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Unread 07-07-2018, 07:55 PM
jcpeng jcpeng is offline
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5 BAG drivetrain gearbox

Hi everyone,

I hear there's been a craze for 775pro gearboxes nowadays. In the spirit of being excessive while still remaining somewhat practical, here's a drivetrain gearbox with BAG motors! It has a bit more power than the "usual" 2 CIMs and only works with 6" wheels due to its unique design.


/CLICKBAIT

There are lots of images, I put them all here: https://imgur.com/a/Cq2o22A

Cad Files: (Inventor and STEP)
Dropbox link

The reasoning:
- better peak power over time performance compared with CIMs and 775Pros, without the power-to-weight loss of Mini-CIMs
- better efficiency than Mini-CIMs
- "with the sealed-case robustness FIRST ģ Robotics Competition teams have come to love & trust."
- 5 BAGs: appx. 2 CIMs of power.
- BAG motors. No more reasons needed.

Notes:
- Shown: 11.5 ft/sec gearing minimal speed, can go up to as high as 24.9 ft/sec by swapping gears (but why would you do that?)
- No idea if the belt will snap (GT2 3mm) little things
- No tensioning, no built in method for tensioning
- Timing pulleys are 12T pulleys intended for the 550 that have to be bored out to 4mm
- A lot of strange fastening techniques
- No lightening: 5.4lb weight, not counting the tube
- Access Hell

Comments are appreciated!

Thanks,
-Jing-Chen
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Unread 07-13-2018, 10:11 AM
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Re: 5 BAG drivetrain gearbox

Just for comparison:
Code:
Approx. 2CIMs worth of power & power delivery-
Motor     Weight     Controller     Weight     Quantity        Sum
775pro     0.80       VictorSPX      0.19          2           1.98lbs
Bag*       0.71       VictorSPX      0.19          5           4.50lbs
MiniCIM    2.16       VictorSPX      0.19          3           7.05lbs
CIM        2.80       VictorSPX      0.19          2           5.98lbs
* Assuming 5 BAG motors = 2CIMs as stated above
** All weights taken form VEXPro Product pages
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Last edited by Skyehawk : 07-13-2018 at 10:16 AM.
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Unread 07-13-2018, 10:16 AM
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Re: 5 BAG drivetrain gearbox

Because who needs PDP slots for mechanisms, right
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Unread 07-13-2018, 10:48 AM
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Re: 5 BAG drivetrain gearbox

Yeah, but when will we get 3D printed differential BAG mecanum caster swerve in wheel?
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Unread 07-13-2018, 11:23 AM
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Re: 5 BAG drivetrain gearbox

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Originally Posted by ClayTownR View Post
Yeah, but when will we get 3D printed differential BAG mecanum caster swerve in wheel?
The same year THIS gets onto Einstein...
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Unread 07-13-2018, 11:39 AM
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Re: 5 BAG drivetrain gearbox

Quote:
Originally Posted by AriMB View Post
Because who needs PDP slots for mechanisms, right
I've never understood that you can legally run multiple motors of certain types off a single speed controller, but not BAGs....

3x Bag is still less current than a CIM, but we're legally allowed to run a single CIM off a speed controller.
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Unread 07-13-2018, 11:52 AM
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Re: 5 BAG drivetrain gearbox

Quote:
Originally Posted by AdamHeard View Post
I've never understood that you can legally run multiple motors of certain types off a single speed controller, but not BAGs....

3x Bag is still less current than a CIM, but we're legally allowed to run a single CIM off a speed controller.
I would beg to differ...

http://motors.vex.com/

CIM has a stall current of 131A, a Bag of 53A. Power-wise, a CIM is 337W, while a Bag is 149W. That makse a Bag motor something like 40% of a CIM.

According to the motor curve, at peak power a Bag motor is pulling somewhere around 24A - running two Bags at peak power is too much for a single 40A breaker.

Generally speaking, those that allow 2 on a single speed controller have stall current at 25A or less, with current at peak power much less. Even with two of them on the same speed controller, you're unlikely to trip the breaker. I can't say the same of the Bag motor.
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Unread 07-13-2018, 12:04 PM
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Re: 5 BAG drivetrain gearbox

Quote:
Originally Posted by Jon Stratis View Post
I would beg to differ...

http://motors.vex.com/

CIM has a stall current of 131A, a Bag of 53A. Power-wise, a CIM is 337W, while a Bag is 149W. That makse a Bag motor something like 40% of a CIM.

According to the motor curve, at peak power a Bag motor is pulling somewhere around 24A - running two Bags at peak power is too much for a single 40A breaker.

Generally speaking, those that allow 2 on a single speed controller have stall current at 25A or less, with current at peak power much less. Even with two of them on the same speed controller, you're unlikely to trip the breaker. I can't say the same of the Bag motor.
I've never understood that you can legally run multiple motors of certain types off a single speed controller, but not BAGs....

3x 2x Bag is still less current than a CIM, but we're legally allowed to run a single CIM off a speed controller.
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Unread 07-13-2018, 04:23 PM
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Re: 5 BAG drivetrain gearbox

Quote:
Originally Posted by Jon Stratis View Post
I would beg to differ...

http://motors.vex.com/

CIM has a stall current of 131A, a Bag of 53A. Power-wise, a CIM is 337W, while a Bag is 149W. That makse a Bag motor something like 40% of a CIM.

According to the motor curve, at peak power a Bag motor is pulling somewhere around 24A - running two Bags at peak power is too much for a single 40A breaker.

Generally speaking, those that allow 2 on a single speed controller have stall current at 25A or less, with current at peak power much less. Even with two of them on the same speed controller, you're unlikely to trip the breaker. I can't say the same of the Bag motor.
Running a single CIM at peak power trips the breaker too, but that’s legal. The restriction of only one BAG per controller makes no sense.



As for actual design advice, if you arrange the five motors in the right way and use a double sided timing belt, you can find a way to get sufficient belt wrap on all of them without using any idlers. I’d play with that a little bit.
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Unread 07-13-2018, 09:00 PM
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Re: 5 BAG drivetrain gearbox

Quote:
Originally Posted by Chris is me View Post
Running a single CIM at peak power trips the breaker too, but thatís legal. The restriction of only one BAG per controller makes no sense.
The most powerful motor you *can* run multiples of on one motor controller is the PG motor, a 43W motor with a stall current of 18A according to the VEX testing. Both of those are about 1/3 of the BAG motor.

My guess--and it's purely a guess--is they're concerned about safe power distribution downstream of the motor controller once they cross the 20A-a-piece threshold. It could be that nobody's pressed the GDC on it though, so who knows?
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Unread 07-13-2018, 10:03 PM
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Re: 5 BAG drivetrain gearbox

Quote:
Originally Posted by AdamHeard View Post
I've never understood that you can legally run multiple motors of certain types off a single speed controller, but not BAGs....

3x Bag is still less current than a CIM, but we're legally allowed to run a single CIM off a speed controller.
The line makes sense in terms of peak power: 9015 and BAG at 139 and 145 W (according to Vex) need separate controllers. None of the motors listed in "up to 2 per controller" row produce are listed above about 50W anywhere I've found. The "line" is most of an order of magnitude wide, and is roughly 100 +/- 50 W*. Should a flood of motors in this range become available, I have no idea where the line will end up; it may be highly dependent on the order in which motors become legal.

* selected as third-decade/factor of 2 mantissas to imply lack of precision rather than any interpretation of precision beyond a third of a decade.

Back on OP: The PDP is separated 8 and 8. I guess if you can find 30A maxi breakers you can make these all equal, but things really start getting strange when you have more than 8 nominally identical motors on an FRC robot.
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Unread 07-13-2018, 11:08 PM
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Re: 5 BAG drivetrain gearbox

Quote:
Originally Posted by Jon Stratis View Post
I would beg to differ...

http://motors.vex.com/

CIM has a stall current of 131A, a Bag of 53A. Power-wise, a CIM is 337W, while a Bag is 149W. That makse a Bag motor something like 40% of a CIM.

According to the motor curve, at peak power a Bag motor is pulling somewhere around 24A - running two Bags at peak power is too much for a single 40A breaker.

Generally speaking, those that allow 2 on a single speed controller have stall current at 25A or less, with current at peak power much less. Even with two of them on the same speed controller, you're unlikely to trip the breaker. I can't say the same of the Bag motor.
So then we need a BAG-like motor that peaks out at 20A instead of 24A, then we can have 2x of them on a single controller? I'd go for that, that wouldn't even be a hard call. I can't even count the number of mechanisms that would simplify.
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Unread 07-14-2018, 12:40 PM
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Re: 5 BAG drivetrain gearbox

Quote:
Originally Posted by Chris is me View Post
Running a single CIM at peak power trips the breaker too, but thatís legal. The restriction of only one BAG per controller makes no sense.

ó

As for actual design advice, if you arrange the five motors in the right way and use a double sided timing belt, you can find a way to get sufficient belt wrap on all of them without using any idlers. Iíd play with that a little bit.
I looked into that briefly but decided to use just the parts from VexPro (in case we ever do want to build this, but why would you? don't want to have to deal with SDP-SI).

Honestly it would probably be better just to use a 32DP gear reduction, but then you wouldn't be able to put the drive shaft in the middle of all your motors
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