pic: 1640 Pivot Module

[Dillon Carey]

Is the bearing at the top of the module the only support you have for turning the module?

If so you might want to think about adding another bearing somwhere on the shaft or around the module.

[Gdeaver]

There are 2 flanged ball bearings supporting the pivot. The top bearing is not shown.

[Siri]

Originally Posted by Dillon Carey

Is the bearing at the top of the module the only support you have for turning the module?

If so you might want to think about adding another bearing somwhere on the shaft or around the module.

Yes, you can see the two bearings in this photo. It's pretty stable and has treated us well this season: very few breaks and and only a couple access issues. (We're working on them for the next iterations.)

[Al Skierkiewicz]

Gary,
What Dillon is hinting at is a method to take some of the strain off the bearings that will be stressed during side loads encountered on the robot frame. With the bearings at the top of a long lever arm, the side loads on the frame will cause some significant friction to build in the bearings. That friction will result in in the need for significant turning forces. Additionally, a bearing surface near the floor will prevent those side forces from bending the assembly and causing the wheel to ride on an edge, thereby loosing friction with the floor.

[Gdeaver]

Al,
What your concerned with is always a concern with co-axial dive modules. If you look at the lower cage design we have allot of metal and bracing along with a captive dead axle for the wheel. We choose not to lighten like 118 does. After this year's experience, I'm glade we did not. The 1" X .5 wall AL tube has proven more than strong enough. Looking at the above diagram there is a 1.5" OD ball type thrust bearing that handles the weight and vertical loads. The 2 1" flanged ball bearings are mounted in a 1/4" plate on the bottom and a 3/16 On the top. The plates are pop riveted to the frame and have 2" of separation. The basic design has proven it's self through some brutal matches and Hours and hours of drive time. We made concessions in places to keep the cost and machining down. The problems we have had are more to do with maintenance than failure. In the off season. We are looking to refine the design. It's not perfect yet. The control software was our biggest night mare with the pivot drive and the control presentation to the driver still needs work. Any team that want's to do swerve should not underestimate the complexities involved. If they want to do swerve next year they should start now.

[Dillon Carey]

Al,
If there is another bearing on the vertical tube then I would probably be ok with the amount of support this module has.

Gary,
As long as you set it up right you can get away without supporting the bottom of the module. I would only try this with a relatively short module, due to the decreased lever arm. Wildstang has steered away from this because they put the motor and gearbox in the module, due to this there module's are relatively tall compared to a co-axial module.

This is the roughly the same method we used this year, only we still had a delrin ring on the bottom for extreme side loads, since this year involved a lot of force on the wheels when hitting the bump, and instead of 2 bearings we used 1 needle roller bearing on the vertical shaft. This worked extremely well.
Most of the time the delrin disk at the bottom didn't even do anything and under normal playing conditions there was very little turning friction.

Also what language did you program your swerve in?

[Brandon Holley]

Originally Posted by Dillon Carey

This is the roughly the same method we used this year, only we still had a delrin ring on the bottom for extreme side loads, since this year involved a lot of force on the wheels when hitting the bump, and instead of 2 bearings we used 1 needle roller bearing on the vertical shaft. This worked extremely well.
Most of the time the delrin disk at the bottom didn't even do anything and under normal playing conditions there was very little turning friction.

Ditto the use of a needle bearing. As long as the thrust load is handled with something like a delrin ring like Dillon mentioned, the needle bearing is an effective quick and dirty solution.

-Brando

[Al Skierkiewicz]

Guys,
The distance from floor to top of module is nearly the same for both our designs. (BTW, we do not use a coaxial drive, opting instead for motors in the module) We have run checks of current draw on the steering motors during a pushing match and found that without the bottom bearing (ours is near the floor) the motor current skyrockets as additional side load is applied to the bearings. It is unavoidable. Obviously, different bearing types produce different frictions, your mileage may vary. Without something to limit the side movement of the module, a strong robot is likely to move the bottom of the drive module at least a 1/4" or more if the wheels are sticky. Something has to give, and with both the steering and drive running in the assembly, something may bind and momentarily fail to perform as desired.
One lost match can screw up your whole day.

[Clem1640]

To Dillon Carey,
We used Labview for the programming. Since each wheel is independently driven and steered, the software development was not trivial. On the other hand, with a choice of crab and snake drive, and two different robot twist modes (around the drive-train centerpoint and around the possessed ball centerpoint), we have a remarkably agile and maneuverable robot. It was worth the effort.

This is basically a fly-by-wire robot.

[Dillon Carey]

Originally Posted by Clem1640

To Dillon Carey,
We used Labview for the programming. Since each wheel is independently driven and steered, the software development was not trivial. On the other hand, with a choice of crab and snake drive, and two different robot twist modes (around the drive-train centerpoint and around the possessed ball centerpoint), we have a remarkably agile and maneuverable robot. It was worth the effort.

This is basically a fly-by-wire robot.

We also programmed our robot in labview. Although since we had them all steered together and the left and right side powered separately we only had 2 different modes. Swerve and 6 wheel.

If you would like any help with code or a copy of our how we programed our bot just send me a personal message.

[Dillon Carey]

Brando,
We used needle thrust bearings to handle the thrust load. the delrin ring went around the bottom of the module, which had an aluminum ring on it. This ring was there only for extreme load situations. Like Al said without bottom support things tend to bind under extreme load.

But I do believe with the correct bearing and support you can nearly remove the lower bearing without much increased turning friction under load. Ex: in a pushing match, if your module is supported enough to have almost no binding until your traction brakes then you should be able to get away without supporting the bottom of the module.