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

[Jay O'Donnell]

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.

[Knufire]

Look at the excel sheet and associated presentation by Paul Copioli here: http://www.fightingpi.org/Resources/Business/MSC%20Seminars/2013%20State%20Seminars.shtml

[Ether]

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.

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".

[Ether]

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".

... 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.

[Oblarg]

Edit: Disregard, figured it out.

[Ether]

I'm curious - how does one derive this?

http://www.chiefdelphi.com/forums/attachment.php?attachmentid=15590&d=1387494073

[JVN]

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.

[Richard Wallace]

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

[AdamHeard]

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.;)

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

[Jay O'Donnell]

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.

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

[Mike Marandola]

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

Depends on your wheels. Which are you using?

[Jay O'Donnell]

Depends on your wheels. Which are you using?

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

[JVN]

Depends on your wheels. Which are you using?

+1

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

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

[Electronica1]

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

[Ether]

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

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