The Idea
In our original sketches, 1 outlines the basic structure and
movement we first thought of to achieve our task. By turning the crank, the
string attached to the bottle would wind around the disk component (positioned
between the upright supports) and thus lift the bottle.
#2 shows our first idea for the winding mechanism. We
planned to create two of such disks, and connect them through the four
outermost ports with Delrin sheet or Delrin rod. The Delrin rod which was
connected to the crank and passing through the uprights would be positioned
through the center aperture.
#3 demonstrates our original idea for the crank component.
We began by creating a handle which would fasten to the central Delrin rod. On
the rectangular side of the handle, we created a hole for another piece of
Delrin to pass through. This was to make winding the string easier, because of
the parallel extension of the handle.
#4 shows a side view of the turning mechanism. At this
point, we had determined that the four connecting supports between the disk
were to be pieces of Delrin rod. It demonstrates how the string would wind
about the four Delrin rods as the central rod turned.
#5 shows our idea for keeping the central rod in place:
Delrin bushings. The bushings would prevent the central rod from slipping about
and causing instability during the winding process.
#6 demonstrates our first idea for our upright frame
components. We understood that a more triangular shape would distribute
pressure and stress more than a rectangular shape. We first thought of creating
triangles with the Delrin sheet.
Physics
The physics behind our design first manifested itself in the
design of the upright supports. Triangles are the strongest geometric shape
because a change in their angles results in a change of one or more of their
sides. We chose triangular supports because of their stability.
Similar to the bottle opener assignment, the windlass
required consideration of the degree of deflection in all materials. The
deflection equation:
Where F is force, L is length, E is Young’s Modulus
(stress/strain, material stiffness), and I is the area moment of inertia.
We used this equation when designing the winding mechanism.
We aimed to maximize the area over which the force (weight of the bottle) was
distributed. This would prevent the single beam that passed through the center
of the structure and attached to the crank from breaking at any one specific
point due to a concentration in weight.
Once we had brainstormed about the general
“two-disks-connected-by-additional-rods” idea, we began to focus upon
distributing the weight of the bottle. The central rod, being held in place
through the center of the winding mechanism with bushings, would deflect to a
comparable degree as the winding mechanism itself. Therefore, we aimed to
minimize deflection in the winding mechanism. The force applied by the bottle
was constant, as its weight remained the same throughout its ascension. The
length of the mechanism was variable. We decided to shorten the Delrin
supporting rods as much as we could while still maintaining a feasible width
for the string to wind around. By minimizing length, we could minimize the
numerator of the deflection equation.
We were unable to control Young’s modulus for the Delrin
rods, as the property of material stiffness is specific to the material used.
We were given a single piece of Delrin rod with a fixed diameter. The
uniformity of the rod caused the area moment of inertia, or, the stiffness of
the cross sectional area of it to be unchangeable. Therefore, we were able to
control the deflection of the winding mechanism only by adjusting the length of
the supporting Delrin rods.
Similarly, we minimized deflection in the central rod by
positioning the two upright supports as closely together as our frame allowed –
slightly greater than the 12cm gap between the tables.
Our further modifications to the winding mechanism kept in
mind the distance of each supporting rod from the center. By increasing that
distance, we would create a winding mechanism with a greater diameter, which
would distribute the string across a greater area and reduce pressure on
individual supports and then the entire mechanism. This also reduced the strain
on the central rod.
Additions
For the winding mechanism, we decided that to have the four
supporting Delrin rods close to the circumference of the disks would put
significantly more strain upon the less wide areas between the rods and the
outer edge. Therefore, we moved the apertures in the disks to be centered at
around 70% of the radius of the entire piece. This gave us a stronger and more
stable winding mechanism.
We then decided to increase the parallel component of the
crank handle to a length of 4cm, thinking that an increase in torque would make
turning it an easier process.
We then returned to the upright components, and decided
against a completely triangular structure. We adapted the structure to look
more like a “V,” and added rectangles at the base for further height.
Eliminating the base side of the triangle gave us inspiration
to create lateral supports to compensate. We decided to put two on either side
at equally spaced lengths apart to create three stabilizing rectangles.
The components of our foam core model:
This part of our foam core model shows the basic upright frame structure and how the central turning rod interacts with it. The winding mechanism is also shown.
A close up of the turning mechanism. The parallel handle component was later extended to 4 cm instead of the original 2cm length.
Someone borrowed our Delrin scrap parts at the time, but we tested out our lateral support idea with the uprights nonetheless. This shows one lateral support on the lower nearer side of the frame. We then intended to place one above on the same side and to mirror the structure on the other side.
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