Maximum Pushing Force

[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?

[Ken Leung]

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.

Consider this:

What good does a drive train do if the wheels are slipping all the time?

If you have horrible traction on your drive train, such as using just skyway wheels without any treads on them, no matter how hard you gear your drive system, you will never push as much as you want.

At the year 2002, Beatty was one of the special case because they got really really good traction at their feet. Mean while, most of the other teams chose to lift up 180 lbs goals to add more weight on their robot so they can have more friction at their wheels. Great examples are team 60 and 45's robot that year.

Another thing to consider is this:

The lower the load on the motor, the less currect they draw. Sometimes you have to gear your motors such that they don't pop the circuit breakers all the time. That may involve gearing the drive train to a stronger torque then you need, but it is still necessary to ensure the health of the robot.

Some other times, people just didnt know the maximum amount of force they can push is the amount of their traction force, so they mistakenly gear their robot to really low torque and waste a lot of the speed they could've gotten from the motors. I know teams who choose to only use 2 drill motor because they realize their strongest pushing force is how much traction they have, and didn't think 2 more atwood motors are necessary.

[JVN]

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]

[KenWittlief]

someone else mentioned you need to be able to spins your wheels if the bot is against the wall - so you dont pop your breakers

but there is a little more to it - if you look at the RPMs vs current draw, you have to not only keep your motors from stalling, you have to keep them spinning above the point where they draw 40 amps (preferrably well above that point so they are only drawing 20 amps or less, or they will heat up quickly)

thats why you want your motors to be geared down lower than 'necessary' - so you can spin your wheels when push comes to shove, and keep spinning them for a while without cooking the varninsh off the windings in your motors.

It has a very pungent aroma - once you've caught a wiff of it, you will never forget what cooked motors smell like.

btw - the reason why you need to be able to spin your wheels is, the point at which they start to spin or slip is when you have reach the max limit of your friction with the carpet - so if you design a drivetrain that has all kinds of friction with the carpet, but you dont have your gear ratio low enough to reach the max point, then it goes unused - if you dont have enough torque to break the wheels loose you cant use your wheels to their max force.