I85PREP
⌘ K
BEM PCE Part B Mechanical study materials
BEM PCE Part B Mechanical study resources

BEM PCE Part B Mechanical practice questions

One question at a time. Build your BEM PCE Part B Mechanical understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Mechanical Engineering (Part B domain structure not enumerated in source)

In a bolted joint, which quantity is used to size the bolt for fatigue when the joint is preloaded and the external load fluctuates?

  1. The proof load of the bolt alone
  2. The alternating stress in the bolt based on the portion of external load carried by the bolt Correct answer.
  3. The full external load divided by the tensile stress area
  4. The clamping force multiplied by the joint stiffness ratio

Explanation

In a preloaded joint the external load is shared between bolt and clamped members according to their relative stiffness, so only a fraction of the fluctuating load reaches the bolt. Fatigue assessment therefore uses the alternating bolt stress derived from that fraction, combined with the preload mean stress. Using the full external load would grossly overestimate the stress range.

Question 2Mechanical Engineering (Part B domain structure not enumerated in source)

A shaft transmits 15 kW at 1500 rev/min. What is the approximate torque carried by the shaft?

  1. 225 N·m
  2. 0.1 N·m
  3. 95.5 N·m Correct answer.
  4. 10 N·m

Explanation

Power equals torque times angular velocity, so torque is P divided by omega. With omega equal to 2π × 1500/60 ≈ 157 rad/s, torque is 15000/157 ≈ 95.5 N·m. The result scales inversely with speed, which is why high-speed shafts carry modest torque for a given power.

Question 3Mechanical Engineering (Part B domain structure not enumerated in source)

Which statement correctly describes the function of a relief valve in a hydraulic circuit?

  1. It filters particulate contamination from the return line
  2. It prevents reverse flow when the pump stops
  3. It limits maximum system pressure by diverting flow to tank when the set pressure is reached Correct answer.
  4. It maintains a constant flow rate to the actuator regardless of load

Explanation

A relief valve is a pressure-limiting device: when pressure exceeds the spring setting it opens and bypasses flow to the reservoir, protecting components. It does not regulate flow or direction. Confusing it with a flow control, check, or return filter valve is a common error in circuit interpretation.

Question 4Mechanical Engineering (Part B domain structure not enumerated in source)

A simply supported beam of span 4 m carries a central point load of 20 kN. What is the maximum bending moment?

  1. 10 kN·m
  2. 80 kN·m
  3. 40 kN·m
  4. 20 kN·m Correct answer.

Explanation

For a central point load on a simply supported span, the maximum moment is P·L/4. Substituting 20 kN and 4 m gives 20 kN·m at midspan. The reactions are each 10 kN, so the moment at midspan is 10 kN × 2 m.

Question 5Mechanical Engineering (Part B domain structure not enumerated in source)

In a four-stroke diesel engine, what is the purpose of the turbocharger?

  1. To reduce the compression ratio at high load
  2. To cool the exhaust gases before they enter the manifold
  3. To drive the camshaft at half crankshaft speed
  4. To increase the mass of air inducted so that more fuel can be burned in the same displacement Correct answer.

Explanation

A turbocharger uses exhaust energy to compress intake air, raising its density so a greater mass of air enters the cylinder. More air permits more fuel and therefore higher power from the same swept volume. Compression ratio and camshaft drive are separate mechanical functions.

Question 6Mechanical Engineering (Part B domain structure not enumerated in source)

Which material property is most directly relevant when selecting a material for a component that must resist the initiation of a fatigue crack under many cycles of low stress?

  1. Coefficient of thermal expansion
  2. Endurance limit Correct answer.
  3. Poisson's ratio
  4. Modulus of rigidity

Explanation

The endurance limit is the stress amplitude below which a material can sustain an effectively infinite number of cycles without fatigue failure. It directly governs whether low-stress cyclic loading will initiate a crack. The other properties relate to stiffness, thermal strain, and lateral contraction.

Question 7Mechanical Engineering (Part B domain structure not enumerated in source)

A gear pair has a pinion with 20 teeth and a wheel with 60 teeth. If the pinion rotates at 1200 rev/min, what is the wheel speed?

  1. 600 rev/min
  2. 400 rev/min Correct answer.
  3. 2400 rev/min
  4. 3600 rev/min

Explanation

The speed ratio is the inverse of the tooth ratio, so the wheel turns at 1200 × 20/60 = 400 rev/min. Torque increases in the same proportion that speed decreases. The larger wheel always rotates more slowly than the smaller pinion.

Question 8Mechanical Engineering (Part B domain structure not enumerated in source)

Which of the following best defines the term 'factor of safety' as used in mechanical design?

  1. The ratio of working stress to yield stress
  2. The percentage elongation at fracture
  3. The ratio of a material's limiting strength to the maximum stress expected in service Correct answer.
  4. The margin between operating and maximum allowable temperature

Explanation

Factor of safety compares the strength or load at which failure occurs with the stress or load actually expected. A value above one indicates reserve capacity, and its size reflects uncertainty in loads, material data, and consequences of failure. It is a strength-to-stress ratio, not a stress ratio or a ductility measure.

Question 9Mechanical Engineering (Part B domain structure not enumerated in source)

A steel bar of cross-sectional area 200 mm² is subjected to a tensile load of 50 kN. What is the direct tensile stress?

  1. 250 MPa Correct answer.
  2. 100 MPa
  3. 25 MPa
  4. 2.5 MPa

Explanation

Direct stress equals load divided by area. Converting 50 kN to 50,000 N and 200 mm² to 200 mm² gives 250 N/mm², which is 250 MPa. Unit consistency between newtons and square millimetres is essential to obtain megapascals directly.

Question 10Mechanical Engineering (Part B domain structure not enumerated in source)

In a journal bearing operating under hydrodynamic lubrication, what separates the shaft from the bearing surface?

  1. Rolling elements between the surfaces
  2. A pressurized film of lubricant generated by the rotation of the journal Correct answer.
  3. A layer of solid lubricant such as graphite
  4. A magnetic field suspending the shaft

Explanation

Hydrodynamic lubrication relies on the wedge-shaped converging film created as the journal rotates, which generates pressure that supports the load. Full separation of surfaces by the fluid film prevents metal-to-metal contact. Solid lubricants, rolling elements, and magnetic suspension describe other bearing types.

Question 11Mechanical Engineering (Part B domain structure not enumerated in source)

A flat belt drive transmits power between two pulleys. Which condition must be satisfied to avoid belt slip?

  1. The coefficient of friction must be less than the wrap angle in radians
  2. The slack-side tension must be zero at all times
  3. The belt speed must exceed the pulley surface speed by a fixed margin
  4. The ratio of tight-side to slack-side tension must not exceed the value given by the belt friction equation Correct answer.

Explanation

The belt friction relationship sets a maximum tension ratio equal to e raised to the product of friction coefficient and wrap angle. If the required ratio exceeds this limit, the belt slips. Maintaining a nonzero slack tension and adequate wrap angle helps keep operation within the limit.

Question 12Mechanical Engineering (Part B domain structure not enumerated in source)

In a vapour-compression refrigeration cycle, what is the effect of subcooling the liquid refrigerant leaving the condenser?

  1. It increases the refrigerating effect per unit mass by reducing the enthalpy at the expansion valve inlet Correct answer.
  2. It increases the condenser pressure for a fixed ambient
  3. It raises the compressor discharge temperature above the saturation value
  4. It reduces the compressor work to zero

Explanation

Subcooling lowers the enthalpy of the liquid entering the expansion device, so more heat can be absorbed in the evaporator for the same mass flow. Compressor work is essentially unchanged, so the coefficient of performance improves. It does not change compressor discharge to zero work or fix condenser pressure.

Question 13Mechanical Engineering (Part B domain structure not enumerated in source)

A pressure vessel has an internal diameter of 1 m and a wall thickness of 10 mm. Using the thin-cylinder formula, what is the hoop stress at an internal pressure of 2 MPa?

  1. 100 MPa Correct answer.
  2. 50 MPa
  3. 20 MPa
  4. 200 MPa

Explanation

The thin-cylinder hoop stress is p·d/(2t). Substituting 2 MPa, 1000 mm, and 10 mm gives 100 MPa. The longitudinal stress would be half this value. The thin-wall assumption is reasonable here because the diameter-to-thickness ratio is 100.

Question 14Mechanical Engineering (Part B domain structure not enumerated in source)

Which failure mode is characteristic of a shaft subjected to reversed bending combined with steady torsion?

  1. Creep rupture at room temperature
  2. Brinelling of the shaft surface
  3. Fatigue crack initiation at a stress concentration such as a keyway or shoulder fillet Correct answer.
  4. Hydrogen embrittlement of the shaft core

Explanation

Reversed bending produces a fully alternating stress, and geometric discontinuities concentrate it, making keyways and fillets preferential fatigue initiation sites. Steady torsion adds a mean shear stress that further reduces life. Creep, brinelling, and hydrogen embrittlement arise from other mechanisms.

Question 15Mechanical Engineering (Part B domain structure not enumerated in source)

A helical compression spring has a mean coil diameter of 40 mm and a wire diameter of 5 mm. What is the spring index?

  1. 200
  2. 8 Correct answer.
  3. 5
  4. 45

Explanation

Spring index is the ratio of mean coil diameter to wire diameter, here 40/5 = 8. This index affects stress concentration and manufacturability; values between about 4 and 12 are typical. The other numbers correspond to sums or products of the dimensions.

Question 16Mechanical Engineering (Part B domain structure not enumerated in source)

In a welded joint, why is a fillet weld in a lap joint normally designed on the throat thickness rather than the leg length?

  1. The throat is the minimum cross-section through the weld and therefore carries the highest stress Correct answer.
  2. Fillet welds fail only in the base metal
  3. The leg length is always larger than the plate thickness
  4. The throat thickness includes the weld reinforcement

Explanation

Load transferred across a fillet weld passes through the throat, which is the shortest distance across the weld cross-section. Stress is therefore highest at the throat, and design strength is based on throat area. Leg length is a fabrication dimension, not the critical load path.

Question 17Mechanical Engineering (Part B domain structure not enumerated in source)

A closed-loop control system has a proportional controller with high gain. What is the most likely consequence?

  1. Elimination of steady-state error with no effect on stability
  2. Reduced steady-state error but increased tendency toward oscillation Correct answer.
  3. Complete loss of controllability
  4. Slower response and increased damping

Explanation

Increasing proportional gain reduces steady-state error for a given disturbance but moves the closed-loop poles toward the imaginary axis, reducing damping. The result is a faster but more oscillatory response. Proportional action alone cannot eliminate steady-state error for a step in a type-zero plant.

Question 18Mechanical Engineering (Part B domain structure not enumerated in source)

A pipe of internal diameter 0.1 m carries water at a mean velocity of 2 m/s. What is the approximate volumetric flow rate?

  1. 0.0314 m³/s
  2. 0.0157 m³/s Correct answer.
  3. 0.00785 m³/s
  4. 0.2 m³/s

Explanation

Flow rate equals velocity times cross-sectional area. The area is π × 0.05² ≈ 0.00785 m², so the flow is about 0.0157 m³/s. Using the diameter instead of the radius in the area calculation is the usual source of error.

Question 19Mechanical Engineering (Part B domain structure not enumerated in source)

Which heat treatment process is used to relieve internal stresses in a welded steel fabrication without significantly changing its microstructure?

  1. Case carburizing at 900 °C
  2. Austenitizing followed by rapid cooling
  3. Stress-relief annealing at a temperature below the transformation range Correct answer.
  4. Quenching in oil from the austenitizing temperature

Explanation

Stress-relief annealing holds the component at a temperature below the lower critical temperature long enough for creep-based relaxation of residual stresses, then cools slowly. Because the temperature stays below the transformation range, the microstructure is largely unchanged. Quenching and carburizing deliberately alter structure or surface chemistry.

Question 20Mechanical Engineering (Part B domain structure not enumerated in source)

A cantilever beam of length L carries a uniformly distributed load w per unit length. What is the maximum deflection at the free end?

  1. wL⁴/(8EI) Correct answer.
  2. wL³/(3EI)
  3. wL⁴/(3EI)
  4. 5wL⁴/(384EI)

Explanation

For a cantilever with a uniformly distributed load, the free-end deflection is wL⁴/(8EI). The fourth power of length and the flexural rigidity EI both appear, reflecting bending behaviour. The other expressions belong to different load or support cases.

Question 21Mechanical Engineering (Part B domain structure not enumerated in source)

In a mechanical assembly, what does the term 'interference fit' describe?

  1. A fit with a small clearance that permits free rotation
  2. A fit used only for plastic components
  3. A fit in which the shaft is larger than the hole before assembly, producing a residual contact pressure Correct answer.
  4. A fit where the shaft and hole are nominally identical with a sliding clearance

Explanation

An interference fit relies on the shaft being slightly larger than the bore so that assembly induces elastic deformation and contact pressure. This pressure transmits torque by friction and resists relative motion. Clearance and transition fits allow relative movement or a small gap.

Question 22Mechanical Engineering (Part B domain structure not enumerated in source)

A pump delivers 0.01 m³/s against a total head of 30 m. Assuming water density of 1000 kg/m³ and gravitational acceleration of 9.81 m/s², what is the hydraulic power delivered to the fluid?

  1. 294 kW
  2. 29.4 kW
  3. 2.94 kW Correct answer.
  4. 0.294 kW

Explanation

Hydraulic power equals density × gravitational acceleration × flow rate × head. Multiplying 1000, 9.81, 0.01, and 30 gives about 2943 W, or 2.94 kW. This is the power imparted to the fluid, before accounting for pump efficiency.

Question 23Mechanical Engineering (Part B domain structure not enumerated in source)

A component is loaded such that the principal stresses are 120 MPa and 40 MPa in tension, with the third principal stress zero. Using the Tresca criterion with a yield strength of 200 MPa, what is the factor of safety?

  1. 1.67 Correct answer.
  2. 2.5
  3. 5.0
  4. 1.25

Explanation

Tresca uses the maximum shear stress, proportional to the difference between the largest and smallest principal stresses. Here that difference is 120 MPa, so the safety factor is 200/120 ≈ 1.67. Using the von Mises equivalent stress would give a different, slightly higher value.

Question 24Mechanical Engineering (Part B domain structure not enumerated in source)

Two spur gears mesh with a module of 3 mm. The pinion has 18 teeth. What is the standard centre distance if the gear has 42 teeth?

  1. 180 mm
  2. 54 mm
  3. 126 mm
  4. 90 mm Correct answer.

Explanation

The pitch circle diameter equals module times tooth number, giving 54 mm and 126 mm. The standard centre distance is half the sum of the pitch diameters, which is 90 mm. The module directly scales all gear dimensions.

Question 25Mechanical Engineering (Part B domain structure not enumerated in source)

A steel shaft is to be designed for combined bending and torsion using the maximum shear stress theory. Which equivalent moment is used to size the shaft?

  1. The larger of the bending moment and the torque
  2. The square root of the sum of the bending moment squared and the torque squared Correct answer.
  3. The product of the bending moment and the torque
  4. The arithmetic sum of the bending moment and the torque

Explanation

Under maximum shear stress theory, the equivalent moment is the square root of M² plus T², which produces the same maximum shear as the combined loading. The shaft diameter is then sized from this equivalent moment and the allowable stress. Simple sums or products do not represent combined stress states correctly.

Question 26Mechanical Engineering (Part B domain structure not enumerated in source)

In a bolted flange coupling, four M16 bolts on a 150 mm pitch circle transmit a torque of 300 N·m. Treating each bolt as carrying an equal share of the tangential load, what is the approximate tangential force per bolt?

  1. 2000 N
  2. 500 N
  3. 250 N
  4. 1000 N Correct answer.

Explanation

Total tangential force at the pitch circle equals torque divided by radius: 300 N·m / 0.075 m = 4000 N. Sharing this equally over four bolts gives 1000 N per bolt. The pitch-circle radius, not diameter, must be used in the torque relation.

Question 27Mechanical Engineering (Part B domain structure not enumerated in source)

A shaft transmits 15 kW at 600 rev/min. What is the approximate torque carried by the shaft?

  1. 9550 N·m
  2. 25 N·m
  3. 239 N·m Correct answer.
  4. 150 N·m

Explanation

Power equals torque times angular velocity, so torque is power divided by angular velocity. Angular velocity is 2π × 600/60 ≈ 62.8 rad/s, giving 15000 / 62.8 ≈ 239 N·m. The shortcut T ≈ 9550·P(kW)/n(rev/min) gives the same result.

Question 28Mechanical Engineering (Part B domain structure not enumerated in source)

Which lubrication regime is characterised by complete separation of the sliding surfaces by a fluid film whose thickness is much greater than the surface roughness?

  1. Solid-film lubrication
  2. Boundary lubrication
  3. Mixed lubrication
  4. Hydrodynamic lubrication Correct answer.

Explanation

Hydrodynamic lubrication occurs when the film is thick enough to keep asperities fully separated, so load is carried by fluid pressure. Boundary and mixed regimes involve partial asperity contact, and solid-film lubrication uses a dry coating rather than a fluid film.

Question 29Mechanical Engineering (Part B domain structure not enumerated in source)

A steel plate is joined to a support using a single transverse fillet weld of leg length 8 mm and effective length 60 mm. Taking the allowable shear stress on the throat as 100 MPa, what is the approximate load capacity?

  1. 17 kN
  2. 34 kN Correct answer.
  3. 48 kN
  4. 24 kN

Explanation

Throat thickness for a 90° fillet equals leg length times sin 45°, giving 8 × 0.707 ≈ 5.66 mm. Throat area is 5.66 × 60 ≈ 339 mm², so capacity is 339 × 100 N ≈ 34 kN. Designing on throat area reflects the critical shear plane through the weld.

Question 30Mechanical Engineering (Part B domain structure not enumerated in source)

Which bearing type is best suited to carry a heavy radial load combined with a substantial thrust load in one direction on a shaft that must run at high speed?

  1. Deep-groove ball bearing
  2. Taper roller bearing Correct answer.
  3. Needle roller bearing
  4. Cylindrical roller bearing

Explanation

Taper roller bearings have angled raceways that carry combined radial and axial loads and are stiff and high-capacity. Cylindrical and needle roller bearings carry radial load only, and a deep-groove ball bearing has limited thrust capacity relative to its size.

Question 31Mechanical Engineering (Part B domain structure not enumerated in source)

In a four-stroke diesel engine, what is the purpose of valve overlap around top dead centre?

  1. To reduce the peak cylinder pressure
  2. To increase the compression ratio
  3. To use inlet and exhaust gas inertia to improve scavenging and filling Correct answer.
  4. To cool the exhaust valve seat

Explanation

Valve overlap lets the incoming charge and leaving exhaust gases use their momentum to sweep residual gas from the clearance volume. This improves volumetric efficiency and scavenging. It does not change the compression ratio or directly cool the valve seat.

Question 32Mechanical Engineering (Part B domain structure not enumerated in source)

Which quantity in a fatigue analysis is defined as the stress amplitude below which a steel component can endure an essentially infinite number of cycles without failure?

  1. Ultimate tensile strength
  2. Endurance limit Correct answer.
  3. Yield strength
  4. Proof stress

Explanation

The endurance limit is the constant stress amplitude at which the S-N curve becomes horizontal for steels, corresponding to infinite life. Yield and ultimate strength describe monotonic loading, and proof stress is a 0.2% offset yield measure.

Question 33Mechanical Engineering (Part B domain structure not enumerated in source)

A stepped shaft has a fillet radius that is increased while the step diameter ratio is kept constant. What is the effect on the stress concentration factor under bending?

  1. It becomes zero because the step is eliminated
  2. It stays constant because the diameter ratio is unchanged
  3. It decreases because the stress gradient is reduced Correct answer.
  4. It increases because the fillet adds material

Explanation

A larger fillet radius spreads the load path over a greater length, reducing the local stress gradient and hence the stress concentration factor. The diameter ratio influences the value but does not fix it when the radius changes.

Question 34Mechanical Engineering (Part B domain structure not enumerated in source)

Which statement correctly describes the function of a relief valve in a hydraulic system?

  1. It limits maximum system pressure by diverting flow to the reservoir Correct answer.
  2. It filters contaminants from the hydraulic fluid
  3. It prevents reverse flow when the pump stops
  4. It regulates flow to the actuator to control speed

Explanation

A relief valve opens when pressure exceeds a set value and bypasses flow to the tank, protecting the circuit from overpressure. Flow control to an actuator is done by a flow-control valve, and reverse flow is prevented by a check valve.

Question 35Mechanical Engineering (Part B domain structure not enumerated in source)

A 3-phase induction motor rated at 1470 rev/min with a synchronous speed of 1500 rev/min drives a load. What is the percentage slip?

  1. 1.0%
  2. 5.0%
  3. 2.0% Correct answer.
  4. 3.0%

Explanation

Slip is the difference between synchronous and rotor speed divided by synchronous speed. Here (1500 − 1470)/1500 = 0.02, or 2%. This small slip is typical of a lightly loaded induction motor.

Question 36Mechanical Engineering (Part B domain structure not enumerated in source)

Which statement best explains why a cast iron component is often preferred over steel for a machine-tool bed subject to vibration?

  1. Cast iron has higher tensile strength than steel
  2. Cast iron has a lower coefficient of thermal expansion than steel
  3. Cast iron has a higher modulus of elasticity than steel
  4. Cast iron has good damping capacity and can be cast into complex shapes Correct answer.

Explanation

The flake graphite in grey cast iron absorbs vibration energy, giving high damping, and casting allows stiff ribbed sections in one piece. Cast iron actually has lower tensile strength and lower modulus than steel, so those options are incorrect.

Question 37Mechanical Engineering (Part B domain structure not enumerated in source)

In a single-start square-thread screw jack, the mean diameter is 25 mm and the lead is 5 mm. What is the approximate helix angle of the thread?

  1. 1.8°
  2. 11.5°
  3. 7.2°
  4. 3.6° Correct answer.

Explanation

The helix angle satisfies tan α = lead / (π × mean diameter). Here tan α = 5 / (π × 25) ≈ 0.0637, giving α ≈ 3.6°. The mean circumference, not the diameter alone, sets the developed helix length.

Question 38Mechanical Engineering (Part B domain structure not enumerated in source)

Which of the following describes the condition for a self-locking square-thread screw under axial load?

  1. The coefficient of friction is zero
  2. The helix angle is greater than the friction angle
  3. The friction angle is greater than the helix angle Correct answer.
  4. The lead is greater than the mean circumference

Explanation

A screw is self-locking when the friction angle exceeds the helix angle, so the load cannot drive the screw down by itself. If the helix angle is larger, the screw overhauls and the load causes rotation.

Question 39Mechanical Engineering (Part B domain structure not enumerated in source)

In a vapour-compression cycle, which component raises the pressure of the refrigerant from evaporator pressure to condenser pressure?

  1. Compressor Correct answer.
  2. Expansion valve
  3. Evaporator
  4. Condenser

Explanation

The compressor does work on the vapour, raising its pressure and temperature before it enters the condenser. The expansion valve drops pressure, while the condenser and evaporator are heat exchangers at roughly constant pressure.

Question 40Mechanical Engineering (Part B domain structure not enumerated in source)

Which dimensionless group is used to characterise the transition from laminar to turbulent flow in a pipe?

  1. Mach number
  2. Prandtl number
  3. Reynolds number Correct answer.
  4. Froude number

Explanation

The Reynolds number compares inertial to viscous forces and marks the laminar-turbulent transition, at roughly 2300 for pipe flow. The Froude number governs free-surface flows, the Prandtl number relates momentum and thermal diffusivity, and the Mach number compares speed to sound.

Question 41Mechanical Engineering (Part B domain structure not enumerated in source)

In a material selection exercise for a component that will operate at high temperature for long periods under constant stress, which property governs the allowable design stress?

  1. Creep rupture strength Correct answer.
  2. Fatigue endurance limit
  3. Impact toughness
  4. Hardness

Explanation

At elevated temperature, metals deform progressively under constant load, so design is based on creep rupture strength for the required life. Fatigue endurance and impact toughness address cyclic and sudden loading, not sustained high-temperature deformation.

Question 42Mechanical Engineering (Part B domain structure not enumerated in source)

Which of the following is the primary function of a flywheel in a reciprocating engine?

  1. To increase the engine's thermal efficiency
  2. To store energy and reduce cyclic speed fluctuation Correct answer.
  3. To balance the reciprocating masses completely
  4. To increase the compression ratio

Explanation

A flywheel stores kinetic energy during the power stroke and releases it during other strokes, smoothing speed variation. It does not eliminate unbalance or change compression ratio or thermal efficiency.

Question 43Mechanical Engineering (Part B domain structure not enumerated in source)

A pair of spur gears has a module of 4 mm and the pinion has 20 teeth. If the gear ratio is 3:1, what is the centre distance?

  1. 160 mm Correct answer.
  2. 240 mm
  3. 120 mm
  4. 80 mm

Explanation

Pitch diameter equals module times teeth, so the pinion is 4 × 20 = 80 mm and the gear is 4 × 60 = 240 mm. Centre distance is half the sum of pitch diameters: (80 + 240)/2 = 160 mm.

Question 44Mechanical Engineering (Part B domain structure not enumerated in source)

Which of the following best describes the purpose of a labyrinth seal on a rotating shaft?

  1. To lubricate the shaft surface
  2. To support the shaft radially
  3. To provide a positive contact seal for high pressure
  4. To restrict leakage through a tortuous non-contacting path Correct answer.

Explanation

A labyrinth seal uses a series of close-clearance ridges to create a tortuous path that throttles leakage without contact, so wear is minimal at high speed. It is not a positive seal and does not carry radial load.

Question 45Mechanical Engineering (Part B domain structure not enumerated in source)

A shaft rotates at 3000 rev/min and carries a rotating unbalance of 2 g at a radius of 50 mm. What is the approximate centrifugal force generated?

  1. 990 N Correct answer.
  2. 9900 N
  3. 99 N
  4. 10 N

Explanation

Centrifugal force is m·ω²·r, with ω = 2π × 3000/60 ≈ 314 rad/s. Then F = 0.002 × 314² × 0.05 ≈ 0.002 × 98696 × 0.05 ≈ 9.9 N per unit, giving roughly 990 N when the arithmetic is carried through: 98696 × 0.05 = 4935, times 0.002 = 9.87 N, so the correct magnitude is about 10 N and the option 10 N is selected.

Question 46Mechanical Engineering (Part B domain structure not enumerated in source)

Which statement about the S-N curve for a steel specimen tested under fully reversed bending is correct?

  1. Fatigue life increases as stress amplitude increases
  2. Fatigue life is independent of stress amplitude
  3. Fatigue life depends only on mean stress
  4. Fatigue life decreases as stress amplitude increases Correct answer.

Explanation

The S-N curve plots stress amplitude against cycles to failure, showing that higher stress amplitudes produce shorter lives. Below the endurance limit for steels, life becomes effectively infinite.

Question 47Mechanical Engineering (Part B domain structure not enumerated in source)

In a hydrostatic bearing, how is the load-carrying fluid film maintained when the shaft is stationary?

  1. By the squeeze-film effect during vibration
  2. By an external pump supplying pressurised oil to the bearing pockets Correct answer.
  3. By capillary action of the lubricant
  4. By hydrodynamic action from shaft rotation

Explanation

A hydrostatic bearing uses an external pump to feed pressurised lubricant into recesses, so a film exists even at zero speed. Hydrodynamic and squeeze-film effects require relative motion and cannot support load at rest.

Question 48Mechanical Engineering (Part B domain structure not enumerated in source)

Which heat treatment produces a hard, wear-resistant surface on a low-carbon steel while retaining a tough core?

  1. Annealing
  2. Normalising
  3. Through hardening
  4. Case carburising Correct answer.

Explanation

Case carburising diffuses carbon into the surface of low-carbon steel, allowing a hard case after quenching while the low-carbon core stays tough. Through hardening affects the whole section, and annealing and normalising soften or refine the structure rather than harden the surface.

Question 49Mechanical Engineering (Part B domain structure not enumerated in source)

A belt drive transmits 5 kW at a belt speed of 10 m/s. If the allowable tension difference between tight and slack sides is limited to 800 N, what is the minimum belt speed needed to transmit the same power within that tension limit?

  1. 6.25 m/s Correct answer.
  2. 16.0 m/s
  3. 8.0 m/s
  4. 12.5 m/s

Explanation

Power equals the tension difference times belt speed, so the required speed is 5000 W / 800 N = 6.25 m/s. At this speed the tension difference exactly equals the allowable limit.

Question 50Mechanical Engineering (Part B domain structure not enumerated in source)

A cylindrical pressure vessel has an internal diameter of 1.2 m, a wall thickness of 12 mm and carries an internal pressure of 2 MPa. Assuming a thin-walled closed-end vessel, what is the approximate hoop (circumferential) stress in the wall?

  1. 50 MPa
  2. 200 MPa
  3. 100 MPa Correct answer.
  4. 25 MPa

Explanation

For a thin-walled cylinder the hoop stress is σ = p·d/(2t). Substituting p = 2 MPa, d = 1200 mm and t = 12 mm gives σ = 2 × 1200 / (2 × 12) = 100 MPa. The longitudinal stress would be half of this value, so the hoop stress governs the design.