Swinging Gripper

ENGINEERING DESIGN 1, FALL 2018

Gripper is mounted to the robotic arm for dynamic testing.

I worked with a team of 5 in an Engineering Design class to design, fabricate, and test a robotic gripper in order to reliably grasp, hold, and release a custom object. The gripper must be able to hold on to the object without displacement in any direction while the robotic arm swings rapidly through a circular arc. Performance testing was based on total gripper mass.

Our design grips the object with two 3D printed arms, one static and one moving. A double gear set transfers torque from the motor shaft to the moving arm. The larger gear is spoked in order to reduce the total mass of the gripper. Both the gear and moving arm are press fit onto a keyed shaft. Our group iterated through several prototypes of each component before reaching the final design in order to optimize performance while minimizing mass. The gripper successfully passed dynamic testing, and weighed 152.4 grams.

See detailed writeup here.

Initial Concept and Prototype

In our initial prototype, the transfer gear was laser cut from acrylic and press fit onto a carbon steel shaft. The driving gear was a steel gear secured onto the motor shaft with a set screw. Both arms and the base plate were 3D printed. This prototype failed at the gear teeth during dynamic testing. This prototype failed at the gear teeth and moving arm due to shear force. The acrylic used to cut the transfer gear was too thin, causing teeth to snap when the arm was rotated past 180 degrees during nominal testing. The gripper was unable to hold onto the bottle due to the low friction between the 3D printed arms and bottle surface. Additionally, the carbon steel shaft made the gripper very heavy.

Initial design concept (before object geometry was known)

First prototype after testing (driving gear not pictured)

Second Prototype

For our second prototype, we changed several parameters in order to account for the incorrect dimensions initially given in the project description. Both the static and moving arms were lengthened and increased in thickness to account for the larger moment. A larger gripping surface and non-slip material was added to the ends of the arms in order to reduce displacement during dynamic testing. The brackets on the base plate were lowered in order to bring the moving arm closer to the bottle and a truss support was added to resist axial forces on the brackets. Unnecessary material was removed from the initial base plate design seen in the CAD assembly in order to reduce mass and create a more ergonomic design. Additionally, the split hub clamp was replaced with rubber gaskets because it was determined that axial forces were not large enough to justify a heavy split hub clamp. Both the transfer gear and driving gear were 3D printed. We used an aluminum shaft instead of carbon steel in order to reduce mass. This prototype successfully passed both nominal and dynamic testing, but was very heavy due to unnecessarily high factors of safety.

Second prototype (transfer gear not pictured). Several tweaks were made in between this prototype and the initial assembly pictured to the left.

For the final prototype, the aluminum shaft was replaced with a 3D-printed PLA shaft in order to reduce weight further. In addition, the transfer gear was spoked instead of completely solid, in order to cut unnecessary material in the gear. This design successfully passed testing, while reducing mass by 70 grams.