Phase IV

Objective

Create a gear train that will rotate the base of our payload carrying/lifting mechanism

Explicit Constraints

Rotary base must be driven by a third servo motor
Base must rotate at least 150° between pickup and drop-off zones
Team may receive a 10" diameter circle of 0.75" thick MDF and two drop zone boards, also 0.75" thick MDF. The circle must have a 1/8" diameter hole in the center, added during manufacturing process

Implicit Constraints

Keep servo motor travel between 155º to 170º (it is unlikely that the servo will be able to rotate the horn exactly to 150º and to the full 180º due to motor design limitation).
Keep the gear train at a minimum (simple but effective design)

Design Brainstorming

Brainstorming was done collectively instead of individually during this phase. The group collaboratively decided the angles used for the servo, pitch diameters of each gear, and the number of gears/teeth while working through this phase.

Two Gear System

Pros:

Less gears, less moving parts
Less material to manufacture
More space to work with on the 10" diameter base

Cons:

Lifter arm post comes into contact with servo mount
Must be aware that Gear 2 will rotate in the opposite direction than that of Gear 1
Less freedom to create different gear ratios

Three Gear System

Pros:

Gear 3 will rotate in the same direction as Gear 1
Adding Gear 3 will remove the contact between servo mount and lifter arm post
Can adjust positions of gears to avoid interference

Cons:

More moving parts
More calculations to precisely get Gear 3 to rotate the way we want it to
More materials, more manufacturing

Final Decision

Three Gear Design

Although we originally intended on utilizing only two gears for simplicity's sake and to achieve the desired ratio, during the 3D modelling process we ran into the issue of the servo mount colliding with the arm's stand. In order to solve this, we added another gear in between the arm and the servo that was the same size as the servo gear, thus increasing the distance between the two parts without modifying the gear ratio. The drawing above shows the three gear setup used. From the variety of options, we chose the gear ratio of 32:27 because it allowed for the servo to rotate 160º while also allowing the arm to rotate 135º.

Torque and Speed:

Our design consists of a larger gear driving a smaller gear. In terms of torque, this means that the final of the torque will be smaller. The speed will be inversely affected, meaning that the final output will be faster.

Gear Train Calculations:

Mechanism Rotation

R(N) = (180 * (N - 2)) / N)

R(8) = 135 degrees

Gear Ratio:

160 deg : 135 deg = 32 Teeth : 27 Teeth

d = p / d

d1 = 1.6in , d2 = 1.35in