22-Mec-B5 Product Design and Development · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams, May 2015 — 07-Mec-B5 Product Design and Development. Three hours. Open book; no calculator is permitted. Question 1 must be completed and is worth 40 marks; four of the six remaining questions are chosen, each worth 15 marks, for 100 marks in total. Only the first five questions as they appear in the answer book are marked. The paper states that most questions require an answer in essay format or the use of tables, figures and charts, and that clarity and organisation of the answer are important.
The paper prints 40 + 6 × 15 = 130 marks and a candidate attempts 40 + 4 × 15 = 100 of them. All seven questions are answered below, because this set is a study resource rather than an examination script. The marking scheme printed on the last source page splits Question 1 as 6 / 9 / 9 / 6 / 4 / 6 and gives the part weights for each 15-mark question, and the answers here are proportioned to that split. Because no calculator is allowed, every calculation is arranged so that it can be carried out on paper in one or two lines.
Check: the exam gives no data of its own — every question asks the candidate to bring a product, a set of numbers and a method. All quantities used below (operating torques, embodied energies, machine rates, process sigmas, material properties) are stated explicitly as design assumptions drawn from the reference texts and from Canadian standards, and each answer is written so that the method stands whatever numbers a marker would prefer.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Given. The product selected is item iii, the kitchen faucet (tap), taken as the incumbent Canadian residential product: a deck-mounted two-handle mixer on 100 mm centres, with a pair of quarter-turn ceramic-disc cartridges operated by 45 mm knurled knobs, fed from a tank set to 60 °C and a cold main at 10 °C, and delivering up to 5.7 L/min at 415 kPa. The measured effort figures, service assumptions and rating scales used in parts A, D and F are collected below.
| Quantity | Symbol | Value |
|---|---|---|
| Breakaway torque, worn knob cartridge | Mknob | 2.4 N·m |
| Knob diameter across the knurl | D | 45 mm |
| Breakaway torque, single-lever cartridge | Mlever | 1.2 N·m |
| Operating-force ceiling, accessible controls | Fmax | 22.2 N (5 lbf) |
| Hot supply / cold supply temperature | Th / Tc | 60 °C / 10 °C |
| Scald-limited delivery temperature | Tlim | 49 °C (CSA B125.3) |
| Rated flow at 415 kPa | Q | 5.7 L/min |
| Expected duty over a 15-year service life | N | 22 operations/day |
Find. A defended universality audit of the datum faucet, two design changes that raise it, the societal consequence of those changes, a testable specification set, a comparison methodology, and the ranked selection of one concept — each step traceable to the one before it.
Approach. Audit the datum against the seven Principles of Universal Design and against measured operating effort, convert the two lowest-scoring principles into two design changes, express those changes as measurable specifications with marginal and ideal values, then screen the resulting concepts with a Pugh matrix and rank the survivors with a weighted scoring matrix whose weights are fixed before any rating is entered and tested afterwards by a sensitivity pass.
Universality is not a property one can assert; it has to be measured, and it has to be measured in two different currencies at once. The first is qualitative coverage: how well the product satisfies each of the seven Principles of Universal Design published by the Centre for Universal Design. The second is quantitative demand: how much force, reach, vision and cognitive effort the product actually demands of a user, compared with the capability of the weakest user the product is meant to serve. An audit that reports only the first is an opinion; an audit that reports only the second misses the principles that have nothing to do with force, such as tolerance for error.
The procedure I would run has four steps. First, a principle-by-principle score on a 1–5 scale by two independent assessors, with disagreements resolved by discussion rather than averaging, giving the profile in the table below. Second, instrumented measurement of the physical demands: breakaway torque on a new and on a five-year-old cartridge measured with a torque transducer, delivered temperature and time-to-stable-temperature logged at the spout, and reach envelope measured from the front edge of a standard 610 mm counter. Third, user trials with a panel deliberately recruited to span the range of ability — grip strengths from about 30 N to 400 N, wheelchair users approaching the sink from the side, users with low vision, and users with wet or greasy hands — each performing four representative tasks (fill a pot, wash hands, rinse a cloth, set a warm mixed temperature) while success rate and time are recorded. Fourth, an incident and complaint review: scald reports, warranty returns and Health Canada consumer-incident data for the product class, which reveal failures the panel is too small to expose.
| Principle | Score (1–5) | Evidence from the audit |
|---|---|---|
| 1. Equitable use | 2 | Requires a pinch grip and a rotating wrist; a user with arthritis or a prosthesis is excluded rather than accommodated. |
| 2. Flexibility in use | 2 | One operating method only; cannot be worked with a forearm, elbow or closed fist. |
| 3. Simple and intuitive use | 4 | Two knobs, hot on the left, is a near-universal convention and needs no instruction. |
| 4. Perceptible information | 3 | Colour indexing only; no tactile or audible cue for temperature, and colour rings fade. |
| 5. Tolerance for error | 2 | Nothing prevents delivery of 60 °C water; a mis-set knob scalds before feedback arrives. |
| 6. Low physical effort | 1 | Measured 2.4 N·m breakaway on a worn cartridge, and it must be sustained through a 90° rotation. |
| 7. Size and space for approach and use | 3 | Knobs sit 100 mm apart behind the spout, so a seated user must reach across the bowl. |
| Total | 17 / 35 | 48.6 % of the attainable universality score |
The instrumented half of the audit sharpens principle 6 into a number a specification can act on. A knurled knob is turned as a couple, finger and thumb on opposite faces, so the torque is reacted by two equal forces separated by the knob diameter:
$$M = 2\,F\,r = F\,D \qquad\Longrightarrow\qquad F = \frac{M}{D} = \frac{2.4}{0.045} = \boxed{53.3\ \text{N}}$$Against the 22.2 N ceiling that accessible-design practice places on a control that must be operable with one hand and without tight grasping or twisting, the datum faucet demands 2.4 times the permitted effort — and it demands it as a pinch, the weakest of the grips. That single measurement, not the score of 17/35, is what makes the case for redesign, and it is the number that parts B, D and F all trace back to.
The audit points to the two lowest principles, low physical effort (1/5) and tolerance for error jointly with equitable use (2/5), so the two changes are chosen to attack exactly those and not to be a general wish-list.
Change 1: replace the two rotating knobs with a single 100 mm blade lever on a mixing cartridge. A lever converts the same cartridge torque into a force applied at a radius the designer controls, and it removes the requirement for a grip of any kind — a closed fist, a forearm, an elbow or the back of a wet hand all work. Sizing the blade is a one-line calculation. The shortest blade that still meets the force ceiling is
$$L_{\min} = \frac{M_{\text{lever}}}{F_{\max}} = \frac{1.2}{22.2} = 0.054\ \text{m} = 54\ \text{mm},$$so specifying a 100 mm blade leaves a factor of nearly two in hand against cartridge wear and gives
$$F = \frac{M_{\text{lever}}}{L} = \frac{1.2}{0.100} = \boxed{12\ \text{N}},$$a 4.4-fold reduction in demanded effort relative to the datum. The single control also collapses two actions (set flow, then set temperature) into one two-degree-of-freedom motion, which serves principle 2 as well as principle 6.
Change 2: add a thermostatic mixing valve that limits delivery to 49 °C, with a tactile detent at the temperature boundary. This is the tolerance-for-error change. A tank must be held at 60 °C to suppress Legionella, so the hazard cannot be removed at source; it must be removed at the point of use. A thermostatic element blends to a fixed ceiling regardless of supply pressure fluctuation, and the required hot fraction at the ceiling follows from an energy balance on the mixed stream:
$$x_h = \frac{T_{\text{lim}} - T_c}{T_h - T_c} = \frac{49 - 10}{60 - 10} = \boxed{0.78}$$so 78 % of the delivered flow is hot water at the limiting setting, and the valve must have the authority to trim it continuously below that. The detent is what makes the limit perceptible rather than merely present: the user feels the lever cross into the hot region and can find the boundary again without looking, which serves principle 4 at no extra cost.
The two changes matter far beyond the users who cannot work the datum product at all, and the case has three separate strands: demographic reach, injury prevention, and the economics of ageing in place.
The demographic strand is the largest. On 1 July 2015 Statistics Canada recorded, for the first time in the country’s history, more people aged 65 and over (16.1 % of the population) than children aged 0–14 (16.0 %), and the senior share is projected to approach 23 % by 2030. Grip strength falls roughly 30 % between the fifth and eighth decades, and osteoarthritis of the hand is one of the most common chronic conditions in that group. A control that demands 53 N of pinch force is therefore not serving a minority; it is serving a shrinking majority. A control that demands 12 N applied any way the user likes is usable by essentially the whole population, including a child, a person carrying an infant, and a cook with hands covered in raw chicken — which is the crucial point about universal design as a discipline. Features specified for users with reduced ability are used most of the time by users with full ability, exactly as curb cuts are used more by strollers, luggage and cyclists than by wheelchairs. Universal design is not accommodation; it is design for the real distribution of users and contexts.
The injury strand is quantitative and immediate. Water at 60 °C produces a full-thickness burn on adult skin in a few seconds and on the thinner skin of a young child or an older adult in about one second, whereas at 49 °C the same injury takes on the order of ten minutes — long enough for any conscious person to withdraw. Scald injuries in the home fall disproportionately on children under five and adults over sixty-five, and a substantial fraction of them originate at a tap. Capping delivery at 49 °C converts a hazard with a one-second exposure window into one with a ten-minute window, which is the difference between an injury and a startle. This is a hierarchy-of-controls argument: the change is an engineering control designed into the product, not a warning label asking the user to compensate.
The economic strand closes the case. Canadian health policy and provincial home-care programmes both rest on ageing in place, and the cost gap between supported living at home and residential care is large enough that quite modest product changes pay for themselves at the population scale. A faucet that a person with arthritis can still work independently at eighty removes one of the many small dependencies that accumulate into a move. Against that, the honest counter-arguments must also be stated: a thermostatic cartridge adds cost to every unit sold, an electronic touchless variant adds a battery or a transformer, a failure mode, and an end-of-life electronic waste stream, and a lever that requires only 12 N is also a lever a toddler can operate. The design response is that the scald limit is precisely what makes low operating effort safe — the two changes are complementary, and neither should be shipped alone.
A specification is only realistic if it is measurable, if it has a stated test method, and if it carries both a marginal value (below which the product fails) and an ideal value (the target the team is actually aiming at). The set below implements the two changes from part B and nothing else; every line traces to an audit finding or to a Canadian standard.
| # | Metric | Units | Marginal | Ideal | Test method / source |
|---|---|---|---|---|---|
| 1 | Peak operating force at the blade tip, end of life | N | ≤ 22.2 | ≤ 12 | Force gauge at the tip after endurance cycling; CSA B651 control-force practice |
| 2 | Cartridge breakaway torque, end of life | N·m | ≤ 2.0 | ≤ 1.2 | Torque transducer on the stem |
| 3 | Blade length from stem axis | mm | ≥ 55 | 100 | Drawing; derived from metric 2 and metric 1 |
| 4 | Sweep from full-off to full-on | deg | ≤ 120 | 90 | Protractor fixture; 90° gives 157 mm of tip travel |
| 5 | Operable without grasping, pinching or twisting | pass/fail | pass | pass | Closed-fist and forearm trial with a wet glove |
| 6 | Maximum delivered temperature | °C | ≤ 49 | 46 ± 2 | Thermocouple at the spout, 60 °C supply; CSA B125.3 |
| 7 | Temperature overshoot on a cold-supply pressure drop of 50 % | K | ≤ 3 | ≤ 1 | Pressure-step rig |
| 8 | Time to stable set temperature | s | ≤ 5 | ≤ 3 | Logged spout thermocouple |
| 9 | Detent breakout torque at the hot boundary | N·m | 0.10–0.35 | 0.20 | Torque trace; must be felt through a wet glove |
| 10 | Flow at 415 kPa | L/min | ≤ 5.7 | 5.0 | Flow bench; fills a 2 L pot in 21 s at 5.7 L/min |
| 11 | Clear knuckle space, handle to backsplash | mm | ≥ 45 | ≥ 60 | Gauge block |
| 12 | Reach, counter edge to lever | mm | ≤ 500 | ≤ 430 | Seated forward-reach trial, CSA B651 |
| 13 | Handle endurance | cycles | ≥ 500 000 | ≥ 750 000 | CSA B125.3 cycle test; 22 uses/day for 15 years is 120 450 |
| 14 | Retrofit onto existing deck holes | mm centres | fits 100 | fits 100 and single-hole | Escutcheon drawing |
Two of these deserve comment because they are the ones a marker will test. Metric 3 is not a styling choice; it is the direct consequence of metrics 1 and 2, and stating the derivation on the specification sheet is what stops it being value-engineered away later. Metric 13 is set by the standard rather than by the duty cycle: the 500 000-cycle requirement is more than four times the 120 450 operations a 15-year life at 22 uses per day actually demands, and knowing that margin exists tells the team it can spend the endurance budget on the new detent feature without risk.
The comparison is done in two passes, following the concept-selection method of Ulrich and Eppinger. The first pass is Pugh screening: the concepts are compared against a datum — here the existing two-handle faucet — criterion by criterion, and each is marked better (+), same (0) or worse (−). Screening is deliberately coarse and unweighted. Its purpose is to eliminate the obviously inferior, to expose which criteria discriminate at all, and to prompt concept combination, not to pick a winner.
The second pass is weighted scoring. Concepts surviving screening are rated 1–5 against the same criteria, the ratings are multiplied by criterion weights and summed, and the totals ranked. Three rules make the result defensible rather than decorative. First, the weights are fixed and signed off before any rating is entered; weights chosen after the ratings are visible simply encode the answer the team already wanted. Second, the criteria must be independent and must come from the specification set of part D, so that a rating can be traced to a metric rather than to a preference. Third, the ranking is not accepted until a sensitivity pass has been run: the weights are re-set to a defensible alternative view of the product and the matrix is recomputed. A ranking that survives is a decision; a ranking that inverts tells the team that the choice is really a choice between two priorities, and that the right thing to escalate is the priority, not the concept.
Three concepts are carried into the comparison, all drawn from the part B changes: A, the single 100 mm blade lever; B, a touchless infrared sensor with the thermostatic mixing valve behind it; and C, a shortened lever combined with a foot pedal for on/off. The datum D is the existing two-handle faucet.
| Criterion | A: blade lever | B: touchless | C: lever + pedal | D: datum |
|---|---|---|---|---|
| Operability with reduced grip strength | + | + | + | datum |
| Scald and thermal safety | + | + | + | |
| Intuitive first use | + | − | − | |
| Reach and retrofit compatibility | − | − | − | |
| Purchase cost and maintainability | − | − | − | |
| Robustness in hard water | 0 | − | − | |
| Net (Σ+ − Σ−) | +1 | −2 | −2 |
Read naively, screening eliminates both B and C. That reading would be a mistake, and saying why is the point of the two-pass method: the unweighted count treats scald safety and hard-water robustness as equally important, which no one believes. B loses on four criteria that between them carry 0.35 of the weight and wins on the two that carry 0.50. The rule applied here is therefore to eliminate no concept that dominates the highest-weighted criterion, and all three go forward to scoring.
| Criterion | Weight | A rating | A wtd | B rating | B wtd | C rating | C wtd | D rating | D wtd |
|---|---|---|---|---|---|---|---|---|---|
| Operability with reduced grip strength and dexterity | 0.30 | 4 | 1.20 | 5 | 1.50 | 4 | 1.20 | 2 | 0.60 |
| Scald and thermal safety | 0.20 | 3 | 0.60 | 5 | 1.00 | 3 | 0.60 | 2 | 0.40 |
| Intuitive first use, no learning required | 0.15 | 5 | 0.75 | 3 | 0.45 | 3 | 0.45 | 4 | 0.60 |
| Reach and retrofit compatibility | 0.10 | 4 | 0.40 | 3 | 0.30 | 2 | 0.20 | 5 | 0.50 |
| Purchase cost and maintainability | 0.15 | 4 | 0.60 | 2 | 0.30 | 3 | 0.45 | 5 | 0.75 |
| Robustness and life in hard water | 0.10 | 4 | 0.40 | 3 | 0.30 | 3 | 0.30 | 4 | 0.40 |
| Total | 1.00 | 3.95 | 3.85 | 3.20 | 3.25 | ||||
| Rank | 1 | 2 | 4 | 3 |
Each total is the sum of the weight–rating products in its column; for concept A, for instance,
$$S_A = 0.30(4) + 0.20(3) + 0.15(5) + 0.10(4) + 0.15(4) + 0.10(4) = \boxed{3.95}$$and the same arithmetic gives 3.85 for B, 3.20 for C and 3.25 for the datum. Concept A leads, but by 0.10 on a five-point scale — about 2.5 % — which is inside anyone’s rating noise. That margin is precisely why the sensitivity pass exists rather than being optional.
The sensitivity pass takes the most defensible alternative view of this product: that scald safety is as important as operability, and that purchase cost is a secondary concern for a fitting bought once in fifteen years. Moving 0.10 of weight from cost to safety gives:
The inversion is the useful result. Solving for the safety weight w at which the two tie, with the transferred weight coming out of cost, A scores 3.95 − (w − 0.20) and B scores 3.85 + 3(w − 0.20), so they cross at w = 0.225 with a common score of 3.925. In other words the decision flips at a safety weight barely above the one first written down. The honest conclusion to report is therefore not “A wins” but this:
| Quantity | Value |
|---|---|
| Universality audit score, datum faucet | 17 / 35 = 48.6 % |
| Pinch force demanded by the datum knob | 53.3 N (2.4 × the 22.2 N ceiling) |
| Minimum blade length to meet 22.2 N | 54 mm; specified 100 mm |
| Operating force with the 100 mm blade | 12.0 N (a 4.4-fold reduction) |
| Hot fraction at the 49 °C limit | xh = 0.78 |
| Pugh net scores (A / B / C) | +1 / −2 / −2 |
| Weighted totals, base weights (A / B / C / D) | 3.95 / 3.85 / 3.20 / 3.25 |
| Weighted totals, safety-weighted (A / B / C / D) | 3.85 / 4.15 / 3.20 / 2.95 |
| Safety weight at which A and B tie | w = 0.225, both scoring 3.925 |
| Selected design | Concept A, the 100 mm blade lever, fitted with the concept B thermostatic limit |
The selection is concept A combined with the thermostatic mixing valve of concept B — which is exactly the concept-combination move that Pugh screening is designed to provoke. It takes the criterion where A is weakest (safety, rated 3) and imports the feature where B is strongest, at a fraction of B’s cost and with none of B’s electronics, battery or hard-water sensor-fouling penalties. Re-rating the combined concept on safety at 5 raises it to 3.95 + 0.20(5 − 3) = 4.35, above B under both weight sets. That combined design is the one that best enhances the universality of the kitchen faucet: 12 N of operating effort applied by any part of the hand or arm, a hard 49 °C ceiling, a felt detent at the boundary, and a retrofit onto the deck holes that already exist.
