Re: #25 Chain
 

6 CIM 2 speed 8" pneumatics 6WD Versa WCD with 25H chain on 18T sprockets. Zero issues.

Re: #25 Chain
 

After our fifth or sixth event, we found a few sprockets starting to fail. Chain was probably under-tensioned for awhile contributing to this.

25H chain, 22T sprockets, 6" wheels. It's the defenses that loaded them like this really.

Going to look into belts next year, since our failures weren't so much chain strength as much as not keeping up and constantly adjusting tension. Belts don't stretch.

Re: #25 Chain
 

They do slip and break though... :rolleyes:

Re: #25 Chain
 

A very important lesson that we learned this year is that Belts DO stretch, but it is a little nuanced.

Belts DO NOT stretch appreciably over time. ie. if your spacing is setup properly you will not need to add tension over time because the length of the belt will be constant.

Belts DO stretch instantaneously. If you shock load the belt hard enough it will flex outward in such a way that it can stretch slightly. Possibly enough to skip some teeth. I have some high speed video of this if you want to verify.

Chain DOES stretch appreciably over time. It will need to be cared for.

Chain DOES NOT stretch instantaneously in an appreciable way. As long as your mounting is done well, the length of the chain should not change during shock loads.

If anyone cares to contradict the above, please do. Right now these are all anecdotal, and I would love something more concrete.

Re: #25 Chain
 

1073 ran both belts and chains in our drivetrain this year. We had #25 chain going from the gearbox to the center wheel (it was placed above it due to space issues) and belts running from the middle wheel to the outer wheels. We didn't have any problems with the belts stretching, but we did implement some static tensioners on the #25 chain due to there being quite a bit of slack. No problems with stretching or snapped chains/belts at all

Re: #25 Chain
 

I'll put in a good word for timing belts. On 1768, we used 23 belts on our 2016 robot with great success. We used exact center-to-center distance runs with no tensioners and had zero failures all season long (5 events, 86 official matches). We used a mixture of 6mm, 9mm, and 15mm wide belts, depending on the strength required in the application. We used 15mm belts on our 8WD (picture) with 8" wheels and 60T versapulleys and never skipped a single tooth.

While chain certainly has its place (arms are a great example) and its advantages, properly used timing belts are lighter, quieter, safer, more efficient, require no lubrication, and are completely maintenance-free. Any ratcheting or failure is indicative of a design flaw, not of some unavoidable problem with belts. Whether used at exact distances like we did this year, or with tensioners as we have in the past, timing belts are fantastic for a variety of FRC applications.

Re: #25 Chain
 

What tolerances are you running for bearing placement on your drivetrain?

Re: #25 Chain
 

The OMAX waterjet our sponsor uses holds tolerances around +/-0.003". I'd say somewhere tighter than +/-0.010" is necessary to give exact distance belts proper tension. I've heard that some teams add a few thousandths to their exact distances to compensate for their manufacturing tolerances, though I've never done this.

Re: #25 Chain
 

Agreed.

We used a #25 chain, six 8" pneumatic wheel, 22 tooth Vex sprocket drivetrain this year and had no major problems. We worked hard to make sure that all of the sprockets were aligned. We also used the Dark Soul chainbreak tool to assemble the chain in order to avoid using links and it worked well. There was some slack that showed up and one chain slipped off so we had to improvise a chain tensioner using large zip ties and encountered no further problems. We were quite pleased with the results.

Now I must also add that our robot was well under the weight limit (90 lbs) so I think that helped.

In the future, depending on the nature of the game and if we are close to the weight limit, I think I'd go with #35 just for peace of mind.

Dan

Re: #25 Chain
 

I don't have the high speed camera to investigate this but I'd be curious if this was the belt stretching or a combination of, pulley shafts deflecting under high load, the teeth on the opposite pulley compressing, and the teeth on the skipping pulley compressing and bending. The way the belt is constructed I'd be much less surprised if it was the neoprene teeth deforming rather than the fiberglass tensile member stretching. I don't think they stretch much (more then negligibly) before failing but I don't know the exact numbers (elongation at break) on that.

Re: #25 Chain
 

In your belt runs, do you add anything to the nominal C-C distance? We had designed in about a 0.002 in our shooter belt (3mm GT3 belt), but since it was waterjet out, I can't be certain it held to .002 anyway. Regardless, it worked out fine for us. I've just noticed the west coast products calculator adds a bit.

Re: #25 Chain
 

The fixed c-c runs in our drivetrain were long by quite a bit, I think we've had a few runs were we've added as much as 50 thou. That was early on though and on runs of about 10 inches, when we first tried belts we tested a few different things and went with what we thought was good. Recently we've been trying to go closer to no addition, I don't know what the current situation is on that.

The way our drive has been set up in the past with 22T pulleys I don't think it's actually possible for the belts to skip without breaking teeth.

As far as gearbox belts, we've always built in some way to tension them, the current multiple mounting hole pattern has each pair vary by 5 thou I believe.

Re: #25 Chain
 

• ANSI 25: 1/4” pitch, 115 lb working load (McMaster-Carr)
• Chain stretches/wears over time
• If lengthening is significant chain will skip teeth
• If this happens, chain will need to be tensioned
• Higher tension causes chain lengthening to occur faster
• If tensioning is not used, it is important to space wheels such that a
whole number of chain links are need to span the distance
The amount of tension in the chains is largely effected by
sprocket size
• Assuming torque is fixed, a larger sprocket is equivalent to a
longer moment arm, and therefore less force (tension)
• Max torque is dependent on the amount of friction, which is a fixed
value
• To minimize tension, choose the largest sprocket that provides
enough ground clearance between the playing surface and chain

Bennett, Ben. "Drivetrain Design." Drivetrain Design (n.d.): n. pag. www.simbotics.org. 22 Oct. 2011. Web. <www.simbotics.org/files/pdf/drivetraindesign.pdf>.

Simbotics did an excellent analysis of many common drive trains used in first and the though process behind a reliable design, I encourage everyone to give it a look

Re: #25 Chain
 

Does anyone have any good info on how much #25 chain stretches over time? As chain stretch is actually wear of the roller axes, it will presumably plateau after a break-in period. That is, once it has stretched, what is the effective pitch length for the free runs between the sprockets? 0.2501"? 0.251"? 0.26"? 0.27?"

Has anyone ever tried "pre-stretching" chain as an alternative to dynamic tensioning? If so, how much tension and time is needed to get on that plateau?

Re: #25 Chain
 

We've used previously used chain before with no problem. The working life of chain is much longer than a single FRC season. However, after a season, the chain is basically as long as it's going to get.