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Intro

The goal of this project was to build a bridge entirely out of balsa wood that could span a gap of 400mm, be able to hold 20kgs of weight, not exceed 85g in total weight, and open to allow an object to pass underneath. Movable bridges are one of the most interesting aspects of mechanical engineering and this project was notably valuable in expanding the understanding of draw bridges through the model that was created.

Initial Drawings

Figure 1: Bascule Bridge
Figure 2: Draw Bridge
Figure 3: Rotating Bridge

Results

Overall, the testing of the bridge showed that it was successful. All of the results are accurate and multiple trials were used whenever possible. The results were valid indicative of the work that was put into the project and if done again, the same results would be achieved. It is exciting to see the project do so well and exceed many of the major milestones it was set to achieve.

Graph 1: Time for the bridge to open to the minimum height

This graph shows the time it took for the bridge to raise 140mm from the initial position measured from the midpoint and the total time for the bridge to open that far and then close. The bridge was able to accomplish both of these aspects very quickly and it reached the minimum height requirement with ease.

This table shows the total amount of weight added in each instance to the bridge. As it can be seen, the bridge held a total of 31 kgs before breaking. This was much higher than the requirement of 20 kgs, over 150% of what it was required to do in fact.

Table 1: Amount of weight added to the bridge before breaking

Analysis

There was a lot of analysis that needed to be done for this project. Statics analysis needed to be done to ensure that the bridge would be able to hold the minimum amount of weight of 20kgs. 

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Figure 4: Types of Trusses Analysis
Figure 5: Truss Strength Analysis

This analysis was important to determine the ideal type of truss to fit the needs of this project. There are many different truss designs but three main types that seemed the most effective. Any of these trusses could be effective depending on the material, but for balsa wood which was notably better in tension, the truss that was in tension the most was chosen. This way the strengths of the material can be utilized.

This analysis was done to determine the thickness of the members of the truss. This was done by finding the compressive and tensile forces in each of the truss members through analysis based on the load of 20kgs the bridge is required to lift. Then by using compressive and tensile strengths of balsa wood, the necessary cross sectional areas were worked out. Finally, those values were compared against purchasable sizes to find the acceptable balsa wood sticks to buy.

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This analysis was done in order to have a better understanding of how much wiggle room there was for the rest of the project after the weight of the wood was applied. This was found by finding the total area of balsa wood that was being used, then multiplying it by the density in order to find the total grams. This influenced future design decisions and validated the work that had been done thus far.

Figure 6: Weight of Balsa wood Analysis

Construction

The method used to construct the bridge pieces up to this point has been cutting the wood down to size lengthwise, and then adding angles to the pieces that need angled cuts.

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The pieces start out as long sticks that were already cut to the correct width and depth as purchased. The first step is to cut them to length by using the miter saw.

These are the pieces after they have been cut. Some of the pieces are finished at this point and can be added to the stack of finished parts. But some need to be cut even more

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Some pieces need an angle cut into them to fit into the truss. Most of these pieces were easy to cut as it was the same motion, just at a 45 degree angle.

After all of the parts were cut, the next step in construction was gluing the pieces together. The glue that was selected for this project was polyurethane glue which was really strong and required the excess glue to be planed off.

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Once the road deck was cut to the correct size, a small hole needed to be drilled into the center for testing purposes. This was accomplished by using the drill press

Video of Construction Methods

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Drawing Tree

Testing

There are many different aspects of testing for this bridge including weight, dimensions, strength, and articulation. There were several materials needed to accomplish these tasks including a scale, ruler, and timer. The weight of the bridge was measured on a scale, the dimensions of the bridge taken with a ruler, the strength of the bridge was measured by using a series of weights attached via the 8mm diameter hole in the middle of the bridge, and articulation measured by both a timer and ruler.

There was one slight issue that occurred during the Strength Testing. It was noticed that the piece that held the weight on the bridge was tilting to one side since it was an "L" shape instead of a hook. This caused the weight to be localized in one thin line across the road deck instead of spread out across the width of the square. This needed to be fixed to prevent to bridge from collapsing prematurely. To fix the issue, rubber bands were added to the hook to keep the weight directly under the hole in the bridge so that the pressure would be more uniform.

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Figure 13 shows the piece that was manufactured so that the weight could be attached to the bridge through the howl drilled in the road deck.

Figure 13: Piece to hold weight

Figure 14 on the right shows the initial issue with the weight of the bucket titling the connector piece and giving an uneven weight distribution. Figure 15 on the right shows the fixed version with rubber bands holding the piece in place.

Figure 14: Angled Connector Piece

Figure 15: Rubber bands holding weight in place

Figure 16: Video showing the bridge holding 20kgs

This video shows the Articulation Testing of the bridge. The bridge was raised by using a reel to pull the bridge up so that the midpoint of the bridge exceeded the 140 mm mark, the the bridge was lowered back down. The bridge was able to reach the minimum mark easily and it performed the full cycle and just the raising of the bridge well under the required time of 60 seconds and 10 seconds respectively.

This Video shows the testing set up for the Strength Testing. The 2 tables were set 400mm apart with the bridge in between them. weight was gradually added to the bucket which was held by the Connector Piece. The video shows the moment when the bridge was holding the required amount of weight, 20 kgs, and it does not show any bending. The bridge would go on to hold a total of 31 kgs before finally breaking

Figure 17: Video showing the Articulation Test

This is a picture of the test to determine the weight of the bridge. The bridge alone, without the lifting mechanism, had a requirement to weigh less than 85g.

Figure 18: Weight of Bridge

The dimensions and articulation test of the bridge have also already been tested. The dimensions testing was done by using a ruler and measuring the width of the road deck and the gap between the two trusses. The requirements for these 2 measurements being 38mm and 32 mm respectfully.

For the articulation test, the required angle for opening the bridge was calculated and determined to be 42 degrees. Then, the number of turns of the reel required for the bridge to reach that mark was found at 1.5 rotations. Finally, a timer was set to record the amount of time it took for the bridge to fully raise to that point by turning the reel 1.5 rotations and then return to a closed position.

Budget/Schedule

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Schedule Summary

The manufacturing processes of the project was ahead of schedule in large part due to the fact that all of the parts had the same manufacturing method which made it easier to do all at once. The testing portion all took place in a timely manner and the only part of the schedule left to complete is the presentation and report.

Budget Summary

In total, $127.62 was spent on parts and shipping which was about 6% more than the total expected cost including excess funds. However, some extra parts were ordered so there was no risk of exceeding this value.

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