Design Strategy
Each team was allowed to participate in one of two competitions. The first competition was to design the crane so that it would lift the cylindrical weight the minimum distance of 2” while using the least amount of material. The second competition was to design the crane so that the maximum amount of material could be used to create a lifting mechanism that would lift the cylindrical weight the highest.
It was decided that the crane would be designed to compete in the lifting competition that involved maxing out the material weight of the crane in order to lift the competition weight the highest vertical distance. Because of this, the design for the crane was made with simplicity in mind. The structure and mount for the servo were designed so that only simple geometric shapes such as triangles and rectangles were used. Any odd angles or unusual lengths were calculated using simple geometry and trigonometry. To account for any structural weakness, bends were made into existing supports and diagonally placed cross supports were placed where space allowed. The servo mount was designed to be a simple platform that would be suspended below the arm of the crane so that the lifting mechanism would be able to interact easily with the bottom of the weight. Despite its simplicity the crane structure was intended to be made rigid in comparison to the servo and its mechanism.
Crane Design
The base was built using the thickest members available for stability. The crane arm was composed of two aluminum strips that were held together by bent strips in a truss-like configuration. This was done to prevent bending in the arm. However, two diagonal members were attached from the base to the arm for added robustness. The structure held fast under its own weight. The servo was attached to the end of the arm. The entire structure, including the lifting mechanism and counterweight came to 20oz.
The lifting mechanism was a T-beam attached to the servo. The T-beam configuration prevented the lifting mechanism from bending. In order to provide assistance with the lifting of the weight, a counterweight was attached to the end of the T-beam. The 2 lb. weight had 1 inch lever arm relative to the servo. The counterweight had a lever of 20 inches. The 20-inch lever arm allowed for the counterweight to be its optimal weight while still keeping the structure under 20 oz. as a whole. The counterweight weighed 4.5 oz. The moment produced by the counterweight in addition to the moment of the servo were more than enough to counter the moment caused by the 2 lb. weight. This is shown in the equations section.
The initial crane configuration was designed to be light. However it was not very robust and did not withstand torsion effects. The servo and lifting arm were normal to the arm and it was difficult to prevent the twisting of the arm. This led to placing the lifting mechanism/servo parallel to the arm which made torsion easier to deal with. Unfortunately, the new design was also much heavier than the new one.
During testing, the servo controller tended to overheat. A heat sink was added to the controller to avoid this problem; the heat sink was not included in the overall crane weight.