Matt Jacob
Kyle Andrews
Base Structure:
We began building by making a sturdy tower with cross beams. For our entire base tower we used the widest strips bent in half to increase their strength. We also have a hole in the middle of our base plate to reduce weight, because there are no stresses going through the middle of the plate. One issue that arose as a result of our design was the diagonal supports connected to the bottom left points of the base. This restricted how close the clamp was able to get to the points of connection between the base and base plate which caused bending in the base plate. The deflection of this bending transmitted all the way to the motor when it attempted to lift.
Support Structure:
Our next feature is the section that extends off the base tower and provided support to the crane arm. The strength of this section is found in the beams that extend from the bottom of the base tower to the top of this section. This section provided a strong connection to the base and shortened the distance between the weight and support contact on the crane arm.
Crane Arm:
Attached to these supports is the arm. The arm extends all the way to the weight, and is triangular in nature with supports crossing the faces. The arm is designed to mimmick the truss structure found in construction cranes that minimizes weight compared to a solid piece while maintaining most of its strength. This piece was very strong in response to twisting and bending moments generated by the motor.
Lifting Structure:
Finally, we have our motor support and lever arm. We attached our motor by housing it in a rectangular loop of strip metal around it, then attaching that loop to the crane arm through two vertical supports and one diagonal support. These supports were designed to deflect the least amount possible in both the vertical and horizontal directions. Attached to the actual rotating part of the motor, we have a lever arm with a counter weight on one side and a ‘Y’ shaped arm on the other side to lift the weight.
Theoretical Calculations:
Due to roller bearings in weight I will assume force on lever arm to always be perpendicular to the arm. Below is a simple FBD of the arm in a horizontal orientation. Consider the arm to be pinned at the applied moment of the motor. The forces and distances on the arm are approximated.
Since this moment is negative and we desire a negative moment on the arm to turn it clockwise and thus raise the weight, the servo and counterweight should provide more than enough moment to raise the weight. If these moments were to sum to 0 then only 36 oz-in of the servo torque would be necessary.
Thus, 36oz-in/42 oz-in = 85.7 % max servo torque used
x/2.5=sin50
x=(2.5)sin50
x=1.91 in Since this accounts for only 50 degrees or half of the servo's 100 degrees of rotation we can multiply this distance by 2 to get the maximum theoretical lifting distance
x= 1.91(2) = 3.8 in
Notes and Observations:
The most unique feature of our design would be the end of our lever that lifts the weight. We used a single piece of the widest aluminum strip bent into a u channel for strength. This long straight u channel was bent into a ‘Y’ shape and screwed at the two joints to hold it in place. The thought process behind this design was the result of prelimnary issues with a straight arm in which the lever arm had difficulty remaining in contact with the weight and resulted in undesired twisting of the weight itself. The Y-shape was designed to always be in contact with the weight and support it symmetrically so as to not create any twisting moments. This device's downfall however was that it provided too much contact with the weight such that when the arm lifted to the point where it was horizontal, the whole face of the y-shape was in contact with the weight. This created a large friction force that the motor and counterweight were unable to overcome.
We were very proud of the housing of our motor and the stiffness of our crane arm. Both structures resisted almost all deflection and were very useful in minimizing weight.