Rescuing a book stand after a failed prototype trial
A Grade 11 learner with juvenile arthritis has limited grip: she can lift about 1 kg and cannot pinch hard. She needs a book stand for a wheelchair-accessible desk 730 mm high that will hold a 1.6 kg textbook open at a tilt between 55° and 70°, adjusted with one hand. Your class has a N$250 budget and three weeks. Prototype 1 used an 8 mm acrylic back panel hinged to a 12 mm ply base with a bought friction hinge rated at 1.2 N·m, an aluminium prop rod with five saw-cut notches, and plastic page clips. In trials the clips snapped during the third lesson, the 55° notch slipped and the panel dropped to about 20°, the base slid 40 mm across the desk when she pushed the book away, and she could not open the clips with one hand. Two learners want to start again from scratch; one wants to keep the panel and change the fittings.
1. Rewrite the brief as four measurable criteria that the trial exposed, and decide whether the class should restart or modify. Justify the decision from the evidence.
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Measurable criteria drawn from the failures
- The stand holds a 1.6 kg book at any selected tilt from 55° to 70° with no more than 5° of drift over 10 minutes.
- Every adjustment and the page retention can be operated with one hand, requiring no more than about 10 N of grip.
- The base moves no more than 5 mm when a 20 N horizontal push is applied at the top of the loaded panel.
- The whole stand fits a 730 mm desk, costs no more than N$250, and survives being knocked off the desk once.
Decision: modify, do not restart. The evidence locates every failure in the interface, not in the structure. The acrylic panel did not crack, the hinge did not break, and the base did not distort - the clips, the notch and the base friction failed. Those are three cheap parts. Restarting throws away two of the three weeks and the acrylic already cut and paid for, and it would very likely reproduce the same fittings because the underlying specification was vague, not the geometry.
The one condition for a restart: if the panel's hinge line is positioned so that 70° is geometrically impossible, the concept is wrong. Check that first with a cardboard mock-up before spending anything.
2. Using a simple moment estimate, decide whether the friction hinge alone can hold the book at 55°, and propose a fitting that solves the problem.
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Estimate the overturning moment about the hinge line.
Weight of book: 1.6 kg × 9.81 = 15.7 N.
Take the book's centre of mass as 0.18 m up the panel from the hinge. At a 55° tilt the horizontal lever arm is 0.18 × cos 55° = 0.103 m.
Moment = 15.7 N × 0.103 m ≈ 1.6 N·m.
The hinge is rated at 1.2 N·m, so it is already overloaded before the page clips and the learner's hand add anything. That explains the observed drop: the hinge was never holding the load - the saw-cut notch was, and a saw cut has no positive stop, so once the rod crept out of the notch the panel fell to whatever angle the hinge could sustain.
Solution: replace the friction hinge and prop rod with a positive locking element - a ratchet quadrant or a spring-loaded pin through 5 mm holes at 55°, 60°, 65° and 70°, rated at least 3 N·m so there is a factor of roughly two on the calculated moment. A pin also gives an audible and tactile click, which matters for a user who cannot see the underside of the stand. Saw-cut notches should never be used where the load can lift the rod out.
3. Describe how you would record this iteration in the portfolio so that it counts as evidence of the design process rather than a list of failures.
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Turn each failure into a numbered evidence chain.
For every fault, record four things side by side: the test as carried out (with the load and the angle), the measured result, the criterion it breached, and the decision that followed. For example: Test 3 - loaded to 1.6 kg at 55°, measured 20° after 40 s; criterion 1 requires no more than 5° drift; decision - replace friction hinge with ratchet quadrant rated 3 N·m. Annotated photographs with a steel rule in frame are stronger than description alone, and the broken clips should be kept, labelled and dated, not thrown away.
Show the loop, not the object. Include a short comparison table of the two competing fittings with cost, grip force and locking security, and a signed note from the user recording what she could and could not do. Add a dated revision of the brief showing the criteria tightening from 'adjustable' to '55°–70° with no more than 5° drift' - that change is itself the strongest evidence that evaluation happened.
What moderators look for: a justified decision, not neatness. A well-documented failure and a reasoned fix usually score higher than a tidy model with no evaluation, because it shows the candidate can act on evidence.