My JEMS Experience

This is the essay my team and I wrote for the JEMS experience

This report is to journal the ‘Flying Monkeys’ progress in building and firing the 2005 Junior Engineering Mathematics Science (JEMS) summer workshop’s task. The task was to build a trebuchet and fire it at a low wall and hit it hopefully damaging it. On Sunday, July 10th, 2005, four strangers: Sandy Kralik, Alex Chvilicek, Mandi Glauser, and Dennis Linn came together from miles apart to form JEMS team four. Representing different cultures, geographical areas, and backgrounds, these four strangers shared one common trait: a strong interest in engineering. The members of the team had not known one another prior to this camp but had plenty of opportunities to get to know one another. The team was informed of the task they would be carrying out for the next two weeks, and right away each member of the team began to start the long thinking process that would be required to build a trebuchet. This report is to cover design considerations, construction, trebuchet performance, and commentary on the effectiveness of the design of the trebuchet project. Our goal was to engineer, design, and build a trebuchet capable of hurling a three-pound projectile about seventy-five feet and hit a wall. Points were to be awarded for: hitting the wall, knocking over the wall, damaging the wall, and landing closely behind the wall. However, no points will be awarded for falling short of the wall even if it roles and hits the wall. On the first night, the members of team four, soon to be renamed the ‘Flying Monkeys,’ had to use teamwork and inductive reason to construct small models of trebuchets or catapults that were to launch a ping pong ball approximately twenty feet. After five minutes of looking at the only parts they were allowed to use, they soon began to turn out dozens of ideas. Right from the start, the ‘Flying Monkeys’ established exceptional communication skills that would critically help the design of the big trebuchet when the time came to construct it. The team came up with several designs; unfortunately, they ran out of time and were forced to rely on a backup plan that took only seconds to construct. The backup plan was simple, but it worked. They would have liked to have used one of their other designs, but they ran out of time. Despite this, the team worked together flawlessly from the start. All they needed was the knowledge and the tools to get started. Before construction or planning on the real trebuchet could begin, the team needed a stronger knowledge of trebuchets. Many of the camp participants had never heard of a trebuchet before. The teams watched a video regarding trebuchets, and then everyone in the JEMS workshop individually built a small model of a trebuchet. The video and the model jumpstarted their creative design ideas and knowledge of how to adjust the plans they would make, to design a more accurate design. The teams were given design limitations for the true trebuchet, which included a $75 budget, a three-pound projectile, the counterweight had to fit in a five gallon bucket, and a time constraint. All of the JEMS students took college classes such as Engineering concepts, CAD computer modeling, and Engineering design. They also took a few classes in leadership to help the individual teams work together more efficiently. With this knowledge once again, each team member of the ‘Flying Monkeys’ started to think of ideas. After running numbers through a trebuchet simulator, the ‘Flying Monkeys’ found ideal dimensions that would accomplish the task at hand. They also had to predict the trebuchets performance before they started planning. The Wintreb program provided to the teams predicted a launch distance of two hundred forty-one feet, under the dimensions of: Height of pivot- five feet Length of lever arm- eight feet Length of counterweight arm- two and one half feet Counterweight weight- 100 pounds Sling length- six to eight feet Release angle-one hundred forty degrees Arm angle when loaded-forty-five degrees The team calculated a theoretical range of about two hundred fifty-one feet. With the dimensions, Alex began working on drawing the plans for the trebuchet in auto CAD. He showed the rest of the team his preliminary drawings, and together, the team made small modifications to the drawings to create a design that the whole team was satisfied with. The team members were able to effectively communicate their ideas with each other. Some of the things the team had to consider were: the budget, the supplies available, the dimensions of the supplies, the forces acting on the different pieces, and the strength of the different pieces. Taking the Physics of the projectile learned in Engineering concepts, and the design steps along with the range calculations learned in Engineering design, the team was able to make necessary steps toward building an excellent trebuchet. The team had to calculate: The Height, width, and length of the trebuchet The drop height of the counter weight so that it would not hit the trebuchet The materials that would stand up to the forces of throwing the projectile The maximum counter weight they could get out of five gallons of any material The best materials to fit the budget After the calculations were finished, the team determined what supplies were required in order for the trebuchet to function without breaking. Some parts were supplied to each team to make certain requirements that the teachers had set. The requirements were: the counterweight had to fit in a five gallon bucket, and the firing mechanism had to be a pin which was provided. The rest the team was required to buy under a seventy five dollar budget. Some parts were expensive, and it was tough to balance the budget that included all of the required supplies. The team decided on purchasing: Four inch PVC pipe to use for the lever arm Three quarter inch four foot long steel pipe for the pivot Two three-eighths inch hex bolts four inches long to hold the pivot in the frame Two one fourth hex bolts four inches long to hold the lever arm in place Two one inch black pipe straight connectors, Six hose clamps Twine to make the sling Hand towel to hold the projectile 3/8-inch particle board four feet by eight feet to let the projectile slide on Three eyebolts for the firing mechanism in which we used a pin Five two by twos eight feet in length, Six two by fours eight feet in length, Box of two and half-inch wood screws. After an hour or two of making more modifications to the drawings, the team decided on a design that not only would work, but would also fit the budget. Construction Before constructing, the team reviewed the drawings and checked each of the pieces they had bought. The first step was to measure and mark the wood where it would need to be cut. The team took the two by fours and constructed the base of the trebuchet with the two by twos to strengthen the base by creating two X’s on the bottom. A piece of plywood was placed on top of them and became a flat surface on which the sling would slide across. The team decided to construct a flat surface on the base, because it was undesirable for the sling to bounce around on uneven ground or grass. The plywood was cheaper than constructing a trough for the sling to slide in. After that the team had to build the upright supports. Due to budget constraints, four by fours were not available and the upright supports had to be constructed out of two by fours. After the supports were up, two by fours and two by twos were placed on either side to provide support. This made the base very sturdy. A third two by two was also placed on the side to prevent swaying. The pieces were difficult to attach by themselves so plywood and triangular pieces of two by fours were used to reinforce the joints. Once the base and uprights were completed the arm was constructed. Two one and one eighth inch holes were drilled into the two uprights to support the arm. The team had barely put the pivot pipe in to measure for drilling holes when someone from another team was trying to do a pull up when they jumped onto the middle of the particle board where there was no support and broke it in half. The ‘Flying Monkeys’ were not completely heart broken since the sling could still slide easily over the board. Having run out of building time for the day the ‘Flying Monkeys’ would worry about the damage in the morning when they would return for more building. The next morning however, reviled that a hand trolley full of Pepsi ran over the weekly supported particle board and completely demolished it. The rest of the trebuchet was luckily fine. The team bartered for more particleboard from another team and was on their way building the leaver arm again. A four-inch PVC pipe was used for the leaver arm. It was cut to eight feet and a hole was drilled two and one half feet from one side. A three quarter inch pipe was inserted into the holes and a bolt was put through it to hold it strait. The lever arm pipe was placed on the pipe and bolts were placed on either side with large metal pipefittings as washers. After the arm was completed the rope for the sling was made. The team used the thin twine and made two, four strand braids. The braids measured ten feet to give the sling plenty of adjustment length in case of it breaking or the calculations for the sling length were wrong. The rope was attached to the hand towel and attached to the lever arm. It was adjusted to a length of about six feet. The trebuchet was then ready for the counter weight. For the counterweight the five-gallon bucket, that each team had received, was filled completely full of cement and a rebar bracket was placed inside to attach it to the arm. The bracket was made to be removable with a pin to hold it onto the leaver arm. Once the cement dried it was hung on the short end of the arm. The other end was equipped with a long bar used to hold the rope to the sling and an eyebolt to link it to the base for firing release. On the first attempt, everything worked well except that the lever arm pivot pipe bent in the center. It bowed a full three inches, the team had apparently miss calculated the strength of the pipe. The arm had to be disconnected and the pipe had to be bent back and pulled out. A new larger one-inch pipe was given to us for no additional cost and larger holes were drilled into the uprights to fit it through. The arm was reattached and the trebuchet was fired again. The pipe again bent but not as badly as the first time. It proved to be stable enough to be fired on more than once and the apparatus was moved to the field Once the field was reached it was too late to do much. Two shots were fired, both turning out unfortunate and a third, which traveled no more than twenty-five feet. The team was then sent in for supper and unable to return to the trebuchet until the next morning. The last day of the competition the team arrived at the field ready to win. Sandy and Mandi prepared the trebuchet in the morning to get it ready for the event. They fired and adjusted the trebuchet until they thought that it was at an adequate firing range. They ran into few errors until the hand towel of the sling broke. They had to post pone testing until they received a new hand towel to attach to the rope. The received the towel and attached it to the rope. The trebuchet was finally ready. At two o’clock that afternoon the competition firing began. The ‘Flying Monkeys’ was second in the order of launching. The first throw was a near miss falling short of the target by only two feet, which made that throw about seventy-three feet. The second throw was loaded the same way hoping the hook angle would bend its self to the correct angle by the force of the sling to gain a little more distance. The shot was fired and once again, fell short no more than two feet. Destined to hit the target minor adjustments were made such as changing the lever arm angle, and a third shot fired. This shot was a little better than the previous but still fell short. From there on out it things were not looking good for the ‘Flying Monkeys’. One more shot was fired and the sling broke the hand towel had ripped where they had attached the rope to it. The women fixed it again and that last shot was fired. It was the worst of all. The ‘Flying Monkeys’ came out of the competition with no points. The team was not too disappointed though; knowing their trebuchet actually fired and that with a few extra feet could have won. Although the trebuchet fired quite accurately and at a very reasonable distance, there were a few things that could have been changed. The PVC pipe used for the arm may have given the trebuchet a little more energy by bending, but could have been replaced by a much cheaper four by four, which would have given the team much more money to spend on the support for the trebuchet. The other thing would have been the sling that with more time could have been more properly tested to find the best possible length and at which release angle would have been used most efficiently to achieve maximum distance.

Links to Pictures

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Building Trebuchets

Firing Trebuchets

Classes

 

 

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