Stress Analysis Design Project
Jonathan Bates, Tyrone Celoza, William Maher
(Base, Boom, and Arm of the mechanism)
Construction of the Mechanism
Our mechanism has a rectangular base built with a truss system. On top of the crane is long boom that is about 2 feet long. This boom sits on top of the base at an angle of 60 degrees north of west. The boom is constructed by two of the aluminum pieces made into an I-beam. The individual strips of aluminum of the boom and base are horizontally bent, most of which are folded twice to look like a “C”, except for a few pieces in the base that have only 1 bend. On both sides of the boom is a brace, each about a foot long, which is connected to the boom and the base. A long strip of aluminum is wrapped in the form of a coil all around the boom. The servo is attached to the end of the boom, and connected to the servo is an arm that is 9.25 inches long. This arm rotates about the servo at 3.50 inches from the left end of the arm. At this end of this arm, there is a hook that is free to rotate and this hook connected to the screw in order to lift the weight. At the other end of the arm is a counterweight that weighs about 5 oz. This counterweight is about 5.75 inches from the point of rotation in the servo.
How the Mechanism Works
The boom of the mechanism goes straight the smaller window of the obstacle course. The base of the mechanism is strategically placed so that the end of the boom is about 0.5 inches to the right of the weight. This allows for the hook to be placed right around the screw of the weight. To lift the weight, the servo provides a torque (maximum of 42 oz-in), and the counterweight adds to the moment about the point of rotation. This action at the end of the boom causes it to twist, but this twisting motion is counter-acted by the coiled piece of aluminum. The base is strong enough to rigidly hold the boom in place as the motor lifts the weight.
The most interesting and original feature in the mechanism was the coil wrapped around the boom. The purpose of this aspect was to resist the twisting motion of the boom when it tried to lift the weight. The coil worked surprisingly well and was very unique compared to the rest of the class because it appeared we were the only group that had that type of feature. Another aspect of our mechanism that was interesting was the hook. We designed it in a way so that it could rotate about a point and extend to a greater distance, based on the motion of the arm. It was designed by attaching the hook to the end of a small bar and attaching this to the arm. The hook and the small bar were loosely screwed so they could rotate in such a way that would smoothly lift the weight. One feature of the mechanism we were most proud of was the base. It was built in a truss system and turned out to be very sturdy. In addition to that, it was very light because we did not need to use many materials for this part of the structure.
These theoretical results assume fact that the boom does not deflect. The end of the hook starts off 3.50 inches away from the servo. Based on how the hook is designed, it can be maneuvered in such a way that the maximum distance between the hook and the point of rotation is 4.25 inches. There is a 1 lb force going downwards at the hook, because of the weight, and a 5 oz force going downwards at the other end, provided by the counter weight.
Using these forces and dimensions, we can calculate the theoretical torque of the motor and the height the weight will go.
Finding the torque of the motor:
M|servo = (16 oz)(3.50 in) – (5 oz)(5.75 in) + Ms = 0
Ms = -27.25 oz-in
In order to initially lift the weight, the motor will need to provide an output of at least 27.25 oz-in.The servo provides a torque of 27.25 oz-in going clockwise. The maximum torque is 42 oz-in, so the servo uses 64.8% of the servo’s maximum torque in order to lift the weight.
Finding the height of the weight:
The screw attached to the weight starts off 3.5 inches from the servo. However, the hook can be maneuvered in such a way that the end of the hook can be at most 4.25 inches from the servo. Given these dimensions, we can calculate the theoretical height that the weight can be lifted.
(3.5 in)² + (h)² = (4.25 in)²
h = 2.41 in
So the weight can be lifted 2.41 inches, theoretically.
The goal of this project was to create a mechanism that was less than 20 oz and could lift the weight 2 inches. Our mechanism was 18.6 ounces, but it only could lift the weight 1.6 inches. The main reason why the weight could not be lifted 2 inches was because the end with the counterweight touched the ground before the weight was lifted all the way. One factor for this was that our arm was too long. Based on the geometry of the arm, the weight started off too low for the arm to be able to lift it all the way up without the end of the arm touching the ground. Had the arm been shorter by about even ¾ of an inch, the weight could have been lifted all the way. Also, there was some deflection at the end of the beam. Our whole structure was fairly strong, but it was not absolutely perfect. During the final testing, but there appeared to be a deflection about a small fraction of an inch, a distance that was lost in the total change of height.