Bearings vs Bearing Blocks
 

Is there an advantage to using bearing blocks instead of just imbedding bearings into the frame of a chassis? If so, what is said advantage?

Historically, our team has just drilled 1.125" diameter holes into the sidewall of our chassis, and then popped the bearings in, and we were done! Worked well, was light, and easy to use. Just had to make sure the holes were in the right place when drilled, but never seemed to be too big of an issue (even for a team with no machining equipment beyond a drill press and a band saw).

I notice a lot of teams with WCD use bearing blocks, and VexPro sells giant blocks to hold bearings -- what's the point? I'm sure there's some big problem that they solve or nobody would use them, so I'm curious.

Re: Bearings vs Bearing Blocks
 

I only find bearing blocks advantageous in two ways:
1.) when there is some form of bump in the field (like the scoring platforms this year)

2.) it is generally easier to make drivetrain repairs

Re: Bearings vs Bearing Blocks
 

There is a third reason to use bearing blocks. And a fourth that is connected to that third one.

3. Because if you don't drill the holes to the right size, the bearing can fall out. (big ouch) Or it doesn't go in, requiring a bigger drill, which increases the risk of oversizing the hole. Bearing blocks have the hole drilled for you (and generally speaking, right the first time).

4. In case of chain tensioning by sliding the bearing around, the blocks often have built-in tensioners. Oh, and did I mention that this sort of tensioning system generally needs some form of oversized hole/slot (see #3), so a bearing in a hole doesn't work as well?

*Note: #4 does not apply to "tensioning by sliding transmission", which is another possible method.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

Using bearing blocks doesn't really have anything to do with obstacles on the field, or being more able to drill a hole in aluminum plate versus tubing, or even ease of maintenance.

First, COTS bearing blocks have the advantage of a precision bore to retain bearings - for teams without machining resources, this is simply the only way they're going to pull off a reliable live axle drivetrain.

Bearing blocks are most often used when you are using a sliding block tensioning system. A sliding bearing block tensioning system moves the axles away from the gearbox until the power transmission is properly tensioned. This allows for perfect tension, adjustable over time, without any loss in efficiency as would be the case with a chain idler or other system.

Additionally, single piece bearing blocks allow for perfect concentricity between the bearings. This improves efficiency greatly. Double piece bearing blocks such as the VersaBlock rely on counterbores and other geometric features to ensure concentricity.

Re: Bearings vs Bearing Blocks
 

This is the reason we have used them. Ordering a precision made piece and then adapting it to whatever you need is really powerful. Found out we need to move the shaft .5" further forward? No need to get new pieces and drill new holes, just slide the block down by .5".

Drilling a hole for a bearing is like soldering your electrical connections. Don't get it right the first time and you'll have to rework things a lot more.

That said, drilling a hole can be a lot cheaper and simpler if you have the resources and tolerances.

Re: Bearings vs Bearing Blocks
 

I believe that this refers to using actual bearing blocks, not just sheets of aluminum pre-drilled. Something like asking Google Images for pictures of bearing blocks.

Re: Bearings vs Bearing Blocks
 

Well put.

This is actually a fairly annoying manufacturing problem for a lot of teams. 1.125" drill bits aren't exactly commonly lying around. Step drill bits and metal hole saws will get you there, but not very accurately in terms of either location or diameter. I think the way 90% of FRC teams solve this is by milling their holes to size as most things with a 1.125" hole in them are done on a CNC of some sort.

The best way is probably to center-drill, drill that out with a variable-diameter bit to something like 1.12", then ream to desired fit (1.124", for example). The tricky thing is, even if you're willing to invest over $100 in a reamer, a lot of common school shop drill presses can't deal with the 1/2" shank. A hand reamer can do the job as well without the drill press, but they are also expensive and can be difficult to use.

Perhaps there's an obvious, easy solution I'm missing.

I think this is an unnecessarily snarky response when the object you're alluding to is often called a "pillow block", and when several FRC specific COTS products that do not have this functionality are called "bearing blocks". It's easy to understand how someone would have trouble associating bearing blocks in an FRC context with increasing ground clearance.

(I do realize that "pillow block" doesn't exclusively mean a bearing block where the bearing sits below / above the flange bolting the block to its surface, I've just seen it used as shorthand in an FRC context like that)

Re: Bearings vs Bearing Blocks
 

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

Re: Bearings vs Bearing Blocks
 

I thought we were talking about the vexpro bearing blocks here.
Even so, I still don't see how bearings blocks can help you go over bumps, whether it be the vexpro variety or a professional "pillow block".

Re: Bearings vs Bearing Blocks
 

Actually for the past three years we, using only a handsaw and hand drill, have built live axle drivetrains without using bearing blocks. 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.

This may be a confusing explaination and I don't have any pictures from home, but if anyone is interested I can try to explain in more detail and get pictures the next time I'm in the shop.

Re: Bearings vs Bearing Blocks
 

We use bearing blocks for slots or other moving connections. Otherwise we just use plates with mounting holes, or bore directly into aluminum box tubing when it's applicable.
It really depends on the application.
We generally aim for a 1.124" or 1.123" hole (the latter for undersized measurements on bearings) by boring it out on the Bridgeport mill, manually. It's arduous but generally only takes around 5-6 minutes with an experienced student. I dream of one day having a 1.124" reamer and 1-7/64" drill bit to do bearing bores. We could go to 1.125", a regular size, but then we'd probably have to locktite it.

Re: Bearings vs Bearing Blocks
 

1678 used to use a 1-1/8th wood cutting spade bit that I hand ground down a little on up to 0.125 aluminum plate using our old tiny drill press(still use it for rapidly iterating prototypes) >_> then we upgraded to a step drill. Man, that was only 4 years ago... Some days I really do miss the shipping crate "machine shop".

Re: Bearings vs Bearing Blocks
 

The pillow block style can be mounted below the frame so you have more floor to frame clearance with the same diameter wheel.

Re: Bearings vs Bearing Blocks
 

We have two of these on our 2015 robot holding a shaft that is passing through a piece of rectangular tubing. Sadly the drilling of the 1.25" hole got a little harry and we needed to spend some quality time with with a file in order to get the rectangular tube smooth enough to rivet the plates on. And locating the plates on either side of the tube so that they lined up perfectly was probably more hassle than it was worth. Would have been a great place to use a VersaBlock or something similar that takes care of the locating problem.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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).

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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

Re: Bearings vs Bearing Blocks
 

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.

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.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

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.

Re: Bearings vs Bearing Blocks
 

Yeah this is what I had in mind when I asked. How is the chain not stretched so much that it needs to be replaced?

Re: Bearings vs Bearing Blocks
 

I have yet to see any really good explanations of chain stretch. That doesn't mean no one understands it (if someone does, please jump in!), but that for the most part the FRC community is interested in phenomena instead of root causes. (Never use scissor lifts, never use steel, always active maintain control of game pieces, etc. are examples of phenomenological "rules" in FRC. We are usually content with fuzzy understanding of the physical motivation behind things we observe to be true. Before anyone gets super mad at me, I'm pretty guilty of this too.)

From my experience, chain stretch has three components. One dominates over the short term, but is dominated by the other two effects over the long term.

First, there can be actual physical stretching in the chain, in the sense that there's some very large spring constant to the chain. You can probably stretch chains just a tiny bit as you're putting them together, but barring any extreme loading, this is a very small amount.

Second, the chain itself will wear over time. Grease in the tiny bearing surfaces in the chain links will dry up, and the chain links will physically wear so that the link to link distance increases very slightly. Unlike component one, this is a long term effect, and isn't reversible.

Third, the sprockets around the chain will wear. This doesn't have anything to do with the chain itself, but will manifest itself in the same way. Over time, the steel chain will wear aluminium sprockets, and the chain will become looser as the sprockets become very slightly smaller. In the absence of hard evidence, I would guess that this is the dominant effect behind long term "chain stretch."

In any case, I can't see a way in which a c-c design would stretch chain any more or less than a tensioned system set to exactly the same tension. The only difference is how you compensate for the stretch. In a sliding bearing block system, you just re-tension and you're good to go. In the case of c-c designs that asid was talking about, the wall of the tubing retains the chain, which keeps it from coming off or ratcheting on the sprockets, which makes the stretch less of an issue.

Re: Bearings vs Bearing Blocks
 

Hhm, this could be a key part of the root cause. Any ratcheting increases stretch. Therefore a system that never ratchets would stretch much slower overtime than one that ratchets ever 50 operations or so.

Re: Bearings vs Bearing Blocks
 

You also have all the backlash in every interface reacting the chain tension being pulled out (which can be a reasonable sum depending on the assembly).

In addition to that, all the parts reacting chain tension have a deflection of some amount, which for a "U" shaped frame could be pretty appreciable (or greatly cantilevered shafts).

Re: Bearings vs Bearing Blocks
 

I don't beleive the sprocket wearing is the main cause of chain stretch. Our elevator and drive chains both stretched over the course of the season, and in both cases, the black anodize coating wasn't scratched off or worn at all. Also, I've noticed that replacing old chain with new chain causes a pretty large increase in tension.

It appears to me that he chain stretch comes predominantly from the fit between the pins and the bushings loosening up over time. If you play with a new and old length of chain, you can feel the difference in flexibility.

Re: Bearings vs Bearing Blocks
 

With #25 chain, you have a good deal of links even in relatively short run.

.0001" of wear per pin quickly adds up.

Re: Bearings vs Bearing Blocks
 

I think I confused you on cause and effect. Ratcheting is caused by loose chain, which is a consequence of chain stretch, wear, etc. Chain-in-tube drives help alleviate some of the symptoms of poorly tensioned chain. If they're designed so that the chain is very close to the inside of the tubing, the chain physically can't ratchet without interfering with the wall of the tubing. Since ratcheting is the thing you actually care about in drives (along with efficiency and to a small degree backlash), if you solve the problem of ratcheting you don't have to worry quite as much about the problem of chain stretch causing a loss of tension over time. It's addressing the symptoms, rather than the underlying condition, but that doesn't mean it's not an effective solution.

Ratcheting certainly causes sprocket wear (and a lot of tooth wear on belts), and it's possible that it stretches chain more as well. However, I've almost always heard it described as an effect of loose chains/belts, not visa versa.

Re: Bearings vs Bearing Blocks
 

Chain stretch is a slight misnomer. The links don't stretch. Or more accurately the load needed to deform the links is very close to its breaking load. What you are seeing is wear in the pin & bushings. During break in the high spots wear quickly which gives you the rapid initial stretch. You take a length of worn out chain and lay it on a flat surface and push in & out you will see a lot more movement than in a new chain. Sprocket wear also looks like chain stretch.

Ratcheting is caused way too loose chain, not enough chain wrap around the sprocket, or the sprockets being in adequately supported (allows the CC distance to shrink under load.

Re: Bearings vs Bearing Blocks
 

I was just suggesting that frequent ratcheting could cause chain to stretch faster as it is a big shock load on the chain. Basically a system already in a bad state, accelerating further into that state.

Ah, so the chain could be very loose but it doesn't matter in that case.

Re: Bearings vs Bearing Blocks
 

So from my understanding C to C chain is a bad idea?

Re: Bearings vs Bearing Blocks
 

If it is chain in tube c to c chain it usually is fine.

Re: Bearings vs Bearing Blocks
 

Not necessarily.

C-C with chain and no understanding of what's going on is a bad idea.

If you plan for it and understand the variables involved it's totally doable.

Re: Bearings vs Bearing Blocks
 

Yup! 955's ran variations with #25 and #35 chain with success over the last 5 years. If you're curious Anthony (Munchskull) I'm sure you could go across Corvallis and ask for a look.

Re: Bearings vs Bearing Blocks
 

The way I see it is: If you needed this CD thread to figure out if it was viable and what variables to consider, it's probably not a safe bet for you.

Regarding pin / bushing wear (the primary contributor in chain stretch) - load is a lot less of a factor on this wear than you might expect. Use in a low load or no load state can stretch chain as well. I suspect that the wear occurs as the chain rounds the bend of the sprocket. I'm just speculating here, but I imagine you could "break in" chain by just running it in alternating dimensions on the bench for a few hours. Once the initial wear happens, chain "stretches" a lot more slowly, so you may be able to get your re-tensioning out of the way early.

In 2015, my old team (2791) ran a 6 foot long chain run (so more than 12 feet of actual chain) at exact center distances. What should surprise no one is that the chain did indeed stretch, but we only had to tension it once. We did end up offsetting our mounting holes for the elevator by one half link after the initial stretch though. All of this stretch occurred before the relatively light load was placed on the chain (chain never lifted more than 1 tote ideally) .

Re: Bearings vs Bearing Blocks
 

Can you expand on what you meant by this statement?

-Mike

Re: Bearings vs Bearing Blocks
 

What I mean by that is that it takes a degree of engineering knowledge, design knowledge, and care to run exact center chain runs (outside of a tube), and if a designer wants to use exact center chain runs, they should do some thorough research to understand the problem as best they can. If their first consideration of this idea was a suggestion in a CD thread, if they don't have the background knowledge it's probably safest to just use a simple tensioner.

I think the statement I made was too broad and sweeping. In short: know what you're doing.

Re: Bearings vs Bearing Blocks
 

I agree mostly that to build an efficient and high quality c-c chain system you should investigate the principles in depth.
I'll offer a counterpoint though in that this past year we ended up having to run our elevator c-c at the last minute when a couple of other things failed shortly before build season ended. It was always our intent to use a tensioner but when that failed, it was the quickest way forward was to just go straight c-c and it worked just fine without any design changes. We had to adjust the chain once in our two competitions (removed a link) to keep it ratchet free. While I'm sure it wasn't as efficient as it could have been (after removing the link it was certainly over tensioned), it worked well and didn't cause us any noticeable problems.

Re: Bearings vs Bearing Blocks
 

I don't understand why I keep hearing references to "outside of a tube". What difference does it make? There should be no difference in how well the shaft is supported in either scenario and your shaft deflection is basically nonexistent in both scenarios.

Re: Bearings vs Bearing Blocks
 

I think the point is that the tube keeps the chain from coming off the sprockets, even if it's not sufficiently tensioned.

Re: Bearings vs Bearing Blocks
 

I still think that will have very, very little effect on whether chain details or not. 233 has done exact center to center for ages in a tube, but a tube that is far larger than the sprocket, so there is nothing constraining the chain top-bottom from coming off.

Chain is pretty tolerant to tension. You can dramatically over tension or under tension it with no issues as long as you maintain good lateral alignment and you aren't right on the upper edge of its load rating. Teams should be focusing on good lateral alignment no matter what style chain drive they run.

There's nothing mystical about running exact c-c chain...

Re: Bearings vs Bearing Blocks
 

You're saying that ratcheting isn't a big issue even with "loose" chain as long as your lateral alignment is good?

Re: Bearings vs Bearing Blocks
 

I don't know what you'd have to even be doing to see chain ratchet on a FRC drive.

I'm not saying it can be flopping all over the place. But chain stretch isn't going to cause your chains to derail if you do exact c-c with Paul's added distance.

Re: Bearings vs Bearing Blocks
 

That would be an overstatement. Consider the chain on the sprocket being driven by the chain. The chain is exerting a torque T on the sprocket. This is exerted as a force at the pitch radius r of the sprocket. The chain is exerting a net force along its length of T/r. This means that the tension on the "pull" side of the chain is T/r greater than the tension on the back side. If the "at rest" tension in the chain is less that T/2r, the tension on the back side when applying torque is less than zero (assuming the instantaneous chain stretch is linear). That's an almost sure recipe for ratcheting unless the chain is otherwise constrained to stay on the sprocket. If the sprockets aren't essentially in the same plane, it's a certain recipe for derailment.

Edit/added:
We had it happen this year, not on a drive, but on our lift. We mounted hex shaft bearings in holes bored directly in our lift frame (with a spade bit, no less), but those did not give us any trouble. In order to accommodate chain stretch, we did not make a full loop of the chain, but secured both ends to the back of the lift plate using end point mounts copied in concept from a garage door opener. We used thumb screws, and drilled holes through the thumb tabs an appropriate size for the pin in a master link. We then secured each end of the chain to these screws with a master link. The thumb screws passed through a hole, and were secured on the back side with a nylock nut. I know they were originally built that way; I inspected them myself. However, it appears that on one of our tournament repairs, the lift was put back together with regular hex nuts (in the interest of speed), which loosened, which caused ratcheting on one side and derailment on the other. (One was apparently lined up better.) We then re-tensioned and used lock nuts again, and haven't had the problem since. That reminds me - it's time to check the tension on that demo robot again.

Re: Bearings vs Bearing Blocks
 

Its probably stemming from people seeing the walls of the tube acting as an insurance policy in that if the chain does stretch or it wasn't machined right there's a larger tolerance and the tube will keep the chains on the sprockets. Outside the tube, not so much and if the frame isn't that rigid it gets even better.

When we did C-C #35 in a plate drive in 2014 we got a lot of funny looks talking to people because for teams using the pre-2014 kitbot it was a norm to have tensioners integrated and most mentors work with machines with tensioned chains so therefore the robot needs it too. Some people talked to us like we found some type of black magic thinking every chain needed to have a built in tensioner.

You are also very correct that having proper alignment on chains is crucial as well and is often overlooked.

If you have the ability to build your drivebase with C-C (chains or belts) its a great opportunity to lose a few parts, headache, and possible failure points. There is plenty of time between now and kickoff to design and build one as a prototype.

Re: Bearings vs Bearing Blocks
 

Yes, that's exactly right. The vast majority of "chain in tube" designs I've seen recently are designed so that the inside opening of the tube is greater than the pitch diameter of the sprocket plus the height of the link as viewed from the side (H in this image), and (much) less than the sprocket outer diameter plus H plus R, where R is the hub diameter (same image). This means that the chain is free to move when it engages the sprocket, but cannot jump from one tooth to another even when there's no tension on the "back side". Cory referred above to tube larger than this, where the "in-tube" advantage does not apply:

While I cannot cite experience, I can see where C-C spacing can work for fairly short runs (where C-C is only ten or twenty full links), but I would absolutely shun C-C without a tensioning option for long runs, like a 6' lift. Maybe someone with more experience in this can better tune my gut "ten or twenty full links" to a more definite number.

Re: Bearings vs Bearing Blocks
 

100 ratcheted in 2012 when we were climbing the bridge. 6" wheels with 16T sprockets. There were more than a few stupid things going on with that drive, but chain alignment wasn't one of them. The chains were properly tensioned too.

I guess I've been too conservative in my comments. I never intended to scare people off of c-c chain runs completely. I just want to caution teams against betting your season on something that has the potential to fail miserably if you don't know what you're doing.

Re: Bearings vs Bearing Blocks
 

How did it ratchet if the chains were properly tensioned? What defines "tensioned"?

Re: Bearings vs Bearing Blocks
 

I'm assuming the issue here was too small of a sprocket.

Re: Bearings vs Bearing Blocks
 

Tension is the force applied to a member in the direction that will cause it to expand. In this case, it refers to the force applied by the sprockets to the chain when there is no work being done (usually no motion at all). Chain tension is rarely measured directly, but estimated by pushing transverse on the midddle of the chain run and watching the "rebound". Just like the string on a musical instrument, a chain under more tension will be harder to displace, will return more quickly to the center, and will oscillate more quickly. Usually we just make sure it feels right; most of us grew up with bicycles and know what a properly tensioned chain feels like. Note that the required at-rest tension does depend on the load tension, and also the length of the runs. If you want to see how well you are tensioned, the real test is to put the chain under peak load and watch the loose side of the chain (the return to the drive gearbox). If its motions don't make you worry about the chain ratcheting or jumping off, you should be good.

Re: Bearings vs Bearing Blocks
 

To echo other comments, I too have seen ratcheting multiple times, twice on my high school team's early drive trains. We were using the 2004 and 2005 kitbot back then and I believe the issue was that we mounted our gearboxes on sliding rails in order to allow us to adjust tension. Unfortunately the gearboxes would slip during matches leaving slightly looser chain. As a result we ratched in multiple matches.

On our 2015 robot we have a chain run of around 3 feet with 12 tooth sprockets on either side. It encounters no issues unless the load of the elevator increases dramatically. The chain is tensioned so as to give a nice twang if plucked but it still ratches when under a lot of load. I can check how well aligned the sprockets are. I feel like the answer is "well aligned" but I have no idea of the tolerance off the top of my head.

Re: Bearings vs Bearing Blocks
 

I was actually asking how one would measure tension. Specifically, if the chain is ratcheting but it properly tensioned, does that mean that the shafts of the sprockets are bending, or just that the chain is stretching to the point where it becomes undertensioned, or something else? Or is "tensioned" for one team different from another?

Re: Bearings vs Bearing Blocks
 

You can measure tension either by the displacement under a known deflection force, or the frequency of the "twang". If you apply a known force F at the mid-point of a span of chain of length l, and it deflects by a distance h, the tension in the chain can be figured out with a force diagram. My quick scratches tell me that if h<<l *, the tension T can be calculated as:

• T = F l / 4h

As I indicated above, at a minimum, the stationary tension in the chain should be more than half the tension differential needed to turn the sprockets at peak load; otherwise the back side will go completely slack under load.

* - and if it isn't, the chain is definitely too loose!

Re: Bearings vs Bearing Blocks
 

After thinking about this a bit... If you are ratcheting a chain drive, you have a design issue. The issue might be using too light of a chain to meet another design goal such as weight. Engineering is about trade offs after all. A couple of chain drives designs to think about that don't ratchet despite the high loads:

Bicycles especially full suspension bicycles. The slack side has a relatively light tension from the rear derailleur. Relatively light side loading will move the chain from sprocket to sprocket.

Motorcycles: You actually set a chain deflection so at rest the chain has no tension. This is needed because the C-C distance changes with suspension travel.

Re: Bearings vs Bearing Blocks
 

Chain that is ratcheting is almost always not properly tensioned. If not, you either don't have enough wrap around the sprocket (a good rule of thumb is 120 degrees), or your sprocket / shaft is deflecting significantly. Both of these should be obvious. It's also possible that your sprockets are just way too small - a 16T #25 chain sprocket going to a 6" wheel is pushing it.

Haven't used chain more than a handful of times in the last few years, but the last exact C-C run we did was very much properly aligned, but the span was so long that the chain stretched enough to start ratcheting. I agree with you that alignment is far more critical than tension and that chain in general is pretty forgiving, but I've just seen a few cases of failure in what looks on the surface like a good exact c-c chain setup. Maybe it's just crummy chain, or the runs are ambitiously long.

I've heard of teams that do c-c that just replace entire chain runs if the chain gets too loose. Other than cost and having to keep track of old vs new chain, I don't see much wrong with that approach.

While I've never done it, I've heard c-c for 35 chain is much easier. I imagine the chain's much greater load capacity reduces the effect of wear, not to mention the smaller number of links for the same length. But I really think 35 chain is overkill in most applications, you're just throwing weight away.

Re: Bearings vs Bearing Blocks
 

Vex and Andymark have it for $1/ft, so it's a very valid approach price wise.

Re: Bearings vs Bearing Blocks
 

My previous team has done that. It is pretty easy to tell an old chain loop from a new one. The side deflection on a used chain is significantly more than on a new one.