PRC Mechanical Engineer common mistakes
PRC Mechanical Engineer · Understand the traps and how to avoid them.
Sizing a shaft, belt, gear, or bearing from the exact calculated nominal load and ignoring the applicable service/shock factor before choosing a standard size.
Board formulas often produce a nominal torque or load, while catalog ratings and design codes assume a service condition; candidates treat the nominal value as the design value.
What to do instead
Write the load path first, classify the drive or driven machine, apply the correct service factor to the nominal load, then compare against the catalog/allowable rating and check the next standard size up.
Mixing horsepower, kW, rpm, and rad/s in one power-torque calculation without converting to a single unit system.
Machine design formulas appear in several unit conventions, and candidates substitute values directly from the problem statement without tracking whether speed is in rpm or rad/s.
What to do instead
Convert all inputs to one consistent set before substitution, write the unit equation beside the formula, and verify that the final unit is N·m or lb·ft before selecting a component.
Using ultimate tensile strength as the allowable stress in a machine-member design problem.
Candidates remember strength values from materials tables but do not distinguish ultimate strength, yield strength, endurance limit, and allowable stress after applying a factor of safety.
What to do instead
State the failure mode first, select yield strength for yielding, ultimate strength for static fracture, or endurance limit for fatigue, then divide by the specified factor of safety before comparing to actual stress.
Choosing a material or shop process only by highest strength, ignoring machinability, weldability, castability, heat treatment, or cost.
Materials questions often list several acceptable properties, and candidates pick the strongest option instead of the one that satisfies the actual manufacturing constraint.
What to do instead
Identify the governing constraint in the question, such as weldability for a fabricated frame or machinability for a threaded part, then eliminate materials that fail that constraint before comparing strength and cost.
Substituting gauge pressure into the ideal-gas law, steam-table interpolation, or compressor/boiler energy balance where absolute pressure is required.
Pressure gauges and problem statements commonly report gauge pressure, and candidates forget that thermodynamic relations use absolute pressure.
What to do instead
Convert every pressure to absolute by adding atmospheric pressure before using gas laws or steam tables, and label each pressure as gauge or absolute in the solution.
Treating turbine or engine shaft work as the net cycle work without subtracting pump/compressor work and without applying component efficiencies.
Candidates memorize the Rankine or Brayton efficiency formula but omit auxiliary work or isentropic efficiency when the problem gives real machine efficiencies.
What to do instead
Draw and label the energy balance for each component, compute net work as work output minus work input, and multiply ideal work by the stated isentropic or mechanical efficiency before calculating cycle efficiency.
Leaving the calculator in radian mode while solving degree-based trigonometry or inverse-trig problems.
Trigonometric functions return different values in degree and radian mode, and candidates rarely check the mode after using the calculator for other subjects.
What to do instead
At the start of every mathematics solution, set and verify the calculator mode against the angle unit in the problem, and perform a quick sanity check such as sin 30° = 0.5.
Rounding intermediate results to two or three decimal places before the final calculation.
Board problems often require several steps, and early rounding accumulates error until the final answer no longer matches any option.
What to do instead
Carry at least four to six significant figures through intermediate steps, store values in calculator memory, and round only the final answer to the precision required by the choices.
Applying the simple-interest formula to a compound-interest cash flow or using the wrong depreciation method for the stated condition.
Engineering economics problems contain keywords such as compounded annually, effective rate, declining balance, or sinking fund, and candidates match the formula by habit instead of by the stated cash-flow pattern.
What to do instead
Draw a cash-flow timeline, underline the compounding and depreciation keywords, write the corresponding formula before substituting values, and check whether the answer requires present worth, annual worth, or future worth.
Mixing mass and weight or failing to convert kgf to N in statics and basic thermodynamics problems.
Philippine engineering problems sometimes use kgf and metric tons alongside SI units, and candidates substitute mass values directly where a force is required.
What to do instead
Write a dimensional check for every equation, convert kgf to N using 1 kgf = 9.80665 N when force is needed, and keep mass in kg separate from weight in N throughout the solution.