My initial concept sketches were focused on devising ways to translate the power from the motor to a gripping motion on the sphere. Since the winner of the competition would be the team that completed the task with the lightest design, I tried to challenge myself to avoid using a gearbox that would add to the overall weight. Manufacturability was also a constraint I had to consider. While certain features might be nice in practice, we had no access to a 3D printer for the project, which limited the complexity.
Our first complete design in CAD consisted of three gears that would drive two d-shafts. The d-shafts were attached to two identical gripper arms made from laser cut acrylic. The gripper system was attached to the mounting interface with 4 hand screws that easily allowed for us to install the device within the 30 second time frame. Upon testing this design, we found that the gripper arms were not strong enough to withstand the swinging of the pendulum arm. We fixed this by revising the design to 3 gripper arms with only one being driven by the motor. We also decided to CNC mill the arms out of aluminum to ensure they were strong enough for the task.
In this design revision, we also decided to change the material of the grippers to aluminum and added dycem rubber to the areas that would contact the sphere to reduce the slipping issue we were having with the acrylic grippers. After testing this design, the device gripped the sphere for longer but still ended up dropping it.
We noticed that our gripper arms were not following the curvature of the sphere below the equator, which was giving the ball space to slide down. After fixing a minor geometry mistake, we manufactured the final gripper arms. This time we sand blasted the arms to give it a rougher texture to also reduce slipping between the dycem rubber and the gripping arms.