Stress Analysis Design Project
Mark Stanard, Daniel Yu, Sophie Zimmermann
“The objective of this project is to design and build a mechanism that is powered by a standard servomotor which will lift a cylindrical weight. Your mechanism is to be clamped in a given position while the other end lifts a weight that slides along a vertical post. Between the clamping area for the base of your design and the post holding the weight sits an obstacle. The mechanism may not touch the obstacle or any other surface besides the base and the sliding weight. All teams must design and build such a mechanism with minimum capabilities: it must weigh no more than 20.0 oz, and it must lift the sliding weight by at least 2”. At the end of the semester, teams will compete on the basis of maximizing the sliding distance and minimizing the weight of a functioning mechanism.”
The crane's base consists of one thin aluminim base-plate, which was clamped to the table during testing. From the base-plate, six U-beams were screwed on to the plate and were perpendicular to the base, raising directly upwards from the base; these six beams were in a formation of 2 rows of 3 beams, front to back. The beams were connected to each other via rods, to connect the 3 beams in each row, and small aluminum sheets and beams, to connect beams from the different rows. The 6 beams were then screwed onto the main triangular-prism shaped arm of the crane.
The arm of the crane is made up of 3 parallel beams which run in a straight line through the hole in the wall, until it then is bent diagonally towards the pole and weight. horizontal trusses connected the two top beams of the arm, while triangular strusses connect the bottom beam of the arm to each of the 2 top beams. Towards the end of the arm, a few extra trusses were added, from the tip of the arm to a point farther back on it, so that the truss could resist the downward tendency of the arm, due to the weight's resistance to the arm's motion, via the tensile stresses it incurred as a result of the weight.
The servo motor was then attached to the tip of the arm. On the servo motor is the lifting arm of the crane. The lifting arm, as opposed to the otherwise aluminum body, is made of plastic. The lifting arm was attached to the servo motor with room on each side of it, leaving room for the counterweight, that is on the side of the plastic arm closest to the base, and the hook, which was on the side farthest from the base. With the servo motor and the counterweight's gravitational force adding up to a large moment, the hook then raises the 1 lb weight up on the pole.
Our mechanism works by utilizing the force of a counterweight to maximize our torque and lift the 1lb weight at least 2 inches.
The 3-pronged structure is rigidly attached to a base through a system that maximizes the distance between the two attachment points so as to reduce moments.
LIFTING MECHANISM CALCULATIONS
M|servo = FwD1 - Tservo - FcwD2 = 0
Tservo = FwD1 - FcwD2
= (1 lb)(16 oz / 1lb)(4.5 in) - (7.5 oz)(4 in)
= 42 oz*in
Tservo,required / Tservo,max = 42 / 72 = 58.3%
H = 2(4.5in)sin(45°)
= 6.364 in
Our crane is bent at one point as opposed to being a conjunction of two straight parts.
At some points, one screw held in place the ends of several different metal connectors, creating a sort of cradle for the beams.
Our main structure was made of three beams in a triangular-prism shaped formation.
Our trusses were placed so as they were in tension, giving our structure more stability.
In order to maximize the effect of the counter-weight, we added a half of a 6-in U-channel to the end of the existing U-channel counter-weight so that we can maximize the weight and distance and thus it’s potential for rotation, without going over the weight limit.
In order to make the beams stronger, we bent them into U-channels, but in order to attach these perpendicularly to the base, we flattened out the ends of a few beams with a hammer. The result of this looked somewhat messy, but was effective as an attachment point. It also allowed us to face the feet of the beams in whichever direction suited us best.