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#16
03-20-2017, 03:02 PM
 tonypan Registered User AKA: Minxing Pan FRC #3175 (Knight Vision) Team Role: Programmer Join Date: Jan 2017 Rookie Year: 2016 Location: Detroit Posts: 33
Re: Calculating Gear Ratio Based on Torque

Quote:
 Originally Posted by FrankJ You do realize that Vex doesn't recommend 63:1 for a CIM? Actually they don't recommend the 9:1 and 10:1 gear sets for the CIM. increasing load by increasing the speed might give you issues with the gear box. Vex planetary load recommendations here.
Thanks so much for the advice. We have 1/2 Hex Output Shaft, but the other end of the output shaft is not secured. Last time we climbed, the climber almost pulled itself off the mount but the gearbox remained intact. We were extremely lucky. Now we are thinking about reducing the ratio to 40:1 and securing the other end of the shaft and the mount.
#17
03-20-2017, 03:12 PM
 tonypan Registered User AKA: Minxing Pan FRC #3175 (Knight Vision) Team Role: Programmer Join Date: Jan 2017 Rookie Year: 2016 Location: Detroit Posts: 33
Re: Calculating Gear Ratio Based on Torque

Quote:
 Originally Posted by Oblarg On the other hand, current limiting can eliminate the possibility of a full stall - gearbox torque ratings become somewhat more nuanced, then.
Thank you for the advice. At our last competition, we put the ratcheting wrench backwards on our climber and we did not have a limiting current, so the CIM almost pulled the motor and gearbox off the mount. Should we set our limit to 130 amps as the stall current is at 131 amps, or should it be lower? How do we determine the limit?
#18
03-20-2017, 03:15 PM
 Oblarg Registered User AKA: Eli Barnett FRC #0449 (The Blair Robot Project) Team Role: Mentor Join Date: Mar 2009 Rookie Year: 2008 Location: Philadelphia, PA Posts: 1,838
Re: Calculating Gear Ratio Based on Torque

Quote:
 Originally Posted by tonypan Thank you for the advice. At our last competition, we put the ratcheting wrench backwards on our climber and we did not have a limiting current, so the CIM almost pulled the motor and gearbox off the mount. Should we set our limit to 130 amps as the stall current is at 131 amps, or should it be lower? How do we determine the limit?
We're currently running a mini-CIM geared 50:1 for our climber (1.125'' drum diameter), and a 50 amp current limit has worked just fine for us. You can probably figure out a ballpark estimate by scaling appropriately to your system.

If you're using a Talon SRX, you can implement a current cap that the motor controller will not exceed - else, you can program the motor to shut off entirely if the current limit is exceeded. We do the latter, but also have releasing and re-pressing the climb button restart the climber.
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#19
03-20-2017, 03:17 PM
 tonypan Registered User AKA: Minxing Pan FRC #3175 (Knight Vision) Team Role: Programmer Join Date: Jan 2017 Rookie Year: 2016 Location: Detroit Posts: 33
Re: Calculating Gear Ratio Based on Torque

Quote:
 Originally Posted by hrench everyone saying 'use the radius of the spool' seems to be neglecting the radius of the rope. if you use a circular rope, wouldn't the center of the tension be at one rope-radius farther from the centerline of the spool, that is, in the center of the rope? I suppose with some very-rigid rope the tension at the spool radius may carry more of the robot than the center of the rope, but I can't imagine any case were this should be ignored completely.
Thanks for those helpful information. We sewed velcro to a strap, so our rope is very thin. Our spool spreads all the way across the side of our robot so it does not fold up too much over itself and change the radius by too much.
#20
03-20-2017, 03:21 PM
 tonypan Registered User AKA: Minxing Pan FRC #3175 (Knight Vision) Team Role: Programmer Join Date: Jan 2017 Rookie Year: 2016 Location: Detroit Posts: 33
Re: Calculating Gear Ratio Based on Torque

Quote:
 Originally Posted by Michael Hill FYI: JVN's Design Calculator (https://www.chiefdelphi.com/media/papers/3188) is extremely useful. Check out the "Linear Mechanism" tab. It conveniently uses 154 lb weight by default (hint: what is the weight of your robot?) For the pulley diameter, you should use your drum diameter + diameter of the rope should be fine, but if you want to do a double-wrap scenario, you can use drum diameter + 1.5 * rope diameter. Change your travel distance to be between your drum and the top of the rope (something on the order of 24 in or so). One other thing I did was to scale the motor specs to 10V rather than 12V (because at the end of the match, your battery capacity will be significantly lower than at the start). Designing with this voltage actually saved us in one match where we found out we ran a match with a battery with a dead cell but were still able to climb. Once you do all of that, you can play with the gear ratio to get something less than and around 50% stall (as measured by dividing the Current Draw per Motor (loaded) by the Stall Current). In the end, we are using a 1.25" drum with velcro wrapped around. We are climbing a 5/8" Poly(propylene?) belt from Home Depot. We are using a CIM with around a 13:1 gear ratio (I think more like 13.33:1). It gets us up there in < 2-3 sec.
I have been looking for this calculator. Thank you. I think we will do 40:1 versaplanatary with a current limit at 65 amps.
#21
04-01-2017, 07:59 AM
 matthewdenny Registered User FRC #6054 (Dukes) Team Role: Mentor Join Date: Jan 2012 Rookie Year: 2012 Location: United States Posts: 371
I will show you our calculations as an example:

First we are using a CIM with a 12:1 VP (3:1 and 4:1 stages) gear box on the same shaft as a 12T sprocket and that connects via chain to a shaft with a 26T sprocket. This connects to a climbing drum 1 1/4" in diameter.

Mechanical:

Find total gear ratio:
(Output teeth/input teeth)
VP Gearbox: 12:1
Chain 26:12
Multiply together to get 26:1 or just 26.00.

The CIM has a listed stall torque of 343 oz in. Our gearing will increase it by a factor of 26 for a total of 8920 ozin. Our climber bar has a diameter of 1.25" and a radius of .625 inches. After the rope wraps on it we measure it to be a radius of .85" (we use strap). So 8920oz in/.85=10500 oz or a lifting force of 655 lbs. we assume 80% efficiency so let's call it 525 lbs. our robot with bumpers and battery weighs 125 or 24% of that.

Which means it will spin at about 76% of its free speed (100%-24%)

So a CIM has a listed speed of ~5300 RPM. 76% of that is 4030 RPM. Our gearing slows the rotation down by a factor of 26 so the climbing drum speed will be 4030/26=155RPM or 204/60=2.6 rev per sec.
the diameter of our drum was 1.25 inches so the circumference is 3.92" which is how far it will climb each revolution. At 2.6 rev per sec this comes out to 10.2 " per sec. Our climber is about 48" from the touch pad so it should take about 4.7 sec to climb.

Electrical: we are operating at 24% of stall torque as previously calculated. Stall current is 133A. 24% of 133 is 32A and our 40A breaker should be fine.
#22
04-02-2017, 11:17 AM
 phargo#1018 Registered User AKA: Peter Argo FRC #1018 (RoboDevils) Team Role: Mentor Join Date: Dec 2013 Rookie Year: 2009 Location: Indianapolis, IN, USA Posts: 72
Re: Calculating Gear Ratio Based on Torque

Here is a copy of the analysis workbook I developed this year.
My intent was to be able to evaluate multiple climbing system options.
My team gravitated towards a worm drive solution, so this work is tailored for that approach; however, the worm and worm gear only represent a gear ratio - so really any ratio combination can be evaluated.

Items that are shaded in blue are numerical entries.

Hope that this is of use for any climber changes.

In order to comply with upload constraints, I had to reduce the simulation duration, eliminate the drag coefficient calculations, and climbing test data.
Attached Files
 ClimbingSimulationWormDrv 170326B.xlsx (2.85 MB, 10 views)

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