pic: Too much power?

[Cash4587]

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More like not enough material. Those thin pieces of sheet wouldn't hold up on their own even without the motors. At a minimum, especially for anything with a CIM attached to it, use 1/4" aluminum plate, or else your gearbox is going to snap before the first miliamp hits your motors.

[Chris is me]

Originally Posted by Andrew Lawrence

More like not enough material. Those thin pieces of sheet wouldn't hold up on their own even without the motors. At a minimum, especially for anything with a CIM attached to it, use 1/4" aluminum plate, or else your gearbox is going to snap before the first miliamp hits your motors.

This isn't necessarily true at all. Tons of gearboxes, including the AndyMark Toughbox, use 1/8" aluminum. The Toughbox Nano mounts a CIM to 1/8" thick material (granted, a box rather than a plate). Lots of teams with sheet metal use gearboxes with plates .125 thick or even lower with appropriate bends. Saying a gearbox with 1/8" plates is going to snap before load is even applied to it is a ludicrous generalization.

...but yeah, I'd step up to .125 if you're not adding bends to that.

I don't know what you're trying to accomplish with an 8 motor drive, but I don't think it will be of much benefit. You're going to really tax your main breaker and the voltage drop you induce in the battery might even offset the gain of the 4th motor to some extent.

[AllenGregoryIV]

Originally Posted by Andrew Lawrence

More like not enough material. Those thin pieces of sheet wouldn't hold up on their own even without the motors. At a minimum, especially for anything with a CIM attached to it, use 1/4" aluminum plate, or else your gearbox is going to snap before the first miliamp hits your motors.

I'm little confused, are you talking about their being a problem because the sheet metal isn't flanged at all? Our drive motor plates are .09" 5052 and we didn't have a problem with them at all, though they have more flanges along the perimeter. The VEXpro 3-CIM single reduction single speed gearbox is also .125" I believe.

Originally Posted by Chris is me

I don't know what you're trying to accomplish with an 8 motor drive, but I don't think it will be of much benefit. You're going to really tax your main breaker and the voltage drop you induce in the battery might even offset the gain of the 4th motor to some extent.

We ran 8 motors this year and loved it. We haven't popped our main breaker once. Depending on design constraints we'll likely go with it again in the future.

[Chris is me]

Originally Posted by AllenGregoryIV

We ran 8 motors this year and loved it. We haven't popped our main breaker once. Depending on design constraints we'll likely go with it again in the future.

Did you guys do butterfly / octacanum? I don't have empirical evidence to back this up, but I would be tempted to say your drivetrain is loaded a lot differently than a typical 6WD and thus things might be different. The drive style lends itself to avoiding defense more than trying to force your way through it. The omni wheels eliminate turning scrub which is a large source of high current draw in tank drive robots. I believe you guys also did 4 CIM / 4 mini, but that shouldn't be a *huge* difference over 6 CIM / 2 mini.

With a gearbox like this, it's certainly trivial to add one more mounting hole and just experiment with it, particularly if this is being built in the off season. I expect 6 or even 4 motors might turn out to be better for performance at a variety of FRC speeds, but there could be something I'm missing here. Did you guys notice a particular advantage to 8 versus 6 motors? (I'm not sure if your drive lent itself well to a number of motors not divisible by 4 so I don't know if you tried 6 CIMs or not)

[Amo10]

No not enough power. I think you should add another mini cim.

[Dragonking]

Where is the 5th motor and why is this not attached to a shifting swerve module?

[Nathan Streeter]

Adding onto Joshua Miller's comments, here are some more considerations:

1) Gearbox efficiency goes down as shafts become misaligned... if you're concerned enough with adding acceleration to add another ~3 pound CIM to each side, it seems logical to increase the thickness of your plates and add some bends. Two .090" plates bolted together with standoffs will seem fairly rigid if you try to twist/bend it by hand; however, with 3.67-CIMS of torque, I'm guessing you'll have significant flexing. This flexing will be no good for your efficiency, shaft strength, or gear wear...

2) Stiffness of a profile due to bending is (b*h^3)/12. The base and height are both in the "cross-section" of a profile, with the base being the side parallel to the axis of bending and the height being the side perpendicular to the axis of bending. For the cross-section of your gearbox, b=5" or so and h=.090" or so. If you compare this to a "standard" .250" plate, you have an h of only 36% of .250". If you raise 36% to the third, you have only 4.67%... meaning your flat .090" plate is only 4.67% as stiff as a .250" plate. If you go up to .125" plate, you'll have 268% of the stiffness of your .090 plate, which is 12.5% of the standard .250". I'd definitely still recommend adding flanges to your .125" plate, though... Using just 1/2" flanges with a 5" wide .125" plate, you'll get 480% of the stiffness of your traditional .25" plate. For still only being 40-50% of the weight, that sounds like a good design...

[Chris is me]

If you're attached to .090, here's an off the wall idea: Make your output shaft a dead axle, and use VersaHubs and bearing bore gears / sprockets to couple everything together. That way you can make your axle a standoff, serving as a structural member of the gearbox. This adds rigidity right where you need it.

Considering it's sheet metal, flanges are basically "free". Some teams use the flanges to stand off the gearbox instead of standoffs - this is only really an option if your sheet metal shop is really good at holding tolerances.

[AustinSchuh]

Originally Posted by Chris is me

If you're attached to .090, here's an off the wall idea: Make your output shaft a dead axle, and use VersaHubs and bearing bore gears / sprockets to couple everything together. That way you can make your axle a standoff, serving as a structural member of the gearbox. This adds rigidity right where you need it.

We run our last reduction outside our gearbox, and run a dead axle on the wheel. While we haven't had to do it on competition, we are careful to design it so that our transmissions can be pulled very quickly with minimal effort. Our last reduction is a gear reduction outside the gearbox to enable this. This also reduces the loads inside the gearbox significantly.

Originally Posted by Chris is me

Considering it's sheet metal, flanges are basically "free". Some teams use the flanges to stand off the gearbox instead of standoffs - this is only really an option if your sheet metal shop is really good at holding tolerances.

After the first bend, the bends are 'free'. By running flat plates, you can pick different materials, and shortcircuit an entire part of the process, making it cheaper for sponsors. Food for thought. We put bends in where we need them, and don't worry too much.

We have been running 090 on our gearboxes for years. Make sure that there aren't unsupported large gearbox faces, and tie your standoffs to the plates close to the CIM bolts to create a better load path. Should be fine after that. Do be aware that VP's flanged bearings have a relief that makes them practically unusable with 090 sheet. They press in up until the flange, and then rattle around...

We ran 4 CIMs last year, and will run 4 CIMs again. There is a good chance that 1678 will join us next year, and 'upgrade' from 6 CIMs. They sat dead at SVR in the finals due to a dead breaker. I'd rather our driver push a slightly slower bot to the limit than have to baby a faster bot to keep it running until the end of the match. Food for thought.

[asid61]

Originally Posted by AustinSchuh

We run our last reduction outside our gearbox, and run a dead axle on the wheel. While we haven't had to do it on competition, we are careful to design it so that our transmissions can be pulled very quickly with minimal effort. Our last reduction is a gear reduction outside the gearbox to enable this. This also reduces the loads inside the gearbox significantly.



After the first bend, the bends are 'free'. By running flat plates, you can pick different materials, and shortcircuit an entire part of the process, making it cheaper for sponsors. Food for thought. We put bends in where we need them, and don't worry too much.

We have been running 090 on our gearboxes for years. Make sure that there aren't unsupported large gearbox faces, and tie your standoffs to the plates close to the CIM bolts to create a better load path. Should be fine after that. Do be aware that VP's flanged bearings have a relief that makes them practically unusable with 090 sheet. They press in up until the flange, and then rattle around...

We ran 4 CIMs last year, and will run 4 CIMs again. There is a good chance that 1678 will join us next year, and 'upgrade' from 6 CIMs. They sat dead at SVR in the finals due to a dead breaker. I'd rather our driver push a slightly slower bot to the limit than have to baby a faster bot to keep it running until the end of the match. Food for thought.

I believe 1678 died due to removing the autoshifting code, resulting in a blow. It's not that hard to add a collision detection to a drive.

[DampRobot]

Originally Posted by asid61

I believe 1678 died due to removing the autoshifting code, resulting in a blow. It's not that hard to add a collision detection to a drive.

No, they didn't. Their main breaker blew after one of three things happened. Either colliding with the other robot (1662?) caused their drive motor current to spike until the breaker blew, or the physical hit jostled the already straining breaker into opening, or the physical hit bent their bumpers just enough to hit the open button on the breaker. I have this from conversations with 1678 members and from their CD posts.

I don't believe 1678 runs auto shift code, and even if they did, I don't think it could contribute to a main breaker blow.

[Abhishek R]

Originally Posted by Chris is me

Did you guys do butterfly / octacanum? I don't have empirical evidence to back this up, but I would be tempted to say your drivetrain is loaded a lot differently than a typical 6WD and thus things might be different. The drive style lends itself to avoiding defense more than trying to force your way through it. The omni wheels eliminate turning scrub which is a large source of high current draw in tank drive robots. I believe you guys also did 4 CIM / 4 mini, but that shouldn't be a *huge* difference over 6 CIM / 2 mini.

With a gearbox like this, it's certainly trivial to add one more mounting hole and just experiment with it, particularly if this is being built in the off season. I expect 6 or even 4 motors might turn out to be better for performance at a variety of FRC speeds, but there could be something I'm missing here. Did you guys notice a particular advantage to 8 versus 6 motors? (I'm not sure if your drive lent itself well to a number of motors not divisible by 4 so I don't know if you tried 6 CIMs or not)

I believe Spectrum used 6 CIMs and 2 MiniCIMs.

[ryaneogilvie]

Originally Posted by Cash4587

Good to know. Last time I had spoke with you guys I was unaware of your caution. I will be careful, and put some LEDs on our bot to have a visual voltage reading like you guys use on yours.

As the driver for this years robot, I never had to worry about tripping the main breaker during the match. LEDs or some indicators are useful but I've never had to pay attention to them because we have an efficient drivetrain, short wiring, and we are traction limited. I know last years robot had only 6 cims but that robot could trip the breaker because of the massive amount of chain which caused the drive train to be very inefficient and pull way more current than necessary. The caution we give is in the design: Direct drive and only one belt to the traction wheels allow the wheels to roll with little friction; Very short battery wires reduce the amount of current that needs to be drawn; and traction limited will avoid a complete stall of the motors that will surely pop the main breaker.

Originally Posted by Chris is me

Did you guys do butterfly / octacanum? I don't have empirical evidence to back this up, but I would be tempted to say your drivetrain is loaded a lot differently than a typical 6WD and thus things might be different. The drive style lends itself to avoiding defense more than trying to force your way through it. The omni wheels eliminate turning scrub which is a large source of high current draw in tank drive robots. I believe you guys also did 4 CIM / 4 mini, but that shouldn't be a *huge* difference over 6 CIM / 2 mini.

We use Tex Coast (which is our name for butterfly), and yes tex coast drive lends itself to both have maneuverability and speed while having traction and power when you need it. For the same reason you mentioned above you can avoid pulling to much current however, the design is a little more complicated. For motors we originally used 4 cim 4 minicim but switched to 6cims 2 minicims which you have to be careful about, but it can be done.

[Chris is me]

Originally Posted by ryaneogilvie

As the driver for this years robot, I never had to worry about tripping the main breaker during the match. LEDs or some indicators are useful but I've never had to pay attention to them because we have an efficient drivetrain, short wiring, and we are traction limited. I know last years robot had only 6 cims but that robot could trip the breaker because of the massive amount of chain which caused the drive train to be very inefficient and pull way more current than necessary. The caution we give is in the design: Direct drive and only one belt to the traction wheels allow the wheels to roll with little friction; Very short battery wires reduce the amount of current that needs to be drawn; and traction limited will avoid a complete stall of the motors that will surely pop the main breaker.

Just out of curiosity, what are each of your wheels geared for? I'm curious as to what kinds of current you're drawing when the wheels slip in each orientation, and I can estimate that from the drive free speed. I've been trying to figure out of the old 4 CIM rule of thumb for low gears ("traction limited at 40 amps") isn't conservative enough for 6+ motor drives, and I could use some data.