pic: Too much power?



I am doing some gearbox design with sheetmetal, and I figured I would try 3 CIMS + 1 Mini CIM. The gearbox uses all VEX Pro parts including: 3 CIMS, 1 Mini CIM, 64t Hex Gear, 12t Pinion(Or 11t, still deciding) 2 1/2" Round Bearings, Hex shaft, and 1/2 axle shaft for spacers. The plate is .090" 6061 T6. Please give some feedback. Some inspiration taken from 971.

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.

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.

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.

You don’t need 1/4" plate… Yes, it is more stable for gearbox application, but I have done my share of research on sheet metal gearboxes and 971 does just fine with what it looks like .090". I looked through their entire album and they use A LOT of .090 for the chassis. Granted I will have 2 more motors per gearbox… Either way. If you don’t believe have a look for yourself. If it is not strong enough, I will A)Put a flange on the plate B)increase the thickness to .125" C)Do Both.

971’s 2014 Gearbox:
https://picasaweb.google.com/117769834305511597729/2014FRCTeam971SpartansFabAndAssembly#slideshow/5981588397646577634

With 8 motors I am trying to get more acceleration (mainly), and more pushing power. Spectrum ran 8 this year and really liked it so I figured I would give it a try. If it doesn’t work out I will go with 3 CIM

I think 3/16" is fine. The one problem our gearbox didn’t have this year was bending (and yes, I checked it with a square post-season) although we did not pocket it at all.
However, you might want to look at that sprocket. Depending on what this is used for, it might a good idea to switch to #35 chain, especially if this is ever used for a crab drive or something similar with one drive chain.
I think this is a little overkill, but JMO.

Keep in mind 971 used a 4-cim drive this year. 8 motors is considerably more power and weight.

As you add CIMs the torque bottleneck shifts to either the amount of current the battery can supply, or the amount of current the main breaker can handle. While you can abuse the batteries a bit, the consequences of tripping the main breaker are significant.

I’m not sure, exactly, what the magic number of CIMs would be before it becomes pointless to add more… but I’d bet that you are within +/- 1 motor.

Perhaps a shifting gearbox with two or three CIMs would actually provide both better acceleration AND a higher top speed.

Jason

Oh, I forgot to mention that is a #35 Sprocket. Thanks for the feedback.

I have seen the Spartan’s transmissions up close, and definitely understnad their reasoning behind them and the proof that they work. 971 holds a high standard to their machines, and they ensure everything is well supported and correctly loaded on their transmissions. Whenever I see a transmission that looks like it could be completely bent in one hit and I don’t know who made it, I try not to assume what the poster knows in the event they need some help. Of course I’m not doubting your knowledge of thin sheet transmissions (and I apologize if it came off that way), I was just being cautious and thought it would be better to give the warning and have you already know it than to not give you the warning and have you not know it.

That being said, as others have mentioned, adding flanges and/or increasing the thickness to 1/8 will make a world of difference. It looks like a design with a lot of potential, and I look forward to seeing future iterations.

I am not too worried about tripping the main breaker. If it does happen, I guess lesson learned. I will take off the mini cims at that point. Seeing Spectrum 3847 using the same combo of motors on their drive train gives me some confidence to run this setup. I do agree that shifting would be ideal but the purpose of this gearbox is to actually go into a tex-coast drive. The “shifting” will actually done by deploying your traction wheels which have a second reduction.

A word of advice, you should be worried. One of the reasons we haven’t tripped our breaker is because we are very worried about tripping our main breaker. We do a lot of things to avoid that situation from ever happening.

Good to know. :stuck_out_tongue: 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.

The 2015 Power Distribution Board will have built-in current monitoring. You could prioritize power to your drivetrain by reducing power to lower-priority loads (e.g. shutting off compressor) whenever necessary to prevent tripping the main breaker. This has been discussed extensively before, but I’m feeling too lazy to search for it.

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)

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

I believe Spectrum used 6 CIMs and 2 MiniCIMs.

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.

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.

Seeing as we’re most likely moving towards more power in future iterations of our drive, what are some of those precautions that you took? Are there any besides the ones that Ryan listed above?

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.

There are some more circumstances that play into this as well. Namely each wheel is independent so we lose some power in pushing matches when wheels lift off the ground.

The complete drive train is setup like this.

2 CIMs on each rear wheel module. 1 CIM and 1 MiniCIM on each front wheel module.

Each module is geared 12:72 for the omni wheel using VexPRO gears and then there is a 2nd reduction to the traction wheel that is 18:42 with VexPRO pulleys and 9mm wide belt. So overall we are 6:1 and 14:1 in the two different ratios. 4 in wheels in both omni and traction. I would like to be 5:1 in high gear but at that point I think we would could stall the main breaker but it’s something we will probably try.

Our current setup has the traction wheels on the inside of the modules and the omni wheels towards the front and back of the robot and we pivot the traction wheels up and down. This provides us with less scrub when we are on the traction wheels but hurts us in pushing matches because we can easily be lifted off our front wheels and engage the omni wheels and traction wheels on the back side which is clearly not ideal. The next iteration will attempt to use smaller traction wheels that are at the front and back of the robot. We will likely continue to fix the omni wheel and pivot the traction wheels so we don’t have to float our motors on the module.