Basque PAU Technology & Eng II common mistakes
Basque PAU Technology & Eng II · Understand the traps and how to avoid them.
When asked to select a material, the candidate compares only one property (e.g. picks the lightest or the strongest) and ignores the trade-off with cost, machinability, corrosion resistance or availability, then justifies the choice with a single number.
Selection tables in textbooks often list one headline value per material, and the candidate treats the exercise as a lookup rather than a multi-criterion decision. They also forget that the PAU statement usually gives several constraints that must all be satisfied simultaneously.
What to do instead
Before choosing, write the constraints as a short list (mechanical requirement, environmental requirement, manufacturing requirement, cost ceiling) and score each candidate against every line. Use a simple decision matrix or Ashby-style property chart reasoning, and in the answer explicitly state which property drove the choice and which alternative was rejected and why. Practise with 5–6 typical statements (bicycle frame, pressure vessel, gear, beam) until the multi-criterion habit is automatic.
Confusing stress and strain, or using the elastic modulus as if it were a strength limit, and misreading the elastic/plastic regions of a stress–strain curve when asked to compute elongation, toughness or safety factor.
The symbols σ and ε look similar in handwritten notes, and the candidate memorises 'E = σ/ε' without internalising that E describes stiffness only. The yield point and the ultimate tensile strength are then used interchangeably.
What to do instead
Redraw a labelled stress–strain curve from memory for a ductile and a brittle material, marking proportional limit, yield strength, UTS, fracture and the area representing toughness. For every numerical problem, state units explicitly (MPa for stress, dimensionless or % for strain) and check that the computed strain is plausible (typically < 0.5 % in the elastic region for metals). Solve at least three problems where the safety factor is defined against yield and three where it is defined against UTS, and note which definition the statement requires.
In mechanism problems, computing the velocity ratio correctly but then applying it in the wrong direction, or ignoring efficiency when the train includes a worm gear or a screw, so the output torque or power is overestimated.
Velocity ratios are treated as pure numbers without tracking which shaft is input and which is output, and ideal (frictionless) transmissions are assumed by default because efficiency is mentioned only briefly in class.
What to do instead
Adopt a fixed layout: draw the shafts left to right, label input and output, write each stage ratio as n_out/n_in with an arrow, and multiply stage by stage. For irreversible or high-friction elements (worm gear, lead screw, belt slip) insert the efficiency η into the power equation P_out = η·P_in before computing torque. Always finish with a sanity check: a speed reducer must increase torque, and the output power can never exceed the input power.
When analysing a mechanism's motion, the candidate mixes up angular and linear quantities (using ω where v is needed, or forgetting the radius) and fails to convert rpm to rad/s before substituting into v = ω·r or P = T·ω.
Formulas are memorised as symbol patterns without unit discipline, and rpm is the unit used in everyday machinery, so it feels natural to insert it directly into a formula that requires rad/s.
What to do instead
Force a unit conversion step at the start of every kinematics or power calculation: rpm × 2π/60 = rad/s. Write the formula with units beside each symbol before substituting numbers. As a routine check, verify that the result has the expected dimension (m/s for velocity, N·m for torque, W for power); if the dimension is wrong, the error is almost always a missing radius or an unconverted rpm.
In DC circuit problems, the candidate applies Ohm's law and series/parallel rules without first reducing the network to a single equivalent resistance, or mishandles the sign of sources in Kirchhoff loops, producing a numerically consistent but physically wrong current direction.
The tendency is to start substituting numbers immediately into the first equation that comes to mind. Loop direction conventions are treated as arbitrary, so signs are assigned inconsistently within the same problem.
What to do instead
Always redraw the circuit and reduce it stepwise (series, then parallel, then source), writing R_eq at each stage. For Kirchhoff, fix a loop direction and a node current convention in writing before writing equations, and mark assumed current arrows on the diagram. If a current comes out negative, state that the real direction is opposite rather than silently dropping the sign. Verify the answer with a power balance: total power delivered by sources equals total dissipated in resistors.
The candidate describes a control system as closed-loop merely because it has a sensor or a controller, and cannot distinguish the controlled variable, the setpoint, the error signal and the disturbance in a described process.
Textbook block diagrams are recognised visually but not translated into the physical variables of a real installation (a tank, a motor, an oven). Feedback is associated with 'electronics' rather than with the comparison of measured value and reference.
What to do instead
For any described process, write four lines: controlled variable, measuring element, actuator, disturbance. Then state whether the measured value is compared with a reference to generate the actuating signal; if yes it is closed-loop, if the actuator acts regardless of the output it is open-loop. Practise on five everyday systems (water heater with thermostat, traffic light timer, cruise control, toilet cistern, lift) and classify each, naming the sensor and actuator explicitly.
When drawing or interpreting pneumatic and hydraulic schematics, the candidate uses the wrong symbol for a valve (e.g. draws a 5/2 valve as 4/2, or misplaces the spring and pilot returns) or cannot trace the circuit to say which port is pressurised in each position.
ISO 1219 symbols are learned by copying diagrams rather than by understanding the box-per-position convention, so the number of ports and positions is guessed from the shape instead of counted.
What to do instead
Learn the rule explicitly: number of squares = number of positions, number of connections per square = number of ways; the rest position is the square to which the return spring or the default pilot is attached. Redraw a set of standard symbols (3/2, 5/2, 5/3, flow-control, non-return, cylinder double-acting) with ports numbered 1, 2, 3, 4, 5 and exhausts 3, 5, then trace the air path square by square for each valve position. Do the same for a full circuit: compressor → filter-regulator → valve → cylinder.
In manufacturing questions the candidate names a process without matching it to the material and batch size (e.g. proposing sand casting for a small precision batch, or welding an aluminium alloy that needs a specific technique), or confuses machining with forming and joining in classification questions.
Processes are memorised as a list rather than as a decision driven by material, geometry, tolerance and production volume. The candidate answers with the first process that comes to mind rather than eliminating options against the stated constraints.
What to do instead
Build a three-column table: process, suitable materials, typical batch and tolerance. For each exam statement, extract the constraints (material, quantity, precision, surface finish, cost) and eliminate processes that violate any of them before naming one. Justify the choice in one sentence referencing the constraints, and mention one rejected alternative with the reason. Revise the classification (machining removes material, forming conserves it, joining adds material or bonds) so that classification items become automatic.
In energy and power questions the candidate computes efficiency or energy conversion without carrying units through (mixing kWh, MJ and J, or kW and kW·h) and ignores losses when stating the output of a conversion chain.
Energy units are used loosely in everyday language, and the candidate treats efficiency as a percentage to be added at the end rather than as a multiplier applied at each conversion stage.
What to do instead
Convert everything to SI (J, W, s) at the start, then convert back only for the final answer if the statement asks for kWh. For chains, multiply the efficiencies of each stage (η_total = η1·η2·…) rather than adding them. Always write the balance: input energy = useful output + losses, and check that the computed efficiency lies between 0 and 1. Include the environmental consequence (CO2 per kWh, or the reason a low-efficiency stage dominates) in the final sentence, as PAU statements usually ask for it.
In technical drawing, the candidate produces a view set that is inconsistent (hidden lines missing, projection method mixed between first and third angle, or dimensions duplicated, missing or placed on a non-visible edge), so the drawing is ambiguous even when the shape is understood.
The drawing is treated as a sketch to communicate the idea rather than as a standardised document, and the candidate does not check the view set against the projection rules or the dimensioning rules before finishing.
What to do instead
Follow a fixed checklist on every drawing: (1) choose the front view showing the most characteristic shape, (2) apply the stated projection system consistently (first angle: view placed on the opposite side), (3) include all hidden edges as dashed lines, (4) dimension each feature once, on the view where it appears true size, avoiding redundant dimensions, and (5) add the scale, units and title block. Redraw one moderately complex part weekly from a real object and compare against the checklist rather than against a model answer.