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  #16   Spotlight this post!  
Unread 03-20-2017, 03:02 PM
tonypan tonypan is offline
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Re: Calculating Gear Ratio Based on Torque

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Originally Posted by FrankJ View Post
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.
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Unread 03-20-2017, 03:12 PM
tonypan tonypan is offline
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Re: Calculating Gear Ratio Based on Torque

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Originally Posted by Oblarg View Post
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?
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Unread 03-20-2017, 03:15 PM
Oblarg Oblarg is offline
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Re: Calculating Gear Ratio Based on Torque

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Originally Posted by tonypan View Post
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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Unread 03-20-2017, 03:17 PM
tonypan tonypan is offline
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Re: Calculating Gear Ratio Based on Torque

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Originally Posted by hrench View Post
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.
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Unread 03-20-2017, 03:21 PM
tonypan tonypan is offline
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Re: Calculating Gear Ratio Based on Torque

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Originally Posted by Michael Hill View Post
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.
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