How it Works
The lifting mechanism is simply a member with a counterweight on one side, and a hook on the other. When the servo is powered, it lifts the one pound weight by providing a torque at the middle of the member, with the help of the counterweight.
Close up of lever arm
We are particularly proud of our outrigger arm. This was made with a triangular truss, as this provided a good balance between ease of assembly, weight, and rigitity in both bending and torsion. Compared to a straight bar, the large cross section of the triangular truss also has a larger moment of inertia, which reduces bending stress.
Our largest concern was to minimize bending, which led to a truss structure with few members on the bottom. Throughout our design, we subtracted and modified members that were not crucial to the stresses it would encounter during testing. This included members in the base structure as well.
- We chose a relatively conservative design in order to minimize unforeseen variables and potential long-term problems.
- Our crane consisted of two major subassemblies: A square cross-section base, and an outrigger arm with a triangular cross section to support the motor and lifting arm.
The base was made with a square cross-section due to ease of fabrication, with a pair of L-shaped cross-members added to resist the deflection caused by the torque of the lifting arm. The other two faces received flat members, as the extra rigidity provided by L-shaped members was not necessary. The outrigger arm was connected to the base by a pair of L-shaped members that extended from the base to enable the outrigger to clear the far (larger) hole in the course obstacle. To support the weight of the outrigger and resist bending in the long members, a pair of L-shaped diagonals were added connecting the far edge of the outrigger with the base.
Finite Element Analysis
Finite Element Analysis using SolidWorks
Due to the complex nature of the structure, we decided that FEA would more accurately capture stresses than we would be able to by hand. Below is a screenshot of a simulation done in SolidWorks. Also, the simulation indicates that torsional effects due to the loading were negligible.
- Counterweight is a point mass
- Servo torque is 3/5 lb-in
- Neglect twisting and bending in lifting arm
- Neglect deflection due to twisting and bending in crane arm