White Paper Discuss: Team 179 conceptual - single speed - 3 motor gearbox

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Team 179 conceptual - single speed - 3 motor gearbox by Swampdude

Here ya go Andy and Andy. Thanks for your contributions, please if you guys have the time, I invite your input and anyone elses that might.

I’ve got a couple suggestions (based on using 3-motor gearboxes for the last three years).

SDP/SI is a good choice for gears–their service is fast and they stock many sizes and materials; however, I know for a fact that they carry 32 pitch, 20° pressure angle gears which will mesh properly with the F-P–why choose to use the approx. 36.29 pitch gears (i.e. 0.7 module) instead? Is it because of compactness? Consider choosing the right gears from their catalogue, and modifying the design to accomodate both the 0.7 (for the drill) and the 32 (for the F-P)–you can also match rotational speeds better this way.

Which 24 pitch gears are you using? Consider the hardened steel ones that SDP/SI sells–they worked nicely for us last year. By all accounts, they’re fantastically strong. For a low-speed, high-torque gear, I’d be wary of regular steel gears, and would absolutely rule out aluminum, brass or plastic (again, experience…we chewed up some rather expensive aluminum gears last year, and they were only in the middle of the gearbox). Edit: To clarify, in terms of 24 pitch gears, I’d be careful about regular steel gears. Regular, un-hardened steel 20 pitch gears will be fine–that’s what we use for the gears beside the shifter, though having them hardened wouldn’t be a bad idea.

You’re welding some of the gears, I see. I’m not a fan of welding, because it can make disassembly a great pain. I’d recommend keyways if possible. (Consider adapting the design for keyways; despite the initial difficulty of putting them in, if you lack a broach, they’re very precise and easy to work with.) If you do weld, make sure that the materials are compatible!

Finally, I’m always a little concerned about those drill motors being held on by a pair of 10-32 or M5 screws alone (in holes with very little thread). While teams have definitely had success using this method, I’ve always preferred to use an aluminum collar around the drill motor. (As it happens, the last two years, I’ve used a “belt and suspenders” approach by using both the collar and the mounting holes–but this may not be suitable, depending on how quickly you want to be able to change a drill motor.)

With any luck, I’ll be posting some specs for an uprated version of 188’s 2004 gearbox (3-motor, two-speed dog shifting) in the near future…as a matter of fact, the design is done, and I’m working on the drawings now.

Tristan, Thanks for your response.

I chose the .7 module originally because I didn’t plan to add the FP motor. But since the .7 plastic was so cheap, light and the right fit I decided to give it a try. Also the stock ID happens to fit the CIM shaft OD perfectly. One of the main themes of this design was to minimize modification to the stock parts. I was going for something you could put together in a weekend. I agree though about maybe making that change. But that to me is almost another design, as this was an attempt to make the cheapest/simplest setup I could.
So you used 24 pitch successfully, great! I called out regular carbon steel, you don’t think those will hold though?
Welding yes, simply because we don’t have a broaching tool. Welding would be much simpler for us. The design is like a clam shell, so disassembly shouldn’t be that complicated.
Apparently everyone else agrees about the motor mounts. It likely will become an addition.

Also, the key way option lets the gears stay intact in a high stress situation. In situations like that, the gears teeth will break faster than the weld will, so if you have a couple spare boxes laying around, I would weld them, but keying is much more reliable and easier to change in a breaking situation.

That is one reason we don’t weld gears onto shafts anymore.

I am glad to see that you have taken the plunge to share your design. Your design has pushed the edge for weight and gear load. I design equipment that has to run 10 years in production so I err on the side of strength. Whose design is better? A good design performs its job as intended, if it lasts 10 times longer than needed than possibly too much was spent on it. If it blows up 10 seconds after it completes its mission than it is still a success.

Having said all that, I would like to add some of my experiences and thoughts to help you have a reliable gear box.
-Do not run the FP 32 pitch and Bosch 0.7M to the same gear. The bending moment on the intermediate gear teeth as they run from 32dp to 0.7M will quickly take them right off. Those that ran a 32 pitch gear to the 0.7M actually produced a new tooth pattern.
-The bearings you are using are low grade. This means that there will be some slop as they run. Our 2003 single speed used these bearings on the output and had about a 1/16" play, measured at the end of the output shaft, at the end of the season. Consider what this may do to the 12 tooth gear when making your final choices.
-I recommend using the hex and plastic kit coupling on your output shaft and then having a self supported sprocket/gear output shaft. The reason is that the moment resulting from chains or gears at the distance you are showing from the gearbox bearings may cause movements that are detrimental to the gears. Do not just add one more bearing in-line, this is hard to align and causes a bending action in your shaft.
-Plastic gears: Team 571 ran these successfully in 2004 using Bosch and FP motors. I will be testing this fall using the plastic gear with the Bosch.
-Welding is positive. It also can cause shafts to bend. We have not used welding due to lack of equipment. Last year I used keys and Loctite #609. Loctite advertises that I really did not need the keys (see my note above about how I design)!
-My gut feeling warns me about the 1/8" plates and the small gears. The only way to truly know is to build and test. Our team fights weight every year, this may be a solution.

Build it! When all is said and done the only way to know is to try it.

We used unhardened 24 pitch gears in our intermediate stage of our 2003 gb without difficulty. I’m pretty sure the face width was 0.2"

We used 20 pitch gears for all of our 2004 gearbox gears. The pinion diameter of the smaller gear became too small to use with the axle stock that we had picked for 24 pitch. We shrunk the face width down to about .1" for the intermediate gear and .4" for the final output gear. None of our gears last year experienced any wear.

I agree with your comments about aluminum gears. I would not use them in any drive system gear box (low strength, galling, expense).

Brass is not much weaker than steel and is stronger than aluminum. However, I seem to remember that brass gears are more expensive than steel gears for the same pitch, etc. Nonetheless, if I didn’t have a choice, I’d use them in an intermediate stage of a drive system gear box without qualms. I’d do a basic strength calculation first, though.

Plastic gears should not be used anywhere except the zeroth stage of a drive system gearbox.

BTW, we buy gear stock from SDP-SI and cut our gears to desired width. It is less expensive than buying standard gears and machining them to our specifications. When trying to accommodate wear, you can always increase face width (if you have space/weight) rather than increasing tooth thickness (reducing pitch or increasing module). If your teeth are wearing or your key ways are wearing, increasing width gives you improved performance in both areas.

Also, we used hex axles and hex broaches in 2004 rather than key ways. The load on each facet of the hex is much lower than the load on a key way for the same sized shaft. You can reduce shaft sizes or even go to aluminum axles versus a similar keyed shaft. This reduces weight in the heaviest part of the robot without sacrificing performance. The machining time to broach hex holes and make bearing fits on the end of hex shafts is less than the time to make similar bearing fits on round axles, broach a key way, and machine a key way in the shaft.

The only down side that I have found is that hex broaches are expensive ($200 for a 1/2" broach) and keeping the broach aligned while broaching takes a little art or a fixture.


Welded gears - you’re pointing out that the key will shear before the teeth come off? I’d like to go with keys, and I don’t want to warp the shafts - but back to the intent of this design as cheap and quick, and can it last a few months - maybe not. I haven’t checked out that loctite #609 Andy mentions yet. That sounds pretty cool if there’s a bond that can take those torques without a key- wow.

Anyhow, this is still more of a risky design, that I want to test to see if like Andy says, will it get me through 4 competitions, not will it run for 6 years. “Cheap and Cheerful” as my colleague puts it.

no 32 pitch on the .7 plastic gear, yah I agree - I think I’m just going to use this as a 2 motor design, as it’s original intent. But I might towards the end of it’s life throw the FP on there just to see what happens. The nice thing was that to accomodate the FP all I had to do was put a hole in the sideplate.

Shaft output, yah - we used this method off the plastic gearboxes the last 2 seasons just coupling straight out to another bridged/bearing shaft. It wasn’t pretty but what we did was shim the motor mounts to the transmission wherever it wound up. But if the design permits I might throw a sprocket right on that output shaft instead of a coupling. The problem with this motor configuration is that you don’t have a clear access side since motors protrude from both sides. I like the look of the new Andymarkbiz setup - with both on the same side.

1/8 side plates should be ok since I’ve got them sandwiched with 6 spacers over a small area. 1/8 plate is tough stuff. Unless we do put the sprocket right on the output shaft, then she’ll see some much more significant loads.

I know my views on all this make conservative folk cringe but I think if it works it’s all worth it- Come on you gotta admit it :wink:

Thankyou guys for your input, it’s all very helpfull. I’ll be sharing it with the build team.

Good so the stock carbon steel 24 pitch gears work :slight_smile:
I think I’m going to switch the bearings to the Sintered Bronze, Oilite flange bearings on Small parts - those things are $1.50 ea.
also going to look into the #609 loctite for gear bonding vs. welds
Now we’re talking!

I strongly recomend staying with roller bearings. I have seen may teams learn this lesson.

The way I look at it, we are all limited by the amount of energy we can put into our motors. A design that has less parasitic losses will put more of that energy to the ground.