Iran Nezam Mechanical Design Exam common mistakes
Iran Nezam Mechanical Design Exam · Understand the traps and how to avoid them.
When a problem gives a shaft diameter and a torque, the candidate computes shear stress as τ = T/A (dividing by the cross-sectional area) instead of τ = Tr/J, and then carries that wrong stress into the fatigue or key-sizing check.
The candidate memorized 'stress = force over area' from axial loading and never internalized that torsion produces a stress gradient peaking at the outer fiber. Because the numerical answer looks plausible (same order of magnitude), nothing triggers a recheck.
What to do instead
Before any torsion problem, write the two governing relations side by side: τ_max = T·c/J with J = πd⁴/32 for a solid round shaft, so τ_max = 16T/(πd³). Do a units sanity check (N·mm·mm / mm⁴ = N/mm²) and one order-of-magnitude estimate before touching the calculator. Practice five solid-shaft and five hollow-shaft problems until the 16T/(πd³) form is automatic.
In beam bending problems the candidate draws the shear and moment diagrams with the wrong sign convention or misses the concentrated-moment jump, then reports a maximum moment that is off by a factor or has the wrong location.
Sign conventions differ between textbooks and Iranian course notes (sagging-positive vs. hogging-positive), and candidates mix conventions mid-problem. The jump in the moment diagram at a pure couple is also easy to skip because it does not appear on the load diagram.
What to do instead
Adopt one convention at the start of every problem and state it in writing (e.g., 'sagging positive, upward load positive'). Build the diagrams by the differential relations dV/dx = −w and dM/dx = V, and mark every discontinuity explicitly: point load = step in V, point couple = step in M, free end = V and M must close to zero. Verify the closing condition before reading off M_max.
When selecting a pump for a system, the candidate reads the duty point off the pump curve at the rated flow only and ignores the system curve, then fails to notice that the installed pump will run far from its best efficiency point or will cavitate.
Catalog data is presented as a single curve with a highlighted rated point, so candidates treat that point as the operating point. NPSH available is computed from a formula but never compared against NPSH required at the actual (higher) flow.
What to do instead
Always plot or tabulate the system curve H = H_static + K·Q² alongside the pump curve and find the intersection. Then check NPSH_a = (p_atm − p_v)/(ρg) + z − h_f against NPSH_r at that intersection flow, and confirm the point lies within roughly 70–120% of BEP flow. If a throttle valve is used, re-solve the system curve with the new K rather than assuming the flow stays at rating.
In first-law energy balances the candidate mixes mass flow with volumetric flow, or uses specific heat at constant pressure for a constant-volume process, producing an answer that is dimensionally wrong but numerically tidy.
Problems often quote flow in m³/h from a datasheet while the energy equation needs kg/s, and the density conversion is skipped under time pressure. The c_p vs. c_v distinction is also blurred because both appear in the same chapter.
What to do instead
Start every thermodynamics solution with a stated control volume, a stated substance, and a units table converting all flows to kg/s and all pressures to absolute. Write the energy equation in rate form with each term labeled (kW). Choose c_v for constant-volume and c_p for constant-pressure processes, and for ideal gases verify with c_p − c_v = R.
In heat exchanger and conduction problems the candidate applies the LMTD method to a case with unknown outlet temperatures, or forgets the correction factor F for multipass and cross-flow arrangements, and reports an oversized or undersized area.
The LMTD formula is the one most rehearsed, so it is applied reflexively. The ε-NTU method exists precisely for unknown-outlet cases, and the F correction is a chart lookup that candidates skip because it feels optional.
What to do instead
Decide the method before computing: if both outlet temperatures are known, use LMTD with F from the chart for the given arrangement; if either outlet is unknown, switch to ε-NTU. Always state whether the exchanger is counterflow, parallel, or multipass, because F = 1 only for pure counterflow. Sanity-check that the calculated area gives U·A within the expected range.
When sizing ductwork or selecting a fan for an HVAC system, the candidate adds pressure losses arithmetically along the longest run but ignores fitting losses and the system effect, then specifies a fan that cannot deliver design airflow.
Friction charts give straight-duct loss per metre directly, while fitting loss coefficients require a separate lookup and judgement about equivalent length. Under exam time pressure the fittings get dropped, and the system effect at fan inlet/outlet is rarely taught explicitly.
What to do instead
Use the equal-friction or static-regain method and tabulate every segment: straight length, fitting K-values, and total. Compute total pressure as the sum of friction plus dynamic losses including fittings, then add a documented allowance for system effect. Check the fan curve against the resulting system curve at design flow rather than accepting the catalog rating.
In bearing and gear selection the candidate uses the basic dynamic load rating C as if it were the allowable load, or computes bearing life without converting revolutions to hours at the actual speed.
The L10 = (C/P)^p formula is memorized as a ratio but the meaning of C (load giving one million revolutions of life) is not, and speed is often given in rpm while life is wanted in hours. The exponent p = 3 for ball and 10/3 for roller bearings is also frequently swapped.
What to do instead
Write the definition of C next to the formula every time: C is the load at which 90% of a group survives 10⁶ revolutions. Convert life with L_h = (10⁶/(60·n))·(C/P)^p, and confirm p = 3 for ball, 10/3 for roller. For gears, separately verify bending strength (Lewis) and surface durability, since passing one does not imply the other.
When specifying a weld or interpreting a welding symbol, the candidate confuses the arrow side with the other side, or applies a fillet weld size without checking minimum and maximum limits for the plate thickness.
The standard symbol places information above and below a reference line, and candidates memorize the shape of the symbol without the side convention. Code minimum fillet sizes and the maximum size near a free edge are table lookups that are skipped.
What to do instead
Redraw the symbol and label it: reference line, arrow, tail, size to the left of the symbol, length to the right, symbol above = arrow side, below = other side. Before finalizing any fillet size, look up the minimum size for the thickest part and the maximum permitted along a free edge (typically t − 1 mm for t > 6 mm). State the electrode and process alongside the symbol.
In fire protection layout the candidate sizes sprinkler branch lines and mains from a single occupancy hazard class without verifying that the available water supply at the base of the riser meets the demand at the required residual pressure, and without checking spacing against the hazard.
Hazard classification drives density, spacing, and hose allowance, and candidates pick the class from the building name rather than from the actual contents and storage height. Hydraulic supply verification requires a supply curve that is often given separately from the layout data.
What to do instead
Classify the occupancy from contents and storage configuration first, then read the design density, maximum sprinkler spacing, and hose allowance for that class. Build the demand point (flow and pressure at the base of the riser) and overlay it on the available supply curve; if the supply does not cover the demand, state the remedy (booster pump, larger main, or storage tank) rather than adjusting the density.
On code and professional-practice items the candidate answers from general engineering intuition or from a foreign code they studied, instead of citing the specific Iranian regulation, its edition, and the clause that controls the case.
Iranian building regulations are updated periodically and the exam is set against a specific edition, but candidates often prepare from translated foreign codes or from memory of older national provisions. The wording of the question frequently turns on which document has jurisdiction.
What to do instead
Build a one-page map of the governing documents and their scope (national building regulations series, fire and plumbing provisions, and the professional engineering law and its disciplinary provisions), noting the current edition of each. For every practice question, write the document number and clause before choosing the answer, and flag any item where the applicable edition is uncertain for review rather than guessing.