

Mentors need to be sure that the team is on track to have a working robot ready. Students can do almost everything, but they need help watching the calender. Even if your ideas turn out bad, if your robot is inspectable and running when you arrive at your first event, anything can happen.  AcesPease [more] 



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#16




Re: Calculating Gear Ratio Based on Torque
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#17




Re: Calculating Gear Ratio Based on Torque
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




Re: Calculating Gear Ratio Based on Torque
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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 repressing the climb button restart the climber. 
#19




Re: Calculating Gear Ratio Based on Torque
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#20




Re: Calculating Gear Ratio Based on Torque
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#21




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




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. Items that are shaded in tan are menu selections. 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. 
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