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-   -   Dual CIMs and #25 Chain (http://www.chiefdelphi.com/forums/showthread.php?t=106244)

jwfoss 03-05-2012 08:22

Re: Dual CIMs and #25 Chain
 
One of the primary rules on FRC2168 is that we have to move. If you can't move you can't do anything.
With that we also believe that testing new things in the drivetrain in season is not allowed.

Our 2012 drivetrain consisted of:
  • 8WD
  • Non-Direct Drive
  • AM Shifters (2 CIMs Each)
  • 5"x1.5" Colson Performas (on WCP hubs)
  • Dead Axles (3/8" Grade 8 Bolts)
  • C-C distance for exact chain length
  • Spacer/Tensioner Combo (1" Nylon rod stock with non-concentric thru hole)
  • #35 Chain
  • AM Aluminum Sprocket
We built the drivetrain during week 3 of build season, and haven't done anything except check tension and grease the gearbox.

Is this fairly old school? Yes. Could we switch to #25 chain? Yes. Why haven't we? Because we know that our tolerances are not always the best, and #35 chain is really forgiving.

Based on your posts, I strongly suggest designing for c-c center distance and integrating a different tensioning method. Most problems people have with #25 chain is misalignment and improper tensioning.

robostangs548 03-05-2012 08:45

Re: Dual CIMs and #25 Chain
 
Quote:

Originally Posted by Mark Sheridan (Post 1166275)
I would recommend this tool:

http://www.team221.com/viewproduct.php?id=70


This allows you to build chain without the master link, greatly strengthening the chain.

Also, with #25 you need to tension the chain. When the chain is loose, the whipping action when the chain is pulled tight greatly increases the load on the chain. You don't want the chain to suddenly accelerate and decelerate from it being loose.

Yes, best purchase I made all season. Worth every penny.

Andrew Lawrence 03-05-2012 09:16

Re: Dual CIMs and #25 Chain
 
Quote:

Originally Posted by sst.thad (Post 1166295)
Another thing I have done for this years demos is make it so only 1 CIM is enabled until we hold a button on the controller, which is when the 2nd CIM is enabled. This will both reduce the stall torque on the chain, and make the drive less jumpy.

While that's cool, it's not fixing the problem. You need to understand that the CIMs are in NO WAY related to your chain failure. They're irrelevant to the topic. My advice: Keep all 4 CIMs running all the time, and be careful when using chain.

That's my advice. Take it or leave it as you choose.

Dad1279 03-05-2012 09:37

Re: Dual CIMs and #25 Chain
 
Quote:

Originally Posted by sst.thad (Post 1166295)
.....
Another thing I have done for this years demos is make it so only 1 CIM is enabled until we hold a button on the controller, which is when the 2nd CIM is enabled. This will both reduce the stall torque on the chain, and make the drive less jumpy.
.....

Explain the drive being jumpy?

For demos, you could switch to a less grippy wheel, so the drivetrain won't be stressed.

I don't believe you want to drive two motors that are connected together with different values. That will stress the driven motor more. We also had a situation where we tried to calibrate one of the victors on a drivetrain with the other disconnected, and blew out the disconnected victor, but I'm not sure that's related, but given the price of victors it's an experiment I don't want to repeat.

If you really want to try driving just one motor, I would either drive the second at a 'reduced' PWM value so it will be coasting, or make sure the brake jumper is not set on the coasting motor.

Can you post a picture of the chassis? Is it possible with the cantilevered axles there is flexing that is causing the mis-alignment? Are the chains near the frame, or on the other side of the frame/bearings?

Thad House 03-05-2012 10:31

Re: Dual CIMs and #25 Chain
 
Quote:

Originally Posted by Dad1279 (Post 1166366)
Explain the drive being jumpy?

For demos, you could switch to a less grippy wheel, so the drivetrain won't be stressed.

I don't believe you want to drive two motors that are connected together with different values. That will stress the driven motor more. We also had a situation where we tried to calibrate one of the victors on a drivetrain with the other disconnected, and blew out the disconnected victor, but I'm not sure that's related, but given the price of victors it's an experiment I don't want to repeat.

If you really want to try driving just one motor, I would either drive the second at a 'reduced' PWM value so it will be coasting, or make sure the brake jumper is not set on the coasting motor.

Can you post a picture of the chassis? Is it possible with the cantilevered axles there is flexing that is causing the mis-alignment? Are the chains near the frame, or on the other side of the frame/bearings?

The jumpiness is when we go to move, the robot lurches forward and sometimes the front wheels come off of the ground. Also when reversing directions the robot almost always goes onto the 2 back wheels.

When in the 1 CIM mode, the other CIM is set to coast mode via CAN, and receives a 0 output. so it is just freespinning.

We have a pillow block in the middle with a bearing on both sides. There is 2 inches of axle coming out of both sides. On 1 side is the wheel, and the other side are the 3 sprockets. The sprocket connected to the transmission is the one closest to the bearing, with the sprocket actually touching the bearing.

The only time it could become misaligned is if the transmission bend and torque down, which would move the output shaft slightly.

And the reason we are thinking 1 CIM vs 2 is that last year we used the exact same drive setup except only 1 CIM per side, and never had any issues at all. As driver I loved the 2 CIMs per side, but the chain breaking caused so many issues that i want to figure out what caused it. And everyone on my team is thinking overtorqueing the chain.

BrendanB 03-05-2012 11:14

Re: Dual CIMs and #25 Chain
 
Our team has used 6wd, 3/16in drop center, AndyMark plactions, 4cim, AndyMark Supershifters, and #35 chain with aluminum sprockets for two years. We used the standard kitbot frame our rookie year, and this year we build a custom chassis out of 1x1x1/8in aluminum box.

Using this setup, we have NEVER had an issue with throwing chain and I can't remember throwing chains on my old team of 3 years and they too used 4cims, AM Toughboxes or Shifters, and #35 chain.

As some have suggested it could be your supplier of #25 chain. We use AndyMark which I'm sure is what a majority of teams used.

We did throw one chain run on our Fall Robot that we built for a one day competition. The reason it came off was because it wasn't tensioned properly. In 2011 we used pieces of pvc to provide tension to our chains and added spacers as needed. 2012 we used pieces of pvc with springs underneath them so we just had to loosen the mounting bolts to tension our chain.

Check the alignment of your sprockets and chain runs, everything should be straight with no bending. If you don't have perfectly straight chain runs they will come off with use or when you go into high torque applications.

I would highly recommend that you do not remove a set of CIMS from your drive. The only year that teams used 1 cim per side was 2009 when the floor was a low friction surface. A four cim drive gives you more torque and is a fail safe in case you loose a cim or jaguar in a match which happens a lot more than people think.

As for your robot being very jumpy as a four cim drive. Is your center of gravity high/on the higher side? Is your robot a full 120lbs? Both of these can contribute to why your robot is jumpy with more power in your drivebase.

My personal recommendation is to upgrade to #35. You'd be surprised by the forces at work in your drivebase.

You are running a very standard setup these days (6wd w/4cims), removing two cims will not fix your problems it sounds like your problem lies in the transferring of power between wheels. If you post some detailed pictures of your drivebase when you get your robot back I'm sure we can offer more help.

Thad House 03-05-2012 11:19

Re: Dual CIMs and #25 Chain
 
Our CoG is on the slightly higher side, and we are right at the limit for weight.

We used AM chain.

Ill take a picture today and post it.

This is our first year ever having chain issues, and last year we had a VERY similar drive system with 1 cim per side instead of 2. We never had problems with that.

Ian Curtis 03-05-2012 11:23

Re: Dual CIMs and #25 Chain
 
Quote:

Originally Posted by SuperNerd256 (Post 1166353)
While that's cool, it's not fixing the problem. You need to understand that the CIMs are in NO WAY related to your chain failure. They're irrelevant to the topic. My advice: Keep all 4 CIMs running all the time, and be careful when using chain.

That's my advice. Take it or leave it as you choose.

I understand what you're getting at Andrew, and I agree that keeping 4 CIMs is probably the better choice. Saying 4 CIMs is in "NO WAY" related to the chain failure is pretty forward and probably wrong. You can put a lot more stress on your system with 4 CIMs than you can with 2.

We ran #25 chain from our center drive wheels to our outer drive wheels from '06-'08* and the only time we ever threw a chain was when we got a POOF ball stuck between our chassis rails. As others have said, #25 is fairly unforgiving to misalignment and spacing issues. Moving is the most important thing you can do in a match, so if you can't get those tolerances in line switching to #35 is probably the best choice.

*Those were before the SuperShifter, and we ran #35 from our Gen2 Shifters to the center drive wheels just to make sure we were going to be able to move no matter what.

BrendanB 03-05-2012 11:30

Re: Dual CIMs and #25 Chain
 
Quote:

Originally Posted by sst.thad (Post 1166414)
Our CoG is on the slightly higher side, and we are right at the limit for weight.

We used AM chain.

Ill take a picture today and post it.

This is our first year ever having chain issues, and last year we had a VERY similar drive system with 1 cim per side instead of 2. We never had problems with that.

This is a big factor in your jumpiness. With the four cims you have more power when accelerating so you will do so at a faster rate. If you have a slightly higher CoG your robot will "jump". The reason one cim per side won't do this is because you are accelerating slower because you have less power in your base. You could try fixing this by altering your drive program to slow down your acceleration rate from a dead stop and while reversing directions. Additionally, if you keep your CoG low, your robot will have increased maneuverability and a lower chance of tipping. The past two seasons we have kept our robots below 3ft with most of the weight within 6in. of the ground.

big1boom 03-05-2012 11:38

Re: Dual CIMs and #25 Chain
 
It seems that people are just skipping over the two most important posts in this thread.

Quote:

Originally Posted by Justin Stiltner (Post 1166294)
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)


Quote:

Originally Posted by Al Skierkiewicz (Post 1166323)
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 03-05-2012 12:36

Re: Dual CIMs and #25 Chain
 
Quote:

Originally Posted by big1boom (Post 1166424)
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?

EricH 03-05-2012 12:46

Re: Dual CIMs and #25 Chain
 
Quote:

Originally Posted by sst.thad (Post 1166394)
When in the 1 CIM mode, the other CIM is set to coast mode via CAN, and receives a 0 output. so it is just freespinning.

Freespinning can have multiple meanings. A motor that is supplying no power to the drivetrain but is connected to it fits none of them, however.

In fact, it's acting as a brake. I know you've set it to coast mode and all, but here's an experiment for you to try. Grab one CIM with minimal gearing (none, ideally). Try to backdrive it with your hands. Takes a lot of torque, right? Now apply torque at the stage of your gearbox where the second (powered) CIM comes in (or an equivalent gear ratio). The torque should be a lot less, but still very noticeable. Now, try the rotating the wheels from the same point. That should be very little torque if you're up on blocks, more if you're on the floor.

My point is, your "live" motor has to backdrive your "freespinning" motor as well as driving the wheels. It's not trivial to do so. There isn't a way around it. Your better offseason fix would probably be to run #35 chain on just the run you're having trouble on while you try to solve the root problem. It's a matter of changing sprockets and chain. (Real bonus if you try using bicycle chain--359 has done that.)

Madison 03-05-2012 13:25

Re: Dual CIMs and #25 Chain
 
Somewhat unrelated, but this thread reminded me...

do y'all still have our master links? :)

It's not a big deal one way or the other; I just forgot about giving them to you until just now.

Thad House 03-05-2012 13:30

Re: Dual CIMs and #25 Chain
 
Quote:

Originally Posted by Madison (Post 1166484)
Somewhat unrelated, but this thread reminded me...

do y'all still have our master links? :)

It's not a big deal one way or the other; I just forgot about giving them to you until just now.

Ill check tonight. I think someone took it back, but ill make sure to check around. Ill make sure you get them back.

Mr V 03-05-2012 13:37

Re: Dual CIMs and #25 Chain
 
We only use #25 chain and usually use a 4 cim drive other than for Lunacy and haven't had a problem with breakage though we direct drive our center wheels except for Breakaway. We also used #25 chain for the lift for Logo Motion w/o any problems.


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