Who
never dreamed of being an astronaut as a kid ? Certainly not me. What,
with my bed tucked under a staircase, it felt like the tight quarters of
a spaceship's cabin like in TinTin's Destination Moon !
So, what if the van's bed was cradled in mobile arms, like Starship in SpaceX's Chopsticks ?
And that's how a race with SpaceX started, to see who finishes their chopsticks first !
On mine, the 2 vertical guide rails are made out of 2" square aluminum tubing,
3/16" thick. The vertical slot in each was cut on the table saw. After
installing rivet nuts in the van's wall, and angled spacers around each
bolt hole to compensate for the wall's curvature, the rails could be
installed:
An ANSI #35 chain is used to move each carriage up and down:
The carriages were cut out of a 1/2" thick aluminum plate on the table saw.
In the pic above, note the slotted 1/2" shaft at the bottom that connects the rails' bottom sprockets together. Detail:
Upper sprocket detail:
Chain tensioner and offset brace for the front arm:
The chopsticks (arms) are made out of a 3/16" aluminum sheet. Each was bent 90° inward, along the bottom, to create a 1" lip that will hold the underside of the bed platform.
Unlike
the front arm, the rear arm does not include an offset bracket as the
head of the bed does not need to extend past the rear guide rail. So, it
is bolted directly to its carriage:
The targeted raise / lower time for the bed was ~30 seconds to provide ample time to the user to react (by letting go of the momentary switch) should things go wrong (and they will, sooner or later). That translates into ~15 rpm for the bottom sprockets (13 teeth, 1.75" OD, ~30 inches arm course).
A worm gear motor powers the system: uxcell DC12V 60W 60RPM 8NM Worm Gear Motor JCF63R
Slowing the rotation down to 15 rpm is accomplished via a combination of 2.5:1 reducer (25 & 10 teeth sprockets) and stepping down the voltage from 12v to ~9v with a DC-DC converter.
Assuming a less than 50KG total bed system, the torque needed at the bottom sprockets is 50 * 10 * 1.75 * 0.0254 / 2 = 11NM. So, the nominal torque experienced by the motor is 11 / 2.5 = 4.4NM. That's well within that motor's 8NM spec, and provides a ~45% design margin.
A simple chain tensioner,
from a mini bike, was installed to smooth out the chain's parasitic
motions. That's always a long term with sprockets presenting a tight
turn radius, like the small 35B10 used for the bottom sprockets.
And voila, a so-far-working lopsided bed lift system. This was not a simple build, and many things can go wrong over time with such a chains & sprockets setup. Especially if not properly maintained:
tension, grease, stop switches, shaft couplers, tight bolts... If there
ever is a next time, I might look further into other mechanisms, like
ball screw rails, rack & pinion or gravity assisted belt drums.
For now though, playing with my own Starship Chopsticks is much fun... and starting to severely impact my focus 😅
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