Re: #25 Chain
 

I'm understanding you to say the failure mode is external impact damage to the cotter pin. It's not wear or normal shock loads or fatigue failure. Is that correct?

Re: #25 Chain
 

Since we run chain-in-tube, it's tricky to get a definitive answer here. But from what we can hear and make educated guesses on, external impact damage is a very real possibility. Runner-up would be shock loads; generally speaking, we didn't get enough runtime on the failed ones to make me believe it's fatigue.

We're running a versachassis with 6 8" wheels on 25 chain with standard sprockets (18t?) and haven't had any issues after a full competition with plenty of defence crossing and hard defence. We did have one failure during practice due to a tensioner backing out but it was an easy swap.

Re: #25 Chain
 

The cotter pin is part of it, but the link is just not as strong.

The zig-zag plates in an "offset link" are set up to have complex loads on them (unlike standard links that have plates that are in very simple tension loads by design - assuming your chain sprockets are aligned properly). Anything you can do to help support that load is going to make the chain stronger.

With the cotter pin type offset link, the pin is a slip fit (the pin that carries the load, not the cotter pin). This type of joint gives less support to the zig-zap plate than in the other type of offset link, where the pin is smashed/mushroomed so that the pin and the plates are more effectively joined. Again, that better joint between the pin and the zig-zag plate translates to a stronger offset link.

And this is completely separate than from the whole cotter pins are subject to mis-application when they are so small and jewelry like.

Dr. Joe J.

Re: #25 Chain
 

@ Billfred, Dr Joe: thanks for the added detail.

Re: #25 Chain
 

Very confusing. Both pictures you show are under the same part number? #6261K105

Re: #25 Chain
 

Yes, sometimes McMaster links to a family of parts page rather than a particular part page. No have why.

But, when you get to the point where you actually put the part in your cart, you can usually look at the CAD link and see what the part really looks like.

The photos are there so folks can know what I was talking about without following links. I did not intend to put this or that part number into play.

Dr. Joe J.

Re: #25 Chain
 

I don't know where Dr. Joe got the pictures, but 6261K105 is the one you want. It has the former's arrangement, even if the picture is of the latter. (I should know, I ordered 15 of them. Whether they arrived in time for Palmetto, welllllllll......)

Re: #25 Chain
 

Moly is a magical woman

Re: #25 Chain
 

Shall we do a post-mortem on our drivetrains this season?

We ran into issues with our #25 drive chain and switched to #25H, no problems since.

Re: #25 Chain
 

We (4901) had issues with our #25 chain early in the season (at Palmetto) thanks to poor quality half links. We switched to continuous chain runs made with the dark soul tool with no half links and manually tensioned it with bolts and tensioner slots. Didn't break a chain since then. I should also mention our controls mentor Ryan implemented a SLEW control onto our drivetrain to make it so that the drivers couldn't slam full forward to full reverse instantly. That was a suspected issue causing chains to snap.

The second team I worked (5632) with continued to use half and master links. They used the "good kind" with the non-cotter pin links. They broke chain very often. They tried upgrading chain type within the spectrum of #25 chain and it only made the problem happen less often instead of solving it completely. We started with Vex Pro chain. It didn't even last 5 minutes from what I was told during testing. We upgraded to a Browning (I think) HD chain and that lasted 10+ matches normally.

Re: #25 Chain
 

Repeatedly people keep mentioning keeping chain tight, which is especially important for #25 chain.

I've looked for a COTs chain tensioner that works like the tensioning device for a chain-saw bar and never seen such an item sold. I want to be able to loosen a locking nut, tighten the chain with a screw, then re-tighten the locking nut.

Does someone make a COTS robot device that can move the axle forward or back by turning a screw? Motorcycles have a similar set-up.

If you google 'roller chain tensioner' you get mostly all idler-wheel add-ons.

This would be an easy item to design and offer for a vendor and for 'dead' axles, you'll have to buy two for each axle bolt that needs adjusted, so potentially, eight on a six-wheel robot. That adds up profit fast.

Re: #25 Chain
 

The vex tensioner blocks do this with a cam. I would recommend this method as it's very easy to use.
http://www.vexrobotics.com/vexpro/al...gblocks-g.html

Or you can just wrap a zip tie around your chain run in the middle to tension it. No, I am not joking about this. It usually lasts for entire competitions as long as you are using quality zip ties.

Re: #25 Chain
 

5188 ran a #25H chain drive, with 8" pneumatic wheels and no tensioners, and it was bulletproof. We ran on a four CIM drive with a 11:72 reduction, then a 18:42 chain run to the wheel. We also added 0.018" to all the chain center-to-center distances. Chains were continuous runs made with the Dark Soul Chain Tool.

Re: #25 Chain
 

We ran straight #25 in a pretty bad way; direct center-to-center with sprockets outside the tube. That being said, we only threw a single chain the entire year, at SVR, due to the looseness getting extreme because of some screw holes widening (haven't checked it out for sure yet though). We did not break any chains. We used the Dark Soul chain tool to rejoin it, which I think helped a lot; we've killed masterlinks in the past.
That being said, if I had to do it again I would use sliding bearing blocks for tensioning and run 25H or 35 chain, just from fear. I was in a constant state of fear. :D