Mecanum Direct drive

[fhsroboticsteam]

We are considering building a direct drive config similar to this one by andymark

http://www.andymark.com/ProductDetai...ctCode=am-0485

These would be using the tough boxes from last year and 6" mecanum wheels.

and for the gear boxes we would need to buy theses

http://www.andymark.com/productdetai...ctcode=am-0211

I was wondering if this configuration could support the weight of the robot. It would be putting a lot of strain on the tough boxes.

And would the gear ratio be too high? 12.75:1 with 6" mecanum wheels?
what could happen if our robot is geared too high?

[dtengineering]

The axles will need to be supported in two places. You could say that the axle IS supported in two places already... the inner and outer bearings in the toughbox, but they are pretty close together. If you keep the wheel really, really close to the toughbox, though... you'll probably be okay.

It is an issue of torque... when using just the toughbox bearings, the axle is a lever and the bearing nearest the wheel is the fulcrum. As the wheel gets further away from the fulcrum, it starts to develop mechanical advantage and pushes "up" on the bearing near the wheel, and "down" on the bearing away from the wheel.

The toughboxes are pretty tough, but I'd still consider putting some outboard bearing support in there. You'll probably be fine with the wheels set up as you plan, but I can guarantee you'll be fine with the wheels set up like this: http://www.youtube.com/dtengineering#p/u/0/xgTJcm9EVnE

Jason

[Stephen of REX]

Your links aren't working for whatever reason, so I'll just tell you what I know about mecanums. 16:1 on 8" mecanums is very well balanced setup.

As for mounting the wheels directly on the toughboxes, I'd strongly recommend against it. A better bet would be to mount the wheels on the chassis, and have them chain driven to the toughboxes which are situated on top of the chassis.

Another alternative to toughboxes is Banebots transmissions, they're what we always use.

[Chris is me]

Mounting wheels directly to a Toughbox Nano should be fine if you keep them close to the box.

[fhsroboticsteam]

Your links aren't working for whatever reason, so I'll just tell you what I know about mecanums. 16:1 on 8" mecanums is very well balanced setup.

As for mounting the wheels directly on the toughboxes, I'd strongly recommend against it. A better bet would be to mount the wheels on the chassis, and have them chain driven to the toughboxes which are situated on top of the chassis.

Another alternative to toughboxes is Banebots transmissions, they're what we always use.

So you think that 12:1 would be to low of a ratio (causing the wheels to spin too fast) on 6" wheels?

[fhsroboticsteam]

The axles will need to be supported in two places. You could say that the axle IS supported in two places already... the inner and outer bearings in the toughbox, but they are pretty close together. If you keep the wheel really, really close to the toughbox, though... you'll probably be okay.

It is an issue of torque... when using just the toughbox bearings, the axle is a lever and the bearing nearest the wheel is the fulcrum. As the wheel gets further away from the fulcrum, it starts to develop mechanical advantage and pushes "up" on the bearing near the wheel, and "down" on the bearing away from the wheel.

The toughboxes are pretty tough, but I'd still consider putting some outboard bearing support in there. You'll probably be fine with the wheels set up as you plan, but I can guarantee you'll be fine with the wheels set up like this: http://www.youtube.com/dtengineering#p/u/0/xgTJcm9EVnE

Jason

What gear box/gear ratio were you using in the video you linked me to?

[rutzman]

So you think that 12:1 would be to low of a ratio (causing the wheels to spin too fast) on 6" wheels?

A standard AM Toughbox at 12.75:1 with a 6" mecanum wheel should give you around 7-8 ft/s.*

*With 8" wheels on the same gearbox, we were measuring 10ft/s consistently. Hopefully there isn't some funky issue with mecanums that would put my 6" wheel estimate off by any significant amount.

[pfreivald]

Direct drive systems absolutely must be supported on both sides of the wheel, and well. Pillow blocks (whether IGUS or from mcmaster.com) are very useful in this regard.

[Chris is me]

Direct drive systems absolutely must be supported on both sides of the wheel, and well. Pillow blocks (whether IGUS or from mcmaster.com) are very useful in this regard.

http://www.chiefdelphi.com/media/img/ccb/ccb3487f5f6257ceb8c7f993e6cdda13_m.jpg

Great drivetrain by 829 - cantilevered wheels

[JesseK]

We direct drove our Mecanum drive last year on cantilevered TB-Nanos (hex shafts). We went over the bump once per match on average, and rarely had a 'soft landing' on the other side. Not only did we do 2 competitions (1 regular, 1 offseason), we also had about 15 hours of little kids driving the robot around at the USSEF. Everything was fine for the most part (dirt kept getting in the gearboxes from the SEF floor).

We had a very slight issue with our Toughbox Nanos that we discovered during the build season. One of the bearings on the output shaft could wiggle as if the gear wasn't spaced properly. We added custom-shaved washers to them, and they worked beautifully after that. I'll note that these were the first iterations of the TB-Nanos, and from what I can tell AM fixed that issue in the iteration after.

I'll also second Chris's statement about keeping the wheels as close to the gearbox as possible. Also use 2 hubs per wheel if you can since the wheel is slightly wider than the output shaft's length.

[scottydoh]

Direct drive systems absolutely must be supported on both sides of the wheel, and well. Pillow blocks (whether IGUS or from mcmaster.com) are very useful in this regard.

I'd like to disagree. 810 has been direct driving mecanum wheels off of the BaneBots P80 transmission, unsupported for the past 4 years with absolutely no problems. Although the P80's do seem a little beefier than the ToughBoxes.

[wilsonmw04]

We used the Banebots P80 trannys in our testbed a few years ago. I think we used 16:1 or 12:1 with AM's 6" mechanums. I forget which. We did support our with the banebot barings.

http://www.youtube.com/watch?v=6VEHNhcsj6Q

[IndySam]

http://www.chiefdelphi.com/media/img/ccb/ccb3487f5f6257ceb8c7f993e6cdda13_m.jpg

Great drivetrain by 829 - cantilevered wheels

We pounded that drive mercilessly over the humps through 4 competitions and many hours of practice without a single failure.

If you keep the load close to the gearbox you will not have any problems, especially on this years flat field.

[pfreivald]

Well, what do I know, then? :yikes:

[jblay]

Also use 2 hubs per wheel if you can since the wheel is slightly wider than the output shaft's length.

Is that with the regular nano or the one with the long outputshaft

[JesseK]

Is that with the regular nano or the one with the long outputshaft

That's the length of the regular nano (It's ~1.5" or so).