Bearings vs Bearing Blocks

[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.

[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.

[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.

[Knufire]

Originally Posted by Monochron

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.

5188 ran a chained drive with exact center-to-center distances this past year. We calculated the hole distance for an even amount of chain links, then added 0.018" to compensate for chain stretch. This number was suggested by Paul Copioli and confirmed by testing by 2363.

[DampRobot]

Originally Posted by Monochron

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.

My personal experience with c-c designs is limited, and a lot of the info I have is from talking to friends on other teams.

The systems I felt comfortable running exact c-c were all reasonably low torque, in manipulators. They were also overpowered, so efficiency was not a major concern. This eliminated two of my biggest worries about c-c designs.

First concern: That chains/belts could slip, or "ratchet" as they stretched over time. Basically, if they chain is a little loose, and you apply too much torque to the system, the angle on the teeth in the sprocket will push the belt/chain away from the sprocket. If the belt is loose enough and/or you apply enough torque, the chain/belt will actually fully disengage from the sprocket, and the system will slip. I was OK with this in the applications I used c-c for, because we never expected to see large torques in the system, and if we ever stalled the system, it wouldn't be the end of the world if the belt/chain slipped. Of course, repeated slipping is bad for the life of the chain/belt, especially belts. As you're often going to be stalling your drivetrain, need it to have a lot of torque, and really, really don't want your DT to break, I don't like the idea of using exact c-c DTs.

Second concern: loss of efficiency. If you're tensioning super hard on a chain/belt, there's going to be more friction. If you have sliding bearing blocks, you can dial this tension in, but if it's an exact c-c system, what you see is what you get. I was OK with potentially having a lot of friction in the system because it was overpowered for what it needed to do.

If you're belt/chain is too loose, you run into concern one. If it's too tight, you run into concern two.

Maybe I'm misreading your questions, but c-c tolerances don't directly effect chain stretch. Sure, if your c-c distance is too big, your chain will likely stretch over time, but that won't necessarily be a bad thing if your system is overtensioned.

Basically, tighter tolerances get you closer to the goldilocks zone of between concern one and two. If your application is very demanding on both sides (like DTs), you will need better tolerances. If you're OK ratcheting sometimes or losing efficiency (like in some types of intakes, for example), a c-c solution may make sense.

I don't mean to blast c-c designs. If your team can pull them off for DTs, awesome! They can be much lighter, and certainly are more simple. When I built them, I really liked them. I just didn't trust 100 to be able to pull off a perfect c-c DT when I was on the team, and doubt that the risk/reward calculus makes sense for most teams in FRC.

[Chris is me]

In terms of viability of an exact center drive design, belts and chains can't be directly compared. 25 chain absolutely does stretch over time (and sprockets wear) and thus an exact center chain drive is not always viable. In a WCD, the small sprocket sizes use combined with the loads involved make exact center chain drive a bad idea.

Exact center belt drives are a lot more viable. Belts will not stretch in an FRC robot's lifespan. If you can machine with decent accuracy, you can hit the tolerances required. If I had to make up a number, I would say +/- .005", but really it's just never been a problem for my (former) team. We just CNC the drive tubes to exact center distances and it's good enough. Basically, if you have a CNC mill, there's no reason you can't do an exact center belt drive if you wanted to.

In fact I think it's easier to mess up tension with a sliding block belt drive than an exact center drive. Exact centers are probably better than the adjustment you can do by hand, and it's easy to over or undertension a belt. I think sliding tensioners for belt drives are almost strictly worse than exact centers in my experience. Counterintuitive, I know.

It is possible to overload a belt, causing ratcheting or belt failure. A rule of thumb is for 24T pulleys or smaller in a drivetrain, you will need 15mm wide belts. The combination of 24T pulleys and 15mm belts has served my (former) team well for several seasons, not once ratcheting, failing, or otherwise ever needing maintenance at all.

Other than for retention purposes I don't think the bearing holes have to be within .002" of perfect to work for exact center belt drives - that tolerance is probably a bit tighter than required. Still not hole saw tolerances though.

[asid61]

Originally Posted by Chris is me

In terms of viability of an exact center drive design, belts and chains can't be directly compared. 25 chain absolutely does stretch over time (and sprockets wear) and thus an exact center chain drive is not always viable. In a WCD, the small sprocket sizes use combined with the loads involved make exact center chain drive a bad idea.

Teams that do chain-in tube use fixed C-C regularly. The way the sprockets are set up to force the chain against the side of the tube may account for this, but I think it's application-specific.

[1493kd]

I can attest to the simplicity and robustness of exact c2c belt drive. Chris's old team 2791 was kind enough to walk our team thru its construction and design this past season. We made use of RPI's cnc and turned out by far our best drivetrain we have ever had. Trust me in the past 1493 has built some of the worst drive trains in the history of FRC and I dont think we will be changing from belt in tube c2c for awhile.

The ability to get the hole spacing correct is 99% of the challenge.