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#42
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Complicated issue...
Ivey,
I am really sorry that I have more or less give up monitoring forums, mainly watching from a distance and only once in a while writing a reponse. I love these forums and would like to be more involved but between my family, my church, my work and FIRST, something had to give in my life. Now to your point. I can't really go into the detail that is needed, but I will say that robust drivetrains are the most important part of a robot. They are really worth all the discussion this thread and others have put into them. That said, I have really become a believer is simple simple simple drive trains (and this from a former 4 wheel drive, 4 wheel INDEPENENTLY driven swerver advocate -- to be honest, I have a 4 wheel swerver, 4 wheel drive SHIFT ON THE FLY design sketched up that I had been noodling with as a future Chief Delphi drive train). The K-3 rule and the push-a-thons that FIRST has been giving us lately have really forced me back to basics. For my money, a simple geartrain with a chain drive is a real winner. As to chain, if you are careful, you can usually use #25 chain, but, I have become a big believer in #35 chain of late (based on our robot last year and based on 3 years of "OCCCRA" robots). It is SO easy to work with and it lets you get away with SO many mistakes and SO much abuse, that I wouldn't even consider #25 chain for my drive system. As to the number of wheels and orientation of them, this is very game dependent. All other things being equal, I like all my surfaces that touch the ground to be powered. But, I will say right now, I am not a big fan of treads, though people of good will can disagree on this point (for me, I think there is almost always a wheeled solution that is simpler and more robust -- but I have already admitted my bias on this point). Back on driven wheels vs. casters, depending on the game (and especially the floor), casters can be an excellent solution (espcially if there is not a ramp). As to 8 wheel drive (4 on a side) or 6 wheel drive (3 per side) or 4 wheel drive. I confess to be totally in love with 6 wheel drive with the middle wheels lowered by 1/4 to 1/2 inch (the machine rocks). This system is a good compromise in my view between 4WD with the wheels in the corner (which can lead to a very difficult machine to turn -- especially if you have grippy tires -- one year we had a 4WD machine that would not turn at all unless one wheel was powered in reverse, full power to one side with the other side off STILL drove the machine in a straight line) and 8WD or NWD (N>6). The ratio of distance between the sides and the distance between the 4 wheels that are touching the ground is very important for determining how much scrubbing occurs when your machine turns (this can be managed with omni-wheels but that is another topic). Bigger is better. This is why I advocate lowering the middle set of wheels on a 6WD system -- it effectively doubles this ratio over a similar 4WD machine. As to types of wheels, I am a fan of pneumatic wheels now that they are legal. 8 inch "Mountain board" wheels worked pretty well for us and a number of other teams. They typically come with a 2 piece hub that has a bolt pattern that makes mounting a sprocket pretty much a walk in the park (the hub itself is your hole template -- how hard is that?) As to transmissions, I am pretty much agnostic on the whole roll your own vs. use the drill tranmission controversy. Good arguments can be made on both sides. For me it comes down to putting your resources where they have the biggest impact. If you simply cannot live with the package space taken up by the drill transmissions OR you are convinced shift on the fly is your ticket to Atlanta this year, by all means roll your own transmission. BUT AGAIN, only make it as complex as you need it to be. I don't think I speak too braggingly to say that I have designed some pretty sweet FIRST gearboxes in my day and I have never had to use anything other than a 2 or at most 3 flat plates to hold the bearings for my shafts. I pretty much use straight spur gear transmission. Most of the complexity of my gearboxes comes from acknowledging that I am not smart enough to always get my ratios right (sometimes, I the game is just not what I thought it was so I need more torque or more speed or both -- if I need both I need a shifter OR more likely I need another motor). The complexity this drives is that I have to think hard and have BERG, Small Parts, Stock Drive Products & Mcmaster catalogs open so that I can make sure that I can do 2 things: buy my gears off the shelf (thus the catalogs) AND change ratio without having to change my gearbox plates (this is the hard thinking part). If I can, I try to put a relatively large set of gears (not wide, but large) no more than one stage after the motor. I attach these gears the shafts using trantorques. I know this will shock a lot of folks, but I love them for 2 reasons #1 The only modifications I have to do to off the shelf gears is to put in the right size hole for the trantorque and #2 I can change my gear ration in a few minutes. If you can use the drill transmissions, I think that it would be hard to justify not using them, especially since they went to the 1/2 inch drill output. One final point is that before you decide you just HAVE to have that multi motor drive system, make sure you are not using the cross axis helical gear system FIRST provides or that you are using some other inefficient stage in your gearbox (if you have bevel gears or worm gears or -- to a lesser extent -- a home brew planetary gearbox or differential in your drive system THIS MEANS YOU). The best you are going to get out a cross axis drive like the ones in the kit is something like 70% effeciency. If you can get that stage out of you drive train it is almost like a 50% increase in output power to the wheels. This often makes the difference between willing a pushing match and constantly popping breakers. Well... ...this message has definitely gotten out of hand. I will end it now but again, not because I don't have more to say but because time is short. Joe J Last edited by Joe Johnson : 22-11-2003 at 18:34. |
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