Maintenance of Machinery Ch 2

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2. FUNDAMENTAL OF THE THEORY OF DAMAGES
Damages are causes for the Loss of functionality and workability of production if proper action
is not taken against them. Even under optimum design and operation conditions damage is
unavoidable. Thus dames are the reasons for the existence of a maintenance organization.
To deal with maintenance on a scientific basis, it is necessary to study the reasons for and forms
of damages and the possibility for reducing the influence of damages.
2.1 CLASSIFICATION OF DAMAGES
The use-value of equipment to a means of production is affected by two processes. These are
technological processes and technical and economical processes
Fig. 1 Classification of damages
These two processes reduce the use value of means of production. Technological processes are
related t change of state of equipment which causes damages, while technical –economical
processes cause loss in the value of machinery due to obsolescence or the existence of new
equipment with higher productivity, lower costs, lower material consumption etc.

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2.1.1 Technical-Economical Processes
These processes are related to development in the technology of equipment which causes loss in
the use value of machinery due to obsolescence or the existence of new equipment with higher
productivity, lower costs, lower material consumption etc.
2.1.2 Technical Processes
The processes are related to change of state of equipment or means production which result from
operational processes. The changes of state are manifested in the form of damages. The focus of
this chapter is to analyze these damages.
2.2 REASONS FOR DAMAGES
Damages influenced by environmental conditions and conditions of use of equipment. For proper
maintenance work, it is necessary to make systematic analysis of damages which includes
discussions on reasons for damages and their consequences. The conclusions obtained are
feedback to designers, manufactures and operational maintenance personnel.
Reason for damages can basically be classified as follow:
a. Objective: Those are damages caused by operational processes and environmental causes.
These damages are unavoidable.
b. Subjective: These are caused by failure in design, manufacturing, use operation and
maintenance. If equipment or a means of production is handled appropriately subjective
damages can be avoided.
2.3 BEHAVIOR OF DAMAGES:
The behavior of damages as related to time under the action of damaging or operational
conditions is either sudden or continuous.
2.3.1 Sudden Behavior
In this type of behavior, the use-value of equipment diminishes instantly; in other words, sudden
breakdown of equipment takes place.

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2.3.2 Continuous Behavior
In such a behavior, the use value of equipment decreases with time due to wear. This is a
degradation process that comes with time under loading conditions.
Fig. 2 Use-value and load behavior
2.4 TYPICAL DAMAGES OF EQUIPMENT
The changes of state of damages as are commonly known that result from technological
processes are basically classified into three. These are: corrosion, wear and tear and fatigue.
Apart from these, the natural phenomenon of aging can also be taken as damage process. These
damages are discussed briefly below.
2.5 CORROSION
Corrosion is the destruction or deterioration of materials by chemical or electrochemical reaction
with the environment. This includes the destruction of metals in all types of atmospheres and
liquids and at any temperature. Corrosion is usually superficial but it sometimes is directed along
grain boundaries or other lines of weakness because of differences in resistance to attack or local
electrolyte action. Under most ordinary conditions of exposure corrosion products consist mainly
of oxides, carbonates and sulphides. In many environments corrosion occurs because most
metals are not inherently stable and lend themselves to revert to a more stable combination of

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which the metallic ores are found in nature. Corrosion reduces the useful life of equipment. And
about 5% of yearly production of steel is destroyed by corrosion.
2.5.1 The Process Corrosion
The process of corrosion takes place due to direct chemical action, when the metal enters into a
chemical reaction with other elements to form non-metallic compound or due to electrochemical
action. Metallic elements when placed in contact with water or a solution have definite inherent
tendencies to go into the solution in the form of electrically charged particles. In general, the
metal and particularly iron which enters a solution is throw down as rust. This formation of rust
affects the continuation of the corrosion process in a number of possible ways two of which are
the following.
i. Direct Chemical Corrosion:
This corrosion is limited to conditions involving highly corrosive environments or high
temperature of both. Examples are: metals left open in damp environments and high
temperature; metals in contact with strong acids or alkalis.
ii. Electrochemical Corrosion:
This characterized by movement of metallic ions I solutions. Examples: when zinc is
placed in dilute hydrochloride acid a vigorous reaction occurs, hydrogen gas is evolved
and zinc dissolves to from zinc chloride. This reaction can be divided into the anodic and
cathodic reactions. During metallic corrosion, the rate of oxidation equals the rate of
reduction.
2.5.2 Kinds of Corrosion
Corrosion is classified by the forms in which it manifests itself, the basis for this classification
being the appearance of the corroded metal, which can be identified by visual observation.
i. Surface corrosion (uniform attack)
This is the most common form of corrosion manifest over the entire exposed surface or
over a large area. This type of corrosion is caused by influence of ambient conditions on
unprotected metal surfaces. Its characteristic features are:
‐ Surface is destroyed nearly parallel to the surface and
‐ Metal becomes thinner and eventually may fail

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From the technical standpoint surface corrosion is not of great concern.
ii. Pitting corrosion
This is one of the most destructive and insidious forms of corrosion. It causes equipment
failure because of perforation with only a very small percent weight loss of entire
equipment. The characteristic features of pitting corrosion are:
‐ Difficult to detect because of small size of pin from which often are covered with
corrosion products
‐ Usually it results in localized destruction materials (surface) and
‐ It can take place below the surface affecting the strength of the component (invisible)
iii. Inter-crystalline corrosion
This type of corrosion takes place below the surface and occurs at the grain boundary of
metal alloys usually causing sudden crack.
iv. Trans-crystalline corrosion
As in the case of inter-crystalline corrosion, trans-crystalline corrosion also takes place
below the surface. But unlike the inter-crystalline corrosion it takes place across the
grains of metal alloys.
v. Galvanic corrosion (two-metal corrosion)
This is corrosion associated with the current of a galvanic cell made up of two dissimilar
metals. A potential difference usually exists between two dissimilar metals when they are
immersed in a corrosion solution. If these metals are placed in contact, the potential
difference produces electron flow between them and corrosion of the less corrosion
resistant metal is increased. The less resistant metal becomes anodic and the more
resistant metal cathodic causing transfer of ions, which results in galvanic corrosion.
2.5.3 Common Locations of Corrosion
Common locations where problems can be found are the following:
‐ Along the water-line in partially filled tanks
‐ In and around drops of water on steel surfaces
‐ Along crack lines
‐ Along joints, particularly in dissimilar metals
‐ Along cold-worked area like bending, sharp ends, etc.

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2.5.4 Factors Stimulating Corrosion
Basic factors that simulate the process of corrosion are the following:
‐ Atmospheric corrosion is simulated by damp atmosphere, since a film of water is
maintained on the surface which is an essential condition for corrosion
‐ Oxygen dissolved in water
‐ Acids, acid gases in the atmosphere, sulphur compounds, coke, coal dust etc…
‐ Salts that dissociate in water producing acid reactions
‐ Contact of dissimilar metals
‐ Cold working process on metals results in increased rate of attack by acids
‐ Fatigue stresses
‐ Rust may accelerate corrosion and cause pitting and
‐ Corrosion on metals under stress is much more severe than under ordinary conditions.
2.5.5 Methods of Minimizing Corrosion
From the point of view of maintenance damages caused by corrosion have to be dealt with
preventively. Some of the methods of minimizing of corrosion are the following.
‐ Use of a coating of protective metal such as zinc, tin, lead etc…
‐ Application of protective paints
‐ Rendering the surface of the metal passive (immersing in nitric acid after it has been
highly poised immersing in fuming sulphuric acid)
2.5.6 Corrosion Problems
Corrosion problems are pronounced to a varying degree in
‐ Steam generating plants
‐ Equipment in chemical plants
‐ Pipes and piping and
‐ Structures

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2.6 WEAR
The phenomenon of wear is treated under the subject matter of tribology. Tribology is the
science and technology of interacting surfaces in relative motion which embraces the general
concepts of all aspects of the transmission and dissipation of energy. Friction, wear, lubrication
come under tribology.
Wear mechanisms that are frequently encountered in practice are schematically represented in
figure 4. The figure shows the system structure interaction between the mating parts, the
tribological action, the type of wear that ensures and the effective wear mechanism that takes
place.
Wear is an undesired change of surface of machine components by the removal of little particles
caused by mechanical reasons and/or tribochemical reasons,. Mostly, wear is caused by friction
of two mating parts. For the formation of wear as a result of friction the following conditions
must be fulfilled.
‐ There must be a pair of contacting wear partners consisting of a basic body and a
mating body
‐ A normal force must act maintaining contact between the basic and mating bodies
‐ There must exist relative motion between the contacting surfaces
‐ Other factors that represent those factors that may accelerate or retard wear processes
Relative motion between the mating parts can be sliding, rolling, drilling or fretting motion. The
amount of motion, direction and time behavior affect the resulting wear
Normal force must be applied to keep the mating parts in contact. The magnitude, direction and
time behavior (static of dynamic) of the normal force affect the resulting wear
Intermediate material can be solid, liquid or gaseous. These materials are mostly used in the form
of lubricants.
Others factors that influence wear include factors like environmental conditions (temperature,
moisture, presence of attacking gases etc…) presence of contaminating materials (chips, dust,
harmful gases etc…)
2.6.1 Kinds of Wear and Tear
The whole field of wear which is diverse is divided into limited areas with similar conditions.
i. Depending on the relative motion of mating parts there are three kinds of wear

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a. Kinematic wear which may possess any one or a combination of the following
motion characteristics:
b. Static wear
c. Impact wear
ii. Depending on the time behavior of wear there are two kinds of wear.
a. Stationary wear in which the wears intensify remains constant over a long period of
time.
b. Non-stationary wear in which the wear intensity depends on time
2.6.2 Mechanisms of Wear
Recent investigations in wear phenomenon indicate that wear of materials occurs by many
different mechanisms depending on the materials, the environmental and operating conditions,
and the geometry of the wearing bodies. These wear mechanisms may be classified into two
groups:
‐ Those primarily dominated by the mechanical behavior of solids and
‐ Those primarily dominated by the chemical behavior of materials, other factors being
temperature and operating conditions
The chain of events that leads to the generation of wear particles and material removal from a
given tribological system is initiated by two board classes of tribological processes.
Classification of these wear mechanisms are shown in figure 3.

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Fig. 3, Tribological interactions and wear mechanisms
(Source: Michael J. Neale, The Tribology Handbook, 2nd
ed., Butterworth – Heinenemann Publishing, 1995)
i. Stress intersections
These are due to the combined action of load forces and frictional forces that lead to wear
processes described broadly as surface fatigue and abrasion. These wear mechanisms are
stress interactions.
a. Surface fatigue wear mechanisms: This phenomenon may occur mainly due to the
action of stresses in or below the surfaces without needing a direct physical solid
contact of the surface under consideration. Surface fatigue effects are observed to
occur in journal bearings. The effect of fatigue wear is especially associated with
repeated stress cycling in rolling contact and is accompanied by the generation and
propagation of cracks.
b. Abrasive wear mechanisms: The effect of abrasive occurs in contact situations in
which direct physical contact between two surfaces is given where one of the surfaces
is considerably harder than the other. The harder surface causes wear of the softer
material. Abrasion is usually caused either by particles which are embedded or

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attached to some opposing force or by particles which are free to slide and roll
between two surfaces.
ii. Materials interactions
Due to intermolecular forces either between the interacting solid bodies or between the
interacting solid bodies and the environmental atmosphere and/or the interfacial medium
and lead to wear processes described broadly as tribochemical reactions and adhesion
a. Tribochemical wear mechanisms: Surface fatigue and abrasion wear mechanisms are
described mainly in terms of stress interactions and deformation properties. But in
tribochemical wear the dynamic interactions between the material components and
the environment determine the wear process where the environment is the third
partner.
b. Adhesive wear mechanisms: Adhesive is the ability of atomic structures to hold
themselves together and form surface bonds with other atoms or surfaces with which
they come into contact. The adhesive wear processes are initated by the interfacial
adhesive junctions, which form if solid materials are in contact on an atomic scale.
Material is then transferred due to adhesive joint formation which leads to rupture.
Wear mechanisms that are frequently encountered in practice are schematically represented in
figure 4. The figure shows the system structure, interacting or mating parts, the tribological
action, and type of wear that ensures and the effective wear mechanisms that takes place.
2.6.3 Wear Process
It is a complex which is dependent on a number of factors such as load, velocity, immediate
material, and ambient conditions etc… wear processes are accompanied by heating in the micro-
range as well as changes in the physical and chemical material properties of the wear partners.
Wear processes can be differentiated into the following
i. Shearing process

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Roughness points will be sheared off if the acting forces are greater than the shear strength.
This leads to the reduction of roughness and increased percentage of contact area which
reduces energy concentration and wears velocity.
ii. Elastic deformation
Big surface roughness results in low percentage o contact area and high energy
concentration in contact point. This may result in high local stresses. In the elastic range
this causes small flattening or bending of roughness points. Repetition of this process will
cause local fatigue of material.
iii. Plastic deformation
If the local stress produced exceeds the elastic limit; plastic deformation takes place in
flattening and bending the material with no loss in mass.

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Fig. 4 Commonly encountered wear mechanisms

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2.6.4 Wear Types
Depending on the presence of lubricants and/or lack of it, wear types are classified into the
following.
i. Wear by solid friction
Solid friction occurs between the contacting surfaces of two bodies having relative
motion where there is no intermediate material. In relation to the surface roughness we
find wear process by shearing, plastic and elastic deformation. Wear by solid friction
results in heating of the surfaces.
ii. Wear by liquid friction
In the case of liquid friction the two mating bodies are completely separated from each
other by an intermediate material mostly lubricant. Friction occurs in the lubricant and
wear is influenced by load, relative velocity, temperature, viscosity of lubricant, the
lubricating gap and the force of inertia are neglected.
iii. Wear by mixed friction
Simultaneous action of solid and liquid friction caused by high roughness or high load for
the lubricating film causes wear by mixed friction. This results in metallic contact at
times.
2.6.5 Time behavior of wear
Amount of wear depends on time according to the equation
Where
– is exponent of wear behavior
- is amount of wear
- is wear velocity and
– is the initial wear
Depending on α assuming the initial wear = 0 the wear behavior with time can be increasing,
decreasing or constant. These three wear behaviors with time are shown in the figure below.

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Fig. 5 Wear behavior with time
2.6.6 Protection Against Wear:
The main protection measure against wear is lubrication of the moving parts. A lubricant is the
intermediate material between two parts with relative motion for the purpose of reducing friction
and/or wear between them. Proper lubrication with timely addition/replacement of lubricant
plays a vital role in maintaining machine accuracy and increasing its working life.
2.6.7 Classification of lubricants:
Lubricants are in the form of lubricating oil, grease and solid lubricants. The use of each one of
these depends on purpose and operational desirability.
i. Lubricating oils
Lubricating oils can be mineral and synthetic oils. In addition to preventing or minimizing wear
lubricating oils perform the following duties:
‐ Cooling by reducing friction and removing excess heat generated
‐ Protection by inhibiting corrosion processes caused by air and water
‐ Cleaning by flushing dirt particles away from lubricated surfaces
a. Mineral oils
Mineral oils are basically hydrocarbons often with some additives to introduce specific
characteristic features in the oil. Mineral oils are classified as:

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‐ Paraffinic: which contain significant amounts of waxy hydrocarbons with little or no
asphaltic matter
‐ Naphthenic: which contain asphaltic matter in least fraction with little or no wax
‐ Mixed base: which contain both waxy and asphaltic materials
The viscosity of mineral oils reduces with increasing temperature and increases
significantly with pressure.
Other important lubricant properties are the following
‐ Anti-wear and EP (extreme pressure) properties
‐ Oxidation resistance
‐ Anti-corrosion properties
‐ Anti-foaming properties and
‐ Demulsibility (ability to separate from water)
Lubricating oils can become unfit for further service by oxidation, thermal decomposition
and contamination. Oxidation is the process of forming oxides by some chemical process
factors that influence oxidation are:
‐ Rise in temperature: rate of oxidation doubles for about 10 C rise
‐ Access to oxygen
‐ Presence of catalysts
‐ Type of oil (compound)
‐ Top-up rate
Mineral oils are relatively stable to thermal decomposition in the absence of oxygen. But at
high temperature over 330 C oils will decompose into fragments. The effect of thermal
decomposition is to produce light hydrocarbons and carbonaceous residues which reduce
flash point and viscosity of oil and formation of hard residues which accentuate overheating.
Contamination is probability the most common reason for changing oil. Contaminants can be
one of the following or a mixture thereof.
‐ Gaseous, like air, ammonia etc…
‐ Liquid, like water, oil of another type or viscosity or both
‐ Solid, like fuel soot, road dust, wear products

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Additives are added to plain mineral oils to improve their properties and/or for the purpose of
imparting new properties. These additives improve the oxidation stability of oils result in
increased operating temperatures and induce higher wear prevention ability. The main types
of additives and their functions are given below.
These additives are combined with oils in various ways so as to obtain the performance
required. The mixing if done indiscriminately however may result in undesired interactions.
b. Synthetic oils
Synthetic oils are products under controlled conditions industrially from chemical base and
additives. These are pure, idealized lubricants whose molecules are uniform in weight and
shape. Synthetic oils are engineered to perform under rigorous conditions and extreme
temperature.
The most important properties of synthetic oils are the following:
Added lubricity, higher film strength, good engine start-up properties, and good resistance to
thinning, improved energy efficiency, less sludge or deposit formation, good thermal
properties, stability and good fire resisting properties.
Because of their properties, synthetic oils are used for high speed lubrication under extreme
loading conditions. However, synthetic oils have some disadvantages and these are:
‐ Compatibility problem with paints, elastomers and certain metals
‐ Reactive in the presence of water resulting in hydrolysis and corrosion
‐ High potential for toxicity
‐ Disposal problem due to degradable molecular structure
‐ More expensive
ii. Grease
It is may be defined as solid to semi-solid lubricant consisting of a dispersion of a thickening
agent in a lubricating fluid. The consistency of grease depends on the percentage of thickener in
the grease. Depending on the degree of consistency, greases are classified as.
‐Semi-fluid
‐Soft
‐Stiff

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This classification is based on the measure of penetration by a ball in the grease when released
from a certain height.
Types of greases are normally determined by thickener type used and these are classified as
conventional soap greases, complex soap greases or non-soap greases.
In selecting grease for use considerations must be given to conditions and nature of use. The first
thing to be decided is the consistency range. Next comes the operating temperature. Use of
greases is limited to very low speeds up to 2m/s. They find good use where sealing against dirt is
sought or for intermittent low speed motions.
Application of grease in machinery has their relative advantages and disadvantages over using
oils. Advantages of using greases include the following:
‐ Convenience: grease is easily retained where it is applied
‐ Persistence: lubricating film is retained on surface during shutdown
‐ Protection: minimum leakage thus encouraging the formation of protective coating
against corrosion
‐ Cleanliness: greases don’t leak nor splash thus maintaining cleanliness of surfaces
where they are applied
The disadvantages of using grease are the following
‐ Poor cooling properties: greases don’t dissipate heat generated easily
‐ Contamination: greases can easily get contaminated by dirt, wear debris, oxidation
products etc…
‐Low speed of application: greases cannot be applied where working speed are over 2m/s
iii. Solid lubricants
A solid lubricant is defined as any solid used as power or a thin film on a surface to provide
protection from damage relative motion so as to reduce friction and wear. Solid lubricants are
used when fluid lubricants, i.e. oils and greases are undesirable or ineffective. Fluid lubricants
are undesirable if they are liable to contaminate product as in food machinery, electrical contacts
etc. fluid lubricants are ineffective in hostile environments, high temperature, extreme pressures
and fretting conditions. A common type of solid lubricant is graphite.
The following are some properties of solid lubricants:

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‐Solid lubricants are incapable of carrying away heat
‐Solid lubricants are immobile and they must somehow be bonded to the surface
‐Solid lubricants are capable of retaining their lubricating effectiveness at high
temperatures
2.6.8 Lubricating Systems:
There are two methods of lubricating.
i. Non-circulating lubrication
This is a process of a single passing of a lubricant through the friction area. E.g splash
lubrication (lie-bath) for transmissions application of grease by hand, packed –type etc…
ii. Circulating lubrication
There must be a circuit of lubricant without leakage. In this case all lubricating points must
be tight. This action is a continuous circuit where the lubricant is forced to flow and the
lubricating points are continuously supplied with lubricant. In the circulation type, tank,
admission, lubricating points, re-circulation is needed.
2.6.9 Planning a Lubrication Maintenance System:
When planning a lubrication system for a plant, the following techniques for sorting the work to
be done are helpful.
a) Divide the work in terms of the frequency of lubricant application
b) Divide the work by method of application and lubricant grade
c) Consider the optimum route for the lubrication personnel
By tabulating the data in this way, simple workloads can be established. As simple model for
lubrication planning is shown in figure below.
The personnel carrying out the lubrication should report back machine defects and the planned
lubrication system can be used to initiate repair work.

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Fig. 6 A model of planning for systematic lubrication
2.6.10 Lubrication Instructions
Lubrication instructions should be clear and easily understandable by lubricating personnel. The
procedures involved are listed below.
i. Cleaning and lubrication of machine should be done by operator himself. In this case,
random checks have to be performed so as to eliminate negligence by operator.
ii. From the document of the machine lubricating; frequency should be set.
At the start of the shift
‐ Machine should be cleaned
‐ Moving parts like grind ways, screws, feed shafts should be lubricated
‐ Oil level should be checked
At the end of the shift

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‐ Machine should be cleaned off chips, dust, foreign materials
‐ Exposed pars must be lubricated to prevent rusting
Periodically
‐ Oil reservoirs, containers of oil including systems should be cleaned and filled up to
level
‐ Replacement of oil should be done. Topping up of oils should be discouraged as
much as possible
iii. Lubrication points should be marked differently for the working ease of operator.
2.7 FATIGUE
Fatigue is the failure (or reduction in strength) of a material under fluctuating stresses which are
repeated a very large number of times. Fatigue failure begins with a hair-like crack which
develops at a point of discontinuity in the material (notches, groves, fillets…). Once a small
crack develops, it propagates under load to cause failure.
Components subjected to fluctuating forces must be designed for fatigue conditions. Surface
conditions residual stresses due to metal working processes of metal-treating process, stress
concentration affect fatigue strength very much. Hence they should be considered properly at
design stage.
Fatigue strength (the endurance limit) of materials is greatly decreased by the presence of a
corroding medium.
Alternating stresses tend to cause considerable damage to any protective film formed in the
normal course of action of a corrosive medium. Continual damage to this film may be a source of
the further development of cracks which accelerate fatigue failure.