Servo - September 2011

PARTS IS PARTS: Gear Terminol gy Meshes With Little Susie

● by Morgan Berry

Every builder — even the most seasoned veterans — have experienced more than one “Oh no, what have I gotten myself into?” moment when working on a bot. Apart from the complications that can happen while building, ordering parts brings up a whole other set of issues. Even a simple wedge bot with no moving weapons can require dozens of parts, and with the dozens of varieties of those dozens of parts, an inexperienced builder might be tempted to just give up and walk away entirely. However ... fear no more! The Combat Zone’s guide to gears is sure to make one of the most intimidating steps in the build process much simpler.

Meet little Susie. After attending a combat robotics competition, little Susie was inspired to build a metal-tearing, pain-bringing death machine of her own. A trip to the local hobby shop left Susie with a bargain box with a variety of parts. Now, all she needs to build her bot is a shell and gears.

The metal shop teacher at her school agreed to help her build a shell. So, smiling at how simple the process is, Susie sits down at the computer to purchase some gears. Susie’s motors run at 1,000 RPM, and she is envisioning a wedge bot, so she needs to increase her torque to provide the pushing power a wedge needs. Susie calculates that her bot’s wheels should turn at 100 RPM. She’ll reduce the RPM using gears. (If she was using her motor to drive a saw blade for a weapon, Susie would instead need to increase the RPM to enhance the cutting ability using the same gear ratio.)

FIGURE 1

spins, she doesn’t need to worry about direction of rotation (but, if Susie’s bot had a blade mounted on top, a bevel gear might be necessary). She also rules out a worm gear (Figure 3) which is used when large reductions in RPM are needed.

Now she must decide between the two remaining types of gear: a spur gear or a helical gear. A spur gear (Figure 4) is the most common gear and has parallel teeth, so when they rotate, each set of teeth on the gears come into direct contact with