2nd Nov 2015
Making *working* caterpillar tracks is hard
Caterpiller tracks are well known for their utility in traversing rough terrain, so seem like the ideal choice for the assault course challenge. With the right materials they can also provide amazing traction on smooth surfaces so could be useful for the speed challenge. Many previous competitors have successfuly used caterpillar tracks so this is not a new idea but, as with many of the elements of our pi wars entry, we’d like to try something a bit new.
(image courtesy of Brian Corteil)
From the motor selection blog post, we’ll need large drive wheels to meet our speed target. The large wheels will also help us climb over the obstacles so this is not a bad thing there either. Trawling ebay, model shops and robotics sites, off the shelf tracks are available but only in relatively small sizes or with hard materials, so it looks like we’ll have to make our own.
Making tracks from scratch usually starts with chain or other interlocking elements, then adding treads or rubber pads. Many small items are required and they’ve often very labour intensive.
(image courtesy of https://hackaday.io/project/6106/logs)
Oogoo, a new, low cost, method of makign soft rubbery items has been shared on instructables and looks promising for making tracks. So far it has been used to make some great looking robot wheels but no one has used it for competitionor fo rmaking tracks. In principle, all we need to do is make a 3d printed mold, mix up the ingredients and cast a track.
(image from http://www.instructables.com/id/How-To-Make-Your-Own-Sugru-Substitute/)
Here’s our design for a track:
Central rib on the inside to align the track and keep it on the rollers/wheels
Drive nubs on the inside so the drive wheels transfer their torque to the track
small paddles on the outside to increase grip when climbing steps.
To design the mold we first had to create a new model of the track in its relaxed, circular state. To make the mold design, we simply subtracted the track design from a solid block, then split it up to allow it to be removed.
Here’s the printed track mold:
To make the track, we first used vaseline as a release agent on the mold. Then we mixed up the cornflour and silicone sealer ingredients and spooned them onto the mold and closed it up:
The main issues we had was that the mixture stuck to the spoon easier than it stuck to the mold, so it was very difficult to evenly fill the mold. The track mostly came out looking great but there are a few missing or thin patches. The biggest issue though was that the track came out too long, so it didn’t stay on the rollers:
But it looked promising enough to try again. The options we discussed were incorporating some form of reinforcement (string or cloth) to stop the track stretching and riding off the rollers, adding side plates to the rollers to keep the track on, or to make the track undersize such that its stretched when in place. Given the first track came out much too large, a new mold was needed either way. Given the thickness of the track, the side plates would have to be quite short or they would be the only thing touching the ground. We though it might still ride over the flanges, so We decided to start with trying the ‘stretchy’ version first. By making the initial diameter smaller, we could make the mold smaller, allowing us to print the inner mold in one piece. We hoped this would make filling the mold evenly much easier.
This time we tried something new with the molding, laying out the mixture on greaseproof paper first to try to make it more even, but otherwise the process was the same:
This track came out better, there were less voids and it stretched nicely into place:
Unfortunately, it still rides off the rollers:
So we tried crowning the wheels, a standard technique for getting flat belts to stay on pulleys:
And that didn’t work either. So it looks cool but its still useless 🙁 Watching the video again, it looked the drive nubs might actually be hindering the track staying on the rollers. As a last ditch attempt, we tried cutting the nubs off. The track stayed on better but still not positive enough to be driven around on a grippy surface 🙁
Looking again, other tracks (large and small) appear to have a more significant centralising feature. For now though, we’re going to drop back to wheels so that we have something to start testing with.