Weight Distribution Mechanism in Tread System?

[Kai Zhao]

My team had used the Breco-Flex green "Super-Grip" backing in our drive system but had quite poor pulling ability at the National's pulling test. The 125 lbs robot pulled a meager 90 lbs, before the tracks lost traction.

Our first thought was that the belt might have been worn out, because it had already been driven at one regional. But how much a part could the pressure exerted on the carpet had to go with it? There are 4 pulleys in each side, 2 of which touch the ground. The track is 2" wide with a 22" length on the ground

Using the wonders of Play-Doh, we found out that there is only a 2" x 1" area where the track was exerting force of the carpet.

What would be the best way to resolve this problem? Add additional pulleys, which are really heavy and expensive? Add a "pressure plate" that is coated with teflon? Or increase tension in the system? Has any teams used any of these devices?

-Kai

[Matt Reiland]

For sure you can add pressure with extra 'bogey' wheels similar to the way tanks have. Don't forget that the bogey wheels don't have to be heavy or expensive. It would be very easy to make your own from Delrin plastic instead of buying them from Breco, since these are just idler wheels they can be smooth without the teeth to engage on the tracks.

[Gui Cavalcanti]

We had that sort of problem in 2001 with Peacemaker. This is what we ended up doing:

http://www.chiefdelphi.com/forums/pi...&quiet=verbose

That little red strip under the lexan panel is a piece of plywood that was cut for our timing belt. It puts pressure on the entire length of the belt, is easy to do, and doesn't require much time to machine. Of course, it is crude, but the belt has never cut into the plywood (just got worn off by carpet). We attached aluminum angle to the side of our chassis and flat-head-screwed the plywood into the angle pointing down.

We've never had to replace it since the 2001 season started, and it's worked very well. I suppose Teflon would work better, though.

[Warren Boudreau]

Kai,
We used the Breco-Flex belts this year with surprisingly different results. We got tremendous traction from those belts. Are you sure that some other part of your robot wasn't touching the floor? Maybe a flange, or something. A 125 lb robot using those belts should have had more than 150 lb of traction.
You are right about the expense though.

Warren Boudreaux
Team 180

[Jim Giacchi]

Originally posted by Matt Reiland
It would be very easy to make your own from Delrin plastic

That's exactly what we did and it worked wonderfully. We just took some three inch delrin stock and then just cut ten 2.5 inch pieces centerdrilled and put them in the middle of our tread. It was easy and effective.

[Madison]

On the two tread-based designs I've had a hand in, we used two different methods of making certain the belt had as much contact with the floor as possible.

In 2001, 271's robot used a design similar to what Gui mentioned. A teflon coated running board sat just above the belt, always keeping it in contact with the floor. There was one small portion with a wannabe-idler pulley that kept the belt from walking off.

In 2002, 810's used a massive tread system that exerted incredible force to the ground. While the belts were considerably wider, at 6" apiece, the real key was in how they made contact with the floor.

They were deployed, as part of our anti-transmission, using 2, 2" bore pneumatic cylinders. Nearly the entire weight of the robot rested on these cylinders, which transfered that weight across the belts. Because the cylinders had some give to them, they also helped to evenly distribute the weight.

To do this without our anti-transmission, as it was designed, might violate FIRST's 'pneumatics as springs' rules, however, though we never ran into any problems.

[sanddrag]

Could you post a picture of this "anti-transmission" you speak of?

[Madison]

Originally posted by sanddrag
Could you post a picture of this "anti-transmission" you speak of?

I'm at work, so I don't have any pictures handy. Ian is seeing if he has anything that'll show what I'm talking about.

It's nothing too revolutionary or anything. Since I have no real idea about how automobile transmissions work, and we recognized the need for a shift-on-the-fly sort of system, we developed an alternate way of achieving both a high speed (11 ft/s) and high torque (most powerful robot in the Newton division, as per team 236).

It consisted of, in reality, simply shifting 'wheels' over shifting 'gears'. It's been done before, but ours was unique because one set of 'wheels' was a large tank-tread mechanism, both sets of wheels were coupled to the same four motor driving mechanism, and it looked supremely cool

::EDIT:: here's a reasonable aerial view of the drivetrain.

http://wolos.d2g.com:89/robot/Pict23.JPG

The wheels were used by for high-speed, low-torque applications. The belts, running down the center, were for low-speed, high-torque, bulldozer applications.

The pneumatic pistons on either side raised on lowered the belts. When I get home, I bet I can provide some video, even, if you or anyone is interested. It's a tad difficult to explain without some visual aids.

[Ian W.]

if you want some more pictures, chop off the /pict23.jpg part. i have a few pictures. that's from the day we took the robot apart to clean it up. so, if you want some more pictures, go to http://wolos.d2g.com/robot . not sure how many are actually good ones though.