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Unread 17-12-2003, 21:35
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JVN JVN is offline
@JohnVNeun
AKA: John Vielkind-Neun
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Re: Shifting Gears

Quote:
Originally Posted by George1083
What if you had treads driven by a 3" pully instead of 6" wheels? I'm pretty sure that would satisfy the 10 fps and 150 lbs requirements without burning up.
Not at all!
If we used the above designed gearbox with a 3" pully... it would be going half the speed, with double the output torque.

We're not talking about coeff of friction at all... sure the treads might end up being a little more "grippy" but... that doesn't matter in this debate. You can have all the traction in the world, but unless your gearbox can output the torque to back it up... you're just wasting your time. Just like you can have all the output torque in the world, but without traction, you're doing the same.

Given a set amount of power output, there is only a certain speed you can go while comfortably (without drawing too much current) pushing with a given force. The above proof I did, was to show that the drill and Chip alone, geared at 10fps cannot produce a 150lb linear force "comfortably".

Sure, gearing the system down to 5 fps would probably do it... but that's besides the point of this debate.

You can't have your cake and eat it to.
You can't have "tons of speed" and "tons of pushing force" at the same time, unless you have more power than our current motors allow (unless... you SHIFT!)



Again, the "competitive" level of speed and pushing force varies each year depending on the game. Some seasons 10 fps is enough, some seasons it isn't (like in 2002 if you wanted to beat 60/71 to the goals). Some seasons 150 lbs of linear pushing force is enough, some seasons it isn't (like in 2002 if you wanted to fight back against 308 when they had the goals lifted).

At the beggining of every drive-design process, we all have to set ourselves some specifications we want to fulfill, decide what the drive must accomplish to play the game to the level we want to play it. We basically need to do some guesswork about what our robots will need to deal with during the upcoming season. Any team in 2002 that thought "wow... our robot can push with 100lbs of force... we're unstoppable!" was... mistaken.

Also, if we decide "we are going to lift the goals so we can push harder". We then need to determine exactly how much torque the drive needs to be able to output, in order to back that up. Otherwise... as stated above, we're wasting our time. Imagine if 60 hadn't downshifted after lifting the goals in 2002... a pushing robot would have gotten in their way, and their motors would have stalled. No good! They needed their low-gear torque.

For those who are having trouble understanding some of the basic principles invovled in this debate, I recommend this presentation:
http://www.teamfordfirst.org/_docume...0(12-7-02).ppt

It's the best one I've seen in terms of throughly explaining drivetrain theory. (Paul's the man. ) If you have any other questions, search around this forum, ask your friendly-neighborhood-FIRST-mentor, or drop me an email/IM.

John
(Who last season was on here learning for himself about drive physics, and never thought he'd be the one doing the ranting/teaching.)
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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
 


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