I’ve been seeing a lot of teams basing their initial designs off of parallel linkage designs that have worked in the past, and ignoring the 40" tall elephant in the game for the time being. I originally came up with this design near the end of the Gateway season, and thought it was the logical extension of the Watt’s 6 bar design that’s become so popular. Seeing as Sack Attack did not require teams to try to reach higher with their lift systems, it never showed up independently. I’ve decided to post it to give teams a starting point for more sophisticated linkages to reach new heights, and try break the mindset that “good enough” is always the way to go.
It’s an 8 bar system, composed of three parallelogram 4 bars connected to each other in a geometry which resembles the Watt’s Parallelogram 6 bar, though not the doubled up 6 bar that’s been typically referred to as an “8 bar” around here. It has a similar effect to the 6 bar design, but allows for much greater height to be gained from the vertical section in the middle.
Even without taking into account extra height from an endeffector, this linkage on its own comfortably reaches the 40" hanging bar while crossing under the barrier. It does so with such comfort that you aren’t forced into very specific geometry as with some other lifts, and can modify things a bit more to suit your design’s exact needs. It does not rely on chains or gears to maintain its motion, which can be sources of slop, and doesn’t have the poor transmission angles or numerous joints of a scissor lift. It should be able to be constructed in a similar manner to other linkages, though I’ll leave implementation to the teams.
But the most interesting thing you’ll find about this linkage if you actually build it is that as drawn, it falls into a small category of linkages known as Gruebler’s Paradoxes. This means that according to Gruebler’s equation, which predicts the degree of freedom of a linkage, it should lock up with zero degrees of freedom. However, due to the exact geometry of the linkage, one pin joint in each of the small diamond shaped regions is redundant. Keeping these redundant joints in place adds more friction, but does an incredible job at keeping the linkage rigid throughout it’s motion, as any slop or deformation will fight these redundant joints. You may want to remove the joints, however, if you want to offset one or more of the stages to produce non-parallel motion.