[Andy Baker]

Here are some basic recommendations for designing and building your own gearbox for FIRST Robotics Applications:

1. Fabrication accuracy is key. No matter how good your design is, if your fabrication is not good, then headaches will persist for the entire season. Learn from experienced people (skilled trades folks, instructors, etc.) with regard to making precise features like bearing press fits, concentric keyed holes, and accurately placed bearing holes on a plate.

2. Use 20 dp (diametric pitch) gears. These are fairly abundant. Industry standard gears of this pitch can be purchased from McMaster-Carr, MSC, Martin, Berg, Boston Gear, Stock Drive Products, Rino Mechanical, and other vendors. They can be found with a pressure angle of 20 and 14.5 degrees. These don't mesh well together (they do, but things get really noisy). I think that the 14.5 degree version is a bit more abundant. Also, Misumi is probably be best resource for metric gears. I recommend module 1 for most FRC applications.

3. In gearbox design, add 0.003" to your center-to-center distance. This allows for a bit of eccentricity of your gears, and only adds a negligible bit of backlash.

4. Find a resource for 1/2" or 3/8" hex broaching, and then use 1/2" or 3/8" hex shafting instead of keyed shafts. However... I'll admit that keyed shaft gearboxes are getting easier these days since McMaster-Carr and other places will sell stock of pre-keyed steel and aluminum shafts.

5. I recommend steel gears to start out with. However, if you can justify using plastic, aluminum or brass gears, then those work also. These, of course, can withstand less force and gears will shear if the force on the gear is higher than the gear tooth's cross sectional area x the yield strength of the gear's material. I would also multiply in a safety factor of 2x.

6. When in doubt, increase your face width. Usually, 3/8" is a good face width, but for higher torque situations you may want to go to 1/2" or 5/8" wide gears.

7. Don't forget to lube. Lithium grease is always a good lubricant on a simple gearbox. If you want things to move back and forth on a shaft, like in a dog shifting gearbox, use a lighter lubricant such as an oil.

8. Buy the parts that are difficult to fabricate, and then make your own parts. Many people ask me "why do you sell spare parts for your shifters and gearboxes... do these parts wear down?" My answer to this is "no... they don't wear down, but we sell individual parts so teams can design and build their own gearboxes by using a few of our parts."

9. Get your design made and take pride in your gearbox.

Andy B.