Stress Analysis Design Project • Spring 2010
Allen Chen • Andrew Chen • Clara Cheung
Background || Structure || Mechanism || Theoretical Performance || Unique Features || Video || Students
The objective of this project is to create a mechanism that will lift a one pound cylindrical weight at least 2 inches. The mechanism is powered by a servomotor and needs to reach around a wooden obstacle. The mechanism is clamped in a given position while the other end lifts the weight vertically to a certain height. It must not weigh no more than 20.0 oz and must be capable of performing two lifts in less than 30 seconds.
The design of our structure consists of a triangular truss projecting through the obstacle to reach the weight and a rectangular truss to attach the arm to the strong upright base. The structure of the base consists of two "L" shaped beams that were bent to resemble the shape of a rectangle and are reinforced with diagonal extensions. In addition, the base was attached to the aluminum sheet with "L" shaped beams to prevent the lifting of the base when the structure is being loaded.
The rectangular truss includes strips bent into an "L" shape, forming squares 3 inches in length along equal distances of the truss. Diagonal strips were also attached in order to decrease the effects of the twisting of the structure. The triangular truss is structured similarly to the rectangular truss. Instead, the cross sections consisted of triangles and there were no diagonal members used.
Side view of the structure
Because the arm was located above the weight, the servomotor needed to be place below the weight to be able to lift it. Therefore, we built a cradle to hold the servomotor in place, preventing it from shifting as it attempted to lift the 1 lb weight. After performing the lifting mechanism, the cradle started to move slightly away from the weight. Due to this result, we placed a "L" shape beam on the back side of the cradle to prevent it from shifting as the servomotor was lifting the weight.
View of the cradle holding the servomotor
The mechanism utilizes a "L" shaped delrin strip to lift the weight. The corner of the delrin arm was filed into a curve to allow the weight to smoothly move up the arm. If the arm was not filed correctly, the weight would be unable to move and would get caught in the sharp corner. In addition, lubricant was put on the delrin strip so the lifting mechanism would not lose efficiency due to the effects of friction.
On the opposite end of the delrin strip, a counterweight is attached to assist the lifting mechanism. Two counterweights were used and attached with a rubberband, due to the difficulty in attaching the counterweights.
The L-Shaped delrin strip with counterweight
The servomotor rotates approximately 100° and has a maximum torque of 42 oz-in. Using statics calculations, the amount of torque the load exerts on the servomotor is approximately 40 oz-in. To decrease the net torque applied, a counterweight is attached on the opposite end of the arm, 3 inches from the servomotor. The counterweight, weighing approximately 4 oz., exerts a torque of 12 oz-in in the same direction as the servo. Thus, the servo's torque output is 14 oz-in.
The length of the delrin arm is 2.5 inches from the servomotor. Therefore, the theoretical distance at which the weight will be lifted will be 2.5 inches as well. In the design competition, our structure lifted the weight approximately 2.5 inches. However, our final structure was over the weight limit.
The "L" shaped delrin arm is one of the unique features of our structure. The smooth edge of the arm ensures the most efficient lift and movement of the weight. Additionally, frictional forces could be neglected due to the lack of roughness of the delrin strip. Another unique aspect of our structure are the beams projecting out of the base. During the first design review, the triangular truss deflected greatly after the load was applied to the structure. To prevent the same result, two "L" shaped beams were attached from the bottom of the triangular truss to the bottom of the rectangular truss. As a result, the triangular truss did not deflect at the final design review (see Side view of structure). The final interesting feature of our structure is the servo horn. This X-shaped gear contained fine teeth that easily meshes with the teeth of the servo. This ensures that the arm would not slip on the servo shaft. As a result, the energy output of the servomotor can be utilized efficiently. In addition, the servo horn could be easily attached at almost any angle, allowing the arm to provide maximum lift. The servo horn was attached to the arm by 4 screws that pass through holes of the horn, allowing easy access to the lifting arm if needed.
The X-Shaped servo horn used
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