View Full Version : Efficent number of Gear ratios...
09-13-2004, 08:39 PM
Well, as I have been thinking about gears, and seeing all the new transmissions that are currently being build in Inventor (or whatever) on CD.com, I have a question.
What do YOU think is an efficient number of gears?
As in just a simple 2 gear selection. High and low? Or are you on the other end and say the more gears you have the better? Some of the ideas that have been posted on CD.com are awesome - please don't stop (you're giving me ideas :D) - but really - where would you need a 6+ speed transmission? (Lets be realistic for a few moments about the competition and the places they are going to be held at.)
Personally I think 3 is an efficient number. You got your Lo speed for pushing/pulling/etc. Got the torque when you need it. You got your high speed for trying to zip across the field as fast as your little wheels can get you. And you have one in between. (Say at the start of a match) and you have to push a goal/balls/etc. on the field - and your fast speed won't have enough torque - but your lo speed it too slow.
What do YOU think?
09-13-2004, 08:45 PM
I really don't see a need for a more than 1 speed transmission if it's well built
However I think up to 3 would be cool, more than 3 would be over kill IMO
09-13-2004, 08:47 PM
I voted 3 because I think that provides enough options without overkill, High Speed, High Torque, and a medium for those situations where you wish you had more options.
EDIT: I'd also like to add that my team has only used a 1-speed drive train in the past two years, my vote is based off of assumptions on what would work.
09-13-2004, 08:58 PM
One worked fine for us, and it'll let you build your weight into other things. Thus I vote one.
(of course, if you've got a few pounds to fiddle around with, then by all means, wedge as many of those suckers as you can into your drivetrain!)
Jay H 237
09-13-2004, 09:09 PM
I would go with either a two speed or three. Our team has used two speed gearboxes the past several years with great success. We have high speed for getting out on the field quickly and low speed for pushing, pulling, or climbing. The addition of a third speed wouldn't be bad for reasons mentioned above but in our experience really hasn't been needed since this year we mated the Bosch and CIM motors together with the gearbox and have more than enough torque. Of course it's really cool to see the six speed shown in the other thread. Heck, why stop at six when you could design a sixteen speed gearbox? :p Now wouldn't that be something to see! :cool:
Oh yeah, we've also discovered on our team that the more ranges we add to our drivetrain, the more chance we have of seeing if it would pass the "Elgin Test" or not. ;)
09-13-2004, 09:15 PM
I'm usually a fan of just one transmission ratio.... Of course, it depends on the game. Games such as 1999 and 2002 have clear advantages to having a transmission, where speed is important to get around the field quickly, AND power is important for moving the mobile goals. In general, if designed correctly a simple one-speed transmission can do just fine. Of course, a two speed transmission, if designed correctly, can do better... but unless you have the time to design these transmissions before the season starts, it's not worth your time during the build season.
So, I vote... one... but sometimes two :) Three I don't see the need for, since a middle speed is rarely an advantage.
09-13-2004, 09:21 PM
for what i think, you basically need only 2. one for everything and a ultra low.. just enuf torque before the wheels start to slip, for pushing... id like to do that. ive only worked with onespeeds
09-13-2004, 09:44 PM
It's not so much the exact number of gears, as having a transmission that usually finds itself in the right gear. A transmission is effective if it can arrange to be in the right gear for whatever the game demands at a certain point.
It might be argued that there are two factors here--being in the right gear, and being in any gear. A 1-speed is always in gear, but it is arguable as to whether it is often the right gear. A 2-speed spends some time shifting between gears, but offers more opportunities to be in the right gear. And so on. The value of a transmission depends greatly on how much time you spend shifting, as opposed to putting some power to the ground. If you shift slowly, perhaps a 2-speed is best. If you shift quickly, or can cope with power transmission under partial engagement of your shifter, you can potentially deal with more gears, without spending too much time in neutral. One exception, of course: a CVT is a little strange, in that it is always in gear, and (if well-designed) can always be in the right gear. In that respect, it is the optimal solution.
Of course, most teams can't build a CVT nor use one effectively. And a CVT will often exhibit complexity and sacrifice significant efficiency for its flexibility, meaning that a straight gear drive might be a better solution for some teams.
Given the robot speeds and torques at work in FIRST, the current state of the art in pneumatic shifting (one piston per pair of gears, most of the time), and my bad experience with electric shifting in 2003, I'd tend to think that 2 or 4 would be preferred (in terms of actually driving the robot). Actually, I really mean 2 or 3, but the way that many of these transmissions work makes having 4 gears the same as 3, only with a duplicated ratio in the 3-speed (222 is an exception--I think that they've got something good going with their 2004 3-speed transmission). The downside--and it's a major one--with more than 2 speeds is generally complexity. If complexity is undesired, then perhaps 2 speeds will suffice.
Aside: the 2003 gearbox from 188 was supposed to be pneumatically shifted, but we decided to ditch the compressor for weight early on, before the gearboxes were finalized. We tried to shift them with the Globes, but the mechanism wasn't especially well-designed (I feel a mea culpa coming on...:( ), and the Globe motors couldn't be controlled with servo-like accuracy--they tended to over-rotate, and didn't return to predictable positions. We ended up modifying it to run in high. Good thing too--that year, our low gear would probably have caused even more #25 chain failures, due to the 6 motors and small sprockets (the chains weren't completely my fault!).
Now, if the leadscrew shifters that have been proposed recently (and perhaps previously implemented) can shift more quickly and smoothly than a dog shifter (a 2004 Woburn gearbox shifting instantly at top speed is really a beautiful sight to behold :D ), then a 3-, 4-, 5-, or more-speed transmission is potentially useful. It just needs to be designed so that it isn't wasting too much time shifting. Is that sort of thing impossible with our resources? Certainly not! But it won't be easy.
Overall, until someone comes up with a capable enough design, I think that the 2-speed will continue to be the most practical number. It is relatively cost efficient and buildable. By doubling the number of gears, you gain a 2x benefit that only diminishes with increasing the number of ratios (1.50x, 1.33x, 1.25x, etc.). Similarly, it spends little time shifting between gears, because each gear has a wider useful range--it can therefore make do with an inefficient shifter (though obviously, a better shifter doesn't hurt a thing!).
Some teams will be able to commit the resources to take advantage of the benefits of more than 2 ratios; there's nothing fundamentally wrong with that--it's just that their designs will have to be that much better (irrespective of the number of ratios), in order to make proper use of those extra gears. And if they figure out how to shift faster, and to build and service it more efficiently, in a couple of years, a significant minority of FIRST teams may be following in their footsteps with designs based on their groundbreaking work.
I think transmissions are interesting to design and fool around with. It is kind of cool to see what could be done. Given the playing field size and 2 minute time limit I can't see the need for more than one speed. I could be wrong, we have never used a multi-speed tranny. I think it would be fun, but not to practical.
09-13-2004, 11:01 PM
I can't see the need for more than one speed.Ever tried to push one or race one?
09-14-2004, 06:47 AM
We have found that a single speed can do almost everything in our two minute match. So why did we build a two speed? It seems that every year we end up pushing other robots. For this you need to have gear that allows efficient use of the motors. This speed is fine for the match but there always seems that it is too slow for part of the match, hence the two speed.
09-14-2004, 08:36 AM
We TechnoKats have been shifting gears since '99, always using 2 speeds.
There are many advantages to shifting gears. Some are obvious, while some are subtle. Here are advantages:
Depending on your wheel or tread traction, you can be in low gear and push other robots around
If you want to drive fast, you can be in high gear and drive quickly
You can use your low speed for more precise driving control and positioning
The high end speed can be faster than you are comfortable with for an entire match, knowing that you can always downshift
If your motors get hot and the circuit breakers begin to trip, you can always shift to low gear and continue the match
But, there are also some disadvantages:
A shifter gearbox usually has more efficiency losses compared to a non-shifting gearbox
More weight is needed in a shifter gearbox compared to a non-shifting gearbox
It takes more time to design and build a shifting gearbox
09-14-2004, 09:42 AM
I voted for 2 speed. This is only good however if reliable (right Tristan?). I believe that our team made the right choice going to 2 speed even with the problems the first year. I will agree that some games might be OK with a single speed. I do like the fact that we can go slow for power pushing or delicate tasks. I know that you can go slow with a 1 speed but there is a larger error margin when in low gear.
The major disadvantage is weight. If you can spare it then it is worth it. As for design and building be a disadvantage that has been overcome by a certain unnamed company. :)
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