Dual wheel per axle set crab-steer concept

[Andrew Schuetze]

Our team wanted to build a crab steer system for this year but we lacked the experience and pre-season work to pull it off and went with the AM omniwheels in a holonomic drive. It worked and we were proud of our accomomplishment but it lacked polish and a feedback system.

This summer, we are fortunate to have a young, dedicated to the students mentor who is working with a graduating senior and an freshman entering into 10th grade on a summer weekend project. That being the development of the desired crab-steer system.

The engineer has revived his early concept of having a dual wheel set-up for each crab unit. This will require a differential but does allow for a tighter drive motor set up.

What kind of thoughts do you crab designers or just all around gearheads think of such a concept? Shouldn't require much larger turning forces plus it increases the traction even if not the doubling that might be expected from having twice the number of wheels on the floor....

[Andrew Blair]

I'm not sure you would gain as much advantage here as you think. Generally speaking, more wheels on the floor does not equal more pushing force. There is a very small advantage gained, but friction theoretically is not affected by surface area.

But, if you do end up building it, I would love to see it. The transmission/differential should come out looking pretty sweet.

[Al Skierkiewicz]

Originally Posted by Andrew Schuetze

Our team wanted the build crab steer system for this year but we lacked the experience and pre-season work to pull it off and went with the AM omniwheels in a holonomic drive. It worked and we were proud of our accomomplishment but it lacked polish and a feedback system.

This summer, we are fortunate to have a young, dedicated to the students mentor who is working with a graduating senior and an freshman entering into 10th grade on a summer weekend project. That being the development of the desired crab-steer system.

The engineer has revived his early concept of having a dual wheel set-up for each crab unit. This will require a differencial but does allow for a tighter drive motor set up.

What kind of thoughts do you crab designers or just all around gearheads think of such a concept? Shouldn't require much larger turning forces plus it increases the traction even if not the doubling that might be expected from having twice the number of wheels on the floor....

Dual wheel doesn't automatically demand a differential. The wheels are fairly close so the speed difference is not that great. However, dual wheels do take up more horizontal space in each module and for that reason, they are rarely used. Cautions for double checking the side forces developed on the modules by drive (both forward and reverse) and by pushing from the side. These side forces (as viewed by the supporting structures) can be large enough to deform and damage the supporting mechanisms.

[sanddrag]

I see absolutely no advantage to having two wheels per crab module. And you want to put a differential between them? Why? Perhaps a picture/drawing of the concept would help me better understand.

[Revolverx7]

For a few years my team (358) has used a crab drive system. It was somewhat crude, but got the job done. We had two seperate drive trains facing 90degrees from one another, one inside the other. the center drive train would be forced down with a pnuematic cylindar thus lifting the other drive train off the ground.

Pros: crab steering, easy programming, good manuevering.
Cons: heavy, and a little less powerful.

Dual wheel wouldn't give you too much advantage for the added weight and complexity. However, if you did want to go with it, you don't need a differential. All you would need is a miter gear at a 90 degree angle from the axel of the dual wheels. This would effectively give you perfect transmission of power (with only a slight loss of efficiency).

[Veselin Kolev]

I have pondered the though for quite a while as well. I see the following pros with the design i came up with: you can use small wheels (about 2" diameter) with a differential in between. Also, you can have a CIM mounted vertically, feeding power straight into the differential. This lets you have a VERY small module turning radius. If you think about it, having bevel gears on a single wheel swerve to have the motor vertical is pretty close in size to having two smaller wheels and a differential, with a vertical motor. Sure, a few more gears, but worth a serious thought.

[Andrew Schuetze]

Here is an image found in a FIRST mechanism library posted in another thread on off-season crab-steer projects.

[Ben Piecuch]

Dating back to 2001, Penn Robotics (Team 135) used a dual-wheel per axle, crab-style drive system. It was extremely fast, and very maneuverable. Those kids obviously had some stick time behind it, as it was a blast to watch.

Here's a pic I dug up from the 2001 KSC regional. Maybe someone from the team can tell us a little more about it?

FRC135, 2001