3 motor gearbox
 

I was wondering what factors to take into consideration when designing a gearbox for 2 CIMS and 1 FP. Obviously reduction off of the output shaft for the motor needs to have a matching RPM, but is there anything else?

Re: 3 motor gearbox
 

You can add the power of the motors in each gearbox. Power is absolute and additive and if you know the relationship between power, torque and speed (P=T*n (with proper units/conversions)) you can define all of the characteristics of your motors in your gearbox. Your power will remain the same as you match output speeds of your motors but your output torque will go up by the same factor that your RPMs fall.

If you look through the CD white papers you can find a topic on 'traction limited' drive trains and what maximum gear ratio can be used so that you don't trip breakers and your wheels don't slip. It suggests finding the power of your motors at a maximum 40 amps of current draw so that you stay within the limits of your circuit breakers.

As far as max speed and max acceleration go, that is something I have been working to understanding myself and I have a spread sheet made up for my own understanding of the topic. Maybe that topic can be answered by someone wiser than I.

JVN's drive-train calculator is also very nice and I recommend it for quick design checks, but it can be a little bit of trouble when you start mixing motor inputs. I wanted to see how the numbers worked so I did my own math and checked it with JVN with mixed results based on my own design assumptions.

Re: 3 motor gearbox
 

You might want to consider that we were only allowed 1fp this year. I would not put your money on being able to use fp's next year. Banebots has a gear box for the rs-775's that will match the CIMs out put RPMs.

Also if you do decide to use 2fp's you might want to make sure you have the right hole pattern for the fp to mount on to.

Re: 3 motor gearbox
 

I've got the math behind how it will work down, I'm interested more in the construction aspect.

What motor characteristics need to align to pull off a gearbox with different motors?

Re: 3 motor gearbox
 

Are you talking about the CIMulator?

Re: 3 motor gearbox
 

You just want to match free speed as close as you can, but don't worry if you're off. I've ran motors ~20% mismatched from free speed without issue.

Re: 3 motor gearbox
 

Yes, yes I am. I know the rs-775 Had given all of us problems but they should be fixed for the 2012 season.

Re: 3 motor gearbox
 

If that's the case then all you have to worry about is matching speeds with the CIMs (whether COTS or custom) and mounting it with proper alignment to gear central to your other two CIMs. That comes down to simple geometry in CAD.

The one thing you are going to have to double check is the added torque and bending stresses to your shafts from the extra motors. What may have been overkill for 1-2 CIMs may be verging on 'just enough' for the added power. Diameters may have to be increased to maintain a reliable factor of safety on par with the standards set up by common designs and COTS items. Your stresses may check out and look very safe but for alignment and efficiency in spur gears it is better to design for deflections.

Re: 3 motor gearbox
 

The rs-775 is about 3x faster than CIMS, so should it's gear be about three times bigger? I'll do the math to get it exact if this is the case, but it seems like the CIMulator is the way to go.

Re: 3 motor gearbox
 

The opposite. Should be three times smaller.

of course, you can't go three times smaller than 12 teeth, hence why CIMulators or other banebots gearboxes are used for an initial reduction.

Re: 3 motor gearbox
 

The CIMulator should work well enough to match speeds.

One thing I have found is that a 775 maxed out @40 amps has the same power output as a CIM @40 amps, although I don't know how many 40A breakers one can use. So the 775 may not be fully understood as far as reliability quite yet but looks as if it is on par with a CIM in power.

Re: 3 motor gearbox
 

Right, that one. I've been studying for my physics AP tests and my brain is fried.

Re: 3 motor gearbox
 

Just be careful running 4 CIMs and 2 RS-775s in one drivetrain. They draw a LOT of electrical power at stall (enough to brown out a cRIO from time to time - hopefully the cRIO-FRC2 will fix this with its 9V minimum supply voltage). Design your gear ratio appropriately.

Re: 3 motor gearbox
 

What do the green boxes in the excel document mean? Is that the recommended gear ratio?

Re: 3 motor gearbox
 

Sorry I didn't make a distinction between inputs and outputs, both are green. The min ratio was found for the time to cover 54 ft, the max ratio is based on 40A traction limited specs including CoF and the last three were found by finding where the ratio/max torque and ratio/max speed curves are equal. My math my be a little off on some of the numbers and the intersections are based on those numbers. All numbers have efficiencies accounted for, that is what the 'gear stages' input is for.

Re: 3 motor gearbox
 

Yes, your inputs on each page are Speed (ft/s) (don't need to modify that one as you are matching to that speed anyways), Torque (ft-lb) (Pulled from motor spec pages, taken for one wheel), Coefficient of friction, Wheel Dia. (in) and Gear stages. The Min Time is pulled from the 'T54 ft.' column and you have to follow that to look up the min ratio.

*The speed conversion isn't mathematically linked to the wheel dia. cell and you may want to modify the equation in that cell to reflect that change*[edit]

Re: 3 motor gearbox
 

Increasing the wheel diameter increases time. Is this right, because at first glance it seems backward?

Re: 3 motor gearbox
 

It's a balancing act of more acceleration time vs. more time at max speed. In some cases going to a larger wheel, with no other changes, will indeed cause a higher time.

It appears to be simulating each side as if the 1 side powered the entire robot going forward, and only 1 CIM is ever used on the 3 tabs. It this intentional? Other than that it looks pretty good.

Though I would caution -- if you replay your matches this year, even on an 'open field' like this year, how far did your robot ever sprint in a straight line? Might be best to optimize it for a distance based upon your strategy (20-40ft for us)

Re: 3 motor gearbox
 

Ah! good point. I do have all other power calculations assumed to be using 1/4 of the robot weigh on one wheel (4 wheels evenly bearing the weight). My mistake is that I assumed the same one wheel for acceleration in that it would be accelerating the entire mass of the robot by itself when it would only be accelerating 1/4th the mass as well, right?

Re: 3 motor gearbox
 

Quite honestly, I'm not sure about the 1/4th mass thing. I've only ever analyzed it as an overall robot system of wheels to the floor so weight transfer during acceleration is somewhat negligible; I'd never thought about it that way.

Attached is what I come up with for a SuperShifter with a 42:34 chain connection to 4" wheels @148lbs total, (our drive train this year). Min time to 27ft is 2.6s; your #'s give me 3.3s or so. I think I use 2.43Nm as stall, whereas you use 1.78 @ the 40A breaker; I honestly don't know which is correct. In the end, the times on field roughly match either calculation since there are a number of other factors such as driver control & software tuning.

The only really important thing to know is how any proposed design changes (4"-6" wheel size, 17:48 gearing vs 20:45 on the output of the SS, etc) effect things relative each other so decisions can be made and bad situations (unable to turn, constantly-popping breakers, etc) are avoided.

As a quick aside -- upping the torque allotted for my spreadsheet in order to account for a properly-matched RS-775 saves ~0.5 seconds for getting to 27feet. Yet it also gives you a narrow sliver of extra power curve to push through a road block during your acceleration. I say "narrow" because eventually (as Jared eludes to) you pull too much current for your 120A breaker.

Re: 3 motor gearbox
 

Anybody have any data on how long a 775 will withstand 40 amps before smoking?