Machine Design

Sloppy design leads to choking death

Infant twins were crawling around the house at their mother's feet when, suddenly, one twin began choking.

Edited by Lawrence Kren

The mother could not stop it and called 911. Paramedics did everything possible, but the child was dead on arrival at the hospital.

When an autopsy revealed nothing obvious, one of the doctors decided to take X-rays of the child's throat and found the problem: a cylindrical object had lodged there and cut off breathing. The soft-plastic cylinder turned out to be part of a cabinet. It pressed into a hard-plastic mount, forming a cushion to keep the cabinet drawers from slamming shut.

As an expert witness in the case, I examined the cabinet drawers and found about one-third of the soft-plastic parts missing. We removed the remaining parts as evidence and to prevent further tragedy.

Engineering drawings of the parts revealed that diameter tolerances could, in the worst case, leave about 0.001-in. clearance between the two parts. However, both parts were injection molded and thought to be consistent in size. Micrometer measurements of parts taken from the home seemed consistent, though I had no confidence in measurements of the soft component past two decimal places.

I took some of the parts to a quality-control laboratory that was equipped to make noncontact and soft-touch measurements accurate to four decimal places. These measurements solved the puzzle. The parts were, in fact, consistent in size, but not to print. The soft-plastic part was too long. The assemblies looked fine initially but, over time, the softer part would push itself out of the hard-plastic diameter and fall to the floor.

During the discovery phase of the case, it came out that the manufacturer hadn't conducted a hazard analysis. It did test the cabinetry as an assembly, but never focused on the cushion system alone. Had the company done so, it may have discovered the problem and come up with a fix.

For example, one possibility is to replace the press-fit cylindrical design with off-the-shelf, adhesive-backed rectangular cushions. If staying with the original design, glue the soft-plastic piece inside the hard-plastic diameter. Another option is to include a large-diameter hole down the center of the soft-plastic part. That way a child who swallowed it could still breathe until help arrived.

is a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a question about safety? You can reach Lanny at [email protected].

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