Safety Review

Since I am designing a desk that is meant to be operated by laypeople without specific training, so it is important to minimize personal injury hazards and making the device as robust to improper operation as possible. Natural selection has plenty of mechanisms through which to operate — no need for engineered products to create new ones.

Tipping

One of major risks associated with the desk is having it tip over due to a lateral load from a user leaning on the edge of the desktop (e.g., leaning in to check out a particularly interesting data point, perhaps). The strategy to address this risk is place the center of gravity and coefficient of friction between the adjustable standing desk and an existing table such that lateral loads would cause the desk to slide instead of tip. The tipping analysis can be found in the “tip vs. slide” sheet in this document. Using nominal values for these parameters, the desk will start sliding when subjected to a lateral load only 60% of that required to tip it over. This translates to a safety factor of 1.6, which doesn’t look like much for structural design, but is sufficient in this case because of the small number of input parameters (coefficient of friction, center of gravity) which are known with high certainty.

Loss-of-Power Events

Another risk is for the desktop to fall when power to the motors is unexpectedly lost. This idea was previously addressed in the concept generation phase where I did some analysis on the self-locking, leadscrew-free design (and ultimately rejected it in favor of a conventional leadscrew design). By using a relatively low-helix, single start, and sliding contact screw, I was able to make my drive system self-locking. Regardless of the loads placed on the desktop, it is impossible to back-drive the screws. The detent torque in the motors provide an additional factor of safety, but is ultimately not needed in this case.

Pinch Hazards

Having moving components in the desk means I also need to consider the presence of pinch points. The two critical pinch points are between the sides of the desktop and the exterior surface of the keeper rails, and between the bottom of the desktop and the cross ties. The clearance gap between desktop and column is approximately 2 mm. This comfortably protects against the ingress of objects above 2.5 mm in characteristic dimension. This is a IP3X rating!

The other pinch point between desktop and cross tie has a gap width that varies depending on where the desktop is within its range of motion. This does mean that bodily appendages can plausibly be trapped in this space. However, this risk is mitigated by the fact that this pinch point is obscured by the 250-mm overhang of the desktop. That is, in order to have a finger pinched within this gap, which occurs when the desktop is within 50 mm of the base, one would have to stick one’s arm below the desktop at a really awkward angle.

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