Here's my standard for-every-new-member-of-Chief-Delphi-Welcome: WELCOME TO CHIEF DELPHI!!!!!! :D - [more]
 Chief Delphi The physics of pushing
 CD-Media CD-Spy
 portal register members calendar search Today's Posts Mark Forums Read FAQ rules

#1
05-15-2012, 01:39 PM
 bardd That loud guy AKA: Bar Danino FRC #2212 (The Spikes) Team Role: Alumni Join Date: Dec 2011 Rookie Year: 2011 Location: Israel, Lod Posts: 347
The physics of pushing

I'm trying to decide on a good gear ratio for our drive system, but I'm kind of stuck on one thing - I can't figure out how to calculate the force the robot could inflict on another robot while pushing (not momentarily). I alraedy have the torqe of each wheel, I'm just missing the next step.
Would the number of wheels make a difference?

Bar #2212
#2
05-15-2012, 01:58 PM
 AlexH Registered User no team Team Role: College Student Join Date: Sep 2010 Rookie Year: 2010 Location: PA Posts: 215
Re: The physics of pushing

from personal experience gearing for about 5fps and using any high grip wheel (plaction, colson, traction ect) will give you a very strong pushy bot. you still won't have enough torque to break traction with the carpet though.
#3
05-15-2012, 02:06 PM
 bardd That loud guy AKA: Bar Danino FRC #2212 (The Spikes) Team Role: Alumni Join Date: Dec 2011 Rookie Year: 2011 Location: Israel, Lod Posts: 347
Re: The physics of pushing

Quote:
 Originally Posted by AlexH from personal experience gearing for about 5fps and using any high grip wheel (plaction, colson, traction ect) will give you a very strong pushy bot. you still won't have enough torque to break traction with the carpet though.
I actually want to do the math... And to know how to. The thing is I already have the built robot and all I want to do is decide whether I want to chaneg the gear ratio or not. Besides, I don't want to have a very strong pushy robot, I want a combination of strength and speed that would be the best for the robot out of several options I have.
#4
05-15-2012, 02:12 PM
 Ether systems engineer (retired) no team Join Date: Nov 2009 Rookie Year: 1969 Location: US Posts: 9,126
Re: The physics of pushing

Quote:
 Originally Posted by bardd I actually want to do the math... And to know how to.
For each drive motor, multiply the motor's torque by the total gear ratio (gearbox and sprockets etc) and divide by the wheel diameter radius. Then add those up. Watch your units. Post your results here and we'll check it for you.

Last edited by Ether : 05-15-2012 at 02:54 PM.
#5
05-15-2012, 02:15 PM
 bardd That loud guy AKA: Bar Danino FRC #2212 (The Spikes) Team Role: Alumni Join Date: Dec 2011 Rookie Year: 2011 Location: Israel, Lod Posts: 347
Re: The physics of pushing

Quote:
 Originally Posted by Ether For each drive motor, multiply the motor's torque by the total gear ratio (gearbox and sprockets etc) and divide by the wheel diameter. Then add those up. Watch your units. Post your results here and we'll check it for you.
I got that step... What I want to know is how to calculate the force the robot can push with using the torqe
#6
05-15-2012, 02:16 PM
 Adam.garcia Design Mentor FRC #0004 (Team 4 Element) Team Role: Mentor Join Date: Aug 2009 Rookie Year: 1997 Location: High Tech LA Posts: 133
Re: The physics of pushing

Quote:
 Originally Posted by bardd I'm trying to decide on a good gear ratio for our drive system, but I'm kind of stuck on one thing - I can't figure out how to calculate the force the robot could inflict on another robot while pushing (not momentarily). I alraedy have the torqe of each wheel, I'm just missing the next step. Would the number of wheels make a difference?
If you'd look at JVN's Mechanical Design Calculator, you can see the effects of gear ratio, # of wheels, # of motors, etc. on the effective pushing power of a robot. It allows for easy "prototyping", as the numbers are as simple to change as the push of a button.

I would defer the mathematical explanation to somebody of more knowledge, but for starters you can look at the formulas that he uses in this spreadsheet. JVN's Design Calculator may be found here:

http://www.chiefdelphi.com/media/papers/2059
__________________
“The object of education is to teach us to love what is beautiful.”

“Never discourage anyone who continually makes progress, no matter how slow.”
#7
05-15-2012, 02:18 PM
 EricVanWyk Registered User no team Join Date: Jan 2007 Rookie Year: 2000 Location: Boston Posts: 1,597
Re: The physics of pushing

Draw a "free body diagram" with all the forces you think are relevant and post it here. In your post, tell us all the assumptions that you have made. From there we can create some simple equations and walk through it. The math is actually quite easy - the hard part is finding all the things that affect it.
#8
05-15-2012, 02:25 PM
 Ether systems engineer (retired) no team Join Date: Nov 2009 Rookie Year: 1969 Location: US Posts: 9,126
Re: The physics of pushing

Quote:
 Originally Posted by bardd I got that step... What I want to know is how to calculate the force the robot can push with using the torqe
Assuming you are not traction limited, that is the force the robot can push.

Or perhaps I am not understanding what you want.

#9
05-15-2012, 02:26 PM
 Craig Roys Coach - Team 1718 FRC #1718 (The Fighting Pi) Team Role: Coach Join Date: Jan 2003 Rookie Year: 2001 Location: Armada, MI Posts: 259
Re: The physics of pushing

Pushing force is basically your static friction force between your wheels and the ground. The max static friction force is equal to your [Normal Force (the weight of the robot on a level surface)]x[coefficient of static friction].

The max weight of a robot including bumpers and battery is around 150 lbs.

Assuming a coefficient of static friction between the wheels and carpet of ~1.5, you can create about 225 lbs of pushing force before your wheels start to slip. Once they start to slip, that number will drop.

Keep in mind that you also can only draw 40 amps per motor before you start popping breakers. The trick is to get your gearing such that your wheels will begin to slip just before you get to 40 amps to avoid popping your breakers. Once the wheels slip, the resistance force goes down and your current draw should decrease.

That's an over simplified look at it, but it's a place to start. There are many other factors such as type of drive-train, number of wheels, type of tread, weight distribution of robot, etc. All can affect your pushing power.
__________________
2018 Gibraltar District - Excellence in Engineering Award
2018 Marysville District - Finalist, Engineering Inspiration Award, Dean's List Semifinalist
2018 MI State Championship - Consumers Energy Division Finalist, Dean's List Finalist

#10
05-15-2012, 02:40 PM
 JamesCH95 Hardcore Dork AKA: JCH FRC #0095 (The Grasshoppers) Team Role: Engineer Join Date: Dec 2004 Rookie Year: 2001 Location: Enfield, NH Posts: 2,538
Re: The physics of pushing

You're looking for:

Torque applied to wheels (in*lbs)/Radius of Wheel (in) = Pushing Force (lbs)

Assuming you're not slipping.

I'd also ding about 5% per gearing stage you have (gears sets and sprocket/chain sets). Easiest way is to multiply your end result by 0.95^(number of gear reductions). You can apply more realistic numbers with some minor research.
__________________
Theory is a nice place, I'd like to go there one day, I hear everything works there.

Maturity is knowing you were an idiot, common sense is trying to not be an idiot, wisdom is knowing that you will still be an idiot.
#11
05-15-2012, 08:03 PM
 DonRotolo Getting ready for the break FRC #0832 Team Role: Mentor Join Date: Jan 2005 Rookie Year: 2005 Location: Atlanta GA Posts: 7,344
Re: The physics of pushing

OK, maybe this will help:

If each wheel gets 20 ft-lbs of torque, and the wheel is 6" in diameter, the force acting on the carpet* is 20 / 0.25 = 80 Lbf. (20 is the torque in ft-lbs, 0.25 is the radius of the wheel in ft, and Lbf is "pounds force" (different from pounds weight)).

*Theoretically. Remove maybe 5% for losses (as suggested above), you get 76 Lbf.

Four wheels, 76 * 4 = 304 pounds of force.

Assumes you don't break traction with the carpet.
__________________

I am N2IRZ - What's your callsign?
#12
05-15-2012, 08:17 PM
 Ether systems engineer (retired) no team Join Date: Nov 2009 Rookie Year: 1969 Location: US Posts: 9,126
Re: The physics of pushing

Quote:
 Originally Posted by DonRotolo OK, maybe this will help: If each wheel gets 20 ft-lbs of torque, and the wheel is 6" in diameter, the force acting on the carpet* is 20 / 0.25 = 80 Lbf. (20 is the torque in ft-lbs, 0.25 is the radius of the wheel in ft, and Lbf is "pounds force" (different from pounds weight)). *Theoretically. Remove maybe 5% for losses (as suggested above), you get 76 Lbf. Four wheels, 76 * 4 = 304 pounds of force. Assumes you don't break traction with the carpet.

Be aware that if you are powering 2 wheels with 1 motor, each wheel gets half the torque. Similarly, if you are powering 3 wheels with 2 motors, each wheel gets 1/3 the torque produced by adding the torques of the 2 motors together (and factoring that torque up by the gear ratio, of course). That's why I worded my post in terms of drive motors instead of wheels.

#13
05-22-2012, 12:44 PM
 JamesCH95 Hardcore Dork AKA: JCH FRC #0095 (The Grasshoppers) Team Role: Engineer Join Date: Dec 2004 Rookie Year: 2001 Location: Enfield, NH Posts: 2,538
Re: The physics of pushing

Quote:
 Originally Posted by DonRotolo OK, maybe this will help: If each wheel gets 20 ft-lbs of torque, and the wheel is 6" in diameter, the force acting on the carpet* is 20 / 0.25 = 80 Lbf. (20 is the torque in ft-lbs, 0.25 is the radius of the wheel in ft, and Lbf is "pounds force" (different from pounds weight)). *Theoretically. Remove maybe 5% for losses (as suggested above), you get 76 Lbf. Four wheels, 76 * 4 = 304 pounds of force. Assumes you don't break traction with the carpet.
Bardd: note that this would require a wheel tread with a coefficient of friction of around 2.0, much higher that commonly used tread materials. When you finish your calculation you should double-check it with other potentially limiting factors such as traction and component strength.
__________________
Theory is a nice place, I'd like to go there one day, I hear everything works there.

Maturity is knowing you were an idiot, common sense is trying to not be an idiot, wisdom is knowing that you will still be an idiot.
#14
05-23-2012, 07:32 AM
 IKE Not so Custom User Title AKA: Isaac Rife no team (N/A) Team Role: Mechanical Join Date: Jan 2008 Rookie Year: 2003 Location: Michigan Posts: 2,280
Re: The physics of pushing

There is also a lot of good info in this thread on designing for pushing from the power electronics aspect:
http://www.chiefdelphi.com/forums/sh...hlight=40+amps

Just because the motors can provide the power doesn't mean the electronics can provide the power (for very long).
#15
05-23-2012, 11:57 AM
 Ken Streeter Let the MAYHEM begin! FRC #1519 (Mechanical Mayhem) Team Role: Engineer Join Date: Feb 2005 Rookie Year: 2005 Location: Team: Milford, NH; Me: Bedford, NH Posts: 509
Re: The physics of pushing

Quote:
 Originally Posted by Ether Assuming you are not traction limited, that is the force the robot can push. Or perhaps I am not understanding what you want.
Ether is right that the torque delivered by the wheels (assuming no wheel slip) is the maximum pushing force that can be provided by the robot.

However, in my experience (at least with 1519 robots), we haven't been able to actually get that much pushing force due to a different limiting factor. However, the limiting factor isn't the wheels slipping (in recent years we have used very grippy wheels) but rather the robot starting to flip itself over. (See photo below for an example.) Once teams design robots with sufficient torque and traction, the robot's own weight distribution and geometry is the next limiting factor.

For robots with high torque capabilities and very grippy wheels, "lift up" seems to be the limiting factor. Figuring this out is something that we've never actually done before on our team, but would probably be good to do. There's little point in having more torque or traction in a pushing contest than the amount that starts to result in the robot being lifted up, as that torque (or traction) can't be effectively used.

(By the way, I was hoping that this thread was taking these other factors into account...)

Example of the problem is shown in the photo below from this old CD thread: http://www.chiefdelphi.com/media/photos/35102

__________________
Ken Streeter - Team 1519 - Mechanical Mayhem (Milford Area Youth Homeschoolers Enriching Minds)
2015 NE District Winners with 195 & 2067, 125 & 1786, 230 & 4908, and 95 & 1307
2013 World Finalists & Archimedes Division Winners with 33 & 469
2013 & 2012 North Carolina Regional Winners with teams 435 & 4828 and 1311 & 2642
2011, 2010, 2006 Granite State Regional Winners with teams 175 & 176, 1073 & 1058, and 1276 & 133
Team 1519 Video Gallery - including Chairman's Video, and the infamous "Speed Racer!"

 Thread Tools Display Modes Rate This Thread Linear Mode Rate This Thread: 5 : Excellent 4 : Good 3 : Average 2 : Bad 1 : Terrible

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts vB code is On Smilies are On [IMG] code is On HTML code is Off
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Announcements     User Announcements FIRST     General Forum         FIRST E-Mail Blast Archive     Rumor Mill     Career     Robot Showcase Technical     Technical Discussion     Robotics Education and Curriculum     Motors     Electrical         CAN     Programming         NI LabVIEW         C/C++         Java         Python     Control System         FRC Control System         Sensors     Pneumatics     Kit & Additional Hardware     CAD         Inventor         SolidWorks         Creo     IT / Communications         3D Animation and Competition         Website Design/Showcase         Videography and Photography         Computer Graphics     National Instruments LabVIEW and Data Acquisition         LabView and Data Acquisition Competition     Unsung FIRST Heroes     Awards         Chairman's Award     Rules/Strategy         Scouting         You Make The Call     Team Organization         Fundraising         Starting New Teams         Finding A Team         College Teams     Championship Event     Regional Competitions     District Events     Off-Season Events     Thanks and/or Congrats     FRC Game Design     OCCRA         OCCRA Q&A         OCCRA Programming Other     Chit-Chat         Games/Trivia             Fantasy FIRST     Car Nack's Corner     College & University Education     Dean Kamen's Inventions     FIRST-related Organizations         Western Region Robotics Forum         Southern California Regional Robotics Forum         The Blue Alliance             Video Archives     FIRST In the News...     FIRST Lego League         Lego Mindstorm Discussion     FIRST Tech Challenge     VEX         VEX Robotics Competition         VEX IQ     Televised Robotics     Math and Science         NASA Discussion ChiefDelphi.com Website     CD Forum Support     Extra Discussion

All times are GMT -5. The time now is 04:05 PM.

The Chief Delphi Forums are sponsored by Innovation First International, Inc.

 -- English (12 hour) -- English (24 hour) Contact Us - Chief Delphi - Rules - Archive - Top