Original Part:

The original part provided the center point which the umbrella’s fabric stretched around. Providing this centerpoint was critically important, since no material is ever truly homogenous. The rigid centerpoint provided more uniform stretching to the fabric, which helped maintain the material longer by preventing stress buildup in specific regions. Additionally, the original part was injection molded ABS, and featured knurling on the cap of the screw for fingers.

Failure Description:

The cap of the screw sheared off one day after dropping to the floor. The screw did not have fillets connecting the threads to the cap. The injection molding process also left a cavity at the connection point, where the sprue had connected into the mold during production. The stress concentrations likely multiplied the sharp impact force, which lead to a catastrophic fracture in the part.

Replacement Part Design:

The main interfaces I wanted to preserve were the screw threads and the human interface for assembling the screw with the umbrella. The screw threads were easy to measure with calipers and thread gauges, but the knurling was harder to solve. I did not trust my 3D printer to have a resolution low enough to print knurls, so I opted to instead cut sections of the cap out to improve finger grip.

Failure Mitigation:

There was no space for fillets in the interface space with the umbrella, but there is no longer a cavity in the threads. The part is also made of PETG, which should give it some flexibility during impact events.