pic: Another concept study



Here’s another concept study I did last summer. I’ve been keeping it to myself because I actually like this one. Couldn’t hold back any longer - just had to share.

Beautiful design and render, as always. One question, though - Any reason to have the traction wheels on the inside instead of the outside?

Shorter turn base?

I thought that, but it seems like a really small wheelbase then for only two 4" wheels.

It’s not too small. If that’s modeled as the long side of the robot, that’d be somewhere around 12"-15" between wheels.

Oh, looked short sided to me.

I might be wrong that it’s long side. I just can’t imagine doing octocanum for a wide robot.

I guess that when driving with Octunum, you would drive it like mecanum with traction wheels. Thus you would be mostly driving with mecanum, using your traction wheels only if you need more traction.

Because of this, and with a much smaller wheel base, You might encounter some rock when on the inner wheels. Since you would want your main drive orientation to be as stable as possible, you could want your mecanum wheels on the outside. Also, for mecanum to work well, you really want all 4 wheels to be in contact with the floor at all times. If the mecanum wheels are the inner sets of wheels, during rocking, one or two of the wheels might lose traction with the floor impeding on your motion. (This rock might be negligible but hey, there is no disadvantage on having the mecanum wheels on the outside (And imo it looks cooler))

I’m not sure if this is the long side or the wide side, but in this render by the same person, he says that his team uses a mini-octocanum because they have a wide robot.

I did the concept study this way because it made sense for last year’s game. We wanted maximum stability on the Mecanum wheels. You would have to decide what made sense based on the game. It could be wide or long - again depending on the game.

That makes a lot more sense. What are the speeds for each wheel? How much does it weigh?

I believe I have the Mecanums at around 13 fps and the (3.25") traction wheels at around 7 fps.

The module we competed with last season weighed 23 lbs for each side so 46 lbs total - very heavy. This one is 18 lbs for each side for 36 lbs total. We sure could have used that 10 lbs last year. For example, we only had 1 motor driving our shooting wheel because we just didn’t have the weight allowance to add a second motor.

Also, last year’s version would have been nearly impossible to maintain if anything had failed. This one is very easy to maintain. The Mecanum wheel can be removed by just removing the one screw. The gear boxes have just 4 screws and the clevis pin. The CIMS stay with the frame rail when you drop the gearbox.

Is there any concern that the shaft on the reduction just prior to the 3.25" traction wheel won’t hold up to the stresses of having 1/4 the robot’s weight on it while transferring higher torque through it?

The shaft looks like it’s 0.375" even though the wheels themselves are on a 0.5" shaft (based upon bearing holes, so it’s just a guess), which is the only reason I bring it up. Seems to me that if the wheels need to be on a 0.5" shaft, then that particular reduction shaft needs to also be 0.5" since it will hold the weight of the robot when the traction wheel is down. Or maybe the wheel shafts are 0.5" since the 1/8" keyway is more preferred for the wheel?

Love the render.

I really like the compact packaging of your design. Do you think, just for kicks, you’d try out a vertically oriented CIM + bevel gears to have all the space in the center free? Might be fun, although not necessary.

What are the distances between mec to mec and traction to traction?

You may want to consider making the macanum’s slower. 13 FPS will not give you a lot of torque on the wheels that is needed to make your roboto strafe. In 2011 we used CIMple boxes on our 6inch mecanum’s and we were unable to strafe. I may be wrong because I am not a Mecanum master or anything, but this is just from my experience.

Yeah, but you forget to mention only 3 of your wheels were on the ground at once. Aaron =/= trusted source on mecanums.

My software-addled brain is having trouble visualizing how face-mounted cylinders can follow the arc the traction wheels make when extended downward.

It looks like they aren’t facemounted, but rather front pivoting cylinders. The crossholes in what appear to be the mounts must be what they rotate on.

You may want to consider making the macanum’s slower. 13 FPS will not give you a lot of torque on the wheels that is needed to make your roboto strafe. In 2011 we used CIMple boxes on our 6inch mecanum’s and we were unable to strafe. I may be wrong because I am not a Mecanum master or anything, but this is just from my experience.

What ratio were you using? As Andrew suggested, That sounds more like a problem with the implementation than with the gearing. Our 2011 mecanum drive was around 8:1 and we had plenty of torque and speed to strafe. And besides gearing for torque on mecanum is like gearing for torque in Lunacy, you have just above no pushing power (in most cases) so gearing for high torque doesn’t really do anything except slow you down and make you easy to defend against.

You are correct - the cylinders pivot.

Last year we did use fixed cylinders with the traction wheels. The arc error was only about 0.010" so we made the holes oversized to accommodate it. With this concept the tractor wheel rotates too much for a fixed cylinder.