often used to analyze the stress state of an elastic gears and try to eliminate these causes.
body with complicated geometry, such as a gear.
Pitting is a surface fatigue failure of the gear tooth.
It occurs due to –
 misalignment;
 wrong viscosity selection of the lubricant used, and
 Contact stress exceeding the surface fatigue
strength of the material.
Material in the fatigue region gets removed and a
pit is formed

II. LITERATURE REVIEW

Ali Raad Hassan [1] studied contact stress analysis
of spur gear teeth pair. He compared finite element
analysis results with theoretical calculations. He
TECHNICAL DATA : developed a programme to plot a pair of teeth in contact.
This programme was run for each 3° of pinion rotation
1. TYPE :- HELICAL PLANETARY GEARBOX from the first location of contact to the last location of
2. INPUT :-FREE (SOLID MALE SHAFT) ,200 contact to produce 10 cases. He made finite element
H.P./1000 rpm models for these cases and stress analysis was done. By
3. MODEL :- 4KT-18HX this study he concluded that, the maximum stress result
obtained from AGMA stress calculation method and the
4. MOUNTING :- FOOT maximum contact stress obtained from the finite
5. REDUCTION RATIO :- 162.43: 1 element contact analysis was nearly same under the
same conditions.
6. OUTPUT SPEED :- 6.15 rpm at INPUT 1000 rpm
7. RATED TORQUE :- 823402 Nm Seok-Chul Hwang et al. [2] presented two
examples of spur and helical gears to investigate the
8. ORIENTATION :- HORIZONTAL respective variations of the contact stress in a pair of
9. LUBRICATION :- OIL. mating gears with the contact position. The strength
10. LUBRICANT :- ISO VG-320 determined from the AGMA and ISO standards is valid
under the assumption that the load is uniformly
11. GEARBOX WEIGHT:- 6.5 TON (approx.) distributed along the line of contact. However, in
12. Material for Sun Gear:- 17CrNiMo6 actuality, the load per unit length varies with the point of
13. Material for Planet Gear:- 17CrNiMo6 contact. The results obtained from finite element
analysis are compared with the stresses yielded through
14. Material for Ring Gear:- En-19 AGMA standards. They have concluded that, the values
calculated by using finite element analysis are below the
Reduction Ratio according to stages: contact fatigue strength of the material; hence, they
Sr. No. Stages Red Ratio yield the appropriate strength and safety.
1 KT- 14 HX 2.05 Sunyoung Park et al. [4] investigated that the
2 KT- 14 5.43 failure of planetary gear carrier of 1200HP transmission
3 KT- 16 3.82 was caused by blowholes formed in the casting process.
4 KT- 18 3.82 A systematic failure analysis of the defect planetary gear
carrier of tracked vehicle transmission was carried out.

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The analysis was focused on the metallurgical and III. HERTZ CONTACT STRESS (INVOLUTES
mechanical points of views. Metallurgical analysis is GEAR TOOTH CONTACT STRESS ANALYSIS)
composed of macroscopic, microscopic, chemical
One of the main gear tooth failure is pitting which
composition and spheroidal graphite rate analysis. The
is a surface fatigue failure due to many repetition of
results showed that the stress distribution around the
high contact stresses occurring in the gear tooth surface
blowholes was irregular and the stress was concentrated
while a pair of teeth is transmitting power. Contact
on the blowhole regions as we expected. They found
failure in gears is currently predicted by comparing the
stress distribution of the defect planetary gear carrier is
calculated Hertz contact stress to experimentally
concentrated around blowhole regions and the
determined allowable values for the given material. The
maximum stress of the gear carrier is increased 26%
method of calculating gear contact stress by Hertz‟s
more than the normal gear carrier.
equation originally derived for contact between two
Osman Asi [5] studied fatigue failure of a helical cylinders. Contact stresses between cylinders are shown
gear used in gearbox of a bus, which is made from AISI in figure1 and figure 2.
8620 steel. An evaluation of the failed helical gear was
undertaken to assess its integrity that included a visual
examination, photo documentation, chemical analysis,
micro-hardness measurement, and metallographic
examination. Results indicate that teeth of the helical
gear failed by fatigue with a fatigue crack initiation from
destructive pitting and spalling. By this study, it was
concluded that the primary cause of failure of the helical
gear was likely a misalignment of the helical gear.

WaldemarTuszynski [6] compared two different

automotive gear oils from the point of view of
vulnerability to deteriorating due to ageing their
properties related to scuffing and pitting prevention. Fig. 1 : Cylinders in contact under compression
Gear oils of API GL-3 and GL-5 performance levels
were tested. They used scuffing test and pitting test to
compare these oils. The results of both test were GL-5
oils are less vulnerable to deterioration due to ageing
than GL-3 oils.

Hayrettin Duzcukoglu and HuseyinImrek [7]

investigated a new method for preventing premature
pitting formation on spur gears. They studied the
formation of pitting in the single tooth meshing region
by decreasing the Hertzian surface pressure by
increasing tooth width. By doing this, service life is
Fig . 2 : Elliptical stress distribution across the width
increased by means of making a tooth width
modification (F/b ratio change), thus decreasing the high In machine design, problems frequently occurs
load at the single tooth contact region. Finally they when two members with curved surfaces are deformed
concluded that to minimize the pitting formation, gears when pressed against one another giving rise to an area
having wider teeth can be manufactured. of contact under compressive stresses. Of particular
interest to the gear designer is the case where the curved
Mr. Bharat Gupta et al. [8] studied contact stress
surfaces are of cylindrical shape because they closely
analysis of spur gear. They analyze contact stress using
resemble gear tooth surfaces.
Hertz theory which was originally derived for contact
between two cylinders. Finally they Compared peak In Fig.1 two gear teeth are shown in mating
values of the contact stresses by considering different condition at the pitch point. Referring to Fig.2, the area
modules by Hertz Theory and ANSYS 13.0. By doing of contact under load is a narrow rectangle of width B
this study they concluded that maximum contact stress and length L. The stress distribution pattern is elliptical
decreases with increasing module and for large power across the width as shown in figure 3.
transmission spur gear with higher module is preferred.

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Fig. 3 : Ellipsoidal–prism pressure distribution Value is Fig. 4 : Equivalent contacting cylinder

given by,
Now by putting following equations in Esq. 3.1

3.1
Where,
Where Ft is the tangential force or transmitted load, b is
the tooth width, R1 and R2 are the radii of curvature at
pitch point, and d1 and d2 are the pitch circle diameters
of the gears.

Putting,
Here, F is the applied force, V1 and V2 are Poison‟s
ratio of the two materials of the cylinders with diameters
D1 and D2, and E1 and E2 are the respective modulii of
elasticity.
Putting the values of B and assuming a value of 0.3 to We get,
poison‟s ratio in Esq. 3.1, and by replacing diameters by
respective radii,

3.3

3.2 Inserting these values in Esq. 3.2 we get the expression

for the maximum contact pressure at the pitch point
The Hertz equations discussed so far can be utilized
to calculate the contact stresses which prevail in case of
tooth surfaces of two mating spur gears. Though an
approximation, the contact aspects of such gears can be
taken to be equivalent to those of cylinders having the
same radii of curvature at the contact point as the load
transmitting gears. Radius of curvature changes
continuously in case of an involutes curve, and it
changes sharply in the vicinity of the base circle. Now by considering service pressure angle, αw

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V. FEM RESULTS FOR CONTACT STRESS OF

PLANETARY GEAR TRAIN BY ANSYS 13.0

To simplify calculations, is written in the form

Allowable Maximum Pp = pp/FOS

From the above equation, Pressure or Stress is obtained

at pitch point in which factor of safety will be included
by dividing pp with factor of safety which can be taken
from ANSYS result or may be from other FOS tables.
After that the allowable max. Pressure or Stress at pitch
point answer will be obtained. Ultimately minimum
factor of safety is taken from ANSYS result in order to
get an accurateresult of allowable max. Pressure or
Stress at pitch point. Fig.5: FEA result for module 15

ym is the material coefficient and yp is the pitch point

coefficient, which are given by

IV. MODELING OF PLANETARY GEAR TRAIN:

Modeling of planetary gear train is carried out using

UG NX 7, is an advanced CAD/CAM/CAE software.

Fig. 6: FEA result for module 15 Magnified view

Following results are obtained by changing element

size.
Element size Stress Max.
No. of Nodes
(mm) ( Mpa )
10 872669 1565.5
11 684194 1038.8
12 574583 1551.2
Table No. 1: Gear Data 13 475392 1804.2
14 413646 1771.8
In this paper work module, pressure angle, numbers of 15 345625 1548
teeth of planetary gear train are taken as input
parameters. Table No. 2: No. of Nodes vs. Stress

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Graph 1: No. of Nodes Vs Max. Stress

Graph plotted for No. of Nodes Vs Max. Stress and

it is found 1804.2 MPa is genuine peak. This contact
stresses is higher than the surface endurance limit of the Fig.7: FEA result for module 16
material which is 1761.2 MPa. Therefore it is found that
higher contact stresses are the cause of pitting failure.

Current planetary gear train is of module 15, but

above analysis result shows that contact stresses are
higher than the surface endurance limit.

Standard values of module are as shown.

Fig.8: FEA result for module 15 Magnified view

Following results are obtained by changing element
size.

The module given under choice 1 is always preferred. If

that is not possible under certain circumstances module
under choice 2, can be selected.

From above table module 16 is selected for gear

and according to this new planetary gear train is
designed.
Table No. 3: No. of Nodes vs. Stress (module 16)

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gear. Maximum contact stress decreases with increasing

module.
Also Hear treatment of gear is also an important
factor.

VII. REFERENCES

[1] AliRaad Hassan, „Contact Stress Analysis of Spur

Gear Teeth Pair‟, World Academy of Science,
Engineering and Technology 58 2009, pp. 611-
616.
[2] Mr. Bharat Gupta, Mr. AbhishekChoubey, Mr.
Gautam V. Varde, Contact stress analysis of spur
gear, International Journal of Engineering
Graph 2: No. of Nodes Vs Max. Stress(module 16) Research & Technology (IJERT) Vol. 1 Issue 4,
June - 2012 ISSN: 2278-0181.
By studying the above graph it is found that 1549
MPa is genuine peak and which is lower than the [3] Faydor L. Litvin and Alfonso Fuentes, Gear
surface endurance limit of 1761.2 MPa. Geometry and Applied Theory, Second Edition,
Cambridge University Press, New York, USA,
Pitting is the surface fatigue failure which occurs 2004.
due to many repetitions of Hertz contact stresses. The
failure occurs when the surface contact stresses are [4] Hayrettin Duzcukoglu, HuseyinImrek, „A new
higher than the endurance limit of the material. The method for preventing premature pitting
failure starts with the formation of pits which continue formation on spur gears‟, Engineering Fracture
to grow resulting in the rupture of the tooth surface. Mechanics 75 (2008) 4431–4438.

17CrNiMo6:- Chrome-Nickel-Moly Carburising Steel [5] 5)M. Rameshkumar, P. Sivakumar, S. Sundaresh

generally supplied annealed to HB 229max. Carburised and K. Gopinath, „Load Sharing Analysis of
and heat treated it develops a hard wear resistant case to High-Contact-Ratio Spur Gears in Military
HRC 60-63. Tracked Vehicle Applications‟, gear technology,
july 2010, pp.43-50.
We can improve the surface endurance limit by
increasing hardness of the material by heat treatment. [6] Osman Asi, „Fatigue failure of a helical gear in a
gearbox‟, Engineering Failure Analysis 13 (2006)
Case Hardening: 1116–1125.
Heat to 780°C – 820°C [7] Seok-Chul Hwang, Jin-Hwan Lee, Dong-Hyung
Quench in oil Lee, Seung-Ho Han, Kwon-Hee Lee, „Contact
stress analysis for a pair of mating gears‟,
After heat-treatment hardness of the material is 63 HRC. Mathematical and Computer Modelling
Surface endurance limit after heat-treatment is 1904 (2011),doi: 10.1016/j.mcm.2011.06.055.
MPa.
[8] SunyoungPark ,Jongmin Lee, Uijun Moon,
VI. CONCLUSION Deugjo Kim, „Failure analysis of a planetary gear
carrier of 1200HP transmission‟, Engineering
KAVITSU Transmission Pvt. Ltd., Satara is facing Failure Analysis 17 (2010) 521–529.
gear pitting failure problem in their 4KT- 18 HX model.
For this purpose contact stress is found out using [9] WaldemarTuszynski, RemigiuszMichalczewski,
ANSYS Work-Bench 12. This contact stress is higher WitoldPiekoszewski, Marian Szczerek, „Effect of
than the material surface endurance limit of 1761.12 ageing automotive gear oils on scuffing and
MPa. This result shows that contact stress is the reason pitting‟, Tribology International 41 (2008) 875–
for pitting failure. 888.

The module is important geometrical parameter

during the design of gear. Therefore selection of proper 
module size is an important factor before designing

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