Re: #25 Chain
 

As a team that has successfully implemented #25 chain in numerous situations, including both drive-trains and serious arm mechanisms, we switched to #35 this year for our drive after snapping #25 a few times.

A few things I've seen in our own robot:

Alignment is key. A chassis/drive pod/drive assembly must maintain that alignment under all game conditions. This includes thrashing around on a defense. A chain that looks well-aligned statically may not be aligned at all when dynamic loading is considered.

Related: if you're not using beveled sprockets, you're going to have a bad time.

Shock loading is brutal this year. With dramatically inconsistent contact with traction surfaces during defense crossing wheels will spin up, then catch, then spin up again. This dynamic behavior can potentially overload chain.

If you use #25 chain, make sure you're using #25 HD chain. Typical #25 chain is quite weak.

As mentioned we changed our drive-train to #35 this year. We are still using #25HD on our main shoulder joint though.

Extremely weak... maybe... weaker... yes.

Re: #25 Chain
 

Brian,

I wanted to comment on a couple of factors that influence chain selection that had yet to be mentioned. Others have already hit on items like larger wheel sizes, sprocket size, alignment, tension, etc.

I know that in at least one past year, 2791 has run their drive gearboxes at the corners rather than the middle of the robot. This can be great for making space for a mechanism low down in the middle of the robot, but it also means more tension on one of your chains. Instead of having 1 chain carrying torque to each corner from the center axle, you now have a chain from the gearbox corner carrying the torque for BOTH the center wheel and the far corner wheel. Not sure whether this is in play with this year's bot, but I wanted to mention this.

In 2013 and 2014, Team 20 consciously used #35 chain to reduce part count. The machining resources were certainly more than capable of hitting the tolerances necessary to run #25 chain, but would still require a proper tensioning method, likely including cams and bearing blocks. Instead we chose to directly press our bearings into the 2 x 1 x 1/8" wall tubing and use dead spacing + 0.012" to take advantage of the extra lee-way afforded by #35 chain to reduce complexity and part count. In 2013, this was with 6" AM HiGrip wheels, and in 2014 this was with 4" colsons.

We threw one chain in 2013 at least in part due to a brain fart in design (my bad...) which led to using a half link to get the proper number of chain links.* But with a direct drive center wheel, even that wasn't a big deal and was easily replaced between QF matches. No issues were noticed for 5 events thereafter.

*When doing center spacing for a chain application, design for a C-C distance with an EVEN integer number of links. An odd number means you will be using half-links.

Re: #25 Chain
 

First, as someone who's admired Grasshopper Machines for 20+ years, I am going to tell you that you really need to give JamesCH95's experience strong weight.

Almost all my experience with successful #25 chain implementations on drive trains comes before anyone used conveyor belt for tire tread and certainly before pneumatic tires with sprockets that bolt on were only a few clicks away. It may very well be the case that with these new fangled grippy tire options, young'ins these days just have to go with #35 chain for their own safety.

Amen brother JamesCH95, amen.

Dr. Joe J.

Re: #25 Chain
 

I didn't see any reference between 25 and 25H in this thread. 25H is a far better solution without the weight of 35. It is still a bear to work with, but way better than standard 25. And yes, you have to run 25 tighter than you would think. 25 is very sensitive to alignment and tension. Somewhere I was told the "7 tooth minimum" rule. No idea if it is folklore or real but it works in general. Keep 7 teeth engaged at all times. This means no drive sprockets smaller than 14 teeth on a simple drive.

Re: #25 Chain
 

I build silly tiny electric cars and race them. I have run #25 chain for hundreds of hours with motors equivalent to 6+ CIMs to big (by FRC standards, 11") pneumatic tires. and it works just fine even under shock loads like getting put into the wall at 20mph. I even did some testing with 6.5hp brushless motors spinning a 20 tooth #25 sprocket and it worked fine for FRC time scales of less than a couple of hours of operation. With that said, I would swap over to #35 if at all possible. In my experience it is much more forgiving in fabrication tolerances and dealing with the unexpected situations like those that are common in FRC like getting repeatedly rammed, stalled motors, slight shifting of sprockets etc.

Re: #25 Chain
 

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Re: #25 Chain
 

Personally, I like to overbuild everything. I don't like things breaking, and the degree to which I overbuild is directly proportional to how critical it is for success. The drive train is probably the most critical part of the robot for success. I would never use less than #35 chain for a drive train. Then again, since we made the switch a couple of years ago I'll jump through hoops to avoid using chain at all on the drive train - direct drive all the way!

Re: #25 Chain
 

IMO, #25 chain is great for mechanisms but unless you know what you're doing it's a pain for drive systems. Some teams can make it work, but most underestimate the level of difficulty to get it right. Essentially #35 chain has the benefit of added tolerance for mistakes, so as others have rightly pointed out, the quick fix for #25 chain acting up, is to just use #35 chain.

That said, issues with chains breaking, skipping, and maintenance are the main reason we no longer use chains on our drive system. Geardrive ftw. :rolleyes:

Re: #25 Chain
 

Thanks for this thread. It's very enlightening.

Does anyone have a reliable source for #25HD chain? It's a bit harder to find, from my cursory looks (the vendors we use don't sell it).

Thanks!

Re: #25 Chain
 

Re: #25 Chain
 

#25 chain is very sensitive to alignment, if one sprocket is 1/8" further out than the other, it'll bind up and have problems. Make sure you don't over-tension your chain, as this also makes it more likely to snap. There should be a little bit of slack in your chains, but not enough for the chain to be noticeably floppy. Especially with a game like this, #35 chain is definitely not overkill, and it's much less sensitive to alignment. We're using #25 for mechanisms, but #35 on the drivetrain is definitely a good option.

Re: #25 Chain
 

I would argue that if you can make a drive unit rigid enough to not mis-align gears, and precisely enough to enable proper gear meshing, then you can make a drive that can easily handle no-maintenance chain or belts reliably.

And, yes, no-maintenance (aside from lubrication, which gears need too) chain drives exist.

VexPro only sells #25HD as far as I can tell, by deduction in that they only sell #25 HD master links and state that only HD master links work with their chain. Comparing it side-by-side with other #25 chain sources makes it even more obvious.

I wish they'd state it on their website though...

Edit: boy, Dr. Joe sure knows how to make an engineer blush...

Re: #25 Chain
 

We are setup with 22T sprockets on 8" pneumatic wheels geared at 8 and 18 ft/s off of 2, 3 CIM shifting gearboxes, and #25 completely doesn't cut it.

We will be spending all of Thursday at week one tearing apart our drivetrain to replace all of the chain.

The spacing and tensioning and alignment of the chain was done absolutely perfectly, every chain had good tension without being too tight, it was by far the nicest drivetrain in team history, until we tried to go over defenses and chains started snapping.

Use #35 if at all possible, #25 really burned us this year.

Re: #25 Chain
 

I believe if you are running the 7.56" wheels in a total reduction that gets you 6 fps, I think that operates outside of the recommended range of #25 chain. We've been fine with it this year after beating up three different robots with it over the last 7 weeks but we are running a single speed north of 12 fps on 22T sprockets.

Re: #25 Chain
 

I've come to reiterate most of what has been said in this thread.

4901 ran #25 chain in tube with 17 tooth double sprockets for our drive system. If you were at Palmetto you saw first hand the time we had with it. We ended up taking the drive rails out 4 times.

What we learned is this, DON'T USE HALF LINKS. We ended up with a problem where we had to use half links to get our chain tension-ed without using a tension-er. We would break chain in 5 minutes after installing it again.

I will say if you use #25 chain is that making your chain runs continuous via Dark Soul tool or whatever other method you know of is a great idea. We managed to do so with the help of 1296 and never broke a chain again after that, of course we had to use our pre-planned tension-er slots after all.