Mechanisms

The goal for our design is for a person to flip a switch which triggers mechanisms to push the switch back, raise a “hand” with a “finger” that wags “no” to chastise the person for flipping the switch, and then lower the hand back down. To achieve this, there will be three main mechanisms: 1) switch flipping, 2) raise/lower, and 3) finger wagging shown in the figures below.

Figure 1. Switch flipping mechanism

Figure 2. Raise/lower mechanism

Figure 3. Finger wagging mechanism

The switch flipping mechanism is a quick return mechanism, and the raise/lower and finger wagging mechanisms are four bar crank-rockers. We required that all mechanism input links have a 360 degree range of motion so we would be able to run the motor continuously in one direction to drive the mechanisms. The quick return mechanism was designed so the slider would hit the switch. The raise/lower mechanism was designed to have a roughly 0 to 90 degree output link range of motion so that the output link for lifting the finger wagging mechanism. The finger wagging mechanism was designed so that the output would move back and forth with a range of motion mimicking an index finger wagging. The link lengths of the mechanisms were set so that the input links would be the smallest links and the rest of the links were set to achieve the desired output range of motion for each mechanism.

Motor Selection

We wanted one cycle (one full input link rotation) of the switch flipping and raising/lowering to occur in about 5 seconds which requires 12RPM, and we chose to use a 10RPM motor since it is a more standard RPM to obtain.

Overall Layout

One of our design goals was for one motor to drive all mechanisms, which was a challenge to achieve for two reasons. First, the finger wagging motion is in a plane perpendicular to the plane that the motion of the other two mechanisms is in, so these mechanisms need to have different axes of rotation. To accommodate this, we planned the raise/lower and switch flipping mechanisms to be driven on one axis by the motor, while a parallel shaft is dedicated to the finger wagging mechanism. To transfer the rotation from the motor to the parallel shaft, we planned to use a 1:1 timing belt. Then to change the direction of rotation, we chose to use bevel gears. Second, we wanted the finger wagging motion to occur multiple times during each switch flipping cycle, so it requires a different RPM than the other two mechanisms. To do this, we intended to use a 1:3 timing belt to drive the finger wagging mechanism at 30RPM. A top view of the layout we planned to achieve our design goals is shown below.

Custom Parts

To integrate everything together, we designed some custom parts to serve particular purposes.

Bearing cup: For the ball bearings to be placed into and inserted into wooden pieces. Some required thru holes for shafts to extend through, and some did not.

Motor mount and supports: To hold the motor in place and provide it support from below.

Shaft support: To support/hold and raise the 8mm shafts to allow the input links clearance to rotate 360 degrees. The shaft supports lift the shaft centers to 60mm.

Shaft link adapter: To transfer 8mm shaft rotation to mechanism input links. Designed to use a 8-32 ½” bolt and nut to secure adapter to links and 6-32 ¼” bolt as a set screw to secure the shaft into the adapter.

Washers: For placement onto 8mm shafts to provide spacing between links as needed.

Quick return

Slider shaft holder: To hold the shafts that the slider slides on in place.

Bearing cuff: To attach the linear sleeve bearings that go on the slider shafts to the slider

Bearing cap and riser: To ground the mechanism. A bearing is placed into the bearing cap, the cap is placed onto the riser, and the assembly is grounded in place.