Machine Design — Part 1 MCQs with Explanations

A cotter joint is a type of mechanical connection used to connect two rods together. 1. Co-axial Rods: The cotter joint is specifically designed to connect co-axial rods , meaning the rods are aligned along the same axis. This ensures that the rods remain in alignment and can transmit motion or force effectively along the same line. 2. Perpendicular Rods: If the rods were perpendicular to each other, a different type of joint would be required, such as a pin joint or a universal joint. The cotter joint is not suitable for perpendicular rods. 3. Parallel Rods: Parallel rods may be connected using other types of joints, but not a cotter joint , as this joint is meant for rods that are co-axial. Therefore, the correct answer is Option A: Co-axial .

Definition: A transverse fillet weld is a weld where the axis of the weld is placed perpendicular to the direction of the applied load. Load action: In a transverse fillet welded joint, the load is applied across the throat of the weld, which results in shear stresses being developed in the weld material. Design basis: Since the primary mode of failure for this type of weld is due to shear across the throat area, the joint is designed considering the shear strength of the weld metal. Reason: Designing for shear strength ensures that the weld can safely resist the applied load without shear failure, thus maintaining the integrity of the joint. Conclusion: Transverse fillet welded joints are always designed based on their shear strength .

Concept: In a journal bearing, the shaft is supported on a thin film of lubricant which separates the metal surfaces during rotation. Slow speed condition: At slow speeds, the hydrodynamic pressure in the lubricant film is insufficient to fully separate the shaft from the bearing surface. Effect of rotation direction: When the shaft rotates in an anticlockwise direction at slow speed, the load causes the shaft to shift within the bearing clearance. Due to the rotation and load combination, the shaft moves towards the left side of the bearing. Metal-to-metal contact: In this condition, the lubricant film cannot maintain full separation, resulting in metal to metal contact between the shaft and the bearing surface on the left side. Conclusion: At slow speed in anticlockwise rotation, the shaft moves towards the left of the bearing, causing direct metal-to-metal contact.

Buttress threads are best suited when power is transmitted in one direction only. Reason: These threads have one face nearly perpendicular to the axis and the other face at an angle. This gives them high strength to transmit force efficiently in one direction, while reducing friction losses. Other options: - Knuckle threads are mainly used in couplings and linkages subjected to heavy jerks and vibrations, not for power transmission. - Square threads are used for power transmission where efficiency is needed in both directions, such as in screw jacks and presses. - Acme threads are easier to manufacture than square threads and are widely used in lead screws of lathes, where motion is required in both directions. Conclusion: Therefore, buttress threads are most suitable when the load or power acts in one direction only.

The torsional shear stress in a circular shaft varies linearly from the centre to the outer surface of the shaft. According to the torsion equation: τ = (T × r) / J Where: τ = Torsional shear stress T = Applied torque r = Radial distance from the centre J = Polar moment of inertia Since the stress is directly proportional to the radial distance ( τ ∝ r ): • At the centroidal axis (r = 0): Shear stress is zero. • At the outer surface (r = R): Shear stress is maximum. Therefore, the correct statement is that the torsional shear stress is zero at the centroidal axis and maximum at the outer surface of the member .

Feather key is a special type of key used when one component (like a gear, pulley, or coupling) is required to slide along the shaft while still transmitting torque. Reason: To allow axial movement, the key is made loose in the shaft so it can slide along with the component, but it is tight in the hub to ensure a firm torque-transmitting connection. Other options: - Tight in shaft and loose in hub would prevent sliding, defeating the purpose of a feather key. - Tight in both shaft and hub would act like a sunk key and not allow axial movement. - Loose in both shaft and hub would result in no proper torque transmission. Conclusion: Therefore, a feather key is generally loose in the shaft and tight in the hub .

For pipes carrying water at low pressures, the most commonly used pipe joint is the Socket joint . A Socket joint is simple to make and provides a tight connection suitable for low-pressure applications. - Option A: Socket joint is widely used in plumbing for water supply lines at low pressure. - Option B: Nipple joint is typically a short piece of pipe used to connect two fittings but is not a joint type by itself. - Option C: Union joint is used where frequent dismantling is required, more suitable for maintenance purposes. - Option D: Spigot and socket joint is similar to the socket joint but generally used for higher pressure or different pipe types. Hence, the correct answer is Option A: Socket joint .

Closed-end pressure vessels experience additional forces due to internal fluid pressure acting on the end caps. Circumferential (hoop) stress acts around the circumference and tends to increase the diameter of the vessel. This is always present in thin-walled cylinders. Longitudinal stress arises only when the vessel has closed ends, because the fluid pressure pushes on the end surfaces. This force tries to pull the ends apart, creating stress along the length of the vessel. Shear stress does not play a significant role in such cases. Conclusion: In pressure vessels with closed ends, the internal fluid pressure induces a longitudinal stress in addition to hoop stress, and the correct answer here is longitudinal stress .

The centrifugal tension in belts increases the belt tension without increasing power transmission . Key Points: Centrifugal Tension: This tension is caused by the rotation of the belt and acts to stretch it. Effect: While it increases the overall belt tension, it does not contribute to the power transmitted between the driving and driven pulleys. Impact on Power Transmission: The centrifugal tension can reduce the effective tension available for power transmission because it adds to the total belt tension, which can affect the belt’s grip and efficiency.

The correct answer is (C) Spindles of bench vices . Let's break down why: Buttress threads are special types of threads designed to handle heavy loads in one direction . Think of them like a ramp on one side and a vertical wall on the other. * Option A: Screw cutting lathes: Lathes use various threads, but not typically buttress threads for their main operation. * Option B: Feed mechanisms: While feed mechanisms require movement, they don't usually need the one-directional, high-load capacity of buttress threads. * Option C: Spindles of bench vices: Bench vices are all about applying a strong force in one direction to hold an object securely. The buttress thread is perfect for this! When you tighten a vice, you want a strong, secure grip *without* the vice easily loosening. The ramp allows for easy tightening, and the vertical face prevents slipping under load. This is the most common and ideal application for buttress threads. * Option D: Railway carriage couplings: Railway couplings require a strong connection, but other thread types (like ACME threads) are generally preferred due to their ease of manufacture and suitability for bidirectional forces.

The correct answer is (A) Spindles of bench vices, (C) Feed mechanism of machine tools, and (D) Screw cutting lathes. Here's why: * Square Threads: Square threads are known for their high efficiency and ability to transmit power with minimal friction. This makes them ideal for applications requiring significant force or precise movement. * Option A: Spindles of bench vices: Bench vices need to apply a large clamping force . Square threads on the spindle allow for efficient force transmission when tightening the vice. * Option B: Railway carriage couplings: Railway carriage couplings typically use knuckle threads or other specialized designs for quick and secure connections , not square threads. * Option C: Feed mechanism of machine tools: Machine tools require precise and controlled movement for cutting operations. Square threads are well-suited for the feed mechanisms because they provide smooth, efficient, and accurate linear motion. * Option D: Screw cutting lathes: Screw cutting lathes use square threads in lead screws for achieving axial motion of cutting tools. Therefore, square threads are commonly found in spindles of bench vices, feed mechanisms of machine tools, and screw cutting lathes due to their efficiency and ability to handle high loads.

When we compare rolling contact bearings (ball/roller bearings) with sliding contact bearings (plain bearings), the biggest difference is in how they handle friction. Rolling bearings use balls or rollers. This means the parts roll over each other, which makes it very easy to start moving (low starting friction) and also keeps the friction low while running (low running friction). In contrast, sliding bearings rely on surfaces sliding against each other, which causes higher friction. However, there is one exception – at very high speeds . Rolling bearings face issues like centrifugal force and lubrication problems, which increase friction. In some cases, they may even perform worse than sliding bearings at those speeds. Why not other options? B (Accuracy of shaft alignment): Both types of bearings need proper alignment. This is not a unique advantage of rolling bearings. C (Small overall dimensions): Rolling bearings are not always smaller. Sometimes they can be larger depending on load and design. So, D (All of the above) is not correct because only Option A is the clear, consistent advantage. Therefore, from an exam/student point of view, the best answer is Option A.

Nitriding is a surface-hardening process that diffuses nitrogen into the surface, creating a hard, wear-resistant case with compressive residual stresses. Fatigue crack initiation: Due to the compressive stresses at the surface, fatigue cracks are generally suppressed at the outer layer. Instead, they tend to originate just below the surface , where the protective effect of compressive stress is lower. Why not surface: Although surface imperfections can sometimes cause cracks in practice, the theoretical and exam-oriented answer is that cracks start just beneath the hardened case . Conclusion: From an exam perspective, the correct answer is Option B: Just below the surface .

Wire ropes are made of high-strength steel wires twisted together, giving them superior strength and toughness compared to cotton ropes. They are capable of withstanding heavy and sudden shock loads without failing, which makes them suitable for applications like cranes, lifts, and mining operations. Option A: Incorrect, because wire ropes are heavier , not lighter, than cotton ropes. Option B: Incorrect, because wire ropes do not provide silent operation ; cotton ropes are quieter in working. Option D: Incorrect, because not all listed features apply to wire ropes. Thus, the correct statement is that wire ropes can withstand shock loads .

The backlash for spur gears depends upon both module and pitch line velocity. Backlash refers to the clearance or gap between mating gear teeth. It is influenced by the module, which is the size of the gear teeth, and the pitch line velocity, which is the speed at which the gear teeth engage and disengage. Therefore, both module and pitch line velocity play a role in determining the backlash in spur gears.

When comparing the deflection of two springs with different mean diameters, we use the formula: $$\left( \frac{\text{Deflection of Spring B}}{\text{Deflection of Spring A}} \right) = \left( \frac{\text{Mean Diameter of Spring A}}{\text{Mean Diameter of Spring B}} \right)^3$$ Given that spring 'A' has a mean diameter of 50 mm and spring 'B' has a mean diameter of 25 mm: $$\left( \frac{50}{25} \right)^3 = 2^3 = 8$$ This implies that the deflection of spring 'B' is one-eighth $$ \frac{1}{8} $$ of the deflection of spring 'A'. Hence, the correct answer is: Option A: One-eighth

When the speed of the belt increases The power transmitted by a belt drive is directly proportional to the product of the belt tension and the belt speed Power (P) = (T₁ - T₂) × v , where T₁ is the tension in the tight side, T₂ is the tension in the slack side, and v is the belt speed As the belt speed increases, assuming the tensions remain constant, the power transmitted also increases However, there's a practical limit to this, because at very high speeds, centrifugal force increases and reduces the effective friction and tension difference But initially, with an increase in speed, the power transmitted increases

A bench vice is a mechanical device used for holding and clamping workpieces securely in place during various machining operations. The vice is equipped with a threaded screw mechanism that allows the jaws of the vice to move closer together or farther apart. The type of threads used in the bench vice's screw mechanism determines its efficiency and load-bearing capacity. Among the options provided, the correct answer is Option C: Buttress . A bench vice often employs a buttress thread mechanism. Buttress threads have one side with a steep angle (known as the "serrated" side) and another side with a flat or slightly angled surface. This design provides efficient load-bearing capability in one direction while allowing quick return travel in the opposite direction. Buttress threads are particularly suitable for applications where high axial loads are involved. Therefore, the correct answer is Option C: Buttress .

Knuckle threads are used in railway carriage couplings because they are designed to withstand heavy loads and shocks. Reason: Railway couplings are subjected to sudden jerks and vibrations during starting, stopping, and motion of trains. Knuckle threads have a rounded profile which reduces stress concentration and makes them suitable for such conditions. Other options: - Square threads are used in screw jacks and presses for power transmission where efficiency is required. - Acme threads are used in lead screws of lathes due to their ease of manufacturing and strength. - Buttress threads are used where force acts only in one direction, like in screw presses and vices. Conclusion: Thus, knuckle threads are most suitable for railway carriage couplings.

Thrust bearings are designed to carry axial loads . An axial load is a force that acts parallel to the axis of rotation of the shaft. Think of a shaft rotating in place: the thrust bearing prevents it from moving up or down along its length. This kind of movement is resisted by handling forces that act parallel to the axis . Option A: “Along the axis” is close in meaning, but the more precise description is parallel to the axis , since the force does not need to be exactly concentrated on the axis itself. Option C: Incorrect, because loads perpendicular to the axis are radial loads , handled by radial bearings, not thrust bearings. Option D: Incorrect, because thrust bearings are specifically for axial loads , not loads in every direction. Thus, in thrust bearings, the load acts parallel to the axis of rotation .

In machine design, keys are used to connect rotating machine elements such as shafts and hubs to transmit torque. A sunk key is a type of key that is rectangular or square in shape and fits completely within the keyway of both the shaft and the hub. It is "sunk" or fully inserted into the keyway, providing a secure connection between the two components. Among the options provided, the correct answer is Option A: sunk key . This type of key is commonly used to prevent relative motion between the shaft and the hub while transmitting torque. Therefore, the correct answer is Option A: sunk key .

Inversely as cube of load is the correct answer to “The rated life of a bearing varies”. This question is standard in competitive examination syllabi for Machine Design.

Zero at the point other than centroidal axis is the correct answer to “The bending stress in a curved beam is”. This question is standard in competitive examination syllabi for Machine Design.

Pitch is the correct answer to “The centre to centre distance between two consecutive rivets in a row, is called”. This question is standard in competitive examination syllabi for Machine Design.

$${\text{d}} = 6\sqrt {\text{t}} $$ is the correct answer to “According to Unwin's formula, the relation between the diameter of rivet hole (d) and the thickness of plate (t) is given by (where d and t are in mm)”. This question is standard in competitive examination syllabi for Machine Design.

Thin film is the correct answer to “In a boundary lubricated bearing, there is a __________ of lubricant between the journal and the bearing.”. This question is standard in competitive examination syllabi for Machine Design.

Fullering and caulking is the correct answer to “The edges of the plates for cylindrical vessels are usually bevelled to an angle of 80° for”. This question is standard in competitive examination syllabi for Machine Design.

Shear stress is the correct answer to “In a flange coupling, the bolts are subjected to”. This question is standard in competitive examination syllabi for Machine Design.

4430 mm as nominal pitch length is the correct answer to “A V-belt designated as B 4430 LP has”. This question is standard in competitive examination syllabi for Machine Design.

15 times is the correct answer to “When the bearing is subjected to large fluctuations of load and heavy impacts, the bearing characteristic number should be _________ the bearing modulus.”. This question is standard in competitive examination syllabi for Machine Design.

Move towards right of the bearing making no metal to metal contact is the correct answer to “When the shaft rotates in anticlockwise direction at high speed in a bearing, it will”. This question is standard in competitive examination syllabi for Machine Design.

All the above objectives is the correct answer to “The objective of idler pulley in belt drive is to”. This question is standard in competitive examination syllabi for Machine Design.

Prevent the belt from running off the pulley is the correct answer to “The objective of 'crowning' of the flat pulleys of belt drive is to”. This question is standard in competitive examination syllabi for Machine Design.

$$\frac{{{{\text{T}}_1} + {{\text{T}}_2} + {{\text{T}}_{\text{C}}}}}{2}$$ is the correct answer to “If T 1 and T 2 are the tensions on the tight and slack sides of the belt respectively and T C is the centrifugal tension, then initial tension in the belt is equal to”. This question is standard in competitive examination syllabi for Machine Design.

Base circle is the correct answer to “The size of a cam depends upon”. This question is standard in competitive examination syllabi for Machine Design.

Larger is the correct answer to “The shearing stresses in the inner face as compared to outer face of the wire in a heavy close coiled spring is”. This question is standard in competitive examination syllabi for Machine Design.

Top side of belt as slack side is the correct answer to “In a horizontal flat belt drive, it is customary to use”. This question is standard in competitive examination syllabi for Machine Design.

All of these is the correct answer to “The coefficient of friction between the belt and pulley depends upon the”. This question is standard in competitive examination syllabi for Machine Design.

Bending moment only is the correct answer to “Which one of the following loading is considered for the design of axles”. This question is standard in competitive examination syllabi for Machine Design.

$${\left( {\frac{{6{\text{V}}}}{\pi }} \right)^{\frac{1}{3}}}$$ is the correct answer to “A thin spherical shell of internal diameter d is subjected to an internal pressure p. If V is the storage capacity of shell, then its diameter will be”. This question is standard in competitive examination syllabi for Machine Design.