chapter 1 introduction - shodhgangashodhganga.inflibnet.ac.in/bitstream/10603/17171/6/06_chapter...
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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.