Testing
Out of all the requirements the bridge needs to meet, only six requirements need to be tested. these requirements are:
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1) The bridge must hold at least 20kg at its weakest point and deflect as much as 25mm.
3) The bridge must not weigh more than 85g of balsa wood and glue.
8) The bridge must allow a 100 mm long object to pass along the 400 mm bridge without it causing the bridge to collapse.
9) The midsection of the bridge must rise a least 140 mm and maintain it above its original starting point for at least 10 seconds.
10) When the midsection of the bridge starts to rise, it should take no more than 55 seconds to rise and close completely.
11) Must allow a 32mm by 25mm block to pass through on the bridge.
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Testing the requirement is not that difficult to test. The most difficult part is to test if the bridge can hold 20kg. To do this, a hole will be cut through the middle of the bridge and a hook will be placed through it. With this hook, a jug of water can hold 20 kilograms of water. So, by placing it on the hook, the bridge will either hold the water up or fail under the weight. The weight of the hook would be measured as well. When it comes to the weight of the bridge, simply put the bridge on a scale that can measure it in grams and record the number. Another test that needs to be done is pulling a 32mm X 25mm block through. Measure the width of the bridge and pull the block through. The final test that needs to be done is how high the bridge lift and how long it took to do it is also simple. This can be done by measuring how high the bridge when with a ruler and having a stopwatch to keep how long the bridge is going to take.
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Summary
For the spring quarter, there were mainly three major tests that needed to be done. These tests were the time test, the torque test, and the failure test. The time test was done to see how long it needed for the bridge to reach the desired height. this is mainly to show that the lifting mechanism is fast enough to lift the bridge and that the bridge is not too heavy. For the torque test, a force of 10 grams will be added to the lifting mechanism to see if it can lift the bridge up just enough to allow a piece of paper underneath the bridge. This is mainly to show that the lifting mechanism does not need a lot of force when working. The final major test s the failure test. This was done by adding a metal plate and hook on the bridge. This will hold the weight being applied and distribute the weight across a small area on the bridge deck. Weight will be added to the hook until the bridge fails due to the weight.
There was one major issue when testing this quarter. That issue was there was no weight that was 10 grams weights. To overcome this issue was simple. All that was needed was the nylon spacers that were bought for this and a precision scale that measures in grams. Using the nylon spacers and the bag that came with it, the student was able to make the precise weight that was needed for this test. Furthermore, the weight can be changed to as small as 5 grams to make the test more repeatable and reliable.
The time test will focus on requirements 9 and 10 from the requirements. Before gathering all of the data, a prediction was calculated to see how long this bridge would take to rise to 140mm in height as shown in Figure 19. By using the kinematic equation:
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DeltaX = Vt +(1/2)at^2
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The predicted value for this test was 55 seconds to do one full cycle of raising and lowering. After completing the test with an average of 5 trials, the bridge took about 33.3 seconds and was 141.1 mm in height.
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Figure 20, it shows the height that was needed for the trial to be considered a success. this was mainly done by having a ruler agency an object to hold it in place.
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The Torque test was to see if the lifting mechanism of the bridge was designed correctly. By applying a 10g of force to the lifting mechanism should allow the bridge to raise a little to allow a piece of paper to get underneath. Figure 22 shows what the weight is going to look like. As to get the value for this test, the torque equation will be used and that is:
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T = F*R
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The predicted value for this was estimated to take 57g of force to move this bridge. Before seeing if this was true, a 10g weight was added to the handle of the lifting mechanism and it move the bridge just enough. Showing my prediction was wrong. turns out the weight of the bridge was much lighter than expect and a new prediction was made. the new prediction was now 7 grams of force being applied. to make sure that the prediction was correct, it started out with 10 grams and going down 1 gram every trial until 5 grams. Figure 23 shows the force being applied to the lifting mechanism lifting the bridge just enough for the paper to fit underneath it.
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The Failure test was to see when the bridge was going to fail as weights were going to be added. To make sure that this is being done correctly, the weights being added are going on a scale to get the correct weight. Then these weights are going to be added. The predicted value that the bridge was going to hold was 20kg of weight added to the center of the bridge. How this value was gotten was when designing the bridge and the stress levels that were going through each truss. On testing day, the bridge was set up with a bucket holding the weight. after adding weight to a while, the bridge collapsed suddenly as shown in Figure 24. After getting the bucket, it was now time to see the total weight that was applied to it. As shown in Figure 25, the total weight applied to the bridge was 23 lbs or about 10.5kg. This was way below the predicted value. This was mainly due to the structure the bridge deck had, since the truss system was in one piece still.
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Test 3: Failure Test
Test 2: Torque Test
Figure 20: Seeing what 140mm looks like
Figure 19: The bridge at 140mm
Figure 21: A trial of the time test
In Figure 21, it shows a trial of what the time test looks like. It shows that the timer was started when the bridge started to rise. Once it hits the 140mm mark, the lap button was hit to indicate this point and had to hold for 10 seconds. then the lap button was hit again to indicate that it was lowering. Once it reaches the original position the time was stopped.
Figure 22: The weight that is going to be applied to the lifting mechanism
Figure 23: The force being applied to allow a piece of paper
Figure 24: The aftermath of the test
Figure 25: The weight that caused the bridge to fail