pic: FRC488's Octocanum Ver 2.0

[kmcclary]

Originally Posted by kmcclary
> IMHO, you should always hard mount the mecanum wheels to the frame, and never have them dangling on the end of a pivot arm.

Originally Posted by RyanCahoon

Definitely good points to raise, but I imagine you would see similar kinds of (and perhaps even greater in magnitude) stresses by putting the traction wheels on the end of the pivot arm and then being pushed sideways by a robot playing defense against you. Either way, you definitely want your frame to be pretty tough.

Ah, but I still stand by my original statement(s) (including the comment in my previous post saying that yea, your design also must consider being pushed sideways... )

Note that when mecanums run, they are ALWAYS pushing sideways on the frame, just as much as they push forwards and backwards. If they're on the end of the descending sub-frame, they will ALWAYS be pushing it from side to side, and trying to bend them. That's a LOT of stress, under just NORMAL operation.

Therefore, I still feel the the BEST choice is to always mount the mec's axles AT the chassis' main plane, and immediately couple this side-to-side force directly to the frame with thrust bearings. This greatly increases the mec's effectiveness, and totally unloads your motors from any mec wheel generated side friction, from its basic operation.

The thrust bearings also helps prevent the mec wheels from BINDING UP when being attacked sideways while using them.

Note also (assuming you are only using the barrels for F/B pushing, and are not trying to skid-turn the bot with them) that significant side forces on the descending barrels ONLY exist when you are being pushed sideways by another robot. In that case, you wish the sub-frame and main chassis to act as a single SOLID.

Therefore, the goal is to eliminate any GAP between the sub-frame and the main chassis, WITHOUT introducing friction while changing wheel types.

There is a simple solution for dealing with that (and side attacks)...
Attach a pair of Delrin (Acetal Resin) blocks to the main frame, one on either side of the descending sub-frame, to take up any gap between the main chassis and the descending sub-frame. (Put a little radius on the leading and trailing edges.)

Now, whenever you are pushed sideways, the descending sub-frame "leans against" the Delrin blocks, preventing deflection, bending, and/or damage, and immediately couples the barrel's tractive force, to resist the attack. (It also helps protect the sub-frames should you DO try to turn the bot on the 4 barrels. )

Using Delrin or HDPE for these Skid Blocks gives you the "slipperiness" necessary to allow you to easily "change wheels" while being attacked without binding, for escape, or other action. Just make sure there are no protruding screw heads on the sub-frame to catch the pads (or simply notch out the blocks to allow them to pass by...)

Given a smidgen of clearance between the sub-frame and the blocks, they only come into play when you're being pushed sideways, to add side support and provide a bearing surface to the sub-frame.

IOW, IMHO it's not worth the time, weight or complexity of messing with adding side "follower wheels" to the chassis (or sub-frame) just to support the far end of the descending sub-frame from side to side forces while changing wheel types. IMHO using Skid Pads that come into play whenever necessary is a much simpler, and lighter solution.

Does this make sense?

- Keith
--- Also the List Dad of "OmniMec" - An omnidirectional and mecanum wheel specialty e-list ---

[Dad1279]

If you look at one of the modules we used last year, we mounted the transmissions directly driving the traction wheels to the chassis. The mecanum wheel modules pivoted on this axle, with acetal slides mounted alongside the wheel modules near the mecanum axle.

This worked fine for two regionals, through considerable robot-robot interaction, and a few off-seasons, demos, and testing this year.

With the higher COF, the traction wheels would put more sideways 'force' on the chassis, when being pushed sideways. I haven't measured it, but it takes considerable more force to push a roughtop traction wheel sideways than a robot on mecanums.