Which Gearbox Should I Use?

[SeanFitz]

So my team wants to build several different chassis this fall to find which we think is best. Anyways, I am leading the 6-CIM drive-train group. The idea I'm working on is a 6 wheeled design with a dropped center wheel direct driven by a 3-CIM gearbox.

I went on VEX to look at my options for gearboxes, and now I'm stuck. I found these gearboxes online, and I think they would work well. The only problem is that I have no idea what gear reductions I should buy. Basically, I want it to be two speeds, one for driving fast, and one (more importantly) with a lot of torque to push other bots around. What's your opinion on what I should get?

And thanks in advance to anyone who posts an answer

[thatprogrammer]

Originally Posted by SeanFitz

So my team wants to build several different chassis this fall to find which we think is best. Anyways, I am leading the 6-CIM drive-train group. The idea I'm working on is a 6 wheeled design with a dropped center wheel direct driven by a 3-CIM gearbox.

I went on VEX to look at my options for gearboxes, and now I'm stuck. I found these gearboxes online, and I think they would work well. The only problem is that I have no idea what gear reductions I should buy. Basically, I want it to be two speeds, one for driving fast, and one (more importantly) with a lot of torque to push other bots around. What's your opinion on what I should get?

And thanks in advance to anyone who posts an answer

If you want something VERY reliable, and easy to use for a WCD use this http://www.wcproducts.net/wcp-shifter-3cim/ Also.. gear ratio calculator! http://www.wcproducts.net/how-to-drivetrain/

[Knufire]

You need more quantifiable goals for the drivetrain. When you can put things in numbers, it's much easier to choose. For example, the calculator that thatprogrammer posted has a rough calculation on time it would take to cover a certain distance at every gear ratio. If it was 2013, a logical choice would be to pick your high gear to minimize the time between the feeder station and your shooting location. If it was 2014, what distance would you want to cover in the least amount of time? Or would there be some other factor that you would want to maximize your drivetrain toward? There is no correct answer; it is both dependent on the game and your strategic choices for the game.

[ehfeinberg]

It's great to see that your team wants to experiment in the offseason with different drive trains! My team tried building a 6 wheel WCD in the off season a few years ago and probably wont go back for a long time.

It's hard to say what gear ratios you need without knowing anything else about your drive train plans. Are you planning to have a directly driven center wheel? Live axle/dead axle? And most important for determining your gearing is what wheel size are you planning on using? Before we recommend a gear box or a gear ratio we need to know more about your set up.

If you are asking for generalized speeds for how fast your high and low gear speeds should be, it is generally considered for a low gear the gear ratio should be set for somewhere around 4-6 (maybe even a little higher) feet per second. And a high gear around 14 -16 feet per second. (Some teams gear their high gear for 18 - 20 fps even). It really depends on the game to determine the exact ratios. If you feel that there will be a lot of defense like this last year then having a low gear on the bottom end could be of value, but if the field is also open and you need to move fast having a upper end of the high gear values is also very realistic.

[SeanFitz]

Originally Posted by ehfeinberg

It's great to see that your team wants to experiment in the offseason with different drive trains! My team tried building a 6 wheel WCD in the off season a few years ago and probably wont go back for a long time.

It's hard to say what gear ratios you need without knowing anything else about your drive train plans. Are you planning to have a directly driven center wheel? Live axle/dead axle? And most important for determining your gearing is what wheel size are you planning on using? Before we recommend a gear box or a gear ratio we need to know more about your set up.

If you are asking for generalized speeds for how fast your high and low gear speeds should be, it is generally considered for a low gear the gear ratio should be set for somewhere around 4-6 (maybe even a little higher) feet per second. And a high gear around 14 -16 feet per second. (Some teams gear their high gear for 18 - 20 fps even). It really depends on the game to determine the exact ratios. If you feel that there will be a lot of defense like this last year then having a low gear on the bottom end could be of value, but if the field is also open and you need to move fast having a upper end of the high gear values is also very realistic.

My plan so far is a 1/8th dropped center wheel direct driven by the gearbox and the two other wheels either belted or chained to the center wheel. All wheels would be 6".

What I really want is a gear ratio that can give me a lot of torque. I've noticed over my years in FIRST that most teams either get pushed around or stalemate with other robots on the field. Teams that can push others around are always recognizable and are almost always picked for alliances, even if it's just to play defense (Team 2013 was a perfect example of that this year). And even in games where speed is important (2013), I always see cases where being able to downshift would have given a team a boost to victory.

[thatprogrammer]

Originally Posted by SeanFitz

My plan so far is a 1/8th dropped center wheel direct driven by the gearbox and the two other wheels either belted or chained to the center wheel. All wheels would be 6".

What I really want is a gear ratio that can give me a lot of torque. I've noticed over my years in FIRST that most teams either get pushed around or stalemate with other robots on the field. Teams that can push others around are always recognizable and are almost always picked for alliances, even if it's just to play defense (Team 2013 was a perfect example of that this year). And even in games where speed is important (2013), I always see cases where being able to downshift would have given a team a boost to victory.

True. AN advantage of most 3 CIM gearboxes (If not all) is that you can also test using a mini CIM and two CIM motors. Test and see what amount of torque works for you.

[SeanFitz]

Originally Posted by thatprogrammer

If you want something VERY reliable, and easy to use for a WCD use this http://www.wcproducts.net/wcp-shifter-3cim/ Also.. gear ratio calculator! http://www.wcproducts.net/how-to-drivetrain/

First off, thanks for that calulator, it's very helpful.

And now for a another question.

The only thing that seems to change between the 2-CIM and 3-CIM gearboxes is the Force @ 40 amps box. (If I'm wrong in my next assumption, please feel free to correct me. I'm a mechanical engineer, not electrical) Now if there are less motors on the robot (I.E. 4 CIMs vs. 6), wouldn't there be a higher amperage for those less motors? If so, wouldn't the 2-CIM gearbox be producing more than the 3-CIM?

[SeanFitz]

Originally Posted by BBray_T1296

That box implies force @ 40 amps per motor. Since the CIM fuses trip at 40 amps, you can draw 120 amps total on a 3 CIM drive vs 80 amps on a 2 CIM.

It is also important to note that the 120 amp main breaker can indeed sustain 240 amps for several seconds

But if the main breaker trips at 120, how does a drive train even run at all. Wouldn't 4 CIMs be running at 160 amps for the whole match?

(Again, not an electrical engineer, just a confused mechanical one )

[Greg Woelki]

Originally Posted by SeanFitz

But if the main breaker trips at 120, how does a drive train even run at all. Wouldn't 4 CIMs be running at 160 amps for the whole match?

(Again, not an electrical engineer, just a confused mechanical one )

The main breaker has to be over 120 amps for a while before it trips. If I recall correctly, it will take about 40 seconds for it to trip at 240 amps and about 5 seconds at 500 amps. The motors in your drive train will probably each spend most of their times under 40 amps of current in a match.

[Michael Hill]

Originally Posted by Greg Woelki

The main breaker has to be over 120 amps for a while before it trips. If I recall correctly, it will take about 40 seconds for it to trip at 240 amps and about 5 seconds at 500 amps. The motors in your drive train will probably each spend most of their times under 40 amps of current in a match.

Datasheet for the breaker: http://www.cooperindustries.com/cont...UITBREAKER.pdf

[SeanFitz]

Originally Posted by Greg Woelki

The main breaker has to be over 120 amps for a while before it trips. If I recall correctly, it will take about 40 seconds for it to trip at 240 amps and about 5 seconds at 500 amps. The motors in your drive train will probably each spend most of their times under 40 amps of current in a match.

About those 40 seconds: is it cumulative or at one time? If it is cumulative, I'm going to have to seriously rethink the whole 6-CIM chassis.

[Knufire]

Originally Posted by SeanFitz

About those 40 seconds: is it cumulative or at one time? If it is cumulative, I'm going to have to seriously rethink the whole 6-CIM chassis.

The main breaker actually trips based on temperature. Saying xx amps will trip the main breaker in xx seconds means that xx amps for xx seconds will heat up the breaker enough for it to trip. This time will be shorter if the breaker starts hot and longer if the breaker starts colder.

[thatprogrammer]

To prevent overheating, you can use compressed air to cool the breaker between elimination matches.

[SeanFitz]

Originally Posted by Knufire

The main breaker actually trips based on temperature. Saying xx amps will trip the main breaker in xx seconds means that xx amps for xx seconds will heat up the breaker enough for it to trip. This time will be shorter if the breaker starts hot and longer if the breaker starts colder.

So if it's about temperature, would the various components near the breaker on the bot heating up affect weather or not it trips? Also, once the breaker does trip, is it true that it becomes more likely to trip again at a lower temp?

[Monochron]

Originally Posted by SeanFitz

So if it's about temperature, would the various components near the breaker on the bot heating up affect weather or not it trips? Also, once the breaker does trip, is it true that it becomes more likely to trip again at a lower temp?

Very little heat is going to be transferred through air to the breaker. So no, as long as your breaker is thermally isolated you won't see any difference.

To your question about the 40 seconds being cumulative, that number is an estimate based on continuous stall conditions. If you stall for 20 seconds, then wait for a minute and stall for another 20 seconds, it likely won't trip but it is absolutely possible. Giving the breaker time to cool down will help, but each breaker is going to be manufactured with slight differences. Doing your best to avoid stall conditions (ie. ensure that you wheels will slip on the carpet instead of freezing) is the best thing you can do to avoid tripping.

As for your second question, I don't believe there have been experiments to track the propensity for earlier tripping after a breaker has tripped once. I believe, though, that anecdotal evidence suggests that they DO trip quicker after tripping once.