Balsa Wood Bridge
Sam Katsuda
Pratt Bridge with Lifting Mechanism Drawing
Pratt Bridge with Lifting Mechanism Sketch
Introduction
The objective of this project was to create and articulating balsa wood bridge that would span across a gap and support a load. The student was tasked to use their engineering knowledge to design, manufacture, and test a bridge. It must span a gap of 400mm, hold a static load of 20 kg at the center, and articulate up to 140mm at the center. The method for articulation can be done in any means, but needs to be able to hold the bridge at max height for 10 seconds, as well as have the weight of the articulation structure and the weight of the bridge under 85g.
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To tackle this project, the bridge design used was a Pratt style. Calculations for cross sectional area of the beams was done to determine both the minimum area as well as the area with a safety factor. Articulation was accomplished using a pully system and Arduino controlled motor. Components necessary for articulation were manufactured using a 3-d printer. The structure for the pully system will form a pyramid that will have a cable running over the top to the top center of the bridge.
Results
This section will outline the results of the project as well as what could have been improved and what fixes would be done in future iterations.
Results
Test 1
Articulation Height Test
What was found in this test was that the bridge was able to articulate to a height of 142.6 mm which was 1.86% over the predicted value of 140mm. As it was over the requirement the bridge does pass this test.
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For future iterations, sensors would be used to allow the bridge to stop at the same spot every time. This would also allow the bridge to be run off one button rather than three which would allow the bridge to have a more precise lift.
Test 2
Articulation Cycle Time
Following the cycle time test it was found that the bridge cycle time was 15.216 seconds which is below the predicted 17.4 seconds and below the 60-second requirement so the bridge does pass this test as well. It was also able to maintain a max articulation for 10 seconds so it does pass that test as well.
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One issue that ran into with this test was that the bridge would lift and lower at inconsistent rates, with sometimes it lowering by slamming down as if done by gravity. To solve this a mechanism could be installed to lower the gear ratio of the bridge to give it an easier time lifting and lowering the bridge. Another solution would be to buy a motor with better low end torque leading to the same result as the lower gear ratio but built into the motor.
Test 3
Load and Deflection Test
The results of this test were that the bridge was able to maintain a load of 9.7kg while deflecting around 4.03mm, so it was below the required and predicted 20kg. This means the bridge did not pass this test. It did meet the deflection requirements as it was below the predicted 6.9mm and 25mm max requirements. The results are inconclusive though as it was unable to reach the 20kg requirement so the deflection at that point is undetermined.
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For future iterations, there are a couple of fixes that could be done. One is that the bridge was built with the grains stacked top to bottom meaning the grains would be split under tension. If they were stacked side to side, then they would be able to maintain a higher load under tension without fail(the break can be seen in Figures 1 and 2 below). Another fix would be to increase the cross-sectional area of the beams as the bridge was far under-weight meaning more material could be used to strengthen the structure.
Figure 1: breaking point
Figure 2: fractures
Result Breakdown
Table 1: Results
