Using calculations & Solidworks, material and form experimention, and material from the curriculum, our team developed a crane that lifted a weight of 4.875 inches. We placed second in the class for maximum height reached.
Crane Project: designing to accomplish a prescribed purpose
Skills:
Hand+CNC Milling
Iterative Design
Lathing
Solidworks
Spring 2018
Group Project for course 24262: Stress Analysis in Carnegie Institute of Technology
Team 26:
Professor:
Weighs no more than 20.0 oz and it must lift the sliding weight by atleast 2"
Teams compete on the basis of maximizing the sliding distace and minimizing the weight of the mechanism.
Minimum Capabilities
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. Our 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 our 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. design.
Technical Objective
Design
The main source of error that we encountered was the fact that the servo we were using was not consistent. Not only did we see different capabilities between different controllers, but also when we began the design review, we realized that the servo was performing at a reduced level compared to the night before. Our suspicions were confirmed when we switched servos and our crane performed better.
Going into the first design review, we assumed that the arm itself was massless. This is neither practical nor possible, but we chose to runw ith it because the center of mass of the arm was so close to the axis of rotation-- thus the moment caused by the arm itself was negligible compared to the torque of the servo, the weight of the counterweight, and the force due to the weight.
Additionally, we assumed that the arm would lift a max of 60 degrees. Although the servo was capable of rotating further, as the arm increased in angle, the change in x position due to trigonometric properties would cause the arm to no longer be ablle to lift the weight. As a result, a 60% lift angle would be a safe bet for the actual max lift angle.
Lastly, we assumed that the max torque by the servo was 57 oz-in. After some prototyping, we discovered that we weren't getting anywhere near that torque despite that was what the manufacturer rated the servo for.
Assumptions
Design Review 1
We were getting essentially 2 inches of height up until the night before the first design review. The main source of error that we encountered was the fact that the servo we were using was not consistent. Not only did we see different capabilities between different controllers, but also when we began the design review, we realized that the servo was performing at a reduced level compared to the night before. Our suspicions were confirmed when we switched servos and our crane performed better. There was also a slight source of error due to the assumption that the arm was massless as the arm in practice was not.
The actual output of our crane was much worse than the calculations that we did to find our theoretical servo torque as well as our theoretical lift height. Initially we calculated that using a counterweight that had its center of mass six inches from the axis of rotation would provide plenty of torque to allow for the servo to easily lift the weight. Unfortunately, when we placed our design on the field, we quickly found that the arm could barely lift the weight and that the crane was nowhere near the 2 inches that we needed to achieve.
Even though we were only utilizing 62% of the servo’s stall torque, the arm was struggling to make any progress. After recalculating, we determined that if we doubled our counterweight arm we could decrease the torque that the servo would have to apply in order to lift the weight. With this new arm configuration, we determined that with only 18.5%, the arm would be able to lift the weight, and even though it decreased the maximum lift height, we would still be able to lift above the 2 inch goal. Unfortunately, when we got it on the course for the design review, the counterweight began contacting the frame on the way down, limiting our lift height to just under 2 inches.
After coming up with our design and running our digital simulations, we took our materials into the shop and began working. We focused on a full metal construction out of cylinders. Using cylindrical rods like this was intended to not only prevent torsion, but also to resist buckling and bending. Additionaly, we minimized weight and material use by hollowing out our 1 inch aluminum stock on the lathe.
Manufacturing
* * CAD rendering of our design
Download a STEP file version of our design here
Coming out of the first review, we added a 45 degree bend in the lever arm and improved our counterweight so it would no longer hit out crane arm. Additionally, we milled a smaller base that allowed for more adjustability when it came to iterating for our final version.
Design Review 2
* we used the student shop to machine all our parts ourselves
* milled the servo bracket out of sheet metal to minimize deflection and assure a perfect fit
* * closeup of design review 2 height
Final Design
Our final crane was extremely adjustable in terms of rotation as well as adjusting our servo mount to get the arm to interact with the weight at the correct angle. Hollowed rods allowed for us to minimize weight without sacrificing structural integrity, especially in respect to bending.The tension strips that were used were instrumental in preventing further bending as it prevented bending for the first 60% of the crane arm length. The angled arm allowed for the weight to be lifted at a larger distance from the servo. It allowed the counterweight to not overpower the servo at the beginning, but apply enough counter torque for the arm to lift the weight completely.
In the end, our design allowed us to reach a height of 4.875 inches.
* hand analysis of crane design after changes
* * we changed the base to a more minimal design allowing for adjustments and provided a mount for a tensioner to prevent flex.