Machine design questions

A list of though provoking questions generally asked related to machine design in engineering interviews

Here are some Intriguing questions about machine design which test the understanding of basics. Not only knowing facts but conceptual understanding.

How can the yield strength of a material be determined from the stress-strain curve, and why is it important in machine design?

Yield strength is the point on the stress-strain curve where a material transitions from elastic deformation to plastic deformation, meaning it will no longer return to its original shape after the load is removed. It is determined by identifying the stress level at the end of the linear (elastic) region. In some materials, where the yield point is not well defined, an offset method (usually 0.2%) is used to estimate yield strength. Yield strength is crucial in machine design because it defines the maximum stress a material can endure without undergoing permanent deformation, ensuring safety and functionality under load.

What is the significance of fatigue failure in machine design, and how is it typically accounted for in mechanical components?

Fatigue failure occurs when a material fails after repeated loading and unloading, even though the applied stresses are below the material's ultimate tensile strength. It is significant because most machine components are subjected to cyclic loading, leading to failure over time. To account for fatigue, designers use S-N curves (Stress vs. Number of cycles) to estimate the endurance limit of materials. Components are designed with fatigue safety factors, and stress concentrations are minimized by avoiding sharp corners, using fillets, and ensuring smooth surface finishes.

Why is the factor of safety different for static and dynamic loads, and how is it selected for machine design?

The factor of safety (FoS) for dynamic loads is typically higher than for static loads because dynamic conditions introduce additional variables such as fatigue, impact, vibration, and fluctuating stresses, which can cause failure over time. The FoS is selected based on the severity of the working conditions, the consequences of failure, and the uncertainty in material properties. For static loading, a lower FoS (typically 1.5 to 2) is often used, while for dynamic loads, FoS can range from 3 to 5, depending on the nature of the application.

What is the role of the stress concentration factor (Kt) in machine design, and how can its effects be mitigated?

The stress concentration factor (Kt) quantifies how much stress is amplified at discontinuities like holes, notches, or sharp corners in a component. Its effect can lead to localized failure, even if the overall stress is within acceptable limits. To mitigate its effects, designers can add fillets, reduce sharp transitions, increase material thickness around high-stress areas, or redistribute the load by redesigning the part's geometry to smooth out abrupt changes.

What does the area under the stress-strain curve represent, and how is it useful in machine design?

The area under the stress-strain curve represents the toughness of the material, which is the total energy per unit volume that the material can absorb before fracture. Tough materials can absorb a lot of energy before failure, making them suitable for applications where impact resistance is critical. In machine design, toughness is important when selecting materials for components subject to dynamic or shock loads, ensuring they can withstand sudden impacts without catastrophic failure.

How does temperature variation affect the material properties in machine design, and what strategies are employed to account for it?

Temperature variations can significantly affect material properties, such as yield strength, ductility, and fatigue resistance. Metals generally lose strength at high temperatures and become brittle at low temperatures. To account for this, materials with suitable thermal properties are selected, often including alloys designed for specific temperature ranges. Thermal expansion is another concern, and thermal stresses can be managed by using expansion joints, selecting materials with similar thermal expansion coefficients, or designing components to tolerate differential expansion.

What is the role of interference fit in machine design, and how does it differ from clearance and transition fits?

Interference fit is used in machine design to ensure that two components are tightly joined, often without the need for additional fastening methods. In interference fits, the shaft or part being inserted is slightly larger than the hole, causing a tight press-fit when assembled. This differs from clearance fits, where there is intentional space between the parts to allow for easy assembly, and transition fits, where the fit can either be tight or loose depending on the tolerance. Interference fits are used in applications where high strength or precision is required, such as in the assembly of gears onto shafts or in bearing housings. Proper control of machining tolerances and surface finishes is crucial to avoid overstressing or damaging the parts during assembly.

When designing a gearbox, under what conditions would you choose helical gears over spur gears, and vice versa?

Helical gears are typically chosen over spur gears when smoothness and quietness of operation are critical, as helical gears engage gradually and distribute load more evenly. They are also preferred for handling higher loads at high speeds. However, spur gears might be chosen in cases where simplicity, lower cost, and ease of manufacturing are prioritized, particularly in low-speed applications where noise and smoothness are not as important. Spur gears also avoid the axial thrust loads generated by helical gears, simplifying bearing design.

How would you decide between using a solid shaft versus a hollow shaft for a specific mechanical application?

The decision between a solid and hollow shaft depends on factors such as weight, strength, and rigidity requirements. Hollow shafts can offer the same torsional strength as solid shafts while being lighter, making them ideal for applications where weight reduction is important, such as in automotive and aerospace industries. However, solid shafts are simpler to manufacture and might be more economical for smaller, lower-weight applications. Hollow shafts also tend to perform better in resisting torsional buckling, while solid shafts might be preferred for applications requiring high bending strength.

In the design of a structural frame, what factors would lead you to select an I-beam over a rectangular hollow section (RHS)?

The decision between an I-beam and an RHS depends on load characteristics and structural performance requirements. I-beams are generally chosen for applications requiring high bending strength in one direction, such as in building construction, as their flanges provide excellent resistance to bending. RHS, on the other hand, offers strength in multiple directions and is preferred in applications where torsional strength and stiffness are important, such as in space frames or where loads act in multiple planes. Aesthetic and fabrication requirements might also influence the choice, as RHS sections are cleaner and simpler to connect.

What considerations lead to the selection of hydraulic over pneumatic systems in a mechanical design?

Hydraulic systems are chosen when high force and precision are required, particularly for heavy-duty applications such as lifting, pressing, or driving heavy machinery. They offer smooth control and can handle higher loads compared to pneumatic systems, which are more suited for lighter loads and faster operations. Pneumatics, on the other hand, are favored in applications where speed, simplicity, and lower cost are priorities, especially in automated systems. The working environment is also a factor; hydraulics are better suited for environments requiring water-tight systems, while pneumatics work well in clean, dry environments with less stringent pressure and precision needs.

Categories: : Mechanism and Machine design