Maintanace of machinery assignment

MAINTENANCE OF MACHINERY ASSIGNMENT
BIT, ETHIOPIA
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BAHIRDAR UNIVERSITY
BAHIRDAR INSTITUTE OF TECHNOLOGY
FACULITY OF MECHANICAL AND INDUSTRIAL ENGINEERING
DEP’T (OF): MECHANICAL ENGINEERING
STREAM: THERMAL ENGINEERING
MAINTENANCE OF MACHINERY INDIVIDUAL ASSIGNMENT
BY: LIJALEM TSIHAYE
ID.No: 0800985
Section: D
Submitted To: Getaye Mekonnen
.

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Q1) What factors stimulate damage of slide bearing?
The factors that stimulate damage of sliding bearing are:
❖ Bearing force
❖ Design of bearing material and lubrication
❖ Manufacturing accuracy
❖ Assembling condition: misalignment causes reduce contact area which produces fatigue
❖ Service and operation condition
Q2) Explain the following:
Q2.1) Defect of bearing
Bearing:
✓ Is a device which constraints the motion of the rotating part to one direction.
✓ Is a machine element which support another moving machine element (known as
journal).
✓ It permits a relative motion between the contact surfaces of the members, while carrying
the load.
✓ Based on its nature of contact the bearing can be classified as roller bearing and sliding
bearing.
❖ Bearing running dray
❖ Misalignment
❖ Damage of one or more ball/roller
❖ Damage of running race
❖ Deformed separation
❖ Radial play
Below are the most typical bearing defects and their identification in the frequency
spectrum:
✓ Outer Race Defects: the spectrum is characterized by the presence of harmonic peaks of
the outer race failing frequency (between 8 and 10 harmonics of the BPFO).
✓ Inner Race Defects: the spectrum shows several harmonic peaks of the inner race
failing frequency (usually between 8 and 10 BPFI harmonics) modulated by sidebands at
1x RPM.
✓ Ball or Roller Defects: they are characterized by the presence in the spectrum of
harmonics of the rolling element deterioration frequency (BSF). In most cases, the
harmonic of greater amplitude usually indicates the number of deteriorated balls or
rollers. They are usually accompanied by defects on the races.
✓ Cage Defects: are characterized by the presence in the spectrum of the cage failing
frequency (FTF) and its harmonics. Generally, a defect in the cage is accompanied by

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defects in the races and the FTF usually modulates one of these race failure frequencies
leading to sums and/or differences of frequencies.
✓ Defects of Multiple Components: it is quite common to find bearings with multiple
deteriorated components, and in such case multiple failing frequencies and their
corresponding harmonics will appear.
✓ Looseness: we can distinguish the following types:
• Looseness between bearing and casing: It presents several harmonics of the
rotating frequency, being the peaks at 1x and 4x RPM the ones with higher
amplitudes.
• Looseness between inner race and shaft.
✓ Bearing Misalignment: spectral signatures are characterized by the presence of vibration
at various harmonics of the rotating frequency, with the amplitude being the most
significant at N x RPM, where N is the number of rolling elements in the bearing.
✓ Inadequate Lubrication: lubrication problems are characterized by high frequency
vibration (between 1 kHz and 20 kHz), with bands of peaks spaced apart from each other,
due to the excitation of the resonance frequencies of the bearings in these frequency
ranges.
Q2.2) Gear failure
Gear (Cogwheel):
✓ Is any toothed designed to transmit or receive motion from another member by
successfully engaging teeth.
✓ Is a rotating machine part having cut teeth, or cogs, which mesh with another toothed
part in order to transmit torque, in most cases with teeth on the one gear being of
identical shape, and often also with that shape on the other gear.
✓ Is a mechanical drive which transmits power through toothed wheel.
A gear has failed when it can no longer efficiently do the job for which it was designed. In
general gear failure can be one of the following:
1) Surface Failure:
✓ It starts forming on teeth contact surface with the running of gear
✓ The cause is over stressing of gear material
✓ The remedy is removing the cause of over stressing
2) Scuffing:
✓ This is the result of disruption of lubricant film
✓ Th tooth surface are severely roughened and torn as the result of
unchecked adhesive wear
✓ Cause is the rise of temperature for contacting surfaces above the critical
temperature for the lubricant
✓ The remedy is to use high grade oil with high critical temperature
3) Abrasive Wear:

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✓ Is a surface injury or damage caused by particles(i.e. May be present in
the lubricants as impurities, may be dirt entering the gear box from
outside or may be flakes of detached from the tooth surface) trapped
between the matting gears.
✓ The remedy is protection of oil from contaminating and preventing dirt
from entering in (i.e. filtration of lubricant, complete enclosure of gear,
increasing surface hardness and use high viscosity oil).
4) Plastic Deformation:
✓ With ductile material, heavy loading exceeding the elastic limit
produce plastic deformation
✓ The remedy is to avoid over load
5) Tooth Breakage (Fracture/ Bending Failure):
✓ Due to repetitive bending stress (i.e. Breakage occur when repetitive
bending stress >bending endurance strength or when total load exceeds
the bending strength)
✓ If a whole tooth breaks away the gear has failed
✓ The cause of fractured has to be assessed for the future performance of
the gear
✓ The remedy is increase bending endurance strength, module, face
width, and pressure angle.
6) Corrosive Wear:
✓ Is the deterioration of the surface of the gear due to chemical action .
✓ It is often caused by active ingredients in lubrication oil, such as acid,
moisture and extreme -pressure additives.
7) Pitting:
✓ Is a fatigue failure which occurs due to repetitive contact stress or when
the endurance limit is exceeded.
✓ The nature depends on surface contact stress and number of stress cycles.
Note: A problem with pitting can be labeled as either
Initial pitting: a type of pitting in which the surface is experiencing small pits and it is due to
surface irregularities, error in tooth profile, and misalignment of gear (gears not fitting together
properly)
Destructive pitting: a type of pitting in which the pits are larger in diameter. Second phase of
pitting starts when the hertz contact stress induced on gear tooth surface exceeds the surface
endurance strength (i.e.is typically an issue with surface overload).
8) Scoring:
✓ Is rapid wear resulting from a failure of the oil film due to overheating of the
mesh, permitting metal to metal contact, this contact produces alternate
welding and tearing which removes metal rapidly from the tool surface.
✓ Is a lubrication failure (i.e. inadequate lubrication metal to metal contact).

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✓ Due to poor surface finish, high tooth load, high sliding velocity results to heat
generation in the localized region of the metal to metal contact.
✓ The remedies are providing adequate lubrication, proper surface finish,
maintain sliding velocity and tooth pressure limit.
9) Frosting:
✓ The wear pattern gives a frosted appearance, which are many micro pits on the
surface.
✓ It happens when the lubricant film is broken down due to excessive heat and
usually shows up in the dedendum area of the driving gear.
✓ It is the reason for tiny pits that are on the outside of a gear. A regular
inspection and upkeep including temperature checks are a must to prevent
frosting.
10) Spalling:
✓ Is considered to be a sort of outrageous pitting which happens in areas where
there is high-contact inside the gear and the problem can be averted by paying
attention to wear and tear at the right time.
✓ It is a common problem when high contact stress exists.
Q2.3) Classification of Damage
The value of an equipment or means of production is affected by two process.
I. Technological processes: are related to change of state of an equipment which causes
damage. The change of state manifests in the form of damage.
II. Technical-economical processes: are related to development in technology of an equipment
which causes loss in the value of machinery due to obsolesce or exitance of new
equipment with high productivity, lower cost, lower material consumption.
Q3) Explain the following …
Q3.1) Kind of Corrosion
Corrosion:
✓ Is the gradual deterioration of metals by chemical, electrochemical or biochemical
interaction with the environment.
✓ Is classified by the form in which it manifests itself.
✓ Uniform (Surface) Corrosion
✓ It is the simplest and most frequently observed form of corrosion.
✓ Formed when items are exposed to the atmosphere, soil, and an enormous
variety of aqueous solution.
✓ Normally affects metallic surface that have uniform chemical composition
and microstructure.
✓ It normally increases with temperature.

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✓ Characteristics are: surface destroyed parallel to surface, metal becomes
thinner and eventually fail.
✓ Pitting Corrosion:
✓ It is localized corrosion phenomenon that produce distinct, well defined
cavity on the component surface
✓ Corrosion of inside the pit is uniform
✓ Any metal that can be passivated can suffer this type of corrosive attack,
which happens when the corrosion resistance of material is borderline
because of localized rupture of oxide film
✓ The protective oxide film can be rupture in number of ways, including local
dissolution or mechanical damage
✓ Characteristics: often undetected and with very little material loss, it results
localized destruction of material, it takes place below the surface affecting
the strength of the component.
✓ Galvanic Corrosion:
✓ Occurs when two dissimilar metal contact with each other while immersed
in an electrolyte, the difference in electrochemical potential results this
corrosion
✓ The higher the potential difference, the faster the corrosion of less noble
metal.
✓ Example: if steel screw corroded when in contact with brass in marine
environment
✓ Inter Crystalline (Inter Granular) Corrosion:
✓ Occurs preferentially along grain boundary
✓ The net result is that a microscopic specimen disintegrates along its grain
boundaries.
✓ Prevalent in some stainless steels
✓ Trans Crystalline (Trans Granular) Corrosion:
✓ Takes place below the surface
✓ Unlike inter crystalline corrosion, it takes place across the grain of metal
alloy
Q3.2) Tribological System
✓ It is defined as the science and technology of interacting surfaces in a
relative motion, having its origin in the Greek word tribos meaning
rubbing.
✓ It is the study of friction, lubrication and wear of engineering surface
with view to understanding surface interaction in detail and then
prescribing improvement in given application
Q3.3) Purpose of Maintenance
The purposes of maintenance are:
✓ Maximize utilization of available recourse
✓ It reduces business risk

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✓ Attempt to maximize performance of production equipment efficiently
and regularly.
✓ Prevent breakdown or failures.
✓ Minimize production loss from failures.
✓ Increase reliability of the operating systems.
✓ It optimizes plant availability at minimum cost
✓ It produces safe environment integrity in energy efficiency, product
quality and customer service
✓ It reduces down time and increase equipment availability
✓ Controlling and directing labor force
Q4) Explain the following….
Q4.1) Preventive Maintenance (PM):
✓ Is the periodical inspection and service activities which are aimed to
detect potential failures and perform minor adjustments or repairs which
will prevent major operating problems in future.
✓ Is a technic of minimizing untimely equipment beak down and/or an
equipment condition failing below required level of acceptability, and it
is applied before the occurrence of failure.
✓ Types of preventive maintenance are: scheduled maintenance,
conditioned based maintenance, reliability-based maintenance, total
productive maintenance
Q4.2) Definition of Maintenance intercalation
✓ It defined as the insertion of atomic or molecular guests into a solid host or their
removal without a major disruption of the structure.
✓ The guests are not normally neutral species but enter as ions along with the charge
compensating electrons. Sometime the guest’s ions are accomplished into the host
lattice by neutral solvent molecules, usually imparting the degree of reversibility
of the process
✓ Example: the insertion of alkali metal atom between the carbon layer of graphite.
Q4.3) Corrective (Breakdown) Maintenance/CM/:
✓ Is the repair which is generally done after the equipment has attained down state.
It is often of an emergency nature which will have associated penalty in terms of
expediting cost of maintenance and down time cost of equipment.
✓ Is a maintenance type which is carried out when equipment fails or fail below
acceptable condition while in operation.
✓ It is an emergency maintenance which is carried out after break down thus it is shut
down activity.
✓ It is an off-scheduled maintenance required by in service failure or mal function.
✓ System operation is restored as soon as possible by replace, re position or adjusting
the component which interrupted service
✓ Costly maintenance service.

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Q4.4) Testing Machine Tool
The accuracy of manufactured parts depends on the accuracy of machine tools. The quality
of work piece depends on Rigidity and stiffness of machine tool and its components.
Alignment of various components in relation to one another Quality and accuracy of
driving mechanism and control devices. It can be classified into:
1) Static Tests:
If the alignments of the components of the machine tool are checked under static conditions
then the test are called static test.
2) Dynamic Tests:
If the alignment tests are carried out under dynamic loading condition. The accuracy of machine
tools which cut metal by removing chips is tested by two types of test namely.
✓ Geometrical Tests
In this test, dimensions of components, position of components and displacement of component
relative to one another is checked.
✓ Practical Tests
In these tests, test pieces are machined in the machines. The test pieces must be appropriate
to the fundamental purpose for which the machine has been designed.
Q4.5) Sliding Bearing
Sliding Bearing ( Plain Bearing):
✓ Is a type of bearing in which the sliding takes place along the surfaces of contact between
the moving element and the fixed element.
✓ One of the machine parts which is exposed to damage by wear (i.e. depends on wear
velocity and operation condition particularly T0
) and fatigue in which static load causes
wear and dynamic load causes wear and fatigue.
✓ High temperature and high wear velocity produce overheating which changes the
properties of the material.
✓ The presence of foreign substances in the lubricant causes increased wear velocity.
✓ Examples of sliding bearing failures are: erosion, wiping, fatigue, excessive interference,
fretting, misalignment, corrosion, faulty assembly, seizure of bearing etc.
✓ The remedy for those failures is reducing the failure by proper lubrication, assembly,
manufacture, operation and maintenance.
Q4.6) Antifriction Bearing (Rolling Contact Bearing):
✓ Is a type of bearing in which the steel balls or rollers, are interposed between the moving
and fixed elements.
✓ The contact between the bearing surfaces is rolling instead of sliding as in sliding contact
bearings.
✓ Is a bearing that contains moving elements to provide a low friction support surface for
rotating or sliding surfaces.

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✓ It is also one of the machine parts which is exposed to damage by fatigue(i.e. Bearing track
and rolling element are loaded with periodic reverse loads.
✓ Wear problem can occur in case of tightening, lubrication and over load and high contact
pressure causes damage of inner and outer races.
✓ The defects of antifriction bearing are: bearing inner race loose on shaft, housing bore loose
on shaft, bearing running dry, misalignment, damage of one or more balls or rollers,
damage of running race, deformed separation, and radial play.
Q5.1) Mutual Exclusive Events
✓ Events that can’t happen at the same time or cannot possibly happen simultaneously.
✓ Events are said to be mutually exclusive if the happening of any one of them precludes the
happening of all the others, i.e., if no two or more of them can happen simultaneously in
the same trial. Symbolically the event A and B are mutually exclusive if E1 ∩ E2= ∅.
✓ P (Union of mutually exclusive events) = ∑ (Probability of events)
✓ Probability that either event one or event two will occur mutually exclusive event.
PE1 or E2=PE1 + PE2, where PE1=probability of event one, PE2=probability of event two.
Figure.1. Mutual exclusive event
Q5.2) Non-Mutual Exclusive Event
✓ When event one or event two or both can occur simultaneously. The two events have
an intersection.
✓ PE1 or E2=PE1 + PE2- PE1∩P E2, where PE1∩ P E2=probability of the intersection of the two
Events.
Figure.2. Non- mutual exclusive event

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Q6) Suppose that we have ajar with there while several black marbles whiles blindfolded.
We draw three marbles one at a times replacing each marble in the jar before dividing the next and
win &1-0 for each while marble how many chances in 1000 tries are there of winning 0, 1,2and.3
dollars.
Solution:
The possible outcomes are represented below:
0 white,3 black and win $ 0
1 white, 2 black and win $ 1
Let picking a white marble be success and drawing a black one be failure
The probability of picking a white marble is given by, P=3
10⁄ =0.3
The probability of picking a black marble is given by, q=1-P=1-0.3=0.7, sample size n= 3
Therefore, the probability distribution is given by the binomial equation.
(P+q)3
=p3
+3p2
q +3pq2
+q3
Where, p3
=3 success, 0 failure, 0.33
=0.027
3p2
q=2 success, 1 failure, 3*0.32
*0.7=0.189
3pq2
=1 success, 2 failure,3*0.3*0.72
=0.441
q3
= 0 success, 3 failure, 0.73
= 0.343
then the total summation becomes,0.027+0.189+0.441+0.343=1

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the chance in 1000 tries of winning 3,2,1or 0 dollar is tabulated as below.
Number of white marbles in one draw Chance in 1000 tries
3 27 chance to win $ 3
We can solve this problem by using binomial theorem
P(r,n,p)=
𝑛!
(𝑛−𝑟)!𝑟!
∗ 𝑝r
(1-p)n
therefore 3 success, r=3 ,p=0.027
The probability for r=2, p=0.189; The probability for r=1, p= 0.441; The probability for r=0,
p=0.343
The chance for winning 3 dollars in 1000 tries is 27 the chance for winning 2 dollars in 1000 tries
is 189 Then the chance for winning 1 dollar in 1000 tries is 441 the chance for winning 0 dollar in
1000 tries is 343
Q7) If the expected numbers of occurrences of an event a =0.5 what is the probability of zero
occurrences?
Solution:
We can calculate the probability of zero occurrences by using poisons distribution formula
Pf=∑
𝑎 𝑖 𝑒−𝑎
𝑖!
𝑘
𝑖=0 , where i=number of occurrences, for zero occurrences, k=0
Pf=e-a
=e-0.5
=0.6065, then the probability for zero occurrence is 0.6065
Q8) Determine the probability of obtaining at teats one tail when flipping three coin
simultaneously. Let ‘p’ be the probability of flipping a tail there for ahead and the of be the
probability of flipping at a fail there for p=0.5, q=0.5’
Solution:
P=0.5, q=0.5
The possible out comes and their probability are shown below
Possible out comes probability
HHH P3
=0.125
HHT P2
q=0.125
HTH P2
q=0.125
HTT Pq2
=0.125
THH P2
q=0.125
THT Pq2
=0.125
TTH Pq2
=0.125

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TTT q3
=0.125
Then the probability of flipping al least one tail is given by
P3
+3p2
q+3pq2
+q3
=1,1-p3
=1-0.125=0.875.
Then the probability of flipping at least one coin is 0.875
Q9) Determines failure rate in % (1000h, for the following tax for non-replacements case
test duration is 1000h, numbers of units tested is 12 and numbers of failed units is 5, failure
history.
Unit No. Hours to failure
2 620
3 390
4 780
8 860
9 950
Solution:
If items are not replaced as they fail for non-replacement condition the cumulative time is given
by: T=t1+t2-------+tk+(n-k)t , where, ti= occurrence of ith
failure. Now determine the failure rate
Observed failure rate=
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑓𝑎𝑖𝑙𝑢𝑟𝑒
𝑡𝑜𝑡𝑎𝑙 𝑐𝑜𝑚𝑚𝑢𝑙𝑎𝑡𝑖𝑣𝑒 𝑜𝑏𝑠𝑒𝑟𝑣𝑒𝑑 𝑡𝑖𝑚𝑒
λ=
𝑘
𝑇
, k=5, (N-k)t=(12-5)*1000=7000
T= 620+390+780+860+950=10600hr
Then λ=
5
10600
=0.0004716
𝑓𝑎𝑖𝑙𝑢𝑟𝑒
ℎ𝑟
⁄ , λ=47.16%
𝑓𝑎𝑖𝑙𝑢𝑟
103 ℎ𝑟
⁄ , λ=471.6
𝑓𝑎𝑖𝑙𝑢𝑟𝑒
106ℎ𝑟
⁄
Q10) Discuss the whole life equipment failure profile or bath tab curve
The Bath tab Curve:
✓ Is a type of model demonstrating the likely failure rates of technologies and products.
✓ Over a certain product lifetime, it shows how many units might fail during any given phase
of a three-part timeline.
✓ Is a graphical model made to represent the failure rate of a group of products over a
period of time.
✓ Is the sum of three separate overlapping failure rate distribution.

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✓ Is represented by three parts – Infant Mortality, Normal Life and End of Life Wear-Out.
✓ Together, these three segments look like a bathtub with two steep edges and a flat bottom.
The whole life of equipment or failure may be divided in to three major distinct periods.
Infant Mortality Period (Early Period):
✓ Shows how a number of units would quickly fail due to defects or other issues.
✓ Is the part of the curve where failures happen at the very beginning to a products life cycle
and accounts for things such as DOA or dead on arrival products, manufacturing errors,
material flaws and so forth.
✓ The decreasing failure rate known as early failure of infant mortality (burn in) is usually
related to manufacture and quality assurance.
✓ During this period the failure rate is high owning to the presence of weak and sub-standard
components.
Useful Life Period:
✓ Is the second part of the curve which indicate failures that occur within the normal
functioning period or “lifespan” of the device.
✓ Is sometimes referred to as the “constant failure rate/random error” is stress related.
✓ Failures within this time usually come when the stresses the device are subject to have
exceeded the strength of its weakest component (i.e. application when a device exceeds
its capabilities).
✓ Most of the failures we see during this part of the curve have happened because of an
unexpected environmental stress or load issues.
Wear Out Period:
✓ It begins with time T2 and characterized of rapid rising failure ass more and more
components brake down.
✓ Is the last part of the curve, is the end of life for the product, where you will see the curve
rise steeply as many of the devices components simply reach the point where they will
fail due to simple age or wear and tear.
✓ Is an end-of-life increasing failure rate.
✓ The increase failure rate is known as wear out is due to damage causing wear process.
Note : The function of the bathtub curve is:
✓ Used to show the likelihood of initial failure with products.
✓ Helps manufacturers predict when failures happen and hopefully identify root causes
and prevent them.
The superposition of those curve failure result in curve which is commonly known as the bath tab
curve.

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Figure.3.Bath tab curve
Q11) When we define block diagram what rules should be kept in mind?
Block diagram:
✓ Is a logical diagram which shows the functional relationship among the system elements.
✓ A pictorial representation of the functions performed by each component and of the flow
of signals.
✓ Is a diagram of a system in which the principal parts or functions are represented by blocks
connected by lines that show the relationships of the blocks.
✓ Is a specialized, high-level flowchart used in engineering.
✓ It is used to design new systems or to describe and improve existing ones and its structure
provides a high-level overview of major system components, key process participants, and
important working relationships.
In defining the blocks, the following rules should be kept in mind.
✓ Each block should be representing the maximum number of components in order to
simplify the diagram.
✓ The function of each block should be easily identified.
✓ Blocks should be mutually independent in that failure in one block should not affect the
probability of failure in other blocks.
✓ There should be only one environment within a block.

Maintanace of machinery assignment

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