24-262 Stress Analysis Project

Team 35

Objective

The designed structure needed to be able to lift a weight of 1lb or 16 oz through a minimum height of 2 in. This had to be done using a standard servomotor with maximum torque of 4.5 lb-in or 72 oz-in. The motor had a range of motion of 100 degrees. The total weight of our structure, had to be under 20 oz.

Team Profile

Jenna Krug- Head of Analysis

Jenna is a sophomore Mechanical Engineeing Major with a minor in Design. Jenna was in charge of the numerical analysis of the data that was collected after each design review. She also worked to determine the real-world feasibility of the various designs that the team proposed.

 

 

Sahira Mann- Head of Design

Sahira is a sophomore Mechanial Engineering Major. Her main task was the compilation and sketching of the design ideas the team decided to implement at the end of weekly meetings. She worked thoroughout the build process to ensure that the original design was adhered to.

 

 

Sean Archie- Head of Construction

Sean is a sophomore Mechanial Engineering Major. He was in charge of the main construction process. After the design was finalised at the end of each period and the analysis of data proved real world feasibility, Sean oversaw the build process and coordinated team members during the build.

 

 

How it works.

Principle

The design is based on the principle that, due to the length of the arm and the inability to provide reinforcement after the initial barricade, the majority of structural support should be provided by the 45-degree support arm that connects the base to the main arm.

Structure

Main

After the team decided to approach the weight through the larger opening, the problem of the 90-degree offset from the line of sight had to be overcome. The team decided to build an initial cubical support structure in order to gain the height needed as well as provide a base to attach the main arm to.

The main arm was designed to have a triangular cross-section in order to counter bending stresses in all directions as well as to allow the arm to be mounted to the central structure on a flat side of triangle.

 

 

Pivot Arm

The pivot arm was made out of plastic in order to maintain rigidity while minimizing weight. The counter-weight was attached with a screw to the end of the pivot arm. The weight was not allowed to hang freely but was instead fixed in a horizontal position parallel to the rest of the arm. This was done in an effort to move the center of mass as far backward as possible and thus maximise the moment caused by the weight.

Hook

The hook was crafted out of a flat strip of aluminum attached to the end of the arm and allowed to pivot freely. This was done in an effort to prevent it from restricting the range of motion of the arm during the rotation cycle.


Dimensions

Total Weight - 19.5 oz

Counterweight - 2.74 oz = 0.171 lb

Base Footprint - 3.5" x 3.5"

Central Tower Height - 10 in

Main Arm Length - 30 in

Pivot Arm Length - 10 in

Servo Max Torque - 4.5 lb-in

Required - 3 lb-in = 67% of Servo Max Torque

Servo Rotation Angle - 100 degrees = 1.75 radians

 

Performance

Torque

Required: T = Force x Distance = (1lb)(3-in) = 3 lb-in

Theoretical:

T = Servo Torque + (Counter-weight)(Distance from Pivot)

T = 4.5 lb-in + (0.171 lb)(7 in) = 4.5 lb-in + 1.194 lb-in

T = 5.7 lb-in

Lifting Height

Theoretical:

Height = r(theta) , where theta is the angle of rotation of the arm

Height = 3 in (1.75) = 5.25 in

Under experimental conditions, the final lifting height achieved was 2.7 in. The discrepancy in the values was caused in part by the reduction of the range of motion of the servo due to the position of the counter-weight.

 

Analysis

Simulation

Computer analysis was performed on a simplified version of the structure before the final build to determine whether the expected displacements were within acceptable limits. This simulation was done using Solidworks. It was determined that the displacement could be reduced and this prompted the addition of the support arm.

 

 

Interesting Features

The most interesting and effective feature of our crane was the support arm that went from the base of our main structure to the midpoint of the main arm. This helped to counteract the majority of bending stresses that were placed on the connection point between the arm and the central structure.

Conclusion

Our crane consisted of a large moment arm with the counter-weight at one end and the lifting hook attached to the other end. Upon final analysis, it was determined that the discrepancies between our theoretical and experimental values were due to a number of contributing factors. Firstly, the lifting mechanism was not allowed to maintain contact with the weight or put any kind of upward force on it before the lift. This led to a significant loss of initial tension. Furthermore, the counterweight hit the base surface during the lift, which inhibited the servo from completely rotating through its range of motion. Finally, the material used to build our structure was not rigid (despite using I-beams to build the main truss). This contributed to bending at the support end of the structure. We corrected this setback by using a diagonal supporting beam pushing up from the base (as described in “Interesting Features”). This helped to reduce a large amount of bending but could not completely stop it. All these small factors in cohesion reduced the bending moment a significant amount, resulting in an upward lift of only 2.5in, which met the goal of our project.