This is my second year in college, and I’ve been looking for something to keep me busy (robots?). I decided to create a mecanum robot roughly half the size of an FRC 'bot as a testbed for a variety of mechanisms and control schemes. I figured making low-cost, easy construction 4" mecanum wheels would be a neat sub-project. Not being particularly mechanically inclined (which is partially why I’m doing this), I enlisted the help of my dad (squirrel).
The design went through a series of iterations, but stayed mostly the same. I planned on 6 rollers per wheel to keep costs as low as possible. At first, the center plate and “fingers” were made of 1/8" steel, because that’s what we have laying around. The idea was to cut slots at 45 degrees on a chop saw and weld 1/2" strap into those slots. After a short time, it became apparent that 1/2" plywood could be used for the center plate and 1/4" for the fingers. This eliminated the need for welding (wood glue now) and made cutting out pieces much simpler (table saw).
I didn’t have many ideas in the way of roller construction, but the robot we made a few years ago used wooden dowels trimmed down on a lathe, so that’s what we’re using for this. We also found that rubber stoppers are available in a variety of sizes at McMaster, so buying two sizes with slopes that approximate the curvature of the rollers might work too (and then we wouldn’t have to wrap them with anything!).
The rollers are held on by tightening two 7/32" aluminum tubes of 1/4" length over a 3" #10 threaded steel rod that passes through each finger. Washers are placed between the tube and the finger, and nuts at each end are tightened to provide a very rigid, low-friction axle for the wooden rollers (see attachment).
The current issue I’m having is the nuts are just a little too big (or the rollers too small). I would love to hear other ideas for attaching the rollers, unless making them slightly larger is the best solution.
My dad says they’ll take forever to make. He’s planning on teaching me how to use the lathe so I can make the other forty-seven rollers.
maybe try lathing them with a machinable nylon polymer? it would seem a touch easier and more consistent. in my opinion wood will not stand the test of time. (meaning the hub and spokes seem brittle and weak) but the idea seems mint!
If you play with the plywood parts some you might change you’re mind, they seem pretty strong.
But setting up the saw and jigs to make all the parts the same size seems to be a chore. I’ll look into what it would take to make them out of welded steel, too.
I like the idea of redesigning it to use the rubber stoppers that already have a hole in them.
The wheels might have to be a bit smaller than this design’s 4" diamter to work well though, as most stoppers seem to be 1" tall, and the current design requires the rollers to be about 1.25" tall
That thought had crossed my mind, but it didn’t seem like much material would be left surrounding the nut. Maybe just recessing it half the width of the nut would work, along with a slightly larger radius for the roller. Time to draw!
The hub and spokes seem strong enough, but you might be right about the rollers. After time the center of the rollers might wear down, so maybe getting some of these and press fitting another aluminum tube inside would work?
I found the cross section of two ellipses of vertical radius equal to the radius of the wheel and a horizontal radius of the wheel radius * sqrt(2). The ones in the vex picture were just estimated, I believe.
heh…I used the compound rest. I suggested that the error would be pretty low if we made the half roller as two cones. Kevin figured the angles at 6 and 16 degrees, I set the compound rest to those angles to turn it. then we sanded the lump in the middle to radius it.
Looking back at it, the dowel comes slightly smaller than an inch at the largest, and our estimations cut it down on the inside by another fraction of an inch, which may have made the end of the roller near the nut too small a diameter. Then again, I haven’t wrapped it yet… maybe I’ll go buy some tape and try that out.
T-nuts are a possibility, although that would require changing the entire setup of the rollers so the axle is live. I’m not sure if that’s a good idea with 3/16" plywood, but press fitting a small piece of the aluminum rod into the hole might give it enough strength.
Hey Matt! How’s MIT?
Mr. Forbes mentioned those as a first solution, too. It may be worth looking into. I just had a crazy idea that may or may not work, but if it’s possible to drill and tap into the end of a #10 threaded rod, maybe I can screw on a small washer onto the ends of the rod.
Sorry, should have elaborated. The blue line is the ellipse I’m currently using for the roller contour, and the purple is the equation found on the last page of the third link. Specifically, the length of the roller L is 2sqrt(7/2)" (from ellipse equation), R is 4", and r is .5". The equations are then:
D = R – r = 4-.5 = 3.5
F = (sqrt(23.5^2+(1/2sqrt(7/2))^2))
G = (sqrt(43.5^2+(1/2sqrt(7/2))^2))
T = (4sqrt(2)/ sqrt(23.5^2+(1/2sqrt(7/2))^2))
A = 32*(2r-G(T-1)) / (L^2*(T+1)^2)
But when I plot it, the roots aren’t +/- sqrt(7/2). Oh well, maybe I’m just no good at copy-pasting.
Thank you for posting a link to the video! I thought about using a coarse abrasive if we used rubber rollers, but that would work well with wood too. It makes a mess, but it’s an easy way to make the parts.
Your 10-24 threaded rod has a 0.138" minor diameter (total diameter minus the depth of the threads). This means you will need to use 4-48 or small fasteners to have any material between the inner screw and the outer threaded rod.
Tapping #4 machine screw holes in steel by hand is a pain and repeating the process 48 (or is it 96?) times would likely result in several broken taps/drill bits.
Also, I believe you get into issues with loading on the screws and threaded rod. Speaking qualitatively #4 screws are at risk of breaking during shock loading.