All questions of Design of Machine Elements for Mechanical Engineering Exam

The factor of safety for steel and for steady load is
  • a)
    2
  • b)
    4
  • c)
    6
  • d)
    8
Correct answer is option 'B'. Can you explain this answer?

Divya Banerjee answered
Factor of Safety for Steady Load in Steel

Factor of safety is a measure of the capacity of a material or structure to withstand loads without failure. It is the ratio of the maximum load that a structure can support to the actual load that it is subjected to. The factor of safety for steel and for steady load is 4.

Explanation:

Steady load refers to a constant load that is applied to a structure over a long period of time. For example, the weight of a building or bridge is a steady load. Steel is a commonly used material in construction due to its high strength and durability. The factor of safety for steel is determined by dividing the ultimate strength of the material by the maximum stress that it can endure under normal conditions.

The factor of safety for steel and for steady load is typically set at 4. This means that the maximum load that a structure can support is four times the actual load that it is subjected to. This provides a safety margin to account for unexpected loads or changes in the environment that could affect the structure's stability.

Having a factor of safety of 4 for steel and steady load means that the structure can safely support four times the weight it is designed to hold. This helps to ensure that the structure remains stable and can withstand any unexpected stresses or changes that may arise.

Conclusion:

In conclusion, the factor of safety for steel and for steady load is 4. This provides a safety margin to account for unexpected loads or changes in the environment that could affect the structure's stability. By using steel with a factor of safety of 4, structures can be built to withstand a wide range of loads and remain stable over a long period of time.

Which of the following ropes will be most flexible?
  • a)
    6 x 7
  • b)
    6 x 19
  • c)
    8 x 19
  • d)
    6 x 37
Correct answer is option 'D'. Can you explain this answer?

Rhea Reddy answered
The wire rope of 6 x 7 construction consists of a few wires of relatively large size. It is too stiff for hoisting purposes. The 6 x 19 or 6x 37 constructions are flexible wire ropes, and are commonly used in hoists.

For power transmission square threads
  • a)
    are least efficient
  • b)
    are less rigid
  • c)
    are expensive to manufacture
  • d)
    wear out very fast
Correct answer is option 'C'. Can you explain this answer?

Dhruv Dasgupta answered
Square thread:
1. Square threads are less efficient than trapezoidal thread.
2. Square threads are difficult to manufacture. They are usually turned on lathe with single point cutting tool.
3. The strength of a screw depends upon the thread thickness at the core diameter since square thread have less thickness at core than trapezoidal this reduces the load carrying capacity.

The dynamic load on a gear is due to:
1. Inaccuracies of tooth spacing.
2. Irregularities in tooth profile.
3. Deflection of the teeth under load.
4. Type of service (i.e., intermittent, one shift per day, continuous per day).
Which of the above statements are correct?
  • a)
    1, 2 and 3
  • b)
    2, 3 and 4
  • c)
    1, 3 and 4
  • d)
    1, 2 and 4
Correct answer is option 'A'. Can you explain this answer?

The dynamic load between the meshing teeth arises due to following factors:
1. Inaccuracies of tooth profile
2. Error in tooth spacing
3. Runout of gear
4. Gear mesh stiffness variation
5. Inertia of rotating masses
6. Deflection of teeth
7. Stiffness of rotating parts
But dynamic factor mainly depends upon tooth error and pitch line velocity.

The objective of caulking in a riveted joint is to make the joint
  • a)
    Leakproof
  • b)
    Stronger
  • c)
    Less stressed
  • d)
    None of these
Correct answer is option 'A'. Can you explain this answer?

Caulking is used to make the riveted joint leak proof or fluid tight in pressure vessel like steam boilers, air receivers and tank etc.

A hydraulic press exerts a total load of 4 MN. This load' is carried by two steel rods, supporting the upper head of the press. If the safe stress is 100 MPa and E =210 kN/mm2, what is the design diameter of supporting steel rods?
  • a)
     160 mm
  • b)
    165 mm
  • c)
    170 mm
  • d)
    175 mm
Correct answer is option 'A'. Can you explain this answer?

Rajat Patel answered
Given data:
Total load carried by the steel rods = 4 MN
Safe stress = 100 MPa
Elastic modulus of steel (E) = 210 kN/mm2

To find: Design diameter of supporting steel rods

We can use the following formula to calculate the stress:

Stress = Load/Area

Let's assume the diameter of each rod as 'd' and the cross-sectional area as 'A'.

The load carried by each rod can be calculated as half of the total load, since there are two rods.

Load carried by each rod = Total load/2 = 4 MN/2 = 2 MN

We can rearrange the formula to find the area:

Area = Load/Stress

Substituting the values, we get:

A = (2 MN)/(100 MPa) = (2 × 106 N)/(100 × 106 N/m2) = 20 × 10-3 m2

The area of a circle is given by the formula:

A = πd2/4

Rearranging the formula, we can find the diameter:

d = √(4A/π)

Substituting the value of area, we get:

d = √(4 × 20 × 10-3/π) = √(80 × 10-3/π) = √(80/π) × 10-3 ≈ 160 mm

Therefore, the design diameter of the supporting steel rods is approximately 160 mm.

Hence, option A (160 mm) is the correct answer.

If P is the pitch of a square thread, then the depth of thread d is given by
  • a)
    0.5 P
  • b)
    P
  • c)
    1.5 P
  • d)
    2 P
Correct answer is option 'A'. Can you explain this answer?

The correct answer to the given question is option 'A', which states that the depth of thread (d) is equal to 0.5 times the pitch (P) of a square thread.

A square thread is a type of screw thread that has a square cross-section. It is commonly used in applications where a large amount of linear force is required, such as in vices, clamps, and jacks. The pitch of a square thread refers to the distance between adjacent threads, measured along the axis of the screw.

To understand why the depth of thread is half the pitch, let's break down the components of a square thread:

1. Pitch (P): The pitch is the distance between two adjacent threads. It is typically measured in millimeters or inches.

2. Depth of thread (d): The depth of thread is the distance from the crest (top) of the thread to the root (bottom) of the thread. It determines the engagement between the male and female threads.

Now, let's consider the geometry of a square thread:

- A square thread has equal height and width, resulting in a square-shaped cross-section.
- The depth of thread (d) is equal to the difference in height between the crest and root of the thread.

Based on the geometry of a square thread, we can deduce that the depth of thread is half the pitch:

- When a square thread is fully engaged, the crest of one thread is in contact with the root of the adjacent thread.
- This means that the depth of thread is equal to half the pitch, as the crest and root of adjacent threads align.

Therefore, the correct answer is option 'A', which states that the depth of thread (d) is equal to 0.5 times the pitch (P) of a square thread.

For bolts of uniform strength, the shank diameter is made equal to
  • a)
    major diameter of threads
  • b)
    pitch diameter of threads
  • c)
    minor diameter of threads
  • d)
    nominal diameter of threads
Correct answer is option 'C'. Can you explain this answer?

Stuti Mishra answered
Bolt of uniform strength are made by
1. Reducing the diameter of shank of bolt corresponding to that of minor diameter.
2. Making a hole and making the area of shank equal to root area.

The ratio of inner to outer radii of a clutch is normally taken between
  • a)
    0.3 - 0.4
  • b)
    0.4 - 0.5
  • c)
    0.5 - 0.6
  • d)
    0.6 - 0.7
Correct answer is option 'D'. Can you explain this answer?

Dhruv Dasgupta answered
If equal importance is given to durability and torsional moment output, (r1/r2) is taken as 0.48. In practice, (r1/r2) is usually kept between 0.6 and 0.7.

Consider the following parameter:
1. Oil pressure
2. Bearing metal temperature
3. Drain oil temperature
4. Bearing vibration
5. Friction coefficien
Which of the above should be monitored for determining safe operation of journal bearing?
  • a)
    1,2 and 3
  • b)
    1,2, 3 and 4
  • c)
    1, 2, 3 and 5
  • d)
    All of the above
Correct answer is option 'B'. Can you explain this answer?

Nayanika Yadav answered
Parameters to monitor for determining safe operation of journal bearing:

1. Oil pressure:
- Low oil pressure can result in inadequate lubrication which can cause excessive wear and tear of bearing surfaces.
- High oil pressure can cause excessive heat generation and damage to the bearing surfaces.

2. Bearing metal temperature:
- Elevated bearing temperature can indicate insufficient lubrication, excessive loading or misalignment.
- Too low bearing temperature can lead to inadequate lubrication and increased friction.

3. Drain oil temperature:
- Indicates the temperature of the oil leaving the bearing.
- High temperature can indicate excessive heat generation in the bearing and insufficient cooling.
- Low temperature can indicate insufficient lubrication.

4. Bearing vibration:
- Excessive bearing vibration can lead to fatigue failure and damage to the bearing.
- Can be caused by misalignment, imbalance, or inadequate lubrication.

5. Friction coefficient:
- Indicates the amount of friction between the bearing surfaces.
- High friction coefficient can cause excessive wear and tear on the bearing surfaces and lead to bearing failure.

Conclusion:
All the above parameters should be monitored to determine safe operation of journal bearings. Specifically, oil pressure, bearing metal temperature, drain oil temperature and bearing vibration are critical parameters that can indicate potential problems with the bearing.

The most suitable bearing for carrying very heavy loads with slow speed is
  • a)
    hydrodynamic bearing
  • b)
    ball bearing
  • c)
    roller bearing
  • d)
    hydrostatic bearing
Correct answer is option 'D'. Can you explain this answer?

Divyansh Goyal answered
The hydrostatic bearings are those which can support steady loads without any relative motion between the journal and the bearings. This condition is achieved by forcing externally pressurized lubricant bewteen the members.

To ensure self-locking in a screw jack it is essential that helix angle is
  • a)
    larger than friction angle
  • b)
    smaller than friction angle
  • c)
    equal to friction angle
  • d)
    such as to give maximum efficiency in lifting
Correct answer is option 'B'. Can you explain this answer?



Explanation:

Friction Angle:
- The friction angle is the angle at which two surfaces meet and resist relative motion.
- In the case of a screw jack, the friction angle represents the resistance to motion between the threads of the screw and the load being lifted.

Helix Angle:
- The helix angle of a screw jack is the angle between the thread helix and the axis of the screw.
- It determines the efficiency and self-locking ability of the screw jack.

Relationship between Helix Angle and Friction Angle:
- In order to ensure self-locking in a screw jack, the helix angle must be smaller than the friction angle.
- A smaller helix angle creates a steeper thread profile, which increases the resistance to motion and prevents the screw jack from self-lowering under load.
- If the helix angle is larger than the friction angle, the screw jack may not be able to hold the load in place and could start to unwind.

Conclusion:
- Therefore, to ensure self-locking in a screw jack, the helix angle must be smaller than the friction angle to provide sufficient resistance and prevent the screw jack from self-lowering.

The relationship between diameter of rivet hole and plate thickness (Unwin formula) is given by
  • a)
    d = 6√t 
  • b)
    d = 1.5t
  • c)
    d = 6t  
  • d)
    d = 2t
Correct answer is option 'A'. Can you explain this answer?

Pallabi Bajaj answered
The relation between diameter of rivet hole and plate thickness is given by unwin as
d = 6√t
where, d = diameter of rivet hole plate thickness

Bearing used for heavy loads are
  • a)
    ball bearings
  • b)
    roller bearings
  • c)
    thrust bearings
  • d)
    any of the above
Correct answer is option 'B'. Can you explain this answer?

Dipika Nambiar answered
Roller Bearings for Heavy Loads

Roller bearings are commonly used for heavy loads due to their ability to distribute weight over a larger surface area compared to ball bearings. This results in reduced friction and increased load-bearing capacity.

Types of Roller Bearings:
- Cylindrical roller bearings: These are suitable for high radial loads and moderate thrust loads.
- Tapered roller bearings: Designed to handle both radial and axial loads, commonly used in automotive applications.
- Spherical roller bearings: Can accommodate misalignment and heavy radial loads.
- Needle roller bearings: Ideal for applications with limited space and high load requirements.

Advantages of Roller Bearings:
- Higher load capacity: Roller bearings can support heavier loads compared to ball bearings.
- Durability: Roller bearings have a longer lifespan due to reduced wear and friction.
- Reduced heat generation: The larger contact area in roller bearings helps dissipate heat more effectively.

Applications of Roller Bearings:
- Heavy machinery: Used in construction equipment, mining machinery, and agricultural machinery.
- Automotive industry: Found in wheel bearings, transmissions, and engine components.
- Industrial applications: Utilized in conveyor systems, gearboxes, and pumps.
In conclusion, roller bearings are the preferred choice for applications involving heavy loads due to their superior load-bearing capacity, durability, and reduced heat generation.

It is seen from the curve that there is a minimum value of the coefficient of friction (m) for a particular value of the Bearing Characterstic Number denoted by a. What is this value of the Bearing Characteristic Number called?
  • a)
    McKee Number
  • b)
    Reynolds Number
  • c)
    Bearing Modulus
  • d)
    Sommerfeld Number
Correct answer is option 'C'. Can you explain this answer?

Sneha Nair answered
Bearing Characteristic Number (BCN) is a dimensionless parameter used in the study of fluid film bearings. It is defined as the ratio of the load-carrying capacity of the bearing to the viscous resistance of the lubricant. The BCN is a measure of the effectiveness of the bearing in supporting the load and reducing friction.

The curve mentioned in the question represents the relationship between the coefficient of friction (m) and the BCN. The coefficient of friction is a measure of the resistance to relative motion between two surfaces in contact. In the case of a bearing, it represents the frictional resistance between the rotating shaft and the bearing surface.

The curve shows that there is a minimum value of the coefficient of friction for a particular value of the BCN. This means that at this specific BCN value, the bearing is operating at its optimal condition, with minimum friction and maximum load-carrying capacity.

The correct term for this specific BCN value is the Bearing Modulus. The Bearing Modulus is a measure of the stiffness of the bearing and is defined as the BCN at the point of minimum coefficient of friction. It represents the ability of the bearing to support the load without excessive deformation or deflection.

The Bearing Modulus is an important parameter in bearing design and selection. It helps engineers determine the appropriate bearing size and type for a given application, considering factors such as load, speed, and operating conditions. By operating the bearing at its Bearing Modulus, the friction and wear can be minimized, leading to improved performance and longevity of the bearing.

In summary, the value of the Bearing Characteristic Number at the point of minimum coefficient of friction is called the Bearing Modulus. It represents the optimal operating condition of the bearing, where the load-carrying capacity is maximized and the friction is minimized.

In case of belt drives, the effect of the centrifugal tension is to
  • a)
    cause the belt to leave the pulley and increases the power to be transmitted
  • b)
    cause the belt to stay on the pulley and increase the power to be transmitted
  • c)
    reduce the driving power of the belt
  • d)
    stretch the belt in longitudinal direction
Correct answer is option 'C'. Can you explain this answer?

Anjali Shah answered
Effect of Centrifugal Tension in Belt Drives

Centrifugal tension is the tension that is produced in a belt drive due to the flinging action of the belt as it rotates. This tension is dependent on the speed of rotation of the pulleys and the mass of the belt.

The effect of centrifugal tension on a belt drive is as follows:

Reduction in Driving Power

When the speed of rotation of the pulleys increases, the centrifugal tension also increases. This tension acts in the outward direction and opposes the tension that is applied to the belt by the pulleys. As a result, the effective tension in the belt reduces, leading to a reduction in the driving power of the belt.

Loss of Contact between Belt and Pulley

If the centrifugal tension becomes greater than the tension that is applied to the belt by the pulleys, the belt may lose contact with the pulleys. This can result in a loss of power transmission as the belt is no longer able to transmit the torque from the driving pulley to the driven pulley.

Conclusion

In summary, the effect of centrifugal tension in a belt drive is to reduce the driving power of the belt and, in extreme cases, to cause the belt to lose contact with the pulleys. Therefore, it is important to consider the effect of centrifugal tension when designing belt drives and to ensure that the belt is properly tensioned to prevent loss of power transmission.

A silent chain consists of
  • a)
    links and blocks
  • b)
    links, pins, bushings and rollers
  • c)
    inverted tooth over-lapping links
  • d)
    oval links
Correct answer is option 'C'. Can you explain this answer?

Avinash Sharma answered
A silent chain is also known as inverted tooth chain. It is designed to eliminate the evil effects caused by stretching and to produce noiseless running.

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