Twin Tweaks ...
GETTING THE WORM GEAR MECHANISM TO MESH.
INITIAL TESTING ON THE MECHANISM.
AN EARLY VERSION OF THE CLAW.
70 SERVO 11.2012
Cobra chassis from Fingertech. Despite the fact that the
brain and the chassis each had four mounting places, they
were not at all aligned. This only ended up being a minor
headache, but the larger scale of this project could mean a
So, it was with some trepidation that we aligned the
VEX frame pieces with some of the frame pieces from the
Minds-i kit. We were crestfallen to discover that the black
frame pieces did not even remotely line up with the VEX
pieces we wanted to couple to it. That disappointment
soon turned to elation when we saw that the white frame
pieces were actually perfectly aligned with the hole spacing
on the VEX parts. Encouraged by this revelation, we
continued to build up the arm mechanism.
After meshing the worm gear with the large gear using
a VEX frame, we needed an arm and a claw for it to
articulate. This is where the extra parts from the Minds-i kit
grabbed the spotlight. The arm design we had in mind
would include a claw with one fixed side and one moving
side. The assembly that moved was essentially a kind of
four bar linkage.
Four bar linkages are a classic device, favored for their
easy design and predictable range of motion. The four bar
linkage that comprised our arm was of the planar variety,
because all of the joints had only one degree of freedom
(i.e., they moved within a single plane). Each joint in a
planar four bar linkage can be characterized as either a
resolute or prismatic joint. A resolute joint is one that spins
around an axis, while a prismatic joint slides along a line. All
of the joints in our four bar linkage were resolute joints,
making our linkage a planar quadrilateral linkage.
That’s a lot of fun vocabulary, but how does it help you
evaluate the effectiveness of your design, or come up with
the design in the first place? Four bar linkages are so useful
because they can transform one type of motion into
another – rotational motion into linear motion, continuous
motion into oscillation, and much more. By understanding
the basics of four bar linkages, a tinkerer will never be at a
loss for how to design a certain mechanism as long as you
know the type of motion you want to achieve.
To illustrate: We wanted to devise a claw and arm
mechanism where the claw would open and close, and
where the arm would raise and lower — all with one input
(one motor). The claw opening and closing is a type of non-continuous rotational motion – the moving half of the claw
does not spin around completely, but rather rotates only in
a limited arc. The raising and lowering of the arm is also a
non-continuous rotational motion. A four bar linkage with
this sort of movement is often referred to as a double
rocker. A double rocker is achieved when the sum of the
lengths of the shortest and longest links are greater than
the sum of the lengths of the other two links. Other
calculations can also be performed to determine the lengths
of the arcs that the linkage will travel.
Thus, some simple addition is all the math we needed
to determine the basic design of the arm and claw
mechanism. The frame pieces from the Minds-i kit are