How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

Hi CD,
I've been working on drivetrain designs lately and I've been looking into using a 2 CIM, 1 Mini CIM gearbox. I've been told by people from multiple teams that this is ok to do and that I shouldn't have to change the pinion gear on the mini CIM. Can someone help me figure out things like torque and free speed of a gearbox like that? I don't really know how to find gearbox calculations besides just plugging into JVN Calculator, so I'd like to learn how to calculate it.

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

Look at the excel sheet and associated presentation by Paul Copioli here: http://www.fightingpi.org/Resources/...Seminars.shtml

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

Add the torques.

Free speed is given by the formula (T1+T2+T3)/(T1/S1+T2/S2+T3/S3),

where Tn, Sn is the stall torque and free speed of motor "n".

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

... or you can plot the torque vs speed curve of each of the three motors, plus the sum of the torques vs speed, and see where the sum crosses the speed axis.

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

Edit: Disregard, figured it out.

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

http://www.chiefdelphi.com/forums/at...0&d=1387494073

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

Depending on your desired level of accuracy...
When I need quick & dirty:
I use a MiniCIM as 0.6 of a CIM. So for the gearbox you're describing I'd plug 2.6 CIM motors into my spreadsheet for 2 CIMs + 1 MiniCIM.

This has been close enough for most of the quick design work used during an FRC season. I save the more detailed analysis for the times I need it.

-John

PS - Paul, Ether & Richard, please don't revoke my "allowed to talk about motors" privileges.

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

There is a rich tradition in engineering of celebrating prolific contributors by naming standard physical units after them. For example, the unit of electrical capacitance is named for Michael Faraday, and the unit of power transfer/conversion is named for James Watt.

I propose naming the fundamental unit of FRC drivetrain "oomf" in honor of JVN.

Let's say the "oomf" of a well designed four CIM drivetrain is one JVN. Then by the approximation John proposed above, a well designed drivetrain with four CIMs and two mini-CIMs will deliver 1.3 JVN of "oomf", while a well designed six CIM drivetrain is good for 1.5 JVN.

Your oomf may vary.;)

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

The resistance of the battery makes the 1.5 JVN number a bit idealized though ;)

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

On an unrelated note, what do you use as your input for coefficient of friction?

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

Depends on your wheels. Which are you using?

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

For the design I'm currently working on, 4"X1.5" Colsons.

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

+1

I haven't done traction testing on those in years. I think they are around 1 - 1.1

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

Is there any benefit of using a different pinion on the minicim to get the free speeds closer together?

Re: How to do calculations with a 2 CIM, 1 Mini-CIM gearbox?
 

It depends on what you want the motor combination to do, and what the duty cycle is on the combined motor curve.