All questions of Design of Machine Elements for Mechanical Engineering Exam

The correct answer is option 'A': Pitch.

Explanation:
Pitch is defined as the distance between the centers of two consecutive rivets in a row. It is an important parameter in riveting as it determines the spacing between the rivets and plays a significant role in the strength and stability of the joint.

When riveting two or more pieces of material together, rivets are used to hold them in place. A rivet is a cylindrical fastener with a head on one end and a tail on the other. To create a strong joint, multiple rivets are placed in a row along the length of the joint. The distance between the centers of these rivets, known as the pitch, is carefully determined based on the material properties, joint design, and load requirements.

The pitch is typically specified in the engineering drawing or design documentation for the joint. It is crucial to follow the specified pitch to ensure that the joint can withstand the intended loads. If the pitch is too small, the rivets may be too close to each other, resulting in reduced strength and potential interference during installation. On the other hand, if the pitch is too large, the joint may not be able to withstand the required loads, leading to failure.

In addition to pitch, other parameters such as rivet diameter, material properties, and joint configuration also influence the strength and performance of the joint. Therefore, it is essential to consider all these factors and carefully design the riveted joint to ensure optimal performance.

In summary, the pitch is the center-to-center distance between two consecutive rivets in a row. It is a critical parameter in riveting that affects the strength and stability of the joint. By following the specified pitch and considering other design factors, engineers can create robust and reliable riveted joints.

Self-Aligning Bearing

A self-aligning bearing is a type of bearing that can accommodate misalignment between the shaft and the housing without causing excessive friction or wear. It is designed to automatically align itself to the shaft, thereby reducing the chances of premature failure and improving the overall performance of the bearing system.

There are several types of self-aligning bearings available in the market, but the most commonly used one is the spherical bearing.

Spherical Bearing

A spherical bearing, also known as a spherical roller bearing, is a type of self-aligning bearing that is designed to accommodate misalignment between the shaft and the housing. It consists of an inner ring with two rows of barrel-shaped rollers and an outer ring with a spherical raceway.

Working Principle

The spherical shape of the rollers allows them to move freely in all directions, enabling the bearing to self-align to the shaft. When there is misalignment between the shaft and the housing, the rollers adjust their positions to distribute the load evenly and minimize friction. This helps to prevent excessive wear and prolong the service life of the bearing.

Advantages

- Self-alignment: The spherical shape of the bearing allows it to automatically align itself to the shaft, eliminating the need for precise alignment during installation.

- Load distribution: The multiple rows of rollers in a spherical bearing distribute the load evenly, reducing the chances of localized stress and improving the overall load-carrying capacity.

- Shock resistance: The design of the spherical bearing provides good shock resistance, making it suitable for applications with high impact or vibration.

- Lubrication: Spherical bearings are typically designed to accommodate lubrication, ensuring smooth and efficient operation.

Conclusion

In summary, the self-aligning bearing that can accommodate misalignment between the shaft and the housing is the spherical bearing. Its unique design allows it to automatically adjust and distribute the load evenly, providing improved performance and longevity in bearing applications.

B)90

Endurance Limit

The stress to which a metal can be subjected indefinitely to stress reversal without rupture is known as the endurance limit. It is also referred to as the fatigue limit or fatigue strength. This property is crucial in designing structures and components that are subjected to cyclic loading or repeated stress reversals.

Importance of Endurance Limit

The endurance limit determines the maximum stress that a material can withstand for an infinite number of cycles without failure. It is particularly important in applications where the material experiences repeated or cyclic loading, such as in machinery, bridges, aircraft, and automotive components. Understanding the endurance limit helps engineers determine the safe operating range for a material under cyclic loading conditions.

Fatigue Failure

When a material is subjected to repeated stress cycles, it can lead to fatigue failure, even if the maximum stress is below the ultimate stress or yield point of the material. Fatigue failure occurs due to the accumulation of microscopic cracks and damage that gradually grow until the material fractures. Therefore, it is crucial to determine the endurance limit to ensure the longevity and reliability of a structure or component.

Determining the Endurance Limit

The endurance limit is determined through fatigue testing, where a metal specimen is subjected to repeated stress cycles until failure occurs. The stress levels applied to the specimen are gradually increased until a stress level is reached where failure does not occur even after a large number of cycles (typically 10^6 cycles). This stress level is then defined as the endurance limit.

Factors Affecting Endurance Limit

Several factors can influence the endurance limit of a material, including:

1. Material Composition: Different materials have different endurance limits due to variations in their microstructure, alloying elements, and heat treatments.

2. Surface Condition: Surface roughness and defects can significantly reduce the endurance limit. Smooth surfaces with no defects have higher endurance limits compared to rough or flawed surfaces.

3. Environmental Factors: The endurance limit can be affected by the presence of corrosive environments, high temperatures, and other environmental factors.

4. Stress Concentration: The presence of stress concentration points, such as notches or sharp corners, can lower the endurance limit as they promote crack initiation and propagation.

5. Loading Conditions: The type and magnitude of cyclic loading, such as tension-compression, bending, or torsion, can affect the endurance limit.

Conclusion

The endurance limit is a critical parameter in designing structures and components that experience cyclic loading. It represents the maximum stress that a material can withstand indefinitely without failure. Determining the endurance limit through fatigue testing helps ensure the reliability and longevity of engineering structures and components.

Positive Clutch: Dog clutch

A positive clutch is a type of clutch that transmits power from the engine to the transmission without any slippage. In other words, the clutch is fully engaged and the power is transmitted without any loss. The following are the types of positive clutches:

Centrifugal Clutch:
The centrifugal clutch is a type of positive clutch that uses centrifugal force to engage the clutch. It is commonly used in small engines such as go-karts, lawn mowers, and chainsaws.

Single Plate Clutch:
The single plate clutch is a type of positive clutch that has only one friction plate. It is commonly used in cars and trucks.

Cone Clutch:
The cone clutch is a type of positive clutch that uses conical friction surfaces to engage the clutch. It is commonly used in industrial machinery.

Dog Clutch:
The dog clutch is a type of positive clutch that uses two or more pairs of teeth to engage the clutch. It is commonly used in manual transmissions and racing cars. The teeth on the clutch engage with corresponding slots in the transmission gears, providing a positive and instantaneous engagement.

In summary, the dog clutch is the only positive clutch among the given options. It provides instantaneous and positive engagement, making it suitable for high-performance applications.

Ball bearing type screws are commonly used in various applications due to their unique design and advantages. Among the given options, the correct answer is option 'D', which states that ball bearing type screws are found in the steering mechanism.

The steering mechanism of a vehicle is responsible for allowing the driver to control the direction of the vehicle. It consists of various components, including a steering wheel, steering column, and a mechanism to transfer the rotation of the steering wheel to the wheels. Ball bearing type screws are used in the steering mechanism for several reasons:

1. Smooth operation: Ball bearing type screws are designed with ball bearings between the screw and the nut. These ball bearings reduce friction and allow for smooth and efficient movement of the screw. In the steering mechanism, this ensures that the steering wheel can be turned easily and smoothly, providing the driver with a comfortable and responsive steering experience.

2. High load capacity: The steering mechanism is subjected to various loads, including the weight of the vehicle and the forces generated during steering. Ball bearing type screws are capable of handling high loads due to the ball bearings distributing the load evenly along the contact surfaces. This ensures the steering mechanism can withstand the forces encountered during normal driving conditions.

3. Precise motion control: Ball bearing type screws provide precise motion control, which is crucial in the steering mechanism. The ball bearings reduce backlash and play, ensuring that the steering wheel movement is directly translated to the wheels without any delay or inaccuracies. This allows for accurate steering control, enhancing the safety and maneuverability of the vehicle.

4. Durability and reliability: Ball bearing type screws are known for their durability and reliability. They are designed to withstand repeated use and high loads without compromising their performance. In the steering mechanism, where continuous steering inputs are required, the use of ball bearing type screws ensures long-lasting and reliable operation.

In conclusion, ball bearing type screws are commonly found in the steering mechanism of vehicles due to their smooth operation, high load capacity, precise motion control, durability, and reliability. These screws contribute to the overall performance and safety of the steering system, providing drivers with a responsive and accurate steering experience.

Trapezoidal thread

A trapezoidal thread is a type of thread commonly used in mechanical engineering applications. It is characterized by its trapezoidal shape, with both the thread flanks and the top being inclined at a certain angle.

Characteristics of Trapezoidal Threads

- Trapezoidal threads have a relatively large thread angle, typically 30 degrees. This angle allows for a larger contact area between the mating threads, resulting in increased load-carrying capacity and improved resistance to wear.

- The flanks of trapezoidal threads are inclined at the thread angle, which provides self-locking characteristics. This means that the thread can resist axial forces and prevent unintended loosening or tightening of the threaded components.

- Trapezoidal threads have a flat top, which allows for efficient transfer of torque between the threaded components. This makes them suitable for applications that require high torque transmission, such as power transmission systems.

Comparison with Other Thread Types

- Acme threads: Acme threads are similar to trapezoidal threads in terms of their trapezoidal shape and self-locking characteristics. However, the thread angle of Acme threads is typically 29 degrees instead of 30 degrees for trapezoidal threads. Both thread types are commonly used in power transmission and linear motion systems.

- Square threads: Square threads have a square-shaped profile, with both the flanks and the top being perpendicular to the thread axis. Unlike trapezoidal threads, square threads do not have self-locking characteristics. They are commonly used in applications that require high efficiency in power transmission, such as lead screws.

- Buttress threads: Buttress threads have an asymmetrical shape, with one flank being inclined at a large angle and the other flank being perpendicular to the thread axis. They are primarily used in applications that require high load-carrying capacity in one direction, such as jack screws and vise mechanisms.

Conclusion

In conclusion, trapezoidal threads are a type of thread commonly used in mechanical engineering applications. They are characterized by their trapezoidal shape, self-locking characteristics, and efficient torque transmission. While other thread types such as Acme, square, and buttress threads also have their own advantages and applications, the correct answer to the given question is that trapezoidal threads are specifically associated with the trapezoidal shape.