Dual CIMs and #25 Chain

[Thad House]

This year was out first year using Dual CIMs for each side of our 6WD, and it has left a sour taste in our mouths. It cost us both a higher ranking in Seattle, and playing in the eliminations on Archimedes. At the beginning of the season we were using 25 chain with aluminum sprockets. During competition we were both shrinking the sprockets and snapping chain at both the master links and cleanly at other pins on the chain. In st louis we replaced the aluminum sprockets with steel sprockets. We worked great through thursday and most of friday, and then our last match friday we snapped a chain at the master link. Then on saturday morning one of the pins in the middle of the chain had a clean, crystalized snap. Every time it was the chain that went from the transmission to the center axle (we use a live axle system). Does anyone have an explanation on why this is happening. We did some testing and realized that Dual CIMs was needed this year to get up the ramp, but its left a really bad taste in our mouths and makes us not want to use it ever again unless we absolutely have to.

[Justin Stiltner]

The problem you were likely encountering is fatigue failure. You are not loading the chain to its tensile limit(although you may be) but you are coming close, and when you come close enough times the damage begins to accumulate and eventually it fails.

You should calculate the load you are putting on the chain(it will surprise you) and provide yourself a factor of safety on that so you do not exceed the abilities of the chain.

In your particular case I wager that if you were to increase the size of both of your sprockets (to maintain the same gear ratio) you would see far less chain failure. You are increasing the length of the moment arm by using a bigger sprocket, and this will reduce the tension in the chain, but provide the same speed and torque at the wheel. For example if you were previously using a 10 tooth sprocket and a 20 tooth one, change them out for a 20 and a 40.

Also, I have found and been told that half links are weaker than master links. Make sure to get a good brand chain, Diamond is a brand I trust when it matters. All #25 chain is not created equally, and I have never seen a use for #35 chain in a drive system, unless you cannot align your sprockets well enough to use #25.

Good luck

I'm real happy for you, and imma let you finish, but you're about to make the worst mistake of your entire team career.

Honestly, the CIMs have been nicknamed the drive motors for a reason. Any team anytime (except 2009 at times) not using 4 CIMs in their drive is at a HUGE disadvantage in power compared to every other team competing on the field. If all else fails, your drivetrain will still work, and with 2 CIMs you're running at half a drivetrain. Half a drivetrain vs. a full one will leave you in the dust, possibly with something broken.

I can almost assure you that your problems were completely unrelated to the use of 4 CIMs. The problem was your chains, your sprockets, anything else, but not the motors. The motors have no relation to the chain. They don't make things go faster than they normally do. Switch to #35 chain. It's stronger than #25 chain by a lot, and many teams (330 to name my favorite) have used them successfully.

This is from friend to friend, FIRSTer to FIRSTer. Making a decision like this will be as good as chopping your c-rio in two.

And I'll ask you this: Why are you blaming it on the extra CIMs? Nowhere in your passage do you say why the CIMs themselves are causing the problem. Reevaluate the event, and what was directly involved in it.

[EricH]

I wouldn't put the blame on the dual CIMs. Teams have been using that setup for years with few problems. (254, you want to chime in here?)

I'd look at your sprocket alignment first. #25 chain is quite finicky about how it's aligned--get it off by too much, and you'll have problems. Hit it right, and you'll work for quite a while with no problems.

Then, I'd look at your chain tension. Are you really loose or really tight? Either one could cause trouble. (It could also be that you had a bad batch of chain.)

And, if all else fails... #35 chain. It's more tolerant of misalignment, and tougher than #25 to some extent. But it's also heavier. You could probably use it on the "weak" link and that should solve the problems.

Like I said, I don't think it's the 2-CIM setup, or inherently a problem with the #25. I think it's alignment or tension that's causing the issue.

[Thad House]

Our thought is that we are putting too much torque on the chain when reversing directions. Either then or when hitting the bridge were when it usually broke. We have good alignment, and we try to have good tension but when looking at it today it looks like the toughbox mini's keep bending then we put a drive load on them, which looks to be changing the tension. Currently we are using 16 tooth sprockets on both ends, but might increase that next year. We are also thinking about using direct drive, because it was only ever the chain from the transmission to the center that ever broke.

[Cory]

Sprocket size is not your problem. We are using 16T AM aluminum sprockets in our drive this year.

What brand/source of chain are you using? We do not use masterlinks anymore, but we rarely if ever saw failures due to them (if we did it was usually because of shoddy installation by us) and definitely never saw pins failing in shear. You could have bad chain, but far more likely you have improper tension/alignment.

[Thad House]

Originally Posted by Cory

Sprocket size is not your problem. We are using 16T AM aluminum sprockets in our drive this year.

What brand/source of chain are you using? We do not use masterlinks anymore, but we rarely if ever saw failures due to them (if we did it was usually because of shoddy installation by us) and definitely never saw pins failing in shear. You could have bad chain, but far more likely you have improper tension/alignment.

We are using this chain
http://www.andymark.com/product-p/am-0370.htm

Our chain from our center wheel to our outer wheels never broke once. These were tensioned using cams. Our centers were tensioned by moving the transmission up with washers to raise the distance. We use e-clips for alignment, so that doesn't change. The only thing i can think of and see is that the transmissions are bending under load, which could be changing the alignment and tension.
Those AM were the same sprockets we used at the beginning, and i can get you a picture of one that is round and is missing teeth.

[Justin Stiltner]

It sounds like you have only one chain running from your 2 cim gearbox to the center wheel, and then other chains run from that center wheel to the front and back correct?

If so I think that is why you are seeing issues, with only 16 tooth sprockets your chain tension will be extremely high, and you have only one strand of chain to deal with it. If you were to run chain from the motor gearbox to the front, and another to the back, with another to the center wheel then you will have 3 chains to transmit the total output of the motor to the wheels, which will (in the case with all wheels having the same traction) reduce the tension in the chain to 1/3 of what it is now.

Also as a reliability concern, I always run separate chains to the front and rear wheels from the transmission, If one chain fails for whatever reason you still have a somewhat functioning drive train. It takes a double chain failure to stop that side.

I did a bit of math, and assuming the following
2 CIM motors in stall, 16 tooth sprocket, and 12.75:1 gear ratio on your tough boxes, you are applying 848 pounds of force to the chain and the sprocket teeth. Diamond brand roller chain in #25 size is rated to have a tensile strength of 875lbs, so it really is not much of a surprise that the chains failed at the master link, which is typically weaker than the chain it holds together. If you change to a 32 tooth sprocket the chain would only experience 427 pounds of force, which is about a 2x factor of safety.

[big1boom]

It seems that people are just skipping over the two most important posts in this thread.

Originally Posted by Justin Stiltner

I did a bit of math, and assuming the following
2 CIM motors in stall, 16 tooth sprocket, and 12.75:1 gear ratio on your tough boxes, you are applying 848 pounds of force to the chain and the sprocket teeth. Diamond brand roller chain in #25 size is rated to have a tensile strength of 875lbs, so it really is not much of a surprise that the chains failed at the master link, which is typically weaker than the chain it holds together. If you change to a 32 tooth sprocket the chain would only experience 427 pounds of force, which is about a 2x factor of safety.

So, based on the math, you WILL have fatigue failure sooner or later. (Assuming the math is correct, I don't feel like actually redoing the calculations)

Originally Posted by Al Skierkiewicz

I have seen many teams use your method to tension chain from the transmission. Often the washers allow the mounting for the transmission to shift, bend and twist under the load of reversing the drive. Without pictures this is pure conjecture but when the trans moves, the chance for the chain to climb up on the sprocket increases. Something has to give. I would bet that a close inspection of your sprockets with a bright light will show some unusual telltale marks. That is where the chain break occurred when it was caught on a high spot or tooth during the stress.

The other major problem that has been pointed out is your transmission wobbles during heavy load. This will significantly increase the amount of stress that the chain is experiencing and possibly cause a tensile failure or at the very least a fatigue failure.


My advice,
1) Make sure everything in your drivetrain is rock solid. Any amount of wobble will just exaggerate the stresses on the system and cause failure.
2) Direct drive the center wheel if at all possible, or have all the chain going off of the transmission output shaft directly.

[Thad House]

Originally Posted by big1boom

It seems that people are just skipping over the two most important posts in this thread.


So, based on the math, you WILL have fatigue failure sooner or later. (Assuming the math is correct, I don't feel like actually redoing the calculations)



The other major problem that has been pointed out is your transmission wobbles during heavy load. This will significantly increase the amount of stress that the chain is experiencing and possibly cause a tensile failure or at the very least a fatigue failure.


My advice,
1) Make sure everything in your drivetrain is rock solid. Any amount of wobble will just exaggerate the stresses on the system and cause failure.
2) Direct drive the center wheel if at all possible, or have all the chain going off of the transmission output shaft directly.

Our team plans on prototyping a direct drive this summer. I already have a cadded version, we just need to find a way to machine the side plates. Ill post both the cad of this transmission and a picture of this years drivetrain when i get home.

If we do have to use chain again, we will most likely use #35. Would anyone be willing to help with the math? Because we use 7/16 hex, the biggest sprocket we can buy is an 11 tooth for 35 chain. With this size sprockets what would be the torque load on this?

[AdamHeard]

Originally Posted by sst.thad

Our team plans on prototyping a direct drive this summer. I already have a cadded version, we just need to find a way to machine the side plates. Ill post both the cad of this transmission and a picture of this years drivetrain when i get home.

If we do have to use chain again, we will most likely use #35. Would anyone be willing to help with the math? Because we use 7/16 hex, the biggest sprocket we can buy is an 11 tooth for 35 chain. With this size sprockets what would be the torque load on this?

Why don't you post some pictures of your current setup so we can better offer help.

[Thad House]

Ok Here are some cad pictures that are accurate.

The first picture is from the top, and the second one is from the back. The sprocket coming off of the transmission is the one that is placed closest to the bearing on the pillow blocks. The transmissions are held together with the aluminum rod in the KOP. Chain runs straight down from the transmission to the axle.

[Mr. Van]

Without pictures, this is something of a guess, but I do have a suggestion.

Replace the chain that runs from the transmission to the center wheel with 35 pitch. Keep the other two chains (from the center to the outer wheels) at 25 pitch.

As was posted earlier, the chain from the transmission to the center wheel is going to have 2-3 times the tension as the other two.

Just about everyone (including most of the "WCD" crowd) uses 25 pitch chain, but they do not have a single chain taking the entire load on one side of the robot. Replace that chain with 35 and you should be good to go with little or no other modifications.

- Mr. Van
Coach, Robodox

[GRT808]

We ran into a frustrating problem similar to the one you are describing in San Diego that probably cost us the regional. We also use #25 chain from AndyMark but have switched to #25 from Mcmaster-Carr after getting some advice from The Holy Cows at SDR.

During the regional at least one chain would break and end up on the field, usually the one from our transmission to drive wheel. This happened at least 7 times at that regional.

Upon closer inspection it almost always broke at the masterlink. It took quite a bit of time to figure out what the problem was. There was a number of things that went wrong.

1: Shaft collar on the transmission output shaft was too close to the chain. If you run the wheels manually it may look fine. But flying at several hundred rpm looks quite different. The shaft collar was basically flicking the master link lock off the masterlink and then the tension on the chain bent the master link pins making the chain looked like it broke under load.

2: Make sure you install the masterlink locks on properly. Another reason why the locks flew off so easily is that we weren't installing them till they "clicked". This was a bit tricky for us to tell, the locks can go on and then stay on for a manually inspection (turning the wheels with your hands) but may fly off under faster speeds and higher loads. For both Mcmaster-carr and AndyMark links you have to make sure the lock is installed well, otherwise it will fly off during a match and then destroy your chain.

3: This was probably not the reason our chains failed during the regional, but we made sure it would not be a problem. Our bearing block bolts were not countersunk, we had a 1/4-20 button head sticking out and with the smallest amount of space from the chain. We added another spacer so that it was a good 0.125" away from the chain so it wouldn't nick the master link.

Not too sure how you broke your sprockets, we abused our AndyMark ones to the max when we ran our robot with slack chain during early testing, we never broke any gears or sprockets and I've seen many teams with 7075 sprockets that never failed them.

Hope this helps, it really sounds as though installing the locks on the masterlinks or having something too close that was contacting it may have been your problem. It was quite a puzzler for us to solve.

[Thad House]

We didnt have a shaft collar on until st louis, so I do not know if that is an issue, but we will look for sure.

For anyone that is wondering, we use a 6wd and live cantilevered axles with 3 chains coming off of the center axle. 1 going to the front, 1 going to the back, and the one closest to the bearing going straight up to the transmission. We have a 1:1 gear ratio coming off of the standard toughbox mini ratio. All sprockets are 16 tooth steel mcmaster sprockets. We also use the KOP wheels, so it is geared for about 9.5 feet per second.