PEC EPE Mechanical common mistakes
PEC EPE Mechanical · Understand the traps and how to avoid them.
Substitutes angles directly into formulas that require radians while the calculator is still in degree mode - arc length, small-angle approximations, trigonometric series, angular velocity, and phase angles all come out wrong by a factor of about 57.
Most exam handbooks print the formula without stating the unit, and the calculator's DEG indicator is easy to miss under time pressure. Candidates who habitually use degrees in statics and surveying carry the setting into dynamics and heat-transfer problems.
What to do instead
Before starting the session, set the calculator to RAD and treat degrees as the exception: whenever an angle appears, write its unit explicitly next to the number, and convert with the factor pi/180 on the same line. For small-angle work, verify that sin(theta) and tan(theta) are within a few percent of theta - if they are not, the angle was in the wrong unit.
Picks the area moment of inertia I for a torsion problem, or the polar moment J for a bending problem, and for hollow sections subtracts diameters instead of fourth powers. The resulting shear stress or deflection is off by roughly a factor of two.
Both symbols appear in the same formula sheet, both are called 'moment of inertia', and shafts and beams are often drawn in the same style of sketch. Under time pressure the candidate grabs the familiar second-moment expression without checking which stress (normal or shear) the question is asking for.
What to do instead
Before computing, state in writing what the stress is: bending gives normal stress with I = bh^3/12 (about the neutral axis); torsion gives shear stress with J = pid^4/32. For hollow sections, tabulate the property from a standard table and cross-check with the parallel-axis theorem, and test the answer's plausibility by comparing the hollow section with the solid one it was bored from (it must be less stiff).
Applies a static yield-based factor of safety to a fluctuating-stress fatigue problem, or uses the modified Goodman line while ignoring the mean-stress component after converting an alternating load.
Yield strength is the number candidates remember, and the endurance limit diagram looks like extra work. When a question gives maximum and minimum load, many candidates average them away and design against the amplitude alone, losing the mean-stress term that controls the answer.
What to do instead
Write down sigma_max and sigma_min, then compute sigma_a and sigma_m explicitly before selecting any criterion. Match the criterion to the material: modified Goodman for ductile steels, Gerber where the question indicates a conservative-to-moderate design, and Soderberg only when yielding must be excluded. Add a separate yield check on sigma_max to confirm the part does not yield on the first cycle.
Applies the open-system (steady-flow) energy equation to a closed piston-cylinder process, or uses cp where cv belongs, producing answers that violate the first law by a factor of the specific-heat ratio k.
Coursework drills the steady-flow energy equation until it becomes the default, and the two specific heats differ only by the symbol subscript on the formula sheet. Questions that describe a sealed cylinder with a moving piston still tempt the candidate toward the flow form.
What to do instead
First identify the control volume: sealed boundary means closed system with Q - W = delta U; a device with inlet and outlet means steady flow. Then write the working fluid's constraint (constant volume gives cv, constant pressure gives cp) above the equation before substituting. Verify with k = cp/cv = 1.4 for air and check that the sign of work matches whether the volume grew or shrank.
Uses the counterflow log-mean temperature difference for a parallel-flow or cross-flow exchanger, or omits the correction factor F when the arrangement is not pure counterflow.
Counterflow LMTD is the formula most often practised, and in a parallel-flow unit the two terminal differences are swapped, which changes the answer while still producing a plausible-looking number. Cross-flow questions look identical except for a single word.
What to do instead
Sketch the two fluid streams with arrows and label both terminal temperature differences before touching the formula. If the streams run in opposite directions, use counterflow directly; if they run parallel, swap the terminal differences; if the arrangement is shell-and-tube multipass or cross-flow, look up F from the R and P parameters and multiply. Sanity check: the calculated LMTD must lie between the two terminal differences.
Uses gauge pressure where absolute pressure is required when computing NPSH available, or the reverse when converting a suction pressure for a pump cavitation check.
Pressure gauges in the field read gauge pressure, so most practice problems are written in gauge, and the vapour pressure is always given as an absolute value. Mixing them shifts NPSH by roughly one atmosphere, which flips a marginal design from safe to cavitating.
What to do instead
Adopt a fixed rule: every pressure in a cavitation or NPSH calculation is absolute. Convert at the start by adding atmospheric pressure (about 101.3 kPa) to every gauge reading, and keep the vapour pressure as given. Write the NPSH available expression with each term's unit and source, then check that the result is a small positive number of metres - a value near 10 m usually means gauge pressure leaked into the sum.
Writes Bernoulli's equation between two points in a pumped pipeline without adding pump head, subtracting turbine/friction head, or checking that the energy balance is complete - then wonders why the discharge pressure comes out below the suction pressure.
The textbook Bernoulli form with only velocity and elevation terms is the version most often memorised, and pump head is usually introduced later in a separate chapter. In exam questions the pump or the friction loss is tucked into the prose rather than the diagram.
What to do instead
Always write the full extended form: p1/(rhog) + V1^2/(2g) + z1 + h_pump = p2/(rhog) + V2^2/(2g) + z2 + h_turbine + h_loss. Underline each term as you find its data in the question, and if a term has no data, state the assumption (for example negligible minor loss) rather than silently dropping it. Check the sign of head loss - it must always reduce the downstream total head.
Selects a material by trade name alone - 'stainless steel' for a corrosive service, 'aluminium' for a weight-critical bracket - without checking the grade, temper, or heat-treatment condition that the question actually specifies.
Familiar names carry strong associations from general reading, and exam questions supply designations like 304, 316L, 6061-T6 or 4340 quenched and tempered that look like decoration but determine the properties. Candidates answer the property question from the name and lose marks on condition.
What to do instead
Build a one-page property card before the exam: for each material family, record the relevant design strength, corrosion resistance in the specified environment, and the temper or heat treatment that produces it. When reading the stem, highlight the exact designation and ask what the condition does - T6 raises aluminium strength by precipitation hardening, 316L resists chlorides better than 304 because of molybdenum. Answer the condition, not the family.
Applies the economic order quantity formula with annual demand in one term and a per-order or monthly holding cost in another, or adds safety stock into the EOQ input instead of treating it as a separate reorder-point calculation.
The EOQ expression is short and easy to memorise, and its radical hides the time basis. Question stems deliberately mix annual demand with monthly holding cost or per-unit-per-year carrying rates, and a single time-unit mismatch changes the answer by a factor of 12.
What to do instead
Before substituting, write a units line beneath each symbol: D in units/year, S in currency/order, H in currency/unit-year. If holding cost is given as an annual percentage of unit cost, convert it to currency per unit per year first. Keep safety stock and reorder point out of EOQ entirely - EOQ sets order size, reorder point covers lead-time variability - and state that separation explicitly in the answer.
When asked to shorten a project duration at least cost, crashes the cheapest activity in the network rather than the cheapest activity that lies on the critical path, and does not re-identify the critical path after each crash.
The instinct is to look for the lowest cost per week on the activity list, and float data is presented in a table that is easy to skim without reading which activities have zero total float. Crashing a non-critical activity consumes budget without moving the completion date.
What to do instead
Draw or redraw the network and compute early and late times, listing total float for every activity. Crash only activities with zero total float, choosing among them the lowest cost slope, and check whether a parallel critical path appears after each compression. Stop crashing when the cheapest critical activity's cost slope exceeds the penalty or overhead saving per week - and state that stopping condition.
Treats an ethics question as a personality test - selects the answer that sounds most virtuous (immediately resign, or report straight to the regulator) instead of the sequence the professional code requires: verify the facts, raise it internally with the responsible engineer, escalate through the chain, and only then go outside.
Candidates prepare for ethics by intuition rather than by reading the code, and the most dramatic option reads as the most principled. The correct answer is usually the measured one that protects public safety while respecting due process and confidentiality.
What to do instead
Learn the code's hierarchy once: public safety and welfare first, then competence and honesty, then confidentiality and conflict-of-interest rules. For any scenario, work the standard ladder in order - confirm the facts, raise the concern with the immediate supervisor in writing, escalate to higher management or the engineering manager, and go to the regulator only if the risk to the public persists. Reject options that skip rungs, and reject options that trade public safety for loyalty to an employer.