Assistive Jar Opener Prototype Failure
An eighth-grade design team is building a one-handed jar opener for a client with arthritis. The client can grip a 25 mm handle and apply about 10 N of force but cannot twist. The team's 3D-printed prototype has a 30 mm handle, a rubber strap, and a 120 mm lever. In testing, it opens a 60 mm lid at 1.4 N·m, but the client needs 2.2 N·m for a new jar. The strap slips when the lid is wet, and the handle flexes 8 mm under load. The team has one week and two spools of PLA; the school printer takes 5 hours per print. They want to add a longer lever and print in the same orientation. The teacher must decide what feedback to give and what the team should test next.
1. Diagnose the prototype's main failure modes and decide which change should be tested first. Justify your priority.
Read the worked answer
The device has three linked problems. First, it delivers only 1.4 N·m against a 2.2 N·m requirement, a shortfall of about 0.8 N·m, or roughly 57 percent more torque needed. Second, the strap slips when wet, so some applied force never reaches the lid. Third, the handle flexes 8 mm, which wastes force and may make the grip feel unstable. The 30 mm handle also exceeds the client's stated 25 mm grip capacity, so ergonomics may be limiting force input.
A longer lever is a reasonable hypothesis because torque depends on lever arm, but it should not be the first or only change. If the strap slips, a longer lever may simply increase slip. I would prioritize the lid interface and handle stiffness, then retest the lever length. The team should change one variable at a time: strap material or geometry in wet and dry conditions, then handle cross-section or grip diameter, then lever length. This order tests whether the device can transmit the client's available force before asking for more leverage.
2. Design a controlled test plan the team can complete in one week. Include what to measure and how to reduce test error.
Read the worked answer
Use the same jar and lid for comparisons. Tighten the lid to a repeatable setting, or use a torque wrench to record breakaway torque. Attach a spring scale perpendicular to the lever at a marked distance and pull gradually; torque equals force times perpendicular lever-arm distance. Record the peak force before motion and calculate torque. Run three trials dry and three trials wet for each prototype version.
Measure four things: breakaway torque, strap slip, handle deflection at the grip, and client comfort or grip security. Photograph or mark the strap starting position so slip is visible. Use the same client or a supervised adult proxy for preliminary tests, and keep the client trial brief and controlled.
Reduce error by marking the lever arm, using the same lid, allowing the strap to settle, and randomizing the order of wet and dry trials. The team should print a small test coupon or partial handle before committing a full 5-hour print. A successful result is not just a higher number; it is reaching at least 2.2 N·m without slip or excessive flex while fitting the client's grip.
3. Explain how print orientation could affect the handle and what tradeoff the team faces with one week and two spools of PLA.
Read the worked answer
In FDM printing, the bond between layers is usually weaker than the material within a layer. If the handle is oriented so bending stress pulls layers apart, it may delaminate or flex more than expected. Orienting the handle so the main bending load runs in the plane of the layers usually improves strength, but it may require supports, leave a rougher surface, or increase print time. The current 8 mm deflection suggests the design needs more section stiffness, not just a different orientation.
The tradeoff is time and material. At 5 hours per full print and only two spools, the team cannot iterate many complete handles. It should print small coupons or a shortened handle section to compare orientations, add ribs or increase thickness where deflection is highest, and reserve one full print for the best candidate. The final choice should be based on deflection and grip tests, not on print appearance. If the week runs short, a temporary reinforced handle or a commercially made grip could be used for client testing while the printed design is revised.