After ruminating on the overall architecture of my desk for the past couple of weeks, I have decided to commit to a desk-mount design with a single linear motion axis. My reason for pursuing a desk-mounted design is so that the new device would fit into current and future spaces that I would live in. I anticipate living in pre-furnished spaces in the near future, so an adjustable desk that complements an existing desk is preferable to an additional piece of free-standing furniture.
I have also decided to limit myself to a single axis of motion in order to control costs and mechanical complexity. In my experience using desks, I have had little occasion to desire adjustment other than height. I am currently considering a single-column cantilever design as well as a twin-column design. Read more about these concepts below.
I am setting a total allowable error of 5 mm along the height dimension for my desk. 10% of this is allocated to the actuator (I think cheap bearings and structural members would contribute significantly more error than purchased leadscrews), and the remainder is evenly split between bearings and structure. As I mentioned previously, user experienced is degraded far more significantly by “free wiggle” geometric errors than load-induced deflections that are associated with significant resistance.
This is an early concept that I started off with, and it has stayed around through the process largely because of its simplicity. Additionally, there is something visually appealing (to me) about a cantilevered desk with a substantial support column made from wood.
The drawbacks are primarily associated with the relatively long cantilever length, which natural translates to larger deflections and higher forces at the bearing.
This alternative design has the tabletop simply supported (an idealization) between two bearings each running in a column. These columns are located centrally at each of the sides of the desk. The advantage of this design is that it halves the moment arm for the worst-case scenario where all of the design load is concentrated at one corner of the desk. This translates to lower forces felt at the bearing and smaller deflections in the tabletop.
However, the need to double up on structural material and drive mechanisms (need to somehow actuate both sides to prevent racking) would increase cost. Additionally, the visually interesting bearing columns are not moved to the sides and out of direct view for the user. And finally, the columns may turn into a nuisance by reducing elbow room.
In the interest of time, I am going to focus on the single-column design for my exploration this week. The twin-column design will serve as a backup that I can come back to if I run into unanticipated and terminal problems with the single-column design.
See my first-order error budget for the single-column design here.
Quick Note on Self-Locking Bearing Concept
Avid readers (there are not that many…) of this site will remember reading about an idea I had to exploit the binding characteristics of sliding bearings to achieve self-locking. The primary reason for doing that was to allow the use of a more compact and lighter belt or chain drive system. Unlike leadscrews, belt or chain drive systems generally don’t have sufficient friction in the drivechain to resist substantial forces when powered off.
I spent some time this week analyzing a concept that has a chain attached some distance away from the center-of-stiffness of the bearing. The idea was to design the bearing so that the weight of the table and things piled on top would cause the bearing to bind up and resist moving downwards. When the actuator is switched on, it pulls on the chain to relieve part of the binding moment, thereby releasing the table to move upwards. Unfortunately, my analysis showed that the reciprocal action to this — when the user wants to move the table downwards — actually increases the binding moment and causes even more friction. I did some what-ifs scenario in a spreadsheet and found that my actuator would have to exert thousands of Newtons to overcome the friction and move the table downwards. This is not only difficult to achieve using a small stepper motor, but also very very difficult to achieve without breaking the structure or bearings due to the large forces involved. Ultimately, I think there are too many drawbacks to this idea for it to be worth pursuing further, so back to leadscrews it is.