pic: 114 is thinking of what?!

[lukevanoort]

Originally Posted by 114ManualLabor

Actually, this is designed for a tank track. We're not really traction limited with around 60 inches square of red linatex on the ground. (I haven't really taken physics yet, so correct me if that's flawed. A discussion with a mentor yielded that answer). There's no middle wheel to be lowered. As for not pushing someone in high gear, you never really know when you're going to get defended against, and not be practiced enough to downshift before the pushing begins.

Your conjecture is, in fact, flawed. On an ideal surface, friction and surface area are unrelated. FIRST carpet is not an ideal surface; however, I have never seen any conclusive evidence to support the idea that surface area and friction on it are related. (Unless you have real rigid 'spiky' traction materials, like 71's 2002 file cards or the old Technokat tread designs, but those have been illegal since 2003?, so that isn't really relevant)

Since red linatex has a mu of 1.6 instead of the mu of 1.3 that I did my earlier calcs with, I need to revise what I said. Most definitely, do not use this design if you intend to push in high gear; the FP will be drawing a bit over 41A, which is 65% of a FP's stall current. I do not think that this will last too well. You may not get a sudden smoking of the FP (well, if you push a lot in high you will, but not if it is just a tad here and there), but I think you will probably slowly loose power as you melt insulation on the windings, which causes windings to short out, reducing power. In low gear, you are still fine; the FPs only draw 15A. If I intended to use a gear for pushing, I would feel comfortable with FPs drawing 16A at the traction limit, but not much more than that.

On a sidenote: when trying to do drivetrain calculations, has anyone else been getting 403 forbidden errors when trying to use Google calculator due to their query looking like something malware would produce.

[maclaren]

Just so everyone knows.

Coefficent of friction is not static value with rubber. As the weight per square inch decreases the coefficent of friction actually goes up. This was proven by several different experiments that are posted in the thread that you mentioned. So assumeing 1.6mu is fine for baseline calculations but will vary depending on how you implement it. The mu vlaue could increase by 20%or more depending on implementation.

So the euqation is:

mu*Normal force="Tractive Force"

If you normal force stays the same but your mu goes up by 20% you will see a 20-30% greater tracitve force thus

1.2mu*Normal Force = 1.2Tractive Force

(I use the term tractive force to mean the force that you must apply to break staic friction and start relative motion between the ineracting materials.)

Just so you know manual labor the outback tracks have a pivot wheel so you don't get the full length of engagement on the tracks. So change 60 inches of engagement to about 30 inches but it is still 5 times the engagment of a 6wd robot.

I hope some day to do an exhaustive experiment to settle this question once and for all but I don't have the time or the money right now.

I really, really don't want this to turn into one of the other 50 threads on CD about surface area v. traction. So please, let's focus on a gearbox, not on a debate that's raging elsewhere.

[Cory]

Originally Posted by 114ManualLabor

I really, really don't want this to turn into one of the other 50 threads on CD about surface area v. traction. So please, let's focus on a gearbox, not on a debate that's raging elsewhere.

I think it's an important distinction here...If you design a mechanism based on an assumption, you may end up finding out later that you didn't need 6 motors, and could have saved time, weight, and effort by just using 4, for example.

It's my personal opinion that 6 motor drives have a very small niche, and are almost never a better choice than 4 motors. The only reason I would ever use 6 motors is if I was making a single speed drivetrain and wanted to keep my speed high, while still having some torque (and I probably wouldn't, even then).

$0.02

[maclaren]

Originally Posted by 114ManualLabor

I really, really don't want this to turn into one of the other 50 threads on CD about surface area v. traction. So please, let's focus on a gearbox, not on a debate that's raging elsewhere.

Sorry that was not my intention. I also desipse when that happens to one of my threads.

How much does the transmission weigh(with motors)?

I tend to agree with cory about adding another motor. I'm not sure that you will get a significant return on the investment of weight.

It would be pretty cool though to have some more power to apply to the tracks.

Originally Posted by maclaren

Sorry that was not my intention. I also desipse when that happens to one of my threads.

How much does the transmission weigh(with motors)?

I tend to agree with cory about adding another motor. I'm not sure that you will get a significant return on the investment of weight.

It would be pretty cool though to have some more power to apply to the tracks.

Oh, no worries. With motors, I recall something around 10 (I don't actually have the CAD right now). Plus, if you're building tank tracks, your primary focus is probably defense, so you might as well put as much power to the ground as you can get.

I'm hoping people will comment here, as this thread has the more updated version of this on it's intended mechanism.

[popo308]

We had a big problem with the FP over heating and burning out so we put a victor fan at the end of it and it works fine now and really doesn't heat up at all.

[pakratt1991]

Last year we used the track system that is modeled, our tracks NEVER slipped against the carpet, we have even pulled a van while I sat down in front of the robot, video is on google video. I never saw our track slip so the extra power might actually be helpful.

[AdamHeard]

Cory has a good point in that in this case the 6 motors are pretty unnecessary. The only advantage I can think of is less load per motor, but that's never really been an issue when using two small CIMs.