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Team 5: Charles Li, Kira Pusch, Taylor Tabb

How the mechanism works:

A long beam, the support shaft, comprised of two manually-bent U-brackets and a flat aluminum strip used to pre-tension the shaft, are supported at the points of 3 triangles formed by 6 manually-bent L-channels, each affixed to a square base. A secondary shaft, the lift shaft, located above and along the length of the support shaft, hosts a counterweight to assist the lift mechanism and is affixed by two inches of aluminum to the servo motor. This lift shaft is likewise pinned to an L shaped member on the opposite end which serves to lift the 16oz. weight. The L shaped member which comes into contact with the 16oz. weight is pinned at the corner of the “L” to the support shaft, creating a lever system between the L shaped member, lift shaft, and servo arm which lifts the 16oz. exactly two inches.

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Panel 1

Theoretical Calculations

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For theoretical calculations, we assumed an ideal servo lift of 57 oz-in and an ideal weight force of 16 oz.  Using a lever attached directly to the servo and an assumed 2 inch lift, we found a max length of 3.56 inches.  With the new length, we calculated the max lift from a 90° sweep.  The theoretical max lift we got was 5.13 inches.

Practice18362680_1344706782243028_1058357632_o

In practice, things were much more complicated.  The direct lever connection was dropped in favor of a moving arm system in an attempt to eliminate the moment on the arm.  In this system, there were L-shaped lifters and counterweights.  We found that the lift of the servo was a bit less than advertised, settling on a practice torque of 44 oz-in.  Also, due to multiple minor resistances due to the construction of the crane and friction points, we added 8.8 oz (generated from testing data) of minor resistances to account for additional forces. In the end, using properties measured from the crane, we ultimately found an actual practice lift of just about 2 inches. Unaccounted for in our calculations are also the minor deflections of, and torque on, the support shaft.

Materials Science Mumbo Jumbo

The components of 3003 Aluminum:

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(http://www.azom.com/article.aspx?ArticleID=6618)

Per the above graph, 300 Aluminum is composed almost entirely of pure aluminum, alloyed with a very small fractional percent of manganese. We considered heat treating the given aluminum pieces, however manganese alloys of the 3xxx aluminum series are not highly responsive to heat treatment due to the fact that the manganese does not become more soluble at higher temperatures. Were the aluminum alloy provided a member of, for example, the 2xxx aluminum series containing alloying copper components, the copper would become more soluble in the solid alloy solution at higher temperatures and thus heat treatment would enable a greater copper inclusion percentage and, consequently, a heat-treatable alloy that could potentially be heated and quenched for increased rigidity and overall greater strength of the aluminum.

That said, the strength of 3003 aluminum can be drastically increased by cold working. This was a motivating factor in the decision to manually bend the provided aluminum pieces into L and U brackets. Permanent plastic deformation of the aluminum introduces and increases the number of defects, namely dislocations, into the material. These dislocations, in addition to the manganese inclusions in the aluminum lattice, eventually pile up in the material, inhibiting movement and ultimately increasing the strength of the aluminum.

Panel 2

Notable Features + Design Advantages

Notable Features:

Lever System & L Member: This is the gem of our crane. We assembled a system to create significant leverage with relatively small counterweight and small negative displacement of lever arm due to reaction forces. Most proud of this!

U Channels: We considered U channels prior to learning about them in the course, and they ended up being extremely effective in reducing the deformation of the members! When screwed back to back, we formed an I beam.

L Channels: Similar to U brackets in motivation, the L channels were more practical for screwing connections than a U, easier to generate, and ultimately stronger and more rigid than the standard flat aluminum pieces.

Design Advantages + Structural Principals Learned In class:
Ideally would have used hollow beams, because, as we learned in class, for same cross sectional area and mass, hollow beams would have been more ridged because the moment of inertia would have been larger — however, the U channels, screwed back to back, form an I beam- which creates more mass on top and bottom, reducing the deformation due to bending moment. However these U-channel “I-beams” are not good in torsion. this was our motivation for moving the servo back closer to the supports.
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