3 motor gearbox

[548swimmer]

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?

[A_Reed]

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.

[MattC9]

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.

[548swimmer]

Originally Posted by A_Reed

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.

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.

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.

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?

[548swimmer]

Originally Posted by MattC9

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.

Are you talking about the CIMulator?

[AdamHeard]

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.

[MattC9]

Originally Posted by 548swimmer

Are you talking about the CIMulator?

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

[A_Reed]

Originally Posted by 548swimmer

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?

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.

[548swimmer]

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.

[Chris is me]

Originally Posted by 548swimmer

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.

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.

[A_Reed]

Originally Posted by 548swimmer

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.

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.

[548swimmer]

Originally Posted by Chris is me

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.

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

[Jared Russell]

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.

[548swimmer]

Originally Posted by A_Reed

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.

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

[A_Reed]

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.