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Unread 30-12-2003, 14:44
Unsung FIRST Hero
JVN JVN is offline
@JohnVNeun
AKA: John Vielkind-Neun
FRC #0148 (Robowranglers)
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Re: Maximum Pushing Force

Quote:
Originally Posted by mzitz2k
I have read much about the pushing force of certain drive trains. What is getting me is that I have heard a few individuals in the forums say that the maximum force/amount that a robot can push is its weight times its coefficient of friction. If this is true, why would teams concern themselves with gearboxes and shifting down to insane low gears. I recall Beatty (Team 71) having ungodly amounts of pushing force two years ago in the game with three goals - yet they didn't lift the goals to increase their effective weight. So, I am at a bit of a loss as to why people are then saying the maximum force that a robot can exert or push is its weight times coefficient of friction.

BTW - This would mean that it would be unnecessary to gear our robots beyond a pushing force of about 1.5*130 lbs = 195 lbs. Our teams current design is going to be near a pushing force of 5 times that. Am I missing something?
You are absolutely right in all that you said above.

Pushing force is limited by the amount of traction the robot gets with the carpet. This is based on friction ( Friction = mu * Normal).

There are 2 ways to increase your traction:
1. Increase your coeff. of friction with the carpet.
2. Increase your normal force (by adding weight to the robot).



In 2002:
Team 60 Chose option 2. They lifted up the 2 goals, and greatly increased their pushing force.

Team 71 chose option 1. They used file cards as their tractive material. Since file cards "dig in" to the carpet, they essentially have a coefficient of friction which approaches "infinite".

Now... since 71 was geared low enough (The Beatty Shuffle...), their infinite coeff of friction allowed them to outpush almost anyone, without increasing their normal force!


So yes... if you have a 130lb robot, with a 1.5 coeff. of friction, you don't need to gear the robot down anymore than that. (Unless you think you will "interlock" with the carpet, or wire-mesh, or whatever FIRST gives us this year).

There are also many other considerations when you're "gearing down" (as Ken mentioned above). It is important that your robot can "slip" your wheels when it is pushing against an immovable object (a wall). Stalling your wheels, and motors is NOT good for the system, and can lead to damage, or popping of circuit breakers.

How much current do you want the motors to draw while you are at "max load"?
How fast do you want the robot to be able to move while still being able to push a set amount?
Does the motor you're using for drive have enough power to accomplish both?
Should you think about using a "combiner" to add on another motor?
Do you need a shifter so you can accomplish both?

The answers to all these questions can be found through simple calculations! *gasp*. If you need help with any of them, I recommend this thread: http://www.chiefdelphi.com/forums/sh...ad.php?t=22604

There are plenty of other good threads like it.
Simply search the forums.
Anything "Paul Copioli" says is drivetrain gold.

Good Luck, and don't be afraid to ask for more help. There are plenty of mentors on here more than willing to answer questions for you.



You may also email or AIM me for more help.
Hope this helps,
John

[edit] This presentation might be helpful to you. http://www.teamfordfirst.org/_documents/Motor%20%20Drive%20Presentation%20(12-7-02).ppt [/edit]
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In the interest of full disclosure: I work for VEX Robotics a subsidiary of Innovation First International (IFI) Crown Supplier & Proud Supporter of FIRST

Last edited by JVN : 30-12-2003 at 14:47.
 


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