pic: Cooney 269's Worm Crab Gearbox Interior

[AdamHeard]

Originally Posted by Barry Bonzack

Wow, I'm also impressed with your lifting ramp. I understand the theory that the worm drive will keep your wheels from back driving. Is there a reason your team chose not use high traction wheels or chose not to put high traction material over the andymarks?

Common "High Traction" wheels in first are usually in the 1.2-1.3 coefficient of friction range. The AndyMarks are 1.0-1.1; So they are still pretty grippy.

[lukevanoort]

Originally Posted by AdamHeard

Common "High Traction" wheels in first are usually in the 1.2-1.3 coefficient of friction range. The AndyMarks are 1.0-1.1; So they are still pretty grippy.

This is very true, and a lot of people seem to think that these AndyMark wheels are equivalent to the old Skyway kit wheels. In practice,the difference between AndyMark Kit wheels and most high traction wheels is so small that driving makes a much, much bigger difference in defensive maneuvers and keeping from being defended. The only reasons I can see for using other wheels are for differences in size, and drive method. (Although I am aware of two materials that can be put on wheels to get a coeff. of friction of about 1.6, but both are a touch pricey, and one seems rather hard to put on a wheel)

[AndyB]

Actually we even added Armoral to the wheels to make them slicker, so they actually acted more like the skyway wheels. We were having turning problems so we decided that mobility would be more important.

The gearboxes turn 90 degrees in both directions, similar to Beatty's 2006 robot, I believe. We looked into a 360 gearbox but it would have required the rotation to be at the top of the motor due to the offset motor placement.

[les chortos]

The problem with Worm drive on the main wheels is that Worm drive is very ineffecient. What this means is that it draws a lot of battery power to drive the wheels. Last year team 772 . used a worm and wheel and at the start of a match could drive up last years ramp- ( the ball shooter ramp) . but at the end of the match after they run the battery down for 2 mins. could not.
Worm and wheel is good for high reduction ratios and if the worm pitch is choosen properly- self locking. - that a bout it.- they are generally high is sliding friction and cannot coast. If you want a "coastable" robot - do not use it.

In a nut shell- don't waste your time.- if it was good all the good teams would be using it.!

[JesseK]

The major problem I see with a worm gear on the main drive wheel is that for the wheels to "lock" and not move, inwards or outwards pressure is directly put onto the motor shaft. There are ways around it (shaft collars is the first thing that comes to mind), but it definitely could wear a motor down over the course of the 30-50 matches your robot has a potential to compete in if it's not accounted for.

Other than that it's a very nice simple design, especially for a defense+ramp robot this year.

[Sean Schuff]

Originally Posted by les chortos

The problem with Worm drive on the main wheels is that Worm drive is very ineffecient.

That depends on if you use a single, double, triple or quad lead worm. Each of these provides a different level of efficiency - roughly 60%, 70%, 80% and 90% respectively.

We have never used a worm/worm gear combination for a drive but we have used it at the elbow of an arm. We needed to maintain a high level of efficiency and went with a 4-lead worm. I would worry less about the efficiency and more about sheering teeth in a pushing match. You may find that the wheels slip before the teeth fail but it can happen. Trust me! We experienced it at IRI in 2005.

Great looking drive Cooney! I was impressed with the prototype video when I saw it a couple months ago. Very smooth!

Sean