Re: WCD vs Standard
 

I am pretty sure that those bearings are steel not aluminum, so welding them to aluminum isn't really possible (As far as I know). but anyways, there are far easier ways to mount bearings than welding them, as Mark already said they can be pressed in or super-glued. On our robots we usually hold the bearings in with a screw and washer that covers part of the flange.

The smaller wheels allow for smaller gear ratios and sometimes fewer reductions in the gearbox, this usually results in a more lightweight gearbox. I do not know about keeping the wheels as far out as possible as a motivation for smaller wheels, I would usually try to do that anyways regardless of wheel size (just my preference no real reasoning on my part behind it).

Re: WCD vs Standard
 

So you do want to put your smaller wheels farther to the outside, to widen your wheelbase? Putting them in the same spot you would put an 8in wheel would be pointless, I am assuming, because the wheel base would be the same, but you wouldn't get better speed?

And what is "press fitting"? Could you elaborate.

Re: WCD vs Standard
 

Press fitting usually means (at least how I have interpreted it in FRC) the hole is slightly undersized such that the bearing needs to be pressed in quite hard, this will result in a tight fit that will hold the bearing in place.

Re: WCD vs Standard
 

Don't weld or weld near bearings. The grease has a bad habit of lighting on fire.


You need less gearing for 4" wheels though, and they have a lower mass (and therefore moment), so it can save you weight even if you don't put them further out on the frame.

Press fitting refers to having a hole that's just slightly smaller than the diameter of the bearing, so when you press it in, it doesn't come out. It's kind of tough to figure out what size hole to make and then make it accurately (within a few tenths).

Re: WCD vs Standard
 

Well, good thing I asked, thanks!

Re: WCD vs Standard
 

Ok, multiple things:

1. If you need to hold in a bearing that is not a press fit the standard approach is to use bearing loctite, not glue or welding. Although glue is a much better idea than welding. But why are so keen on holding in your bearings?

2. A press fit is not necessarily undersized, it could also be exact size. The difference between press and slip fit depends on surface finish as well as hole size. If you have a hole that you made exact size via a rat tail file chances are that's not going to be a slip fit. I caution you if you press in your bearings, too tight a fit can make a bearing seize up.

3. I would not suggest having a drivetrain with swap-out wheels with the size variation that you're talking about. If you have clearance for a 8" wheel but you use a 4" you're just not utilizing your frame space, the idea is to get the wheels close to the ends of the frame. There is no way you can swap out a 8" wheel with a 4" without changing the gear ratio. The fact that you want to do this makes me think that you didn't fully plan out your ratios and wheel sizes to your desired torque/speed. I suggest you calculate your drivetrains speeds using your different wheel sizes and you'll see how big of a difference wheel size makes.

4. The advantage of small wheels is that they're light, they lower your CG and they require less reduction. Don't use them unless you calculate your robots speed using them, or you might end up with a very slow bot.

Re: WCD vs Standard
 

Live axle = Axle turns, wheel is keyed to the axle.
Dead axle = Axle is fixed, wheel turns on the axle.

Re: WCD vs Standard
 

So, doing more research on CD, I have come across bearing mounts called "Sliding Bearing Mounts". What are these exactly. It seems as if they are used for chain tightening, but I have no details about how they do this.

Re: WCD vs Standard
 

Many teams build bearing blocks for their drive train which hold the bearing and axle. This bearing block can slide forward and backward in the frame (moving the wheel with it) in order to adjust chain or belt tension.

Unfortunately, 254's website is under construction, so I can't pull up any of the great pics I know they had there. I'm sure someone else here can dig up some instructive photos.

Re: WCD vs Standard
 

The "Sliding Bearing Mounts" are blocks that house the bearings for the live axle and they slide in slots in the frame. This allows you to move the axles slightly further appart tensioning the chain.

Re: WCD vs Standard
 

It's interesting to hear so many people say that canilevered, direct driven systems are the lightest and best when some of the most successful teams in the world do neither of those things (most notably 67 and 1114).

I know Jim Z has done annalysis on 254, 1114, and 67's frames. I *think* 1114's frame was lighter than 254's by about two pounds.

We used .06 sheet metal this year and will probably go down to .05 sheet metal next year. I am pretty sure that our frame weight beats out 254's by a pound or two. What get's 254's weight so far down is there use of tiny wheels which takes weight out of both their gearboxes (which are custom and very light) and wheels (also custom and very light). Direct diving helps too I'm sure.

Perhaps, some 254 people could chime in and correct me here if I'm wrong on any of these points?

In any case, I'm just stirring the pot.
Regards, Bryan

Re: WCD vs Standard
 

For the best example of a well executed sliding bearing block I would strongly suggest downloading some of 973's CAD models.

Re: WCD vs Standard
 

The idea is that the wheel, axles, sprocket, and bearings are all part of a block that slides back and forth along the chain path to adjust tension.

Here is a section view from our offseason project last fall. If you look carefully inside the tube, you can see the space the bearing block has to slide back towards the gearbox and loosen tension (to the right).

Also, here's a view of how we kept the bearing block in place. http://i.imgur.com/VrDEJ.png
It worked, but it really wasn't the greatest method. It should get you thinking though, and we came up with it halfway through building it.

Re: WCD vs Standard
 

I will side with the live axle cantilevered drivetrain not on the grounds that it "weighs less", but by the fact that maintaining one can be easier. When the wheel is supported by live axles on both sides by the frame, working on the drivetrain becomes easily more painful and irritating when frame members are in the way. With the average west coast drive, replacing components like wheels, axles, and chain is a snap because there's only one frame member to deal with.

Re: WCD vs Standard
 

I can't find a good picture of the bearing blocks themselves, but here's a shot of the rectangular holes in the frame where they go. You might get a better idea from browsing the rest of the 2012 build season album. Basically, there are two halves that come in from either side of the rail and fit inside each other, each having a bearing pressed into it. Long bolts through the rail are used to hold the two halves together, and there's a cam mounted to the rail that is torqued once the chain is installed to push the block which tensions the chain.

I wouldn't say that weight is the top factor in 254's choice of using a cantilevered drive year after year. It probably only comes in fourth after ease of maintenance, robustness, and aesthetics (you wouldn't believe how many design decisions come down to aesthetics).