Iran Nezam Mechanical Design Exam flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Parallel-axis theorem

The moment of inertia about any axis equals that about a parallel centroidal axis plus the area times the squared perpendicular distance between the axes: I = I_c + A·d².

Newton–Raphson method

Root-finding iteration x_{n+1} = x_n − f(x_n)/f′(x_n). It converges quadratically near a simple root but can diverge if the derivative is near zero or the initial guess is poor.

Euler buckling load

Critical axial load for a slender column: P_cr = π²EI/(KL)². The effective-length factor K depends on end conditions, from 0.5 for fixed-fixed to 2 for fixed-free.

Maximum shear stress

In plane stress, τ_max = √[((σ_x − σ_y)/2)² + τ_xy²], equal to half the difference of the principal stresses. Used with the Tresca yield criterion.

Beam shear–moment relation

The shear force equals the slope of the bending moment diagram, V = dM/dx. A concentrated load causes a shear jump and a slope change in the moment diagram.

Reynolds number criterion

Re = ρVD/μ classifies pipe flow: below roughly 2300 laminar, 2300–4000 transitional, above 4000 turbulent. It compares inertia to viscous forces.

Pump affinity laws

For a geometrically similar pump at constant impeller diameter, flow varies with speed, head with speed squared, and shaft power with speed cubed.

Net positive suction head available

NPSH_a = (P_atm − P_v)/(ρg) + z − h_f on the suction side. It must exceed the pump's required NPSH to avoid cavitation.

Darcy–Weisbach head loss

Friction head loss h_f = f(L/D)(V²/2g). The friction factor f comes from the Moody chart or Colebrook equation and equals 64/Re in laminar flow.

Carnot efficiency

Maximum efficiency of any heat engine between two reservoirs: η = 1 − T_L/T_H, using absolute temperatures. No real cycle between the same reservoirs can exceed it.

Log mean temperature difference

ΔT_lm = (ΔT₁ − ΔT₂)/ln(ΔT₁/ΔT₂) drives heat exchanger sizing q = U·A·ΔT_lm. A correction factor F applies to multipass and cross-flow arrangements.

Biot number

Bi = h·L_c/k compares internal conduction resistance to external convection resistance. Values below 0.1 justify a lumped-capacitance analysis with uniform body temperature.

Stefan–Boltzmann law

Radiant heat transfer between a surface and its surroundings is q = ε·σ·A·(T_s⁴ − T_sur⁴), with σ = 5.67×10⁻⁸ W/(m²·K⁴) and absolute temperatures.

Rankine cycle efficiency

Thermal efficiency equals net work divided by heat added, (W_t − W_p)/Q_in. Reheating and regenerative feedwater heating raise the average temperature of heat addition.

Sensible heat ratio

SHR = Q_sensible/(Q_sensible + Q_latent) for a cooling coil. It fixes the slope of the coil process line and the resulting room supply air condition.

Fan laws

For a given fan at constant air density, flow varies with speed, pressure with speed squared, and power with speed cubed. The operating point is where fan and system curves intersect.

Equal friction duct sizing

Duct sections are sized for a constant pressure loss per unit length along the run. Velocity limits are then checked to control regenerated noise and carry dust.

Sensible cooling load equation

Q_s = 1.23·V̇(L/s)·ΔT(°C) in watts, or 1.08·cfm·ΔT(°F) in Btu/h. The constant embeds air density and specific heat.

Goodman fatigue criterion

Failure line 1/n = σ_a/S_e + σ_m/S_u for mean stress σ_m and alternating stress σ_a. It is less conservative than Soderberg, which uses yield strength in the mean term.

Bearing rating life L10

L₁₀ = (C/P)^p million revolutions, where C is basic dynamic load rating, P the equivalent load, and p = 3 for ball and 10/3 for roller bearings.

Gear module

Module m = d/z relates pitch diameter to tooth number; circular pitch p = π·m. Mating gears must share the same module and pressure angle.

Marin endurance limit factors

The endurance limit of a steel part is estimated as S_e = k_a·k_b·k_c·k_d·k_e·(0.5·S_u), correcting surface, size, load, temperature, and reliability effects.

Carbon equivalent

CE = %C + %Mn/6 + (%Cr + %Mo + %V)/5 + (%Ni + %Cu)/15 estimates weldability. Values above about 0.40 generally require preheat to avoid cold cracking.

Austenitic stainless sensitization

Chromium carbide precipitation at grain boundaries in the 425–870 °C range depletes chromium and promotes intergranular corrosion. Low-carbon or stabilized grades mitigate this in welds.

Riser design principle

A riser must remain liquid longer than the casting section it feeds, so it solidifies last. Risers are placed at hot spots and sized using the modulus method.

Fixture unit demand

Water supply pipes are sized from total fixture units converted to probable demand, not the sum of individual fixture flows, because fixtures are used intermittently.

Trap seal protection

A water seal of about 50–100 mm in each trap blocks sewer gas. Venting limits pressure swings that would siphon or blow out the seal.

Hazen–Williams equation

Empirical pipe friction formula h_f = 10.67·L·Q^1.852/(C^1.852·D^4.87) in SI units, with roughness coefficient C typically 100–150 for water pipes.

Sprinkler design density

Required flow equals discharge density times the design area, with density and area set by occupancy hazard class. Hose stream demand may be added separately.

Mabhas 14

The volume of the Iranian National Building Regulations covering HVAC installations. It sets design conditions, ventilation, and system requirements for building mechanical services.

Mabhas 16

The Iranian National Building Regulations volume on sanitary plumbing. It addresses water supply, drainage, venting, and fixture requirements for buildings.

Standard 2800

The Iranian code for seismic-resistant design of buildings. It defines design spectra, base shear, and detailing requirements applied together with the national loading code.