Original Part:

The original part physically connected the left ear cup to the head band. The two sets of interfaces (one set to the ear cup, one feature to the headband) were comprised of concentric circles. Radial cuts to these outer circles became motion stops when assembled, limiting the rotational range of motion for each interface. It also had wire paths to run cabling across to the right ear cup. The original part was injection molded ABS, and as such had pockets removed across the piece to maintain cross-sectional area while molding.

Failure Description:

The “top” stem, which interfaced with the headband, sheared off one evening while donning the headset. The lack of fillets around the base of the stem likely lead to stress concentration around the base. Additionally, repeatedly flexing the joint through simply using the headset likely lead to fatigue fractures. The stress concentrations probably catalyzed the fatigue fractures, and led to the inevitable failure.

Replacement Part Design:

The most important features to get right were the connection interfaces. I measured the inner and outer circles on both to ensure the replacement would function the same as the original. After ensuring the interfaces were spaced correctly, I added the ear cup clearance and cable clearance to ensure the replacement would fit into the same space.

Failure Mitigation:

There was no space on either interface to add fillets to their bases, but 3D printing the replacement provided two benefits over the original design:

1. There was no need to cut pockets in the material, so the body of the material has a slightly higher maximum stress before it fractures.

2. I designed the part to print in an orientation where shearing off the stem requires more stress than cracking down the stem. The part’s functions can survive a crack propagating down, and it can be seen easily, so replacing the part before catastrophic failure is easier now.