Re: #25 Chain
 

Whaaaaaaat you don't have a 221/DarkSoul tool?
#25 is a total dream if you use it right, we used it wrong once after a couple former mentors tried to sell some fiction one year and of the 4 chain runs we had on that robot at Alamo in 2014, we threw all of them at least once. In 2014 because of snow we were allowed to bring in 45 pounds of withholding so we left the drivetrain out of the bag before Virginia and rebuilt the whole thing.

Every other time the chain has been properly tensioned and spaced and we have loved it.

Re: #25 Chain
 

We don't have one. We just bought one Yesterday. I was only brought into the light about that tool this week.

Re: #25 Chain
 

The recommended working load of #25HD is around 90lbf. This is fine to exceed for shorter lifespans, but with the tiny sprockets teams are running this year... :eek:

In round numbers:

8in wheel diameter
2in chain wrap diameter
µF = 1.3
Robot weight = 140lbf

Assume:
All weight on one wheel (possible, if not likely, during defense crossing)
Drive is traction-limited (certainly the case at 6ft/s with 3 CIMs per side!)

Chain tension:
140lbf*1.3*8in/2in = 730lbf

Breaking strength of #25 is around 780lbf, giving one a FoS of 1.1, and virtually guaranteeing yielding in the chain.

This case is quite conservative, but being conservative in drive-train stress considerations is probably a good idea.

For consideration: in our chain-snapping 2016 drive train the calculated tension was 550lbf. Changing to #35, with a breaking strength of about 1900lbf, did the trick. In prior years we've had chain tensions in the 150-200lbf range. Having a FoS of 3+ versus chain breaking strength is probably a good idea.

Re: #25 Chain
 

My philosophy, especially when it comes to drive trains, is, "do the math, then use the math to go for total overkill."

The importance of robustness in a drive train cannot be overstated, literally--if your robot can't drive around, you can't compete***, no matter how awesome your upper mechanical systems happen to be.

***Except last year, maybe. But let's not talk about last year, like, ever. Let's file it away in the Mental Folder of Things that Do Not Actually Exist, like HIGHLANDER 2. Ahhhh, that's better.

Re: #25 Chain
 

I think I recall a similar thread a couple of years ago which went in a different direction, with a lot of folks emphasizing the qualities of #25. My team has been using #25 for last several years with no problems, and we used it on our drive train this year. I guess we will see how well it holds up next week. Maybe I should bring sullied to do a "quick" swap in AZ if necessary.

Re: #25 Chain
 

Looking at this site here: http://www.partsforscooters.com/PFS-Chain-Chart

I was able to see a difference of thickness between #25 and #25H chain (roughly .24mm per plate, for overall thickness of about 1mm).

I am looking for a side-by-side tensile strength comparison to determine if it's truly worth making a jump from mcmaster #25 to a #25h change. If I find one, I'll post it here.

Re: #25 Chain
 

It is tough to find good manufacturer data for 25 series chain. Have a look at the table at the bottom of page 1 here.

The max allowable load column is synonymous with max recommended working load. Converted to lbf, it is 165 lbf for #25, 242 lbf for #25H. Average tensile strength is 992 lbf and 1323 lbf, respectively. These values are a little higher than I recall from past research, but at least gives you a side-by-side comparison.

We have used #25 chain successfully in drive and manipulator applications since our rookie year in 2004. I would definitely recommend #25H chain over #25 in all applications, the weight penalty is marginal for the added strength. I generally avoid #35 chain at all costs, due to weight, but if you are having failures with #25, I would say switching to 25H alone will likely be a marginal improvement - try to increase sprocket size as well. If you have the weight, consider #35 for the added reliability.

Drivetrain failures are are the worst possible failure point, as it leaves you useless on the field. Whenever possible, I encourage direct-driving at least one wheel, so that no chain is a single-point-of-failure to half your drive. If you must have chain directly from your gearbox, then I would consider #35, as failures there are unacceptable. This year, with 8WD using 8" pneumatic tires, we chose to direct drive one center wheel, use #35 to the other center wheel, and #25 to the outer corner wheels. The idea is a compromise between weight and reliability - no #25 chain failure will leave us unable to drive reasonably well, as it would only cut power transfer to one corner tire.

I think a huge part of the problem people are seeing is the dramatically higher forces involved with the larger tires paired with smaller sprockets. For years FRC drive trains have been evolving toward smaller wheels and less gearbox reduction, which pairs favorably with transitioning to #25 chain. In James' example case, if you were instead running 4" wheels, the max possible chain tension would drop to 365 lbf, and SF would increase from 1.1 to 2.1. I agree that his scenario is conservative, but I think most teams (running live-axle WCD, at least) are using even smaller sprockets than he calculated. Consider that the largest hubbed #25 sprocket Vex sells is 22T, with a pitch diameter of 1.75".

Re: #25 Chain
 

Where did you find it? Everywhere I've looked is sold out.

Re: #25 Chain
 

A google search for a "221 chain tool" (that's what we call it since all of ours are 221 branded) will lead you to 221's page for the item which they no longer sell....

...but the page has a link for the tool sold elsewhere and is available from that seller.

Re: #25 Chain
 

We usually use #25 chain in our drive, but not this year. We decided not to push our luck with 8" wheels and a field that promises to provide more drive train abuse than usual.

Re: #25 Chain
 

Thanks for the tip on how to google... Looks like Dave's Motors got them back in stock since I last checked their site.

Re: #25 Chain
 

Wasn't trying to push any buttons, just telling you how I got there since I didn't even know how to search for the current supplier's tool without knowing the name.

:)

Re: #25 Chain
 

I agree with your diagnosis but you lost me there on the recommended treatment.

Are shafts somehow immune to problems? Live axles bring there own problems. Gear teeth are not infinitely strong. And so on.

Make sure you have a reliable drive train. That is the take away. There are a lot of ways to get there, but you really need to get there.

Dr. Joe J.

Re: #25 Chain
 

True, though gears are slightly more tolerant to misalignment along the shaft axis than small chains (you just risk damage to the gears if the misalignment is too extreme). Our problems in the past usually stemmed from getting chain that had inconsistent tension (possibly due to using old chain?) and making shaft spacers for the wheels that were the correct size. As for lubrication, for external (not inside of a gearbox) gears we just spray on Teflon dry lubricant during assembly and don't worry about it again for the rest of the season.

Of course I never meant to imply no-maintainace chain systems weren't possible, just harder get right with smaller chain.

Interesting, I didn't know Vex sold #25HD. Guess next time I'll have to put an order in for some and replace all the stuff we have that's like 10 years old that used to come in the KoP. :rolleyes:

I think the point he was trying to make was that typically having a direct drive wheel eliminates total drive failure due to a chain or belt breaking. I also think that for most teams, you're FAR more likely to have a failure in a chain or a belt than you will with a gearbox output shaft (if you do, you've done something terribly wrong).

Re: #25 Chain
 

I was looking forward to switching to dry lube similar to this this year, given its the first time in many years that we are running open gearboxes. After bad signs of premature wear on the practice bot, we have played it safe by sticking with Red & Tacky, and adding covers to keep the grease from getting flung everywhere.

I'll have to make it a goal in the off-season to properly test a dry lube, so I can have confidence in it.


Chris pretty much has it dead on. Direct-drive of course does not eliminate all possible failure modes, but it at least eliminates one. If you are designing for live axles already then direct-drive is almost trivial.

Of course very reliable dead-axle drivetrains are possible without direct-drive.

Attention to design details, such as well-supported shafts, ridgid/robust frame construction, careful component selection (chain/belt type, sprocket/pulley diameter, etc.) and reliable method of chain tensioning are all still critical for success, no matter what drive style you choose. In this era where there are a selection of proven COTS gearboxes to choose from, I would generally agree with Chris that most teams will have more issues with assembly and implementation than with the gearbox components themselves.

We have run both live and dead-axle drivetrains, with varying degrees of success. In my time in FIRST, we have experienced:
- Bent drive shafts (2007, poor steel grade selection)
- Stripped gear teeth (2012, inadequate lubrication, fortunately happened post-season)
- Sheared shafts at poorly placed E-clip grooves
- Set screw failure (2007, never use set screws and D shafts in reversing load situations, and never in drive trains!)
- More chain tension issues than I can count
And many other drive failures ranging from nuisance maintenance items to crippling season-killers.

Experience can teach you what not to do, but it also proves that there are many ways to get it right.