Bearings vs Bearing Blocks

[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.

[Monochron]

Originally Posted by asid61

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.

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?

[DampRobot]

Originally Posted by Monochron

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?

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.

[Monochron]

Originally Posted by DampRobot

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.

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.

[AdamHeard]

Originally Posted by DampRobot

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.

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).

[DampRobot]

Originally Posted by Monochron

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.

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.

[FrankJ]

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.

[Monochron]

Originally Posted by DampRobot

I think I confused you on cause and effect. Ratcheting is caused by loose chain, which is a consequence of chain stretch, wear, etc.

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.

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.

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

[Munchskull]

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

[Peyton Yeung]

Originally Posted by Munchskull

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

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

[AdamHeard]

Originally Posted by Munchskull

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

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.

[Dunngeon]

Originally Posted by AdamHeard

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.

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.

[Chris is me]

Originally Posted by Munchskull

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

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) .