Making a 2-Speed Custom Gearbox

This tutorial will show how I created a custom 2-speed shifting gearbox. As a student in the FIRST Robotics Competition (FRC), I became heavily involved with designing and building my team’s drivetrain and chassis. Though the chassis was custom designed and fabricated, we always used commercially produced gearboxes due to time and resource constraints. As I looked at some other teams’ robots, I was amazed by their beautiful custom gearboxes that perfectly fulfilled their design goals. Seeing these other designs inspired me to try making one of my own, even though producing it would be impractical for my team during the actual build season.

This project was really just me having fun before I graduated from my high school, where I had access to the machining equipment necessary to complete the project. Unfortunately, I did not have enough time to finish all of the parts. However, I do have enough to give a very clear idea of what it will look like. Though the gearbox was designed to be used on a FRC robot, it will probably never see use.

This tutorial was made through the Autodesk FIRST High School Intern program.

Materials
Aluminum Plate
Aluminum Tube
Aluminum Hex Bar
Gears
Bearings
Motors
Bolts
The PDF attached to this step is a complete Bill of Materials for this project. It includes information such as the quantity, cost, material type, and vendor for many of the required parts.

Tools
CNC Mill
Lathe
Band Saw
3D Printer
Socket Wrench
Hex Wrench
Pliers

Step 1: Designing the Gearbox

The design of this gearbox was heavily inspired by other FRC teams who posted pictures or full 3D models of their designs. Admittedly, I am not the most creative designer (steal from the best, invent the rest, right?), so the inspiration is fairly obvious. In particular, the design was inspired by Team 254 and Team 973, who have been gracious enough to share their designs.

I designed my gearbox entirely in Autodesk Inventor before purchasing a single part. I started by choosing a gear ratio (which you can learn more about here), and then ensured that there would be no clearance issues between the rotating parts using the sketch shown in the second picture. This sketch also helped me define the exact shape of the gearbox plates. From there, I designed the rest of the gearbox, part by part. Throughout the design process, I had to ensure that I could make each part that I designed on the tools available to me.

首先，你得有台3d打印机

Are you required to use DC motors? Just curious, because although I've always been interested in building robots, I've never built one (directed my free-time to things of a combustible nature lol). I'm asking because my company manufactures high torque (in the hundreds of ft/lbs) AC brush-less motors of comparable size, that I can envision being quite effective in applications such as this, especially with torque multiplication.

The motors and speed controllers you are allowed to use are very limited. Most of those brushless hobby motors are infact DC.

Is the role of the 3D printed spacer in the output shaft bearing block just to ensure that it is concentric to the shaft?

You've done a very nice job of implementing this gearbox - it looks beautiful! A technology woth looking at for the gear-shift is a 'ball clutch' which can be engaged & disengaged / gear change while moving. They are used in sequential shift gearboxes - but are very simple to make. You leave all the gears engaged all the time - and just select which one to take the drive from. They require little force to change gear - well within the range of an RC servo for example. Just a thought! Si

Now that i just might need... hmmm... (thinking) thanx for sharing. - chase -

Hi, ever thought of using a planetary gear to connect both motors to the wheel so you dont need to switch between them ?

Great "ible" by the way!

No, I actually hadn't. The biggest problem with planetary gear-trains is that they are generally less efficient than traditional spur gears. It's also not the motors that I'm switching between, but instead gears. It's just like changing gears in a car - as you shift up, your car is able to go faster, but doesn't have as much torque.

Good lord, if the school I mentored had those tools we would have done much better. About all we had was a drill press and various power tools. The crap any Joe Schmo might have at home. It was so bad, I actually bought a cordless dremel and various attachments for them to use. However, having said that, you're certainly head and shoulders above what any of them were capable of. Just please don't let the absence of tools discourage you.

Why have you used two motors ?

Using two motors doubles the gearbox's torque output. This allows the robot it drives to accelerate faster and push harder. In the FIRST Robotics Competition, teams are allowed to use four of this type of the motor. Since it is the most powerful motor allowed, teams generally dedicate all four to driving the robot, where the power is needed.

There are power limits on your motors then ? Are you going to shift it, or use EM clutches ? Done on the high speed size, EM clutches would easily shift the torques you will get from those motors.

Yeah, we're essentially given a list of motors we are allowed to use for the competition. This design uses a pneumatic piston to shift between a high torque, low speed gear and a high speed, low torque gear. I don't think an EM clutch (had to look them up) would be appropriate for the application - it seems like they either transfer full torque or no torque.

That's right, thereby shifting power from the motor through one gear train or the other. Sure, you'd need two, but they are very compact, and you wouldn't have any chance of wrenching teeth off your gears, since your pickup would be relatively gradual.

Outstanding! Best of luck with your studies in college. We have experienced problems with young graduate engineers who test well but probably have never built a doghouse or model airplane, You should be head and shoulders above them.

Kid, you are amazing, did you stay back 30 years or something, i applaud you, your parents and your school system. We couldnt even get supplies in auto shop and our machine shop was left overs from the Civil war. How you can be so smart and have skills to pull off close tolerenes on a lathe is amazing. Let us know when you develop the first cyborg. keep up the great work

Thank you so much for posting an Instructable associated with FIRST! This makes me incredibly happy. I'm with team 1332 in Collbran, Colorado. Although I don't think it would be legal to use this exact gearbox on a robot, since it was designed during the offseason. It would have to be modified and build during a season for it to be legally used on a robot...but no one follows those rules anyway! Thanks for the design!

I'm glad you thought it was cool! As for using the design, I believe that would be allowed, since it is now publicly available. Definitely can't use the actual thing, but I think the design (or a modified version of it) could be used.

I'm 37 years old and have never designed nor built a gear box. Well done! The engineering and science departments at every university should be clamoring for you.

Thank you for your compliments! I am glad that you find my writing understandable - it's a skill that I've been working to improve the past couple of years. As for designing a gearbox yourself, it is a fun challenge that I would recommend trying if you're interested in mechanical design. I learned a lot by looking at gearboxes that other people designed.

If I may further heap praise, you write very well for someone your age. This is priceless in any profession, but especially so in STEM disciplines (science, technology, engineering, math).

Your work is impressive, congratulations! I don't understand why you don't use an electronic volts-amps controller to vary the speed and power of the DC motor.

An electronic speed controller is in fact used to control these motors on the robots in the FIRST Robotics Competition, but often times a 2-speed gearbox is a good solution for more torque in the event of a brief pushing match. Similarly, it's good to be able to get into a higher gear if your robot needs to get from one side of the playing field to another (54 feet).

OK, thanks for the axplanation.

Radioactive_Legos hit the nail on the head - you use H-Bridge motor controllers to control the DC motors and then use the shifting to alternate between high torque and and high speed. For this gearbox, low gear is about 6 ft/s while high gear is about 15 ft/s. However, low gear can push about 2.5 times harder before blowing the circuit breakers that regulate current to the motors.

Very nice! What team were you on? I'm currently a member of Team 100

Thanks, I was a member of Team 2374 up here in Oregon, but since I've graduated I haven't really been associated with a team. I'm planning to mentor one of the teams near my college if I have time.

oh, its interesting!