Problem with CIM-VersaPlanetary Setup (Choo-Choo motor)

[AM694]

Hi, our idea, for the choo-choo motor, was to have a modified CIM motor (shaft cut to approximately 1/2") mounted to a VersaPlanetary 50:1 reduction, and to use the key (approx. .4") as opposed to a set screw, as we thought a set screw would not be able to adequately transfer the torque.

Eventually, after a day of testing, the setup became unable to pull down the Choo-Choo. My gut feeling, from what my ears were telling me, was that something was skipping, as there was this grinding sound that seemed like something was hitting against something else.

This could possibly be because the 2mm key is failing to transfer torque to the gearbox (e.g, it's skipping or breaking somehow), it could be because of a failing CIM motor (on a previous one that broke, the CIM operated almost perfectly, but on occasion, its rotation faltered).

It was very easy to put one's hand on the "output sprocket", and rotate the VP output shaft, albeit with the knocking sound occurring.

We've tried increasing the length of the key (by artificially pushing out the CIM's keyway relative to the keyway of the gearbox with extra face plates), and that helped, but this setup needs to be more sustainable and rugged than it is. What are your thoughts? Did anyone have similar issues? Thanks a lot for your help!

[Thad House]

Have you taken the gearbox apart to see if there is any problem with the gears? Also, what ratio's are you using?

[jblay]

Have you taken the gearbox apart to see if there is any problem with the gears? Also, what ratio's are you using?

We are running a 50:1 and we have taken it apart. The problem almost surely lies in the key skipping because when we opened things up there was a mark on the shaft of the CIM all the way around that was likely made by the key spinning round and round.

Also the main issue is the free spinning of the motor which is related to the connection between the shaft of the CIM and the collar from the gearbox. We are trying to figure out a better way to connect the two than the set screw and key options that we have already played with.

I will say that the key held up for a solid amount of testing and practice before giving way.

[Exla357]

We've had a very similar problem with our choo-choo mechanism.

We have a bag motor on a 250:1 reduction and our problem has been that the orbital gears inside the box, somewhere, have been shattering. The box still functions (as the pieces of the gears are held in place by friction) but there is a skipping motion as well as some wierd noises. We had to replace it twice in the 3 days before bag day. I would suggest opening up the VersaPlanetary and checking it out.

Also, make sure you've got a hard stop on your catapult that ISN'T the choo-choo. This causes incredible torquing and can eventually break the whole assembly. We use a steel cable.

[theawesome1730]

It sounds to me like you have damaged splines on the output shaft of the planetary or broken a sun or pinion gear. You should take it apart to assess the issue to know for sure. If there are broken gears, the torque applied to the planetary exceeds its point of failure and you should look into alternate solutions.

[Lil' Lavery]

Use a motor/gearbox combination that more naturally interfaces with each other. Consider replacing your CIM motor with a 775, or replacing the VersaPlanetary with a GEM500 (http://www.andymark.com/product-p/am-0401.htm). Perhaps the fine folks at Vex can make suggestions as to how to modify a CIM to better fit their planetaries, but it is not designed to integrate with a CIM off the shelf.

[jblay]

Use a motor/gearbox combination that more naturally interfaces with each other. Consider replacing your CIM motor with a 775, or replacing the VersaPlanetary with a GEM500 (http://www.andymark.com/product-p/am-0401.htm). Perhaps the fine folks at Vex can make suggestions as to how to modify a CIM to better fit their planetaries, but it is not designed to integrate with a CIM off the shelf.

We want to get the power that a CIM provides us, the other motors won't do the job at the speeds we need if at all with our current ratio and a ratio change would mean major mounting adjustments in something that has holes setup at the base of our robot which is pretty impossible at this stage in the game.

It was working really well for a while before the problem so we are pretty confidant that if we can connect the collar to the CIM better it will hold up through competition. We just need the clever engineers of the CD world to give us a better idea than mine of, just put a pin through both.

[Thad House]

If using the correct type key, it does fit into the opening on the collar, and the only thing that would cause it to fail is if the key actually breaks, or the key moves. Maybe putting the key in, then using some Loctite 609 to hold it in place could stop it from moving.

Also, with that ratio off of a CIM, at stall you are actually over the torque rating of the gearbox. With the 10:1 gearset, it can only take 74 ft-lbs, where a CIM at that ratio can output 89 ft-lbs. So I would defiantly be worried about actually breaking the insides of the gearbox with that much reduction.

[thefro526]

We want to get the power that a CIM provides us, the other motors won't do the job at the speeds we need if at all with our current ratio and a ratio change would mean major mounting adjustments in something that has holes setup at the base of our robot which is pretty impossible at this stage in the game.

It was working really well for a while before the problem so we are pretty confidant that if we can connect the collar to the CIM better it will hold up through competition. We just need the clever engineers of the CD world to give us a better idea than mine of, just put a pin through both.

It sounds like pinning the CIM motor could be your best option at this point.

Without knowing the exact details on your setup, it's hard to give specific (and relevant) advice, but here are some ideas worth thinking about and/or questions worth asking:

Which CIM motor are you using? One of the newer ones, or one of the older ones? I'm not entirely sure when - but at one point or another (I think somewhere around 2007/2008, maybe earlier) CIM motors went from having a 'full'(ish) keyway on the output shaft, to the current milled slot sort of setup. I don't have a motor in front of me at the moment, but I'd imagine that if you cut the CIM output down to 1/2" long, then you're only getting a 1/4" key there - if you're lucky. If this is the case, finding an "older" cim motor with the full (More full) keyway might be worthwhile. Or, on the same train of thought, if you've got a halfway decent mill, you could always extend the keyway another .125-.25" towards the CIM's mounting boss, but that could get a bit tricky (Need to fixture the CIM just right). The longer keyway would allow you to run a longer key, which should be able to take a bit more abuse - assuming that the versaplanet's input isn't the limiting factor in key length. **see edit below

Otherwise, since your failure seems to be rooted in the CIM to VersaPlanet interface, you're only real option seems to be to make the interface stronger via some other mechanical method. Pinning the CIM to the input doesn't seem like a half bad idea to be quite honest - I'd imagine you can get a reasonably large pin through both shafts without issue. At that point assembly/dis-assembly might become problematic/annoying/etc, but if you've got the money/resources to have multiple motor/gearbox assemblies on hand, then it could be a non-issue.

Edit: Did some digging on older CIM motors, and came up with this drawing from ~2002. It appears that back then, CIM motors had a keyway that was ~.125" (3mm) longer than the current ones, at least when looking at where the keyway stops relative to the CIM mounting boss.

Old CIM: http://www.team1322.org/chip.jpg

Current CIM: http://files.andymark.com/CIM-motor-curve.pdf

Also, there's part of me that thinks that the 2007/2008ish BaneBot's Supplied CIM motors may also have a longer keyway too. Can't find a drawing to get conclusive proof, but it might be worth looking through old CIM motors, just in case - there are a bunch of legal part numbers that are legal, so you may get lucky and find the 'right one.'

[ryaneogilvie]

Im pretty sure (you can check the wattage) You can get more power by using the dual motor mount for versa-planetary with two 775's if it fits with your setup.

Assuming that your key is shearing you may try finding a key made of stronger material that wont shear. you might also be using the incorrect key in which you should find the correct one. ( the correct key should be a press fit in the keyway of the motor).

[roystur44]

Check to make sure the tiny tiny pin in the sun gear didn't fall out. The pin is staked in and will easily fall out. After you'll hear a grinding noise and the transmission will bind and then free rotate.

We had that problem on our intake transmissions.

To solve the high power needed for the winch we are using the dual 18V -RS-775 on our Choo Choo at 300:1 plus the 18:84 gears. It takes 8 seconds to wind but it is strong. We designed our whole ChooChoo mechanism to be a drop in module cause we know how much torque is needed for these high powered catapults. They are bound to break.

[Ether]

Im pretty sure (you can check the wattage) You can get more power by using the dual motor mount for versa-planetary with two 775's if it fits with your setup.

That depends on the load and the gearing. You can't just compare the max power ratings of the motors. You have to figure what the operating point would be, given the load and the gearing.

[Ether]

we are using the dual 18V -RS-775 on our Choo Choo at 300:1

What's the output torque rating of that 300:1 gearbox?

Stall torque of 775-18 is 113 ozin.

113*2*300 = 67,680 ozin = 352 ft_lb

[roystur44]

What's the output torque rating of that 300:1 gearbox?

Stall torque of 775-18 is 113 ozin.

113*2*300 = 67,680 ozin = 352 ft_lb





Here is our spec for our Choo Choo. We know the rating is out of spec that VexPro recommends. But it works


http://www.chiefdelphi.com/forums/attachment.php?attachmentid=16399&d=1393529241

[Ether]

We know the rating is out of spec that VexPro recommends.

What does VexPro recommend for a 300:1 setup?

[geomapguy]

What does VexPro recommend for a 300:1 setup?




http://content.vexrobotics.com/vexpro/pdf/VEXpro_VersaPlanetaryLoadRatings_20140211.pdf

[Ether]

http://content.vexrobotics.com/vexpro/pdf/VEXpro_VersaPlanetaryLoadRatings_20140211.pdf

Thanks Graham.

Roy, you said "They are bound to break". Have you guys broken one yet with this setup?

[AM694]

So a couple days back, I tried to put a hole in the CIM shaft corresponding to the position of the set screw hole on the Versa collar. This didn't work at all! Because I was using the Versa as a sort of guide for the hole I was drilling in the CIM shaft (and I did center punch it), I couldn't see that it was far too close to the edge of the shaft. By the way, the reason I was doing this was to put a roll or spring pin in through the Versa hole and the hole in the CIM shaft.

This resulted in a chipped 5/32" black oxide drill bit, a CIM shaft with a diagonal hole in it (luckily I was using an old one to test with), and all-around poor results.

Today I tried putting a tapped 6-32 set screw hole right on top of the Versa's keyway. This endeavor was more successful. The reasoning behind this try was that if we compress the key between the keyways of the CIM and Versa, it is less likely to slip.

Did anyone come up with anything similar? Did it work? Thanks a lot guys!

[Ether]

This resulted in a ... diagonal hole

What is a diagonal hole?

[Answer42]

We want to get the power that a CIM provides us, the other motors won't do the job at the speeds .

Actually two 775's would do the job. According JVN design calculator, your current setup has a stall torque of 80 foot pounds and a free speed of a little under two revolutions per second. Two 775's plugged into the dual motor input that vex pro sells+ plus swapping out the 5:1 stage you currently have for a 10:1 stage, gives you a gearbox with a free speed of a little over 2 revolutions per second, and a stall torque of 104 foot pounds. Provided you are using a half inch hex output shaft and not cantilevering the load too much, I would imagine that you'd still be safe. Especially providing that the key, and not the gearbox was failing in your current setup. But as you said you're running more torque than vex recommends already.

[AM694]

Yeah that was a bad description. It was just a really badly drilled hole...like the bit was walking even as it drilled or more likely was being forced to the side by something, though I can't imagin what.