pic: Too much power?
 

Re: pic: Too much power?
 

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.

Re: pic: Too much power?
 

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.

Re: pic: Too much power?
 

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.

Re: pic: Too much power?
 

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/1177698...88397646577634

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

Re: pic: Too much power?
 

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.

Re: pic: Too much power?
 

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

Re: pic: Too much power?
 

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

Re: pic: Too much power?
 

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.

Re: pic: Too much power?
 

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.

Re: pic: Too much power?
 

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.

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Good to know. :p 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.

Re: pic: Too much power?
 

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.

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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)

Re: pic: Too much power?
 

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