I’ve come into possession of a set of AM 8" HD mecanum wheels. Now, I’m trying to figure out mounting a hub on these. I’ve got the AM 375 hex hubs and they certainly fit. However, I’ve seen several different mounting scenarios where the hub was on the same side of the wheel as the gearbox, hub was on the opposite side of the wheel and the hub axle went through the wheel and where there were two hubs to help support both sides of the wheel. I’m looking to try and keep things simple for my squad’s preseason experiments in drive trains. Can anyone offer a suggestion for mounting the hub to the wheel?
It seems to me that having the hub on the same side of the wheel as the gearbox would be OK if we’re not putting a lot of weight on the frame. Thoughts?
My team built a Mecanum chassis which had the hubs on the same side as the gearbox. The reason we did that was because the extended shaft on the hub made it so that we didn’t need spacers to prevent the screw-heads from the mecanum wheel from scratching the gearbox. We also went with a cantilevered wheel mounting, primarily because we didn’t have the resources to build our own drive shaft. It wasn’t a problem for us because the robot is meant to be a practice and demonstration chassis and nothing else (no extra weight). Apparently AndyMark does some decently comprehensive tests on all their gearboxes (prior to building, one of our members found a datasheet showing how much weight each box could handle; it was more than we needed). This probably isn’t up to par with best-practices, but it’s working so far
We used mecanum for Rebound Rumble (6"HD) and Logomotion (6") and mounted them both the same (ish) way. The hub was mounted as far inward as possible and nearly touching the toughbox nano bearing. We put the assembly as far outward as possible with a bearing support still on the outside (cutting the TBn output shaft). It worked just swimmingly both years, we even had a really hard hit off the coopertition bridge (front wheels on the ramp, back wheels off, ramp was in extreme opposite position, and we fell off [a glitch, really] with the front wheels falling 2.5’ to the ground) and hopping over the bump with no issue.
In 2010, we built a mecanum drive where the axles were cantilevered as in each of the scenarios you mentioned. We simply stuck each wheel on the output shaft of an AM Toughbox with the hubs between the wheel and the gearbox. The setup worked fine except for the fact that driving over the bumps on the field started to actually bend the gearboxes outwards since they were sitting underneath the robot with no support. You can see that robot here.
In 2011 we supported our axles on both sides, which seemed to work out just fine. Whatever solution you choose, just make sure the wheels and gearboxes are mounted very sturdily, otherwise you may see issues with uneven contact among your 4 wheels.
Hub in one side, bearing in the other is the standard method I’ve seen. The bearing doesn’t actually function as a bearing, mind, it just keeps the other side of the wheel supported on the shaft.
As a side note, the 8’’ AndyMark mecanums are…quite bad, really. We used them last year on 4464, as they were donated and money was tight, but they definitely cost us more in utility than simply buying a set of the 6’’ wheels.
Hmm… OK, thanks for the tips. As we’re just using them as a pre-season project, I suspect that we’ll be able to muddle through. However, it sounds like these wheels will not make it onto the real bot anytime soon.
… could you recommend some good ones? My team had a set of 10 inch mecanums laying around, so we built our practice chassis with them. They weigh 51lbs on their own, so, if we use mecanums in the competition, we will go down a size or two. I just saw that AndyMark revamped a few mecanum models to allow for more weight capacity and durability. Also, we’ve never really looked at the gigantic VEX Pro wheels. Any thoughts on that?
it sounds like these wheels will not make it onto the real bot anytime soon.
I don’t know if this is the case for other teams, but I think it’s going to be REALLY hard to go back to skid-steering after using Mecanum wheels… they’re so much fun
Generally, I’ve found that the same holds for mecanums as holds for pretty much all wheels - unless there are pressing design constraints that require otherwise, smaller is better (and if there are pressing design constraints requiring big wheels, you probably don’t want a mecanum drive anyway).
The standard AndyMark 6’’ mecanums right now are very satisfactory; they’re durable, don’t cause too much vibration, and are easy to service. They’ve come a long way in recent years; I remember the models from when they were introduced back in 2008, which was my first year in FRC, and the amount of maintenance they required back then was gratuitous (the plate was thin and flimsy and would bend in on the rollers, the bolts holding the rollers on were too short, and the quality control wasn’t too great).
The super high-load 6’’ AndyMark mecanums are overkill for just about all FRC applications, and they’re less versatile in terms of replacement parts as the left and right wheel assemblies are not the same. I have not seen a structural failure of the normal versions since they thickened the plate from the original 2008 specs.
I can’t speak from personal experience on the VexPro 6’’ mecanums, but I have heard good things about them from everyone who has used them.
In 2012 (rebound rumble) we used mecanum wheels for the second time in a row. We used the same code, identical gearboxes and layout. We did change from the AndyMark 6" to the Andymark 6" HD wheels, mainly for this reason. Even though we only had 6" wheels, we could go over the 4" bump, quite simply. Our mounting assemblies (1/8" alum) had ram plates in front of and behind the wheels. These were built-in at 45 degrees (above the floor) and extended heightwise from 3" to 5". We just rammed the bump with a good amount of momentum, and the ram plates deflected the end of the robot up enough to get the front (or back if reversing) wheels over the ledge. this worked surprisingly well. After 2 regionals, the orange powder coating on the ram plates hadn’t even been chipped off, and we only suffered a minor cave-in of the structure when we accidentally hit the bump at full speed. The HD wheels were chosen to withstand the impact of the robot falling from each bounce, back onto the wheels (we calculated each wheel pair [front or back] to experience +/-200lb of force upon landing). We also had the advantage of a light drivetrain compared to other mecanum bots we saw (10" mecanums are just ridiculous)
