pic: Jester Drive:Mecanum Wheel Drive Train

[psquared]

Same thing. I know some teams are able to make these molds themselves because they have the resources available, but for smaller teams with less resources, where would you go to buy such wheels, and about what is the price? Our team hasn't decided on a drive system for next year yet, but I would really like to propose this system and these wheels as a possibility for the team, thanks.

[EricH]

I don't know if mechanum wheels are sold at the size a FIRST team would want. There are commercial models--used to unload those baggage containers from airplanes and very large.

[techtiger1]

A few questions for 190 and 357 what Material were the rollers made out of and what material is around them. Also what are the diameters of your wheels. Finally, how did you attach the hub of the wheel to the shaft.

After reading the problems with these wheels I think they are a great
concept however if people would take a closer look at the commercial ones. They have less rollers then the ones I have seen produced and they seem to by at slightly less then a 45 this i think would help going sideways. Just a thought. To 190 and others who have tried this thank you. Thank you for giving them a chance and seeing them in action really helped. OH almost forgot u might want to lose traction actually off the rollers to help sideways movement.

Drew Disbury The Tech Tigers
team #1251

[EricH]

I talked to 357 and they said they made theirs out of the same stuff skateboard wheels are made from. I think 190 used rubber but am not sure. 330 used nylon on their wheel set. Yes, nylon.

[ahecht]

190's wheels were cast from urethane. 60A durometer Urethane, if I remember correctly.

[BrianBSL]

Originally Posted by EricH

BEFORE you commit to doing the mechanum drive, be sure you know what will happen and have a backup plan. We almost did it, but we decided that our design, which is closer to 190's than 357's, did not meet our requirements, so we are currently getting ready (very slowly) to try the next stage of design. The problems observed both on ours and 357's: a) lack of pushing power, and b) slow sideways motion. Both problems may be fixed in later editions and/or by other teams. So, test now, and if you decide to do it, good luck!

We didn't have a huge issue with lack of pushing power, although we rarely got into pushing matches because of the maneuverability it gives you (unless they have you pinned up against something you can almost always get out by going sideways. As far as sideways speed, because our drive was field oriented (pushing the joystick moved the robot in the direction you pushed the joystick, not in the direction relative to the front of the robot), we actually limited our forward speed to equal the speed we could move sideways. That way going full speed, you can move along a vector while spinning without it looking like you changed speed at all. I suppose that we could have added a "turbo" button to allow you to go full speed when you were moving forwards, but we never really had an issue with speed. In fact, we actually decided that we needed to gear it down a bit for our demo season as it is too fast for the average elementary/middle schooler who drives it during one of our demos.

As far as making them, it was quite a task to take on. We didn't know that there would be 4 identical motors in the kit this year (a requirement for this type of drive), so we had no plans on how to do it until after our 1 week of planning. We spent nearly 4 weeks actually manufacturing the wheels including making the molds and casting the molds (casting them was quite the learning experience). There is no affordable source to buy them that we found - in fact it would probably be cheaper to draw up prints and have a machine shop make the parts. The couple places we found that sell them are all custom runs in the thousands of dollar range for a set (which would blow you way out of budget as far as the $3500 limit). The most expensive part is the labor - especially in machining a hub like 357's uses.

[the_short1]

im glad that fixed the whole pushing issue..for us, we are a strong robot we can put forward reverse VERY well, but ppl notice our omni wheels, and just push us on our side.. on that corner and we get pushed around really easy..(the omni wheels just roll). those are awsome.and a great fix to that problem... once i get home i will post a picture of those wheels in real life. (from 357). i took a picture with my camera .

me along with a few others on our team are thinking on an alternative to those wheels.. as they dont translate so fast etc..but normal omni wheels caused us problems with defence... if we find a better solutn i will post it ..

also.. would you guys (357) be willing to make a few sets of those wheels and sell them?

[Alex357]

Hey all.

Just wanted to clarify a few things. Our rollers were cast from urethane rubber, at a derometer of 80. We had to play around with the derometer a bit until we settled on this one, it gives a good balance between pushing and release of friction.

Also, the type of mecanum wheel we decided on came after looking at all of the different designs, including those used by 190. Here are the reasons we chose the type we did:
-Our wheels require a significantly less amount of parts
-The orientation of 190's rollers have them being held externally on either end. This gives significantly less ground clearence and also more exposed materials. The fact that our rollers are held on the inside give us much more clearence and keeps parts unexposed.
-The design we chose has two separate rollers at one position, while the other designs call for a single large roller. This large roller prevents friction from being released as steadily, and therefore draws more power without delivering more pushing power.

Keep the questions rolling...

[Denman]

Thats awesome... i was truley scared when i saw that video

so to slide right you put the front 2 wheels going back and the front 2 going foward?
Am impressed
Well done

[techtiger1]

Keep the questions rolling...[/quote]

lol as per request. I got another one in looking at your rollers 357 has two separate rollers, where as 190's are one whole roller. Would this make a significant difference?

Thanks again,Drew

Team 1251 The Tech tigers

2004- rookie all star award winners UCF regional
2005-finalist at UCF with 845 and 1270
2005-Palmetto Finalist with 25 and 301
2005-Palmetto Xerox creativity award.

[pakrat]

did making those wheels take up a lot of your build season time?

[Alex357]

The one roller vs. two roller difference is indeed significant. Two rollers allow for more controlled motion because of better release of friction. One roller wheels have a 50% higher coefficient of friction causing for more draw and no extra pushing power for the cost.

To answer another question, building the wheels was a very long process. We prototyped throughout the summer in our SRP (Summer Robotics Project)... our team doesn't have an off-season really. Anyway, deciding to make and use these wheels during the six weeks isn't really feasible. There's a lot that goes into the process, from hubs to molding rollers. You cant go out and buy these wheels pre-made, so there's a lot of hardworking and trial-and-error stuff that goes into it. But it's a fun process, and when you finish, you feel like you've accomplished something awesome (cause you have).

[BrianBSL]

Originally Posted by Alex357

The one roller vs. two roller difference is indeed significant. Two rollers allow for more controlled motion because of better release of friction. One roller wheels have a 50% higher coefficient of friction causing for more draw and no extra pushing power for the cost.

We haven't done any tests on the two roller model, as we decided that the machining difficulty for the hub was be far too great and supporting the rollers from only the middle would be difficult. I highly doubt that it is a 50% higher coefficient of friction - the only way it would hurt you is if one roller was going to be spinning at a significantly different speeds. I'm no ME (I'm an EE), but doesn't the coefficient of friction deal with the actual force you apply to the ground, meaning you would actually have more pushing power? I agree that the two roller design is better, obviously Airtrax had a reason for using it, but I think you are blowing it out of proportion a bit. The single roller design is much easier to manufacture, although it is more parts. (The hub is simply a piece of square 3x3 aluminum stock with the corners cut off). We had absolutely no issues with pushing power.

[techtiger1]

Thanks Alex. as the engineer from beach bots said think and have a back up plan before u get into doing this. Making these is extremely labor intensive and you have to make them from scratch I have seen prints for wheels like 357 and there a whole off season's worth of work as Alex stated.

[Alex357]

Originally Posted by BrianBSL

We haven't done any tests on the two roller model, as we decided that the machining difficulty for the hub was be far too great and supporting the rollers from only the middle would be difficult. I highly doubt that it is a 50% higher coefficient of friction - the only way it would hurt you is if one roller was going to be spinning at a significantly different speeds. I'm no ME (I'm an EE), but doesn't the coefficient of friction deal with the actual force you apply to the ground, meaning you would actually have more pushing power? I agree that the two roller design is better, obviously Airtrax had a reason for using it, but I think you are blowing it out of proportion a bit. The single roller design is much easier to manufacture, although it is more parts. (The hub is simply a piece of square 3x3 aluminum stock with the corners cut off). We had absolutely no issues with pushing power.

It's not coefficient of friction... sorry i hadn't had coffee yet. The 50% figure comes straight from the studies Airtrax performed. It's that the single roller model has 50% less release of friction. The two roller model has two independent rollers, meaning that either side can move either forward or backward, so one could be spinning in a different direction than the other allowing for more release of friction.
The one roller design does not limit pushing power, but it draws more juice to get the same pushing power as the two-wheeled design.