Team 3 - Design Process

To accomplish a 3-dimensional pick-and-place operation, we started with a planar four-bar linkage, added an electromagnetic end-effector to grab the object, and then mounted the assembly on a rotating gear.  Since we had specified two stages at the same height, and a third stage at a different height, we also needed to have intermittent motion between the rotating base and four-bar linkage.

One common way to disengage and re-engage a geartrain is through a clutch.  A clutch can be controlled manually, or through a cam-follower mechanism, as shown in the below schematic.  A clutch system provides flexibility for where the stages can be placed but also requires several complex joints.

We needed something with fewer interfaces and decided on an inverted spherical Geneva mechanism.  A Geneva mechanism provides intermittent motion while only requiring two components and one interface.  The added benefit of a spherical Geneva mechanism is that it can also transmit motion at a right angle, thereby replacing the bevel gears. All of the components that are shown in the red-dashed box in the previous schematic are replaced with two components: a crank and a sphere, as shown below.

Geneva mechanisms are not as flexible as a clutch, but we used their restrictions to guide the placement of the stages.  Our spherical Geneva mechanism is also technically inverted because the "crank" is motionless while the "sphere" rotates around it.  The sphere constantly rotates with the main gear in the horizontal plane, but for half of this horizontal rotation, the crank is engaged and drives vertical rotation in the sphere.  A pulley system transfers torque to the pick-and-place mechanism.

We sized the mechanism to fit nicely on a desktop and show how everything works clearly.  We knew we needed a large gear ratio between the motor and main gear to make it run slow and smooth and we also knew we needed the pick-and-place mechanism to be small and light since a full cycle was powered by one 90-degree turn of the Geneva sphere.  Press-fits were not enough to prevent some of the shafts from slipping, so we tried to add hubs later, but the slip is still an issue for either high-speed or high-torque scenarios.

Before diving into the CAD, we also created some schematics showing the different stages of operation.

Modeling Components

Most of the components didn't take very long to model, but we usually had to make multiple revisions.  A good example was the pulley wheels, which needed to mesh with the grooves in the timing belt because any slip would cause the four-bar motion to be out of phase.  Knowing that it would take a few iterations, we tested some very narrow spokes to decrease printing time, and they proved very strong - we probably could have also used 3 spokes instead of 5.

The geneva mechanism was rather large because we wanted to use a track roller at the interface, and the smallest size we could find was 1/2", but the size worked well for demonstration purposes.

An example of our joint interface for the four-bar mechanism is shown below. When two links make a pin joint, we use a collar in between the surfaces to avoid sliding friction. In this case, we use bearings on both links as we want them to rotate freely on the shaft. Our joints had to be low friction for a smooth mechanism. For all the revolute joints, we used press-fit ball bearings.