Intermediate Mechanical interview questions

150 questions with answers (MEC-151 to MEC-300). Try each one aloud before reading the answer.

Engineering Mechanics

MEC-151 · A 10 kg block rests on a surface with μs = 0.4. What horizontal force starts it moving?

F = μs·m·g = 0.4 × 10 × 9.81 ≈ 39.2 N. Any force below this is resisted by static friction of equal magnitude.

MEC-152 · What is the principle of virtual work?

For a body in equilibrium, the total virtual work done by all external forces during any small virtual displacement consistent with constraints is zero. It solves equilibrium of connected systems without calculating internal reactions.

MEC-153 · How do you analyze a truss by the method of joints and method of sections?

Method of joints applies ΣFx = ΣFy = 0 at each pin, working from joints with at most two unknowns. Method of sections cuts through up to three members and uses equilibrium of one part, which is faster for finding a few specific member forces.

MEC-154 · What is the work–energy principle?

The net work done on a body equals its change in kinetic energy: ΣW = ½mv₂² − ½mv₁². It avoids finding acceleration and is convenient when forces depend on position.

MEC-155 · What is the impulse–momentum principle?

The impulse of a force equals the change in linear momentum: ∫F dt = mv₂ − mv₁. It is useful for impacts and short-duration forces where the force history is unknown.

MEC-156 · What is the coefficient of restitution?

e = relative velocity of separation ÷ relative velocity of approach along the line of impact. e = 1 is perfectly elastic, e = 0 perfectly plastic; kinetic energy is lost whenever e < 1.

MEC-157 · What is D'Alembert's principle?

Adding an inertia force −ma (and inertia couple −Iα) to the real forces turns a dynamics problem into one of static equilibrium. It is widely used in machine force analysis.

MEC-158 · How do you find the moment of inertia about a parallel axis?

Parallel axis theorem: I = I_G + Ad² for areas (or I_G + md² for masses), where I_G is about the centroidal axis and d is the distance between the axes.

MEC-159 · What is the perpendicular axis theorem?

For a plane lamina, I_z = I_x + I_y, where z is perpendicular to the plane and x, y lie in it, all through the same point. It gives the polar moment J of a circle as twice its diametral I.

MEC-160 · What is the second moment of area of a rectangle and a solid circle?

Rectangle about its centroidal axis parallel to the base: I = bh³/12. Solid circle about a diameter: I = πd⁴/64, and its polar moment J = πd⁴/32.

MEC-161 · What is the mechanical advantage and efficiency of a simple machine?

Mechanical advantage MA = load/effort; velocity ratio VR = effort distance/load distance; efficiency η = MA/VR. A machine is self-locking (irreversible) if its efficiency is below 50%.

MEC-162 · What is centripetal acceleration and what force produces it?

A body moving in a circle of radius r at speed v accelerates toward the center at a = v²/r = ω²r. The net inward force, F = mv²/r, is supplied by tension, friction, normal force or gravity.

Strength of Materials

MEC-163 · What is the relationship between load, shear force and bending moment?

dV/dx = −w and dM/dx = V, where w is distributed load intensity. Hence the bending moment diagram slope equals shear force, and moment is extreme where shear force is zero.

MEC-164 · What is the maximum bending moment of a simply supported beam with central point load and with UDL?

Central point load W on span L: M_max = WL/4 at midspan. Uniformly distributed load w per length: M_max = wL²/8 at midspan.

MEC-165 · What is the maximum deflection of a cantilever with an end load and a simply supported beam with central load?

Cantilever with end load W: δ = WL³/(3EI). Simply supported with central load: δ = WL³/(48EI). Simply supported with UDL: δ = 5wL⁴/(384EI).

MEC-166 · How is shear stress distributed across a rectangular beam section?

Parabolically, τ = VQ/(Ib), zero at the top and bottom fibres and maximum at the neutral axis, where τ_max = 1.5 × average shear stress (V/A). For a solid circle τ_max = 4/3 × average.

MEC-167 · What are principal stresses and principal planes?

Principal planes are planes with zero shear stress, and the normal stresses on them are principal stresses: σ₁,₂ = (σx + σy)/2 ± √(((σx − σy)/2)² + τxy²). Maximum in-plane shear stress = (σ₁ − σ₂)/2 on planes at 45°.

MEC-168 · What is Mohr's circle used for?

A graphical method to transform stresses. Its center is at (σx + σy)/2 and radius gives maximum shear stress; rotating 2θ on the circle corresponds to rotating θ on the element, so principal stresses are read directly.

MEC-169 · What are hoop and longitudinal stresses in a thin cylinder?

For internal pressure p, diameter d and wall thickness t: hoop stress σ_h = pd/(2t) and longitudinal stress σ_l = pd/(4t). Hoop stress is twice longitudinal, so cylinders tend to split lengthwise. A thin sphere has σ = pd/(4t).

MEC-170 · What is Euler's buckling load for a column?

P_cr = π²EI/L_e², where L_e is effective length: L for pinned–pinned, 2L for fixed–free, 0.5L for fixed–fixed, and about 0.7L for fixed–pinned. It applies to long, slender columns.

MEC-171 · What is slenderness ratio?

λ = L_e/k, where k = √(I/A) is the least radius of gyration. High λ means buckling governs (Euler); low λ means crushing governs, and intermediate columns use empirical formulas like Rankine's or Johnson's.

MEC-172 · How is a bar's elongation under axial load calculated?

δ = PL/(AE). For stepped bars, sum each segment's PL/(AE). For a bar hanging under its own weight, δ = ρgL²/(2E), half that of an equal end load.

MEC-173 · What is the strain energy stored in a bar under gradual, sudden and impact loading?

Gradual load: U = P²L/(2AE). Suddenly applied load gives twice the gradual stress. A load W dropped from height h gives σ = (W/A)[1 + √(1 + 2AEh/(WL))].

MEC-174 · What is the power transmitted by a shaft?

P = 2πNT/60, with N in rpm and T in N·m, giving watts. Equivalently P = Tω.

MEC-175 · How do you calculate the stiffness of a helical spring?

k = Gd⁴/(8D³n), where d is wire diameter, D mean coil diameter, n number of active coils and G modulus of rigidity. Shear stress τ = 8FD·K_w/(πd³), with Wahl factor K_w for curvature and direct shear.

MEC-176 · How do series and parallel springs combine?

Parallel springs: k_eq = k₁ + k₂ (same deflection, loads add). Series springs: 1/k_eq = 1/k₁ + 1/k₂ (same load, deflections add).

Thermodynamics

MEC-177 · What is Carnot efficiency?

η_Carnot = 1 − T_c/T_h, with temperatures in kelvin. It is the maximum possible efficiency of any heat engine operating between those two reservoirs, achieved only by a fully reversible cycle.

MEC-178 · What is the steady-flow energy equation?

For a control volume per unit mass: q − w = (h₂ − h₁) + (V₂² − V₁²)/2 + g(z₂ − z₁). It simplifies to w = h₁ − h₂ for an adiabatic turbine and V₂ = √(2(h₁ − h₂)) for a nozzle from rest.

MEC-179 · What is the work done in isothermal and polytropic processes of an ideal gas?

Isothermal: W = p₁V₁ ln(V₂/V₁) = mRT ln(p₁/p₂). Polytropic pVⁿ = C: W = (p₁V₁ − p₂V₂)/(n − 1). For n = γ it is the reversible adiabatic case.

MEC-180 · What is the Clausius inequality?

∮δQ/T ≤ 0 for any cycle, with equality only for a reversible cycle. It leads to the definition of entropy and the principle of entropy increase.

MEC-181 · What is exergy (availability)?

The maximum useful work obtainable from a system as it comes into equilibrium with the environment (dead state). Exergy is destroyed by irreversibility, I = T₀·S_gen (Gouy–Stodola theorem).

MEC-182 · What is the Rankine cycle?

The ideal vapour power cycle: isentropic pumping of liquid, constant-pressure heating in the boiler, isentropic expansion in the turbine, and constant-pressure condensation. It is the basis of steam power plants.

MEC-183 · How do superheating, reheating and regeneration improve a Rankine cycle?

Superheating raises mean heat-addition temperature and reduces turbine exit moisture. Reheating allows higher boiler pressure without wet exhaust. Regeneration with feedwater heaters preheats feedwater using bled steam, raising mean heat-addition temperature and efficiency.

MEC-184 · What is the Brayton cycle?

The ideal gas turbine cycle: isentropic compression, constant-pressure heat addition, isentropic expansion and constant-pressure heat rejection. Efficiency η = 1 − 1/r_p^((γ−1)/γ), where r_p is the pressure ratio.

MEC-185 · What is throttling and why is it isenthalpic?

Throttling is a pressure drop through a valve or porous plug with no work, negligible heat transfer and negligible kinetic energy change, so the steady-flow energy equation gives h₁ = h₂. It is irreversible, so entropy rises.

MEC-186 · What is the Joule–Thomson coefficient?

μ_JT = (∂T/∂p)_h, the temperature change per unit pressure drop during throttling. It is zero for an ideal gas; real gases cool on throttling below their inversion temperature, which is used in gas liquefaction.

MEC-187 · What is the entropy change of an ideal gas?

Δs = c_p ln(T₂/T₁) − R ln(p₂/p₁) = c_v ln(T₂/T₁) + R ln(v₂/v₁). For an isentropic process this gives T₂/T₁ = (p₂/p₁)^((γ−1)/γ).

MEC-188 · What is isentropic efficiency of a turbine and a compressor?

Turbine: η_T = actual work ÷ isentropic work = (h₁ − h₂)/(h₁ − h₂s). Compressor: η_C = isentropic work ÷ actual work = (h₂s − h₁)/(h₂ − h₁). Both are below 1 due to friction and irreversibilities.

MEC-189 · What is the difference between saturated, subcooled and superheated states?

A saturated state lies on the liquid–vapour dome at the saturation temperature for its pressure. Subcooled (compressed) liquid is below saturation temperature at that pressure. Superheated vapour is above it.

MEC-190 · What is a T–s diagram and why is it useful?

A plot of temperature versus entropy. The area under a reversible process curve equals heat transferred, and the enclosed area of a cycle equals net work, so it shows clearly how a cycle's efficiency can be improved.

Fluid Mechanics

MEC-191 · What is the hydrostatic force on a submerged plane surface and where does it act?

F = ρg·h_c·A, where h_c is depth of the centroid. It acts at the center of pressure, at depth h_cp = h_c + I_G sin²θ/(A·h_c), which is always below the centroid.

MEC-192 · What is metacentric height and how does it relate to stability?

GM is the distance from the center of gravity to the metacenter of a floating body. GM > 0 is stable, GM = 0 neutral, GM < 0 unstable. GM = BM − BG, with BM = I/V.

MEC-193 · How does a venturimeter measure flow?

It accelerates flow through a converging throat, dropping pressure. From Bernoulli and continuity, Q = C_d·A₁A₂√(2gh)/√(A₁² − A₂²), with C_d typically about 0.95–0.98.

MEC-194 · What is the Darcy–Weisbach equation?

Head loss in pipe flow h_f = fLV²/(2gD), where f is the Darcy friction factor. For laminar flow f = 64/Re; for turbulent flow f is found from the Moody chart or the Colebrook equation.

MEC-195 · What is the Hagen–Poiseuille equation?

For laminar flow in a circular pipe, Δp = 128μLQ/(πD⁴) = 32μLV/D². The velocity profile is parabolic and the maximum velocity is twice the average.

MEC-196 · What are minor losses in pipes?

Losses at fittings, bends, valves, entrances, exits and sudden area changes, expressed as h = K·V²/2g. Sudden expansion loss is (V₁ − V₂)²/2g, and exit loss to a reservoir is V²/2g.

MEC-197 · What is a Pitot tube?

A tube facing the flow that measures stagnation pressure. Combined with static pressure, it gives velocity V = √(2(p₀ − p)/ρ), used in aircraft airspeed and duct flow measurement.

MEC-198 · What is the momentum equation and where is it used?

ΣF = ṁ(V_out − V_in) for a control volume in steady flow. It gives forces on pipe bends, nozzles, jets striking vanes, and thrust of propulsion devices.

MEC-199 · What are the main dimensionless numbers in fluid mechanics?

Reynolds (inertia/viscous), Froude (inertia/gravity, open channels and ship waves), Mach (velocity/sound speed, compressibility), Weber (inertia/surface tension), and Euler (pressure/inertia).

MEC-200 · What is the Buckingham π theorem?

If a physical problem involves n variables with m fundamental dimensions, it can be expressed with n − m independent dimensionless π groups. It reduces experiments and underpins model testing.

MEC-201 · What is cavitation?

Formation and violent collapse of vapour bubbles when local pressure falls below the vapour pressure of a liquid. It causes pitting, noise, vibration and performance loss in pumps, propellers and valves.

MEC-202 · What is the difference between impulse and reaction turbines?

In an impulse turbine (Pelton) the whole pressure drop occurs in the nozzle and the runner is driven by jet momentum at atmospheric pressure. In a reaction turbine (Francis, Kaplan) pressure drops partly in the runner, which runs full of water.

MEC-203 · What is NPSH in a pump?

Net Positive Suction Head is the absolute suction head above the liquid's vapour pressure. NPSH available must exceed NPSH required by the pump, usually with a margin, to prevent cavitation.

Heat Transfer

MEC-204 · What are the three modes of heat transfer?

Conduction through a medium by molecular interaction, q = −kA dT/dx (Fourier). Convection between a surface and a moving fluid, Q = hAΔT (Newton). Radiation by electromagnetic waves without a medium, E = εσT⁴ (Stefan–Boltzmann).

MEC-205 · How is heat conducted through a cylindrical wall?

Q = 2πkL(T₁ − T₂)/ln(r₂/r₁). The thermal resistance is R = ln(r₂/r₁)/(2πkL), so resistance grows logarithmically, not linearly, with thickness.

MEC-206 · What is the critical radius of insulation?

For a cylinder, r_c = k/h; for a sphere, r_c = 2k/h. Adding insulation to a pipe of radius smaller than r_c increases heat loss until the outer radius reaches r_c.

MEC-207 · What is the Biot number and when is the lumped capacitance method valid?

Bi = hL_c/k, the ratio of internal conduction resistance to surface convection resistance, with L_c = V/A. If Bi < 0.1, internal temperature gradients are negligible and T(t) − T∞ = (T_i − T∞)e^(−hAt/ρVc).

MEC-208 · What is fin efficiency?

η_f = actual heat transfer from the fin ÷ heat transfer if the entire fin were at base temperature. For a long fin, Q = √(hPkA_c)·θ_b; fins work best with high k and low h.

MEC-209 · What is log mean temperature difference?

LMTD = (ΔT₁ − ΔT₂)/ln(ΔT₁/ΔT₂), where ΔT₁ and ΔT₂ are terminal temperature differences. Heat duty is Q = UA·LMTD, with a correction factor F for multi-pass and cross-flow exchangers.

MEC-210 · Why is counter-flow more effective than parallel-flow?

Counter-flow maintains a more uniform temperature difference along the length, giving a higher LMTD for the same terminals, and lets the cold fluid outlet exceed the hot fluid outlet temperature. This needs less area for the same duty.

MEC-211 · What is the Stefan–Boltzmann law?

A black body emits E_b = σT⁴, where σ = 5.67 × 10⁻⁸ W/m²·K⁴ and T is absolute temperature. Net exchange between two surfaces depends on T₁⁴ − T₂⁴, emissivities and view factors.

MEC-212 · What is a view factor?

F₁₂ is the fraction of radiation leaving surface 1 that strikes surface 2 directly. It obeys reciprocity, A₁F₁₂ = A₂F₂₁, and summation, ΣF₁ⱼ = 1.

MEC-213 · What is Kirchhoff's law of radiation?

At thermal equilibrium, a surface's emissivity equals its absorptivity at the same temperature and wavelength (ε_λ = α_λ). For a grey surface, ε = α overall.

MEC-214 · What is Wien's displacement law?

The wavelength of maximum emission of a black body is λ_max·T ≈ 2898 μm·K. Hotter bodies peak at shorter wavelengths: the sun (~5800 K) peaks near 0.5 μm, room-temperature objects near 10 μm.

MEC-215 · What is the Grashof number?

Gr = gβΔTL³/ν², the ratio of buoyancy to viscous forces. It governs natural convection the way Reynolds number governs forced convection; Rayleigh number Ra = Gr·Pr sets the laminar–turbulent transition.

Theory of Machines & Vibrations

MEC-216 · What is the instantaneous center of rotation?

A point about which a link appears to rotate purely at a given instant, with zero relative velocity there. Velocity of any point is ω × distance from the I-center; Kennedy's theorem says three I-centers of three links lie on a straight line.

MEC-217 · What is Coriolis acceleration and when does it occur?

It arises when a point slides along a rotating link: a_c = 2ωv, directed perpendicular to the sliding velocity. It appears in quick-return mechanisms and must be included in their acceleration analysis.

MEC-218 · What is the law of gearing?

For a constant velocity ratio, the common normal at the point of contact of two teeth must always pass through a fixed point on the line of centers, the pitch point. Involute and cycloidal profiles satisfy it.

MEC-219 · What is interference in involute gears and how is it avoided?

Interference occurs when the tip of one tooth contacts the non-involute flank below the base circle of its mate, causing undercutting. It is avoided by using enough teeth (theoretically about 17–18 for a 20° full-depth pinion meshing with a rack), a larger pressure angle, or profile shift.

MEC-220 · How do you find the speed ratio of an epicyclic gear train?

Use the tabular or relative velocity method: (N_ring − N_arm)/(N_sun − N_arm) = −Z_sun/Z_ring. Fixing one member and driving another gives different ratios from the same train.

MEC-221 · What is the coefficient of fluctuation of speed?

C_s = (ω_max − ω_min)/ω_mean. The flywheel energy fluctuation is ΔE = Iω²C_s, so I = ΔE/(ω²C_s) sizes the flywheel.

MEC-222 · What are sensitivity, stability and hunting in governors?

Sensitivity is the ratio of mean speed to the speed range. A stable governor's sleeve radius increases with speed. An over-sensitive governor hunts, oscillating above and below the mean speed; an isochronous governor has zero speed range and is prone to hunting.

MEC-223 · What is the friction circle and friction torque in bearings?

In a journal, the resultant reaction is tangent to a friction circle of radius μr. In a flat pivot under uniform wear, friction torque T = ½μWR; under uniform pressure T = ⅔μWR.

MEC-224 · What is the ratio of belt tensions?

For flat belts, T₁/T₂ = e^(μθ), where θ is the angle of lap in radians. For V-belts, T₁/T₂ = e^(μθ/sin(β/2)), with β the groove angle. Power P = (T₁ − T₂)v.

MEC-225 · What is the logarithmic decrement?

δ = ln(x_n/x_(n+1)) = 2πζ/√(1 − ζ²), measured from successive amplitudes of free damped vibration. It is a practical way to determine the damping ratio ζ.

MEC-226 · What are underdamped, critically damped and overdamped systems?

Damping ratio ζ = c/c_c, where c_c = 2√(km). ζ < 1 oscillates with decaying amplitude, ζ = 1 returns fastest without oscillation, and ζ > 1 returns slowly without oscillation.

MEC-227 · What is the static deflection method for natural frequency?

For an undamped spring–mass system, f_n = (1/2π)√(g/δ_st). With δ in mm, f_n ≈ 15.76/√δ Hz, a quick way to estimate isolator and shaft frequencies.

MEC-228 · What is whirling of shafts?

At the critical speed, the rotating shaft's speed equals its lateral natural frequency, so small eccentricity causes large deflection. Critical speed for a shaft with one disc is N_c = (60/2π)√(g/δ) rpm, where δ is static deflection under the disc weight.

Machine Design

MEC-229 · What is stress concentration and how is it reduced?

A local rise in stress at holes, notches, keyways, threads and sharp corners, expressed by K_t = σ_max/σ_nominal. Reduce it with generous fillets, gradual section changes, relief grooves and moving holes away from high-stress regions.

MEC-230 · What is notch sensitivity?

q = (K_f − 1)/(K_t − 1), where K_f is the fatigue stress concentration factor. q ranges from 0 (insensitive) to 1 (fully sensitive); high-strength steels are more notch sensitive than soft steels or cast iron.

MEC-231 · Name the main static failure theories.

Maximum principal stress (Rankine), maximum shear stress (Tresca), maximum principal strain (St. Venant), total strain energy (Haigh) and distortion energy (von Mises). Tresca and von Mises suit ductile materials; Rankine suits brittle materials.

MEC-232 · What is the von Mises equivalent stress?

σ_vm = √(σ₁² + σ₂² − σ₁σ₂) for plane stress, or √(σx² − σxσy + σy² + 3τxy²). Yield is predicted when σ_vm reaches the uniaxial yield strength.

MEC-233 · How is a shaft sized for combined bending and torsion?

Using maximum shear stress theory, the equivalent torque is T_e = √(M² + T²) and d³ = 16T_e/(πτ_allow). Using maximum normal stress, M_e = ½(M + √(M² + T²)) and d³ = 32M_e/(πσ_allow).

MEC-234 · What are endurance limit modifying factors?

Marin factors adjust the test-specimen endurance limit: S_e = k_a·k_b·k_c·k_d·k_e·S_e′ for surface finish, size, load type, temperature and reliability. For steels, S_e′ ≈ 0.5 S_ut up to about 1400 MPa.

MEC-235 · What are the Goodman, Gerber and Soderberg criteria?

They handle combined mean and alternating stress. Soderberg: σ_a/S_e + σ_m/S_y = 1/n (most conservative). Goodman: σ_a/S_e + σ_m/S_ut = 1/n (linear, widely used). Gerber: a parabola through S_ut, closest to test data for ductile steels.

MEC-236 · How is bolt preload chosen?

Typically about 75% of proof load for reusable joints and 90% for permanent ones. Preload keeps the joint from separating and reduces the alternating stress carried by the bolt under fluctuating loads.

MEC-237 · How is rolling bearing life calculated?

L₁₀ = (C/P)^p million revolutions, with p = 3 for ball bearings and 10/3 for roller bearings, where C is dynamic load rating and P equivalent dynamic load. L₁₀ is the life that 90% of bearings reach or exceed.

MEC-238 · What is the Petroff equation?

For a lightly loaded concentric journal bearing, friction coefficient f = 2π²(μN/p)(r/c), with N in rev/s, p load per projected area and c radial clearance. It shows friction rises with viscosity and speed.

MEC-239 · What is the Lewis equation for gear tooth bending?

σ = F_t/(b·m·Y), treating the tooth as a cantilever, where F_t is tangential load, b face width, m module and Y the Lewis form factor. It is modified by velocity and dynamic factors in practice.

MEC-240 · What is self-locking in a power screw?

A screw is self-locking if the load cannot rotate it backwards, which requires friction angle ≥ lead angle (μ ≥ tan λ). Self-locking screws have efficiency below 50%.

Manufacturing Processes

MEC-241 · What are the differences between casting and forging?

Casting pours molten metal into a mould and suits complex shapes and large parts, but may contain porosity, shrinkage and coarse grains. Forging plastically deforms solid metal, giving aligned grain flow, fewer voids and higher strength and fatigue resistance.

MEC-242 · What is Chvorinov's rule?

Solidification time t = B(V/A)ⁿ, with n ≈ 2. Risers are designed with a larger V/A (modulus) than the casting section they feed so they solidify last.

MEC-243 · What are common casting defects?

Blow holes and gas porosity, shrinkage cavities, cold shuts, misruns, hot tears, sand inclusions, scabs, and mismatch (shift) between mould halves.

MEC-244 · What is Merchant's circle in metal cutting?

A force diagram relating cutting force, thrust force, shear force and friction force on the chip. Merchant's shear angle relation is 2φ + β − α = 90°, where β is friction angle and α rake angle.

MEC-245 · State Taylor's tool life equation.

VTⁿ = C, where V is cutting speed, T tool life and n an exponent (about 0.1–0.15 for HSS, 0.2–0.5 for carbides). Tool life falls sharply as cutting speed increases.

MEC-246 · What types of chips form in machining?

Continuous chips (ductile materials, high speed, large rake), continuous with built-up edge (low speed, ductile, poor lubrication), discontinuous chips (brittle materials like cast iron) and serrated chips (titanium, hardened steel).

MEC-247 · How is material removal rate calculated in turning?

MRR = V·f·d, with cutting speed V (mm/min), feed f (mm/rev) and depth of cut d (mm), giving mm³/min. Cutting speed V = πDN.

MEC-248 · What are the main non-conventional machining processes?

EDM (spark erosion of conductive materials), ECM (anodic dissolution), laser beam, electron beam, abrasive water jet, ultrasonic machining and chemical machining. They suit hard materials and complex shapes regardless of hardness.

MEC-249 · What is the heat-affected zone in welding?

The region of base metal next to the weld that did not melt but had its microstructure and properties changed by heat. It can harden, soften or crack, and is often the weakest zone of a joint.

MEC-250 · What are common welding defects?

Porosity, slag inclusions, lack of fusion, incomplete penetration, undercut, overlap, cracks (hot and cold) and distortion.

MEC-251 · What is powder metallurgy?

Producing parts by compacting metal powders in a die and sintering below the melting point. It gives near-net shape, controlled porosity (self-lubricating bearings) and materials hard to melt, such as tungsten carbide.

MEC-252 · What is injection moulding?

Molten thermoplastic is injected under high pressure into a closed mould, cooled and ejected. It produces high volumes of complex plastic parts; key defects include sink marks, short shots, flash and warpage.

MEC-253 · What is additive manufacturing?

Building parts layer by layer from a digital model. Main methods: FDM (extruded filament), SLA (UV-cured resin), SLS (sintered polymer powder), and powder bed fusion and DED for metals.

MEC-254 · What is the difference between deep drawing and stretch forming?

In deep drawing a punch pushes sheet into a die cavity while the flange draws in, making cups with near-constant thickness. In stretch forming the sheet is clamped at the edges and stretched over a form, so it thins.

IC Engines

MEC-255 · Otto is more efficient than Diesel at the same compression ratio, so why are diesel engines more efficient in practice?

Diesels run much higher compression ratios, about 15–22 versus 8–12, because they compress only air and have no knock limit. They are also unthrottled, which avoids pumping losses at part load.

MEC-256 · What is the air-standard efficiency of the Otto cycle?

η = 1 − 1/r^(γ−1). With r = 8 and γ = 1.4, η = 1 − 1/8^0.4 ≈ 56.5%. It depends only on compression ratio.

MEC-257 · What is the air-standard efficiency of the Diesel cycle?

η = 1 − (1/r^(γ−1))·[(ρ^γ − 1)/(γ(ρ − 1))], where ρ is the cut-off ratio. The bracket term exceeds 1, so Diesel efficiency falls as cut-off ratio increases.

MEC-258 · What is the dual (limited pressure) cycle?

Heat is added partly at constant volume and partly at constant pressure. It models real high-speed diesel engines more closely than either the Otto or Diesel cycle.

MEC-259 · What is mean effective pressure?

The constant pressure that, acting on the piston over one stroke, would produce the same work as the actual cycle: MEP = W_net/V_s. Brake MEP allows comparison of engines of different sizes.

MEC-260 · How is brake power calculated from BMEP?

BP = p_mb·L·A·N·k/(60·n_R), with N in rpm, k cylinders and n_R = 2 for four-stroke, 1 for two-stroke. Brake power is also 2πNT/60 from measured torque.

MEC-261 · What is volumetric efficiency?

The actual mass of air inducted per cycle ÷ mass that would fill the swept volume at ambient (or intake) conditions. It is reduced by flow restrictions, hot walls and residual gas, and raised by tuned intake, multiple valves and turbocharging.

MEC-262 · What is the stoichiometric air–fuel ratio for petrol and diesel?

About 14.7:1 by mass for petrol and about 14.5:1 for diesel. Equivalence ratio φ = actual F/A ÷ stoichiometric F/A; SI engines run near φ = 1 and diesels run lean overall.

MEC-263 · What is diesel knock?

Excessive ignition delay lets a large quantity of fuel accumulate before ignition, which then burns at once and produces a steep pressure rise. It is reduced by high-cetane fuel, higher compression temperature and controlled injection rate.

MEC-264 · How do you reduce knock in SI engines?

Use higher octane fuel, retard spark timing, lower compression ratio or boost, improve cooling of the end gas, use compact combustion chambers with central spark plugs, and promote turbulence for faster burning.

MEC-265 · What is ignition timing advance?

Firing the spark before TDC so peak cylinder pressure occurs shortly after TDC (about 10–15° ATDC). More advance is needed at higher speed and lighter load; too much causes knock.

MEC-266 · What is a heat balance sheet of an IC engine?

An account of fuel energy split into brake power, cooling water, exhaust gas and unaccounted losses. In a typical engine roughly a third or less goes to brake power and the rest is lost as heat.

Refrigeration & Air Conditioning

MEC-267 · What is the Carnot COP of a refrigerator?

COP_R = T_L/(T_H − T_L), with absolute temperatures. Between −10 °C and 40 °C, COP = 263/50 ≈ 5.3; real systems achieve much less.

MEC-268 · How is a vapour compression cycle shown on a p–h diagram?

Compression 1–2 rises along constant entropy, condensation 2–3 is a horizontal line at high pressure, throttling 3–4 is a vertical line (constant enthalpy), and evaporation 4–1 is horizontal at low pressure. Refrigerating effect = h₁ − h₄, compressor work = h₂ − h₁.

MEC-269 · Why are subcooling and superheating used in refrigeration?

Subcooling the liquid below condensing temperature increases refrigerating effect per kg. Superheating at the evaporator exit ensures no liquid enters the compressor, protecting it from liquid slugging.

MEC-270 · What is the difference between sensible and latent heat load?

Sensible load changes air temperature without changing moisture content. Latent load changes moisture content at constant temperature. Sensible heat factor SHF = sensible ÷ total load.

MEC-271 · What is bypass factor of a cooling coil?

The fraction of air passing through the coil that does not contact the coil surface, BF = (t₂ − t_ADP)/(t₁ − t_ADP), where ADP is apparatus dew point. More rows and lower face velocity reduce BF.

MEC-272 · What is the difference between cooling with dehumidification and evaporative cooling?

Cooling with dehumidification passes air over a coil below dew point, lowering temperature and humidity ratio. Evaporative cooling adds water, lowering dry bulb at nearly constant wet bulb while raising humidity, which suits hot dry climates.

MEC-273 · How does a heat pump work in heating mode?

It runs a vapour compression cycle with the outdoor coil as evaporator and the indoor coil as condenser. A reversing valve switches the flow direction between cooling and heating modes.

MEC-274 · What is the function of an accumulator and receiver in refrigeration?

The suction accumulator traps liquid refrigerant before the compressor. The liquid receiver stores refrigerant after the condenser so the system handles load changes and service pump-down.

MEC-275 · What types of compressors are used in refrigeration?

Reciprocating, rotary (rolling piston), scroll, screw and centrifugal. Scroll and rotary dominate small AC units, screws suit medium to large plants, and centrifugals suit large chillers.

MEC-276 · What is ODP and GWP of a refrigerant?

ODP (ozone depletion potential) compares ozone-layer damage relative to R-11 = 1. GWP compares radiative warming over a period (usually 100 years) relative to CO₂ = 1. Modern refrigerants target zero ODP and low GWP.

MEC-277 · What are the components of cooling load in a building?

External: conduction through walls and roof, solar radiation through glass, infiltration and ventilation air. Internal: occupants, lighting and equipment. Each has sensible and, for people and moist air, latent components.

Engineering Materials

MEC-278 · What are the key phases in the iron–carbon diagram?

Ferrite (α, BCC, low C solubility), austenite (γ, FCC, up to 2.1% C), cementite (Fe₃C, 6.67% C, hard and brittle), pearlite (ferrite + cementite lamellae) and ledeburite. The eutectoid point is 0.76–0.8% C at 727 °C, the eutectic 4.3% C at 1147 °C.

MEC-279 · What is martensite?

A supersaturated, body-centered tetragonal phase formed when austenite is quenched fast enough to prevent carbon diffusion. It is very hard and brittle and is tempered before use.

MEC-280 · What is hardenability and how is it measured?

The ability of a steel to form martensite to a depth under a given quench. It is measured by the Jominy end-quench test; alloying elements such as Cr, Mo, Mn and Ni increase it.

MEC-281 · What are TTT diagrams?

Time–temperature–transformation diagrams show when austenite transforms at constant temperature into pearlite, bainite or martensite. The nose of the curve sets the critical cooling rate for full hardening.

MEC-282 · What are case hardening processes?

Carburizing (adding carbon), nitriding (nitrogen, no quench), carbonitriding, induction and flame hardening. They produce a hard, wear-resistant surface with a tough core for gears, cams and shafts.

MEC-283 · What are the types of cast iron?

Grey (graphite flakes, good damping and machinability), white (cementite, hard and brittle), malleable (heat-treated white iron, graphite rosettes), and ductile or SG iron (spheroidal graphite via Mg, good ductility).

MEC-284 · What are the main families of stainless steel?

Austenitic (304, 316: non-magnetic, most corrosion-resistant, not hardenable by heat treatment), ferritic (430), martensitic (410, 420: hardenable), duplex, and precipitation-hardening (17-4PH).

MEC-285 · What is age (precipitation) hardening?

Solution treatment, quenching to form a supersaturated solution, then aging to precipitate fine particles that obstruct dislocations. It strengthens Al alloys such as 2024 and 6061, and some steels and nickel alloys.

MEC-286 · What are dislocations and how do they relate to strength?

Line defects in a crystal whose movement causes plastic deformation. Strengthening mechanisms (grain refinement, solid solution, precipitation, work hardening) all work by impeding dislocation motion.

MEC-287 · What is the Hall–Petch relation?

σ_y = σ₀ + k_y/√d. Smaller grain size d gives higher yield strength because grain boundaries block dislocation motion; fine grains also improve toughness.

MEC-288 · What is the ductile-to-brittle transition temperature?

The temperature below which BCC metals such as carbon steel change from ductile to brittle fracture, measured with Charpy impact tests. FCC metals like aluminium and austenitic stainless steel show no sharp transition.

MEC-289 · Why is aluminium widely used in engineering?

Density about one-third of steel (2.7 vs 7.85 g/cm³), good corrosion resistance from a natural oxide film, high thermal and electrical conductivity, and good formability. Alloys reach high strength-to-weight ratios.

Metrology, CAD/CAM & GD&T

MEC-290 · What are the ISO system fundamental deviations and IT grades?

Letters give the position of the tolerance zone relative to the basic size (capital for holes, lower case for shafts). IT grades (IT01 to IT18) give tolerance magnitude; IT5–IT7 suit precision fits and IT12+ suit rough work. H7/g6 is a common sliding fit.

MEC-291 · What is the difference between form, orientation, location and runout tolerances?

Form controls a feature's own shape (flatness, straightness, circularity, cylindricity) without datums. Orientation (parallelism, perpendicularity, angularity) and location (position, concentricity, symmetry) need datums. Runout controls surface variation during rotation about a datum axis.

MEC-292 · What is Maximum Material Condition (MMC)?

The condition where a feature contains the most material: smallest hole or largest shaft. Specifying a position tolerance at MMC permits bonus tolerance as the feature departs from MMC, aiding functional assembly.

MEC-293 · What is the Taylor principle of gauging?

A GO gauge should check all dimensions and form of a feature simultaneously (full form), while a NO-GO gauge checks only one dimension at a time. A plug gauge checks holes and a snap or ring gauge checks shafts.

MEC-294 · How is surface roughness specified?

Ra is the arithmetic mean deviation of the profile from the mean line, in μm. Rz is the mean peak-to-valley height. Typical Ra: turning 0.8–6.3 μm, grinding 0.1–1.6 μm, lapping below 0.1 μm.

MEC-295 · What is a coordinate measuring machine (CMM)?

A machine that measures 3D geometry by touching or scanning a part with a probe and recording coordinates. Software fits features and evaluates sizes and GD&T tolerances against CAD nominals.

MEC-296 · What is the difference between wireframe, surface and solid modelling?

Wireframe uses edges only and is ambiguous. Surface models define skins without volume. Solid models (B-rep or CSG) define enclosed volume, giving mass properties, interference checks and direct use in FEA and CAM.

MEC-297 · What is parametric and feature-based modelling?

Geometry is built from features (extrude, revolve, hole, fillet) driven by dimensions and constraints. Changing a parameter regenerates the model, with design intent kept through relations and a history tree.

MEC-298 · What is the basic workflow of finite element analysis?

Pre-processing (geometry, mesh, material, loads, boundary conditions), solving the stiffness equations [K]{u} = {F}, and post-processing results such as stresses and deflections, followed by a mesh convergence check.

MEC-299 · What is the difference between absolute and incremental programming in CNC?

Absolute (G90) gives every coordinate from the program origin. Incremental (G91) gives each move relative to the previous position. Absolute is less prone to cumulative error.

MEC-300 · What are tool radius and tool length compensation in CNC?

Radius compensation (G41 left, G42 right, G40 cancel) offsets the toolpath so the part is programmed to its actual contour. Length compensation (G43) accounts for different tool lengths so all tools reference the same Z zero.

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