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CHAPTER 1

INTRODUCTION

1.1 GENERAL

Quality product at reduced cost is the challenge to all the

manufacturers across the world. The increasing global competition is a

challenging job with the conventional machines and tools. The wear and

fracture of machine parts like gears, shafts, keys and pulleys are the major and

widely recognized industrial problems. In the last three decades, many

researchers have extensively studied and analyzed the problems on the failure

and damage detection techniques in mechanical components. However,

failure occurs at any time on the rotating components of machinery which

leads to harmful results or delay in production. It is important to detect the

problems at an early stage to prevent unexpected breakdown of the

mechanical components.

Planetary gears are widely used in power transmission applications,

such as automobiles, heavy machinery, textile machine tools, aircrafts and

marine vehicles. The advantages of planetary gears are large torque-to-weight

ratio, compactness and high power efficiency. In spite of these advantages,

planetary gears have many practical problems like noise and vibration.

Since a planetary gear box has internal and external meshes in

several gear pairs, power distribution is unequal in gears. Furthermore,

planetary gears have more complicated structures than spur gears (Woohyung

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Kim et al 2012). The pictorial view of an industrial planetary gear system is

shown in Figure 1.1.

Figure 1.1 Pictorial view of a planetary gear system (Source: SACL, India)

In this research work, failure studies and analysis of a planetary

gear system used in Sakthi Auto Component Limited (SACL), Tirupur,

Tamilnadu, India has been done. This planetary gear system consists of three

stages of power transmission, namely stage I, stage II and stage III. It is

observed that the sun and planet gears in the stage I and stage II fail

frequently due to wear and fracture. Therefore, the failure studies and

mechanical properties of the above gears in stage I and stage II of the

planetary gear box are studied experimentally. A simulation study using

ANSYS software has been done to find the suitable coating material to

withstand the applied load. The determined coating material is applied on the

tooth material and the properties of the gear material are studied.

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1.2 GEAR NOMENCLATURE

Gears are rotating machine elements that transmit rotary motion

and power by the successive engagements of teeth on its periphery. It

constitutes an economical method for high power transmission with more

accuracy requirements. Gears have been in use for more than three thousand

years. Figure 1.2 shows the schematic of a typical gear nomenclature and the

most common terms are discussed below.

Figure 1.2 Schematic of typical gear tooth nomenclature

(Source: Davis 2005)

Addendum : The height of the tooth above the pitch circle.

Bottom land : The surface at the bottom of a tooth space adjoining

the fillet.

Centre distance : The distance between the axes of rotation between

two mating gears.

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Circular pitch : Length of the arc of the pitch circle between

corresponding points on adjacent teeth.

Dedendum : The depth of the tooth below the pitch circle.

Face width : The length of the gear teeth in an axial plane.

Fillet radius : The radius of the fillet curve at the base of the gear

tooth.

Gear blank : The workpiece used for the manufacture of a gear,

prior to machining the gear teeth.

Gear, pinion : When two gears mesh together, the smaller of the two

is called the pinion; the larger is called the gear.

Gear ratio : The ratio of the larger to the smaller number of teeth

in a pair of gears.

Involute gear tooth : A gear tooth whose profile is established by an

involute curve outward from the base circle.

Pitch circle : The circumference of a gear measured at the point of

contact with the mating gear.

Pitch diameter : The diameter of the pitch circle. In a cross section of

a rack, the pitch line corresponds to the pitch circle.

Pressure angle : The angle formed by the common tangent to pitch

circle and common tangent to Dedendum Circle

throughout the pitch point is known as Pressure

Angle. The pressure angle gives the direction of the

normal to the tooth profile. Standard values of

pressure angle are 14.5, 20 and 25 degrees.

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1.3 BACKGROUND OF PLANETARY GEARBOX

Sakthi Auto Component Limited (SACL) is one among the Sakthi

group of industries situated at Tirupur District, Tamilnadu, India. Presently

this industry has a capacity of 24000 tonnes of mould / annum and was

established in the year 1983. It is one of the major producers of S.G. iron

castings, meeting the needs of most of the automotive and other general

engineering industries. Sakthi Auto Component Limited (SACL) is a major

supplier of critical components to passenger car manufacturers. The

components manufactured by them are steering knuckles, brake drums, brake

discs, hubs, brake calipers, carriers, differential castings, manifolds etc. SACL

is equipped with DISAMATIC foundry with the state of the art manufacturing

technology. Moulding is one of their latest and the most efficient processes

and they have the capacity to produce 440 flawless moulds / hour.

Synchronized belt conveyor system transfers the mould to the DISA Cool

drum (model CDR 3400) where the castings are separated from sand. The

DISA cool drum is rotated by using multi stage planetary gear system. It is

one of the speed reduction gearboxes. Then the sand is removed from the

drum by the conveyor system for recycling. The castings then undergo the

shot blasting process.

1.4 DETAILS OF PLANETARY GEAR SYSTEM

In general, a planetary gear system consists of a centrally pivoted

sun gear, a ring gear and three planet gears which rotate between the sun gear

and the ring gear. This assembly concept explains the term planetary

transmission, as the planet gears rotate around the sun gear as in an

astronomical sense the planets rotate around the sun. Planetary gears are

typically classified as simple and compound planetary gears. Simple planetary

gears have one sun, one ring, one carrier and one planet set. Compound

planetary gears have one or more of the following three types of structures:

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meshed-planet, stepped-planet and multi-stage structures. Compared to simple

planetary gears, compound planetary gears have the advantages of larger

reduction ratio, higher torque-to-weight ratio and more flexible

configurations. The axes of all gears are usually parallel.

There are many distinct advantages of using an epicyclic gear train

than a single load path parallel gear train:

1. A planetary gear drive system has smaller and stiffer

components which help in reduced noise and vibration. Also,

it occupies less space, has lighter weight, helps in efficiency

improvement as they support shared load between several gear

sets of teeth which mesh at a time.

2. The input and output shafts of epicyclic trains are concentric,

so no bending moment or torque is created from radial forces.

3. The input and output shafts of epicyclic trains are concentric,

so driver and driven equipment can be mounted in line,

providing additional space savings.

4. The reduced size of epicyclic components often offsets the

cost of additional parts, especially when many gear trains are

manufactured. The components of parallel shaft designs

become bulky due to very high horse power, so it is

economical.

5. Despite the higher number of engaged teeth, the noise level

remains low as on traditional gear types because the smaller

dimensions give lower peripheral velocity on the pitch

diameter.

6. Despite the higher number of engaged teeth, the smaller tooth

size makes it possible to obtain high efficiencies.

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1.5 PLANETARY GEAR MATERIAL

Even though a variety of cast irons, powder-metallurgy materials,

nonferrous alloys and plastics are used in gears, steel is the most widely used

gear material because of the high strength-to-weight ratio and relatively low

cost for heavy duty and power transmission applications. Carburizing is a

process used in hardened steel with carbon content between 0.1 and 0.3 wt %.

Carburization can be used to increase the surface hardness of low carbon

steel. In general, the steel selected for gear applications should satisfy two

basic requirements namely those involve fabrication and processing and those

involving service. En 353 (15NiCr1Mo12) is a widely used fine-grained steel

billet material found in manufacturing of these critical components. The

applications of En 353 are manufacturing of heavy-duty gears, shaft, pinions,

camshafts, gudgeon pins, heavy vehicle transmission components etc.

1.6 COMMON FAILURES IN PLANETARY GEAR SYSTEM

Sun and planet gears can fail in different ways and increase the

noise and vibration. There is often no indication of difficulty until total failure

occurs. In general, each type of failure leaves characteristic clues on gear

teeth and detailed examination often yields enough information to establish

the cause of failure (Alban 1985). A list of six general modes of gear failure,

of which the first four most common is given below.

1. Bending Fatigue 2. Contact Fatigue 3. Wear4. Scuffing 5. Overload 6. Cracking

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1.6.1 Bending Fatigue

This is a common type of slow and progressive failure caused by

repeated loading. It occurs in three stages:

1. Crack initiation: Plastic deformation occurs in areas of stress

concentration or discontinuities such as notches or inclusions,

leading to microscopic cracks.

2. Crack propagation: A smooth crack grows perpendicular to

the maximum tensile stress.

3. Fracture: When the crack grows large enough, it causes

sudden fracture.

1.6.2 Contact Fatigue

Another failure mode is called as Contact or Hertzian fatigue where

repeated stresses cause surface cracks and detachment of metal fragments

from the tooth contact surface. The most common types of surface fatigue are

macro pitting (visible to the naked eye) and micro pitting (visible to the

microscope).

1.6.3 Wear

Gear tooth surface wear involves removal or displacement of

material due to mechanical, chemical or electrical action. Some common

types of wear include adhesive wear, abrasive wear, surface fatigue, fretting

wear and erosive wear. It is a surface phenomenon in which layers of metal

are removed or “worn out”. The wear is more or less uniform from the

contacting surfaces of the gear teeth. Surface deterioration of the active

profile of the gear teeth is called wear.

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1.6.4 Scuffing

Severe adhesion or scuffing transfer’s metal from the surface of one

tooth to another. Typically, it occurs in the addendum or dedendum in bands

along the direction of sliding though load concentrations can cause localized

scuffing. Surfaces that have a rough or matte texture, under magnification,

appear to be torn and plastically deformed. The maximum load that can be

carried by gear teeth also depends on the velocity of sliding between the

surfaces. If both pressure and sliding speeds are excessive, the frictional heat

developed can cause destruction in tooth surfaces. The pressure-velocity

factor, therefore, has a critical influence on the probability of galling and

scoring of gear teeth. The permissible value of this critical factor is influenced

by gear metal, gear design and lubricant.

1.7 PLASMA-SPRAYED COATING TECHNOLOGY

Plasma spray processes are widely used in industries to improve the

strength of worn surfaces in the power transmission system (gears). Plasma

spray is one of the most versatile methods of the thermal spray processes.

This helps in saving the worn parts by reusing them after they have been

thermally sprayed by suitable wear-resistant coatings. As there are several

factors to be controlled in this process, it is necessary to get the best

combination of process parameters to provide the better wear resistance and

improve the life of gear. Plasma technology is suitable for spraying all types

of coating materials.

Depositing one material over another material or the same material

is referred as coating. Coatings are applied to the surface of the substrate in

order to improve its life by protecting the surface from wear and corrosion. It

is also an economical technique (Pranevicius 2000).

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Figure 1.3 Principle of arc plasma torch (Source: Claire Tendero

et al 2006)

The arc plasma torch is fed by a DC power supply. The plasma arc can

be classified into two categories: current-carrying arc and transferred arc.

These are shown in Figure 1.3. Plasma is a chemically active media.

Depending on the way of activation and their working power, the arc can

generate low or very high ‘‘temperature’’ and is referred as cold or thermal

plasma. This wide temperature range enables varies applications for plasma

technologies namely: surface coatings, waste destruction, gas treatments,

chemical synthesis, machining etc. Nevertheless, many of these techniques

have not been industrialized, albeit environmental norms are strictly followed.

Thermal plasmas (especially arc plasma) are extensively industrialized in

aeronautic sector.

1.8 FINITE ELEMENT METHOD – PROCEDURE

Finite element method is one of the computational methods used to

determine the behaviour of gear material and probable location of failures.

For the structural stress analysis, the engineer seeks to determine the

displacements and stresses throughout the structure in equilibrium. Using the

conventional methods, it is difficult to determine the distribution of

deformation and thus the finite element method is necessarily used. There are

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two approaches associated with the finite element method which are applied

to structural mechanics problems. Among the two, one approach is called as

the force, or flexibility method. This approach considers the internal forces as

unknown in the problem. Starting with the equilibrium equations the

necessary additional equations are found by introducing compatibility

equations in order to obtain the governing equations. The result is a set of

algebraic equations for determining the redundant or unknown forces. The

second approach is called as the displacement or stiffness method. This

approach considers the displacements of the nodes as the unknowns. For

instance, compatibility conditions requiring that elements connected at a

common node, along a common edge or on a common surface before loading

remain connected at that node, edge or surface after deformation takes place

are initially satisfied. Then the governing equations are expressed in terms of

nodal displacements in equilibrium.

1.9 OBJECTIVES OF THIS WORK

This research work aims at achieving the following objectives.

1. To carryout experiments on the gear samples of the heavy

duty industrial planetary gears at stage I and stage II to predict

the reasons for fractures in the gear tooth.

2. To determine the causes for fractures in gear tooth by varies

experimental techniques such as metallurgical studies,

chemical composition analysis, Vickers hardness test and

Wear studies.

3. To develop a finite element model of the planetary gear

assembly to predict the level of stress and displacement in the

gear tooth of sun gear, ring gear and planet gears.

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4. To develop a finite element model for simulating the Vickers

hardness test and static surface wear of base metal for the

prediction of stress and strain components.

5. To apply coating on the gear tooth material and test the

coating material experimentally.

6. Validation of experimental and simulation results.

1.10 OUTLINE OF THE THESIS

This thesis is composed of five chapters. Introduction about the

planetary gear, gear nomenclature, failure of planetary gear, coating

technologies, wear behaviour, finite element method and research objectives

are discussed in Chapter 1.

In Chapter 2, the research outcomes of various scientists and

researchers working in the area of planetary gear, failures in the gear tooth

due to varies loads, contact stress development, metallurgical defects,

improvements of the wear resistance, plasma spray coating and finite element

simulations of the gear failures are discussed. Literature summary is also

given in this chapter.

Chapter 3 is devoted to the description of experimental studies

devised for the measurement of gear tooth failure of the planetary gear.

Theoretical calculations are carried out for the determination of the planetary

gear terminologies at varies stages such as stage I and stage II. Experimental

and simulation details involved in this research work for the gear tooth

failures are discussed in detail.

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Chapter 4 provides a detailed discussion on the results of the

experimental investigations carried out on the gear samples of the planetary

gear tooth of the sun and planet gears. The results of the simulation studies

obtained are also discussed in an elaborate manner.

Finally Chapter 5 summarises the conclusion of this research work

and recollects the main contributions to the area of planetary gear system,

gear life enhancement using tooth surface modification such as surface

coating on the gear materials.