Bearings vs Bearing Blocks

[carpedav000]

Originally Posted by Chak

Can you further explain this point please? I don't understand how bearing blocks would help bumps, since they should be just as rigid as popping bearings in.

Rigidity is not what I was looking at. I was looking at the fact that bearing blocks can make the wheels stick farther down from the frame and give the robot more clearance between the floor and the bellypan.

And no, I did not search "bearing block" in google images.

[Michael Hill]

Originally Posted by nuclearnerd

There is an easy solution. Drill a 1.25" hole and rivet over one of these

We used something very similar. Our chassis in 2015 was made of 0.050 2024-T3, which is structurally sound, but .050 is a little thin to retain bearings, so we made up some similar parts as above out of 0.090 6061-T6 sheet to rivet on to help relieve the bearing stresses. They seemed to work out really well adding mass only where it needed to be.

[FrankJ]

Assuming you don't have a CNC handy, drilling 1-1/8 bearing holes in thin aluminum (<1/4") a step drill works well. You can pilot drill a small hole through the channel to locate the step drill. You don't the spring back or grabbing that you get with a twist drill. For anything under .1" I would recommend a doubler plate to get more thickness.

What I like about the VEX bearing blocks

1. You don't have to be near as accurate drilling holes in the channel
2. It allows for tension adjustment for belts & chains
3. It locates the hole in the Z direction.
4. bearing retainer built in
5. Greater distance between bearings gives you better support moments.
6. smaller holes in channel gives you more strength. (Since you are taking material out of the center, this is a small thing.

The biggest disadvantage is there are a lot more expensive.

[JCharlton]

We got one of these a couple of years ago. A bit fiddly but does an accurate hole through a single sheet or stacks of custom bearing blocks. At $70 don't know why I didn't buy it ages ago.

http://www.busybeetools.com/products...ank-arbor.html

[GeeTwo]

Originally Posted by JCharlton

We got one of these a couple of years ago. A bit fiddly but does an accurate hole through a single sheet or stacks of custom bearing blocks. At $70 don't know why I didn't buy it ages ago.

http://www.busybeetools.com/products...ank-arbor.html

Let me be the first to admit that I have no idea what this piece is, does, or fits into. Nothing on it looks like a cutting surface/edge to me.

[Steven Smith]

Originally Posted by GeeTwo

Let me be the first to admit that I have no idea what this piece is, does, or fits into. Nothing on it looks like a cutting surface/edge to me.

https://www.google.com/search?q=bori...eF4Gbs20_JM%3A

Sorry for messy link and brevity, on my phone. The tool is held in a mill.You can adjust the offset of the boring bar (cutter) to get a precisely dimensioned hole.

[asid61]

Originally Posted by Steven Smith

https://www.google.com/search?q=bori...eF4Gbs20_JM%3A

Sorry for messy link and brevity, on my phone. The tool is held in a mill.You can adjust the offset of the boring bar (cutter) to get a precisely dimensioned hole.

We use these to get press fits for our bearings. I can usually get +/-0.0005 with it even in a rush. They produce very round and smooth holes. I highly recommend purchasing one and learning how to use it if you have a mill.
They will not work in a drill press, however. Not tight enough. It depends on the rigidity of the mill quill.

[GeeTwo]

Originally Posted by Steven Smith

https://www.google.com/search?q=bori...eF4Gbs20_JM%3A

Sorry for messy link and brevity, on my phone. The tool is held in a mill.You can adjust the offset of the boring bar (cutter) to get a precisely dimensioned hole.

Thanks. I used that to find this link, which shows the a boring head with the boring bar installed. Three images further down on that page there is an animated gif that shows it in action.

And no, we don't have a mill. I've never actually seen a mill (or if I have, I didn't know it was a mill).

[JCharlton]

If you don't have a mill you might want to give this a try:

http://www.trick-tools.com/Slugger_S...inch_SM112_402

Note that you also need to buy the arbor (http://www.trick-tools.com/Slugger_S...rbor_18255_449) too.

I have not used one myself, and can't find out how accurate a hole they make. My guess is "good enough", especially for a team working with simple tools. [Update: one site says 0.0005!]

Step drills in thin (0.60-0.125) material doubled up and rivetted together could be another solution for you.

[MrRoboSteve]

this is available in store at Home Depot, if you know where they are kept (hint, not in power tool aisle).

[Chris is me]

Hole saws and similar hole cutters can get you a hole close to 1.125", but it's going to be inaccurate, both in size and in center position. Probably good enough position for an intake roller or something, but not good enough for a gearbox or exact centers on a drivetrain. The real problem is size though - you're going to have a really loose fit on the bearings, and they'll easily fall out. Bearing holes are almost always a circumstance where precision is warranted.

Originally Posted by Ari423

The tricks is to use a piece of plastic block (I believe we use HDPE) with a semicircle cut into it on one side. The block is bolted onto the chassis semicircle side down and the live axle sits in the semicircle. The weight of the robot and the bolts holding on the gearbox keep the axle seated in the cutout, and the plastic has so little friction that the axle turns as if the plastic were a bearing. This also allows us to easily remove the gearbox, wheel, and axle without having to remove the bearing, while avoiding cantilevering the drive axles.

Honestly, I'm glad it works for your team, but this is a really crude and sloppy method of supporting an axle that adds a non-negligible amount of friction. It's not a good solution to this problem.

[DampRobot]

Tensioning is the primary reason for using bearing blocks (or wheel trucks, or whatever you want to call them). There are a number of ways to tension your chain/belt/whatever in your drivetrain, but in my mind bearing blocks are far and away the best solution.

Solution 1: exact center to center design. I think this is what the OP is referring to in terms of "just drilling a hole," but if it was, he left out a lot of the necessary detail. Basically, you design holes into your DT frame that are exactly the diameter of your bearings, at exactly the right distance apart to keep your drive belts/chains perfectly tensioned.

Advantages: low part count, lighter, simpler. Low maintenance (potentially).

Disadvantages: very tight tolerances. You need to get bearing holes to withing -.002/+.000 IIRC (it's been a while) to get a good fit, and center to center distances probably need to be +/- .005 for belt and +/- .01 for 25 chain. (It's been a while, and I'm mostly pulling these numbers out of my behind, but these should give you an idea of the tolerances required.) If you get it wrong, you have to remake everything. Generally harder to assemble and to maintain if it breaks. It often requires a heavier drivetrain, as you must use .125" tubing to properly support the bearings instead of much lighter .0625" tubing. Getting an efficient system is pretty hit and miss.

Solution 2: Tension the belt/chain without sliding a bearing. You can put an idler in to change the chain path and adjust the tension by changing the position of the idler. You can also physically change the length of the chain belt by putting a tensioner in instead of chain links (see the 221 product, or for example the chain that moved 971's 2012 intake arm). Some teams like to shove a floating sprocket into the middle of their chain runs to spread the chain apart and tension the chain run.

Advantages: A lot lower tolerances than solution 1. You can choose exactly where you want the endpoints of your drive system. Easy to do "sloppily", so it often works well for prototypes.

Disadvantages: higher part count than solution 1, and almost always the lowest efficiency of the three solutions (you have an extra idler just adding drag). Lacks a lot of elegance. Depending on the idler design, can be more complex, and the idler can slip over time.


Solution 3: slide one of the endpoints of your system. Almost always, this means a sliding bearing block. See VersaTrucks for a COTS way to implement this system, or 254's DT for the design that continues to inspire teams. Often synonymous with WCD in DTs.

Advantages: You can dial in tension (which means efficiency) after everything is machined. Lower tolerance requirements than solution 1, more localized tolerances (for example, +/- .002 over 2", instead of over 14"). More elegant than solution 2. Easy to fix/modify. Used by a lot of top teams.

Disadvantages: higher parts counts, you can't choose exactly where both endpoints are. Sometimes requires maintenance if you don't use cams/screws to keep the bearing blocks from slipping.

Maybe I'm biased, but solution 3 always appealed the most to me. You get an efficient system that's easy to maintain and easier to machine than exact c-c designs, at a minimum cost of parts count and complexity. COTS solutions like the VersaTruck have made this so easy and accessible that many of the tolerance/machining time constraints have been eliminated.

[Knufire]

Originally Posted by DampRobot

It often requires a heavier drivetrain, as you must use .125" tubing to properly support the bearings instead of much lighter .0625" tubing.

Can 0.0625" thick tubing handle the clamping force of tradition style bearing blocks (i.e., those sold by WCP)? Gut feeling feels like it might deform the material around the edges of the milled slot.

[Monochron]

Originally Posted by DampRobot

Solution 1: exact center to center design
.
Disadvantages: very tight tolerances. You need to get bearing holes to withing -.002/+.000 IIRC (it's been a while) to get a good fit, and center to center distances probably need to be +/- .005 for belt and +/- .01 for 25 chain. (It's been a while, and I'm mostly pulling these numbers out of my behind, but these should give you an idea of the tolerances required.) If you get it wrong, you have to remake everything. Generally harder to assemble and to maintain if it breaks. It often requires a heavier drivetrain, as you must use .125" tubing to properly support the bearings instead of much lighter .0625" tubing. Getting an efficient system is pretty hit and miss.

Maybe you could shine a little light on how center to center behaves with a chain that stretches over time. For instance, we have attempted center to center with success in the past, but we have no where near the tolerances the list as "needed". Would better tolerances cause less stretch in the chain? Basically, I'm still skeptical that you can run chain without ever planning to tension it. I have heard of it being done, I'm just not sure how.

[DampRobot]

Originally Posted by Knufire

Can 0.0625" thick tubing handle the clamping force of tradition style bearing blocks (i.e., those sold by WCP)? Gut feeling feels like it might deform the material around the edges of the milled slot.

If you're talking about VersaBlock style, then yes. We ran .0625" drive frames in 2013 and 2014 (and that was Arial Assault!) with that type of wheel truck.

If you're talking about the type of bearing blocks, like the kind that 254 uses on their "OG" WCD, then I'm not sure. I know they and others (1323, eg) traditionally used .125" in their drive frame, but I'm not sure whether or not their designs (perhaps with small modifications) could handle .0625" drive tubing. If I was designing a 254 style drive, I wouldn't hesitate to use the VP .100" tubing to get weight savings.

In any case, an advantage of using bearing blocks is you can enclose the entire bearing in the block, which is a much better way to load bearings in general. Even with super thick .125" tubing supporting your bearing, you're still cantilevering half of your bearing.