Hopefully I can clear up some of the confusion on naming of linkages…
A 4 bar linkage is called that because there are 4 separate links (or bars), each of which is connected to 2 pivots, one at each end. The bars are counted as they appear in the plane in which movement occurs. For example:
Two of the links are the black arms.
The third link is made up from part of the vertical support that is attached to the robot’s chassis; this is the ground link.
The fourth link is the yellow part which the robot’s manipulator (claw, conveyor, whatever) is attached to.
The pink diamonds are pivots (pin joints).
Many “4-bar” vex robots in fact have 2 of these linkages, one on each side of the manipulator (claw, ramp, conveyor). This is what I think of when I hear “double 4-bar”.
Now for the 6-bar:
Astute observers will note that this is made of 2 four-bar linkages. The first is made up from bars 1, 5, 3, 4. The second is made up from bars 1, 6, 2, 4.
There are 7 pivots, but only 6 bars.
Bar 1 is the longest part of the arm, and has 3 pivots.
Bar 2 is the top part of the arm, and bar 3 is the bottom part. Both have 2 pivots.
Bar 4 is a vertical bar with 3 pivots.
Bar 5 is made up from part of the vertical support that is attached to the robot’s chassis; this is the ground link.
Bar 6 is the yellow part which the robot’s manipulator (claw, conveyor, whatever) is attached to.
Again, most robots that use these mechanisms will have 2 of them, side by side. Our Fush and Chups used this configuration.
It is not the same as a scissor lift on its side because a scissor lift normally uses sliding joints at the top and bottom, although it does share some similarities. The 6 bar uses only pin joints.
Hopefully this goes some way to explaining why we decided to call this mechanism a “6-bar linkage”