Gearbox for mechanism or appendage

[Andrew Duerner]

Let me start with a little background. I'm a professional mechanical engineer and I love robotics! I mentored for a team two years and went to several regionals and two championships. I also love solving problems and inventing. About seven years ago I came up with a new eccentric gearbox, it worked pretty well but it was too costly to produce. After being involved with FRC I started adapting the design to fit the competition requirements, low cost, available motors, ease of use, etc. After many iterations, kudos to the inventor of the 3D printer, I believe I have a design that can help a lot of teams improve their performance of the competition challenges.

Here are a few details on the gearbox. Its’ approximate size is 5”x5”x2” and weighs around 2lbs. It directly mounts a 2.5” diameter motor with a keyed 8mm shaft (CIM family). The base mounts to 1” 80/20 extrusion on four sides. The output has the same mounting provisions to make multi degree of freedom arms and gimbals easier. Right now there are three ratio options, 552:1, 264:1, and 168:1. There is an integrated bearing track approximately 4” in diameter to support the output load. Right now gearbox is made of ABS plastic fabricated on an FMD 3D printer. I believe this will be plenty strong for competition if it is installed correctly and used to manipulate game pieces, not for taking the load of an opposing robot crashing into you (which is true for many mechanisms in FRC). If the demand is warranted I could see transitioning to injection molded parts with better plastic to be stronger and cheaper. As for cost for the current design, I’m trying to keep it under $100.

This effort was directed at the general FRC community. I know that some “elite” teams fabricate all of their own gears and can machine anything they want ending up with something better suited for the competition, but there are many teams that I saw coming to the competition with a drive base and limited game functionality. I really want to help elevate teams to have success with their available resources.

Special thanks to Chase (also an FRC mentor) for help with cost cutting ideas and general improvements along the way.

I attached a picture of one mounted with some 80/20. If anyone has feedback or is interested in more information please post below or send me a PM.

Thank you,
Andrew

[asid61]

$100 for this is a very good price, although I'm wondering how you're getting past the cost of what I assume is a 2 stage planetary transmission. Best of luck in your endeavors.

[MrForbes]

Look up eccentric gearbox...it's not planetary. I'd love to see the insides of this....

[Travis Schuh]

Do you have estimates on backlash and efficiency numbers?

[Mike Marandola]

Originally Posted by MrForbes

I'd love to see the insides of this....

I second this. This looks pretty slick.

[asid61]

Originally Posted by MrForbes

Look up eccentric gearbox...it's not planetary. I'd love to see the insides of this....

Oh whoops, I missed that part of the description. Planetary seems more "normal" to me for large reductions.

[MrForbes]

I'd like to see a "harmonic drive" that's affordable enough to use on one of our robots....

[billbo911]

Originally Posted by MrForbes

... I'd love to see the insides of this....

Originally Posted by Travis Schuh

Do you have estimates on backlash and efficiency numbers?

Excellent questions.
Any chance you can provide these?

[Andrew Duerner]

Good questions.

The backlash is somewhere around 1-3 degrees.

As for the efficiency, I really don't have the right equipment to measure that right now. It is abs plastic and it's not melting, if that's saying anything.

Attached is a picture showing whats going on inside. The gearbox ended up being pretty simple, kind of ironic considering the path it took to get to this point.

-Andrew

[Michael Hill]

Do you have any maximum torque figures?

[AdamHeard]

How can we buy one?

[Tottanka]

Originally Posted by AdamHeard

How can we buy one?

2nd that.

[Richard Wallace]

Originally Posted by AdamHeard

How can we buy one?

Try Harbor Freight. Link to old thread. Other link.

[Paul Copioli]

This gearbox is known in the industrial robotics industry as an epicyclic gearbox and is very common in robots that require low backlash and high gear reduction.

A couple of business and technical points:

1. There are MANY patents on this design. Several of them have expired but a few key ones still exist so do a patent search before selling this.

2. This design looks to only have one stage going in a single direction. This will induce severe oscillations that cause torque ripple which are detrimental to high load arm applications. There are ways to solve this but it adds significant cost.

3. In order to get acceptable efficiency the oscillating gear must have a very smooth surface finish. Grinding that surface is a must or I would not use it. Plastic gearing for this will severely limit the load carrying capability.

That's all I can think of now.

Paul

[Andrew Duerner]

Michael, I torqued the gearbox unpowered with some lever arms until it broke apart. I would estimate that it reached around 50 ft-lbs.

Adam, I built a website, I'm a mechanical engineer so no making fun of it looking like its from 2004, I need to go through the rules on mentioning trademarked names or just delete them, after I do that I'll post the website address and they can be purchased through the Paypal payment system with a credit card.

Paul, I did some patent searching, I couldn't find anything that the design violated, If you know something in particular that may be a problem I would welcome any information. I haven't experienced oscillations besides having to counterweight the input cam/oscillator. Plastic gears do have their limits, obviously I wouldn't recommend this for an industrial robot, but for FRC robot that needs to run for limited amount of time, I believe it can work.

-Andrew