1) Define the term stress concentration and methods of
mitigating it.
Stress concentration is a phenomenon where the magnitude of stress at a specific point in a
material is significantly higher than the average stress applied to the material. This occurs due
to geometric discontinuities, such as holes, notches, fillets, or abrupt changes in crosssectional area. These discontinuities disrupt the smooth flow of stress, causing it to
concentrate at these points.
Effects of Stress Concentration
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Increased localized stress: This can lead to premature failure of the material even
under relatively low applied loads.
Fatigue failure: Cyclic loading can accelerate crack initiation and propagation at
stress concentration points.
Brittle fracture: Materials with low ductility are particularly susceptible to brittle
fracture due to stress concentrations.
Methods of Mitigating Stress Concentration
1. Design Modifications:
o Avoid sharp corners and abrupt changes: Replace sharp corners with fillets or radii
to smooth out stress distribution.
o Optimize hole and notch geometries: Use larger radii at holes and notches, or
consider elongated or elliptical shapes.
o Gradual transitions: Introduce gradual changes in cross-sectional area to reduce
stress concentration.
o Relief notches: Create small notches near stress concentration points to redistribute
stress.
2. Material Selection:
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High fracture toughness: Materials with high fracture toughness can better resist
crack initiation and propagation.
Ductile materials: Ductile materials can better accommodate stress concentrations
without failure.
3. Manufacturing Processes:
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Improved surface finish: A smoother surface reduces stress concentrations caused
by surface imperfections.
Heat treatment: Certain heat treatments can improve material properties and
reduce stress concentration sensitivity.
4. Load Management:
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Reduce peak loads: Lowering the magnitude of applied loads reduces the overall
stress level and the impact of stress concentrations.
Modify loading conditions: Changing the direction or distribution of loads can
alleviate stress concentrations.
5. Stress Analysis:
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Finite element analysis (FEA): Use FEA to predict stress distributions and identify
critical areas.
Experimental stress analysis: Employ techniques like strain gauges or photoelasticity
to measure actual stress levels.
2) Discuss the factors to be investigated while selecting a
bearing for an application.
Selecting the right bearing for an application is crucial for its optimal performance,
reliability, and longevity. Several factors must be carefully evaluated before making a
decision.
Load Characteristics
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Type of load: Radial, axial, or combined.
Magnitude of load: Peak, average, and dynamic load.
Direction of load: Constant or varying.
Shock or impact loads: Presence and severity.
Operating Conditions
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Speed: Operating speed and maximum permissible speed.
Temperature: Operating temperature range and lubrication compatibility.
Environment: Corrosive, dusty, or contaminated environments.
Vibration and shock: Levels of vibration and shock.
Misalignment: Angular and axial misalignment.
Bearing Requirements
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Required life: Desired bearing life under specific operating conditions.
Accuracy: Required level of precision and rotational accuracy.
Rigidity: Stiffness requirements for the application.
Sealing: Necessary sealing to prevent contamination or lubricant leakage.
Lubrication: Type of lubricant, lubrication method, and frequency.
Mounting and dismounting: Ease of installation and removal.
Maintenance: Required maintenance intervals and procedures.
Bearing Types and Materials
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Bearing type: Ball, roller, cylindrical, spherical, tapered, or other types.
Bearing material: Steel, stainless steel, ceramic, or other materials.
Cage material: Steel, brass, or polymer.
Cost and Availability
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Bearing cost: Initial purchase cost and potential maintenance costs.
Availability: Supply chain considerations and lead times.
Additional Considerations
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Noise and vibration levels: Acceptable noise and vibration limits.
Space limitations: Available space for bearing installation.
Reliability and safety: Required level of reliability and safety.
Maintenance requirements: Ease of maintenance and lubrication.
3) Explain the basic governing equation for a power screw
which correlates the applied torque with the load to be
raised or lowered, angle of friction and helix angle.
Understanding the Components
Before diving into the equation, let's define the key terms:
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Torque (T): The rotational force applied to the power screw.
Load (W): The axial force to be raised or lowered.
Angle of friction (θ): The angle between the resultant force and the normal force acting on
the thread.
Helix angle (α): The angle between the thread and a plane perpendicular to the screw axis.
The Equation
The fundamental equation relating these parameters is:
T = (W * d) / (2 * π * η)
Where:
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T is the torque required (N-m)
W is the axial load (N)
d is the mean diameter of the screw (m)
π is the mathematical constant (approximately 3.14159)
η is the screw efficiency
Note: The efficiency (η) is a factor that accounts for friction losses and is typically less than
1. It depends on the angle of friction (θ) and the helix angle (α) through a complex
relationship involving trigonometric functions.
Incorporating Friction and Helix Angle
While the above equation provides a basic understanding, a more accurate analysis requires
considering the effects of friction and helix angle. This leads to a more complex equation:
T = (W * d) / (2 * π * tan(α - θ))
Where:
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T, W, d, and π have the same meanings as before.
α is the helix angle (radians)
θ is the angle of friction (radians)
This equation directly relates the torque to the load, helix angle, and friction angle, providing
a more precise calculation.
4) Discuss the various types of gear tooth failures.
Major Categories of Gear Tooth Failures
1. Surface Fatigue:
o Pitting: Small, round depressions on the gear tooth surface caused by
repeated stress cycles.
o Spalling: Larger and deeper pits compared to pitting.
o Scoring: Grooves or scratches on the gear tooth surface due to insufficient
lubrication or excessive load.
o Scuffing: Severe adhesive wear leading to the transfer of metal from one
tooth to another.
2. Wear:
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Abrasive wear: Removal of material from the gear tooth surface due to hard
particles in the lubricant or environment.
Adhesive wear: Transfer of material from one tooth to another due to high
contact pressures and insufficient lubrication.
Corrosive wear: Chemical attack on the gear tooth surface leading to material
loss.
3. Plastic Deformation:
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Tooth root bending: Excessive load causing plastic deformation of the tooth
root.
Tooth flank plastic deformation: Excessive contact stress leading to plastic
flow on the tooth flank.
4. Tooth Breakage:
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Tooth root fracture: Fatigue or overload leading to fracture at the tooth root.
Tooth tip breakage: Impact loading or overload causing fracture at the tooth
tip.
Other Types of Failures
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Micro-pitting: Smaller pits than regular pitting, often caused by high contact stresses.
Flank wear: Gradual wear of the tooth flank due to sliding contact.
Edge chipping: Removal of small chips from the tooth edge due to impact loading.
Factors Affecting Gear Tooth Failures
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Load: Magnitude, direction, and frequency of loading.
Speed: Rotational speed of the gears.
Lubrication: Type, quantity, and quality of lubricant.
Material properties: Hardness, strength, and toughness of the gear material.
Manufacturing defects: Imperfections in the gear tooth profile or material.
Alignment: Misalignment of the gears.
Operating environment: Temperature, humidity, and contaminants.
5) Discuss design on the basis of strength and design on
the basis of rigidity with suitable examples.
Design on the Basis of Strength
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Objective: To ensure that the component can withstand applied loads without yielding or
fracturing.
Focus: Material properties, stress analysis, and factor of safety.
Example: A connecting rod in an engine must be strong enough to withstand the high tensile
and compressive loads during the combustion cycle. The material selection (e.g., highstrength steel) and cross-sectional design (e.g., I-beam shape) are crucial for preventing
failure.
Design on the Basis of Rigidity
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Objective: To minimize deformation or deflection of the component under load.
Focus: Stiffness, deflection calculations, and allowable deformation limits.
Example: A machine tool spindle must maintain high precision during machining operations.
Therefore, it must be designed to minimize deflection under cutting forces. This requires
careful selection of materials with high modulus of elasticity and a stiff cross-sectional shape
(e.g., hollow shaft).
Balancing Strength and Rigidity
In many cases, a component must meet both strength and rigidity requirements. For instance,
a shaft in a gearbox must be strong enough to transmit torque without breaking and stiff
enough to prevent excessive deflection, which can lead to misalignment and gear wear.
Key Considerations:
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Material Selection: The choice of material significantly affects both strength and rigidity.
High-strength materials often have higher stiffness, but other factors like weight and cost
must be considered.
Cross-Sectional Shape: The shape of the component's cross-section influences its strength
and rigidity. For example, I-beams are efficient for bending resistance, while hollow shafts
provide high stiffness-to-weight ratios.
Support Conditions: How the component is supported affects its deflection. Fixed, pinned,
and roller supports have different stiffness characteristics.
Load Distribution: The location and magnitude of applied loads influence the component's
response. Concentrated loads generally cause higher stresses and deflections than
distributed loads.
Factor of Safety: A suitable factor of safety is applied to account for uncertainties in material
properties, load estimates, and manufacturing tolerances.
Example: Shaft Design
When designing a shaft, engineers consider both strength and rigidity:
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Strength: The shaft diameter is calculated based on the maximum allowable shear stress to
prevent yielding.
Rigidity: The shaft's deflection is calculated to ensure it meets the required tolerance for
alignment and bearing performance.
6) Describe various types of plain carbon steels with their
applications
Types of Plain Carbon Steels
1. Low Carbon Steel (Mild Steel):
o Carbon content: 0.05% to 0.30%
o Properties: Good ductility, weldability, and formability. Relatively low strength
and hardness.
o Applications:
▪ Structural components (beams, columns, plates)
▪ Automotive body panels
▪ Wire and nails
▪ Pipes and tubing
▪ General fabrication
2. Medium Carbon Steel:
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Carbon content: 0.30% to 0.60%
Properties: Good balance of strength, ductility, and hardness. Heat treatable.
Applications:
▪ Shafts and axles
▪ Gears and sprockets
▪ Connecting rods
▪ Railway tracks
▪ Forgings
3. High Carbon Steel:
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Carbon content: 0.60% to 1.40%
Properties: High strength and hardness, but lower ductility and weldability.
Applications:
▪ Cutting tools (knives, chisels, saws)
▪ Springs
▪ Wire ropes
▪ Files and drills
Factors Affecting Properties
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Carbon content: Higher carbon content generally increases strength and hardness
but reduces ductility.
Alloying elements: While not considered alloy steels, small amounts of elements like
manganese, silicon, phosphorus, and sulfur can influence properties.
Heat treatment: Processes like annealing, normalizing, quenching, and tempering
can significantly alter the microstructure and properties of plain carbon steels.
7)Explain the following steels compositions:55C4,
25Cr4Mo2, X15Cr25Ni12, 25C12S14
1. 55C4
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55: Indicates the average carbon content is approximately 0.55% by weight.
C: Stands for carbon.
4: Might refer to the specific grade within the carbon steel classification.
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Manganese (Mn): Typically found in small amounts in carbon steels.
Other Elements: May contain small amounts of silicon (Si) and possibly other alloying
elements.
2. 25Cr4Mo2
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25: Indicates the average carbon content is approximately 0.25% by weight.
Cr: Stands for chromium.
4: Indicates approximately 4% chromium content.
Mo: Stands for molybdenum.
2: Indicates approximately 2% molybdenum content.
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Other Elements: May contain small amounts of silicon (Si) and manganese (Mn).
3. X15Cr25Ni12
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X: Typically used to denote a high-alloy steel.
15: Indicates the carbon content is approximately 0.15% by weight.
Cr: Stands for chromium.
25: Indicates approximately 25% chromium content.
Ni: Stands for nickel.
12: Indicates approximately 12% nickel content.
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Other Elements: May contain small amounts of silicon (Si), manganese (Mn), and possibly
other alloying elements.
4. 25C12S14
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25: Indicates the average carbon content is approximately 0.25% by weight.
C: Stands for carbon.
12: Indicates the chromium content, which can be around 12%.
S: Stands for sulfur.
14: Indicates approximately 0.14% sulfur content (likely meant to be a typo, as sulfur content
is usually much lower; it might indicate 0.014%).
Approximate Composition:
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Other Elements: May contain small amounts of silicon (Si) and manganese (Mn).
8)Define various types of fits with neat diagram.
Types of Fits
A fit is the relationship between two mating parts, specifically the
allowance (difference between the nominal sizes of the hole and
shaft) and the tolerance (permissible variation in size) of the
components. There are three primary types of fits:
Fit Type
Clearance Fit
Interference Fit
Transition Fit
Definition
Characteristics
Applications
The internal
feature (hole) is
Bearings, sliding
larger than the Loose fit, free
mechanisms, shafts
external feature movement, play
in housings, gears
(shaft).
between parts.
on shafts.
The internal
Press fits, shrink
feature is
Tight fit, requires force fits, bushings on
smaller than the to assemble, parts are shafts, wheels on
external feature. held together by friction. axles.
A fit between
clearance and
interference,
with both
Shaft-key
possibilities
Intermediate fit, can be assemblies,
depending on
assembled by hand or coupling ring and
specific
with light force, provides recess, bearing
tolerances.
accurate alignment.
assemblies.
9) Describe the size of the bolts if two plates are fastened by
means of two bolts as shown in Fig. The bolts are made of plain
carbon steel 30C8 (Syt = 400 N/mm2) and the factor of safety is 5
and P = 5KN.
10) Explain the difference between rigid and flexible
coupling.
FEATURE
PURPOSE
ALIGNMENT
TOLERANCE
TORQUE
TRANSMISSION
VIBRATION
DAMPING
BACKLASH
MAINTENANCE
TYPICAL
APPLICATIONS
ADVANTAGES
DISADVANTAGES
RIGID COUPLING
Connects two
perfectly aligned
shafts
Zero or minimal
misalignment
Efficient torque
transfer
Poor vibration
damping
Minimal or no
backlash
Requires precise
alignment
High-speed
applications, pumps,
compressors
High torque capacity,
efficient power
transfer
Susceptible to
misalignment, high
vibration transmission
FLEXIBLE COUPLING
Connects two shafts
with slight
misalignment
Can accommodate
angular, parallel, and
axial misalignment
Some torque loss due
to flexibility
Good vibration
damping
Potential for backlash
Less critical alignment,
easier maintenance
General machinery,
motors, fans,
gearboxes
Absorbs misalignment,
reduces vibration,
quieter operation
Lower torque capacity,
potential for increased
wear