pic: Shifting Drivetrain with Transmissions
 

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Why 6 and 13.5? That seems pretty slow. There is more room to make low gear faster while still being traction limited. We ended up with a 6fps low gear in an off-season bot last year, and found how slow it was a limit to how useful low gear was.

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With the 6 fps low gear, we get 48.7 amps per motor during a pushing match, which is why I chose that ratio. If we want to go faster, the CIM pinion could be replaced, increasing the speed to 7.38 feet per second in low gear and 15.81 feet per second in high gear, but we'd have 56 amps per motor in a pushing match.

What do you think ideal free speeds for a 2 CIM gearbox are?

I can also take high gear up to 18.27 feet per second (with low still at 6), but I think this might be too fast. Low gear could also be increased to 9.41 feet per second (71 amps/motor), but this is too high.

I can also do 6.33 low, 17.5 high, or 5.8 low 16.11 high.
If anybody is interested, here is the CAD.
https://drive.google.com/folderview?...&usp=sha ring

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Why are the wheel wells indented so much for just 1" wheels? You're greatly decreasing your available bellypan space.

What's with the double set of sprockets on each wheel? Just lazy to make spacers, or do you have a reason for doing so?

What is the reasoning behind not having your gearbox plates backed by the frame? The spacers there make me iffy about the strength. If your goal is to save space with the inverted CIMs, ask yourself if it is really necessary that you save that much space. Because I feel as if lessening your wheel wells will save plenty of extra room and you won't require a design more complex than you need. I'm sure a 2 CIM WCP DS would work perfectly in its place and save you a lot more time and trouble than this custom design is worth. If you still insist on doing this kind of transmission, I suggest you take a look at this one I made.

If you really want that low a low gear, shoot for a high gear around 15 to 16 ft/s.

If you have any more questions about drivetrains or transmissions, feel free to hit me up with a pm. This has a lot of potential, but you need to have more focus on what you want to accomplish with it and how it will fit your team's needs and resources.

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I wonder if you could get a little more size and efficiency out of your gearboxes by putting both CIMs around one idler gear that went to the cluster shaft, rather than the two idlers both going to the cluster shaft as you have now. I assume you have to use idlers because the cluster gear can't be made big enough for the CIMs to direct drive it.

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I can move the wheels out 3/8". The frame was just an example, so I just made the indent an even number.

I made the spacer a configuration of the sprocket to save time, but I forgot to make the changes in this assembly.

We used this design with a 3 CIM transmission last year and we saw no problems. Inside those spacers are two 1/4-20" grade 8 bolts that screw into threaded inserts welded into the frame. These same bolts also retain the outer plate too. It lets you take out the transmission by unscrewing two bolts and it just drops straight down out the bottom of the robot. We managed to remove a transmission, disassemble it, replace a dog, put it back together, and install it in under 30 minutes, with no practice doing this repair. This is way faster than any typical transmission, which require that there is room to slide the transmission out (which we usually don't have) in order to do maintenance.

Also, see a size comparison. https://drive.google.com/file/d/0Bzf...s3WFROeTA/view
Moving out the wheel wells saves me .875". Switching to the WCP gearboxes makes me lose over 9".

I would also argue that this design is no more complicated than the WCP gearbox, is significantly cheaper to our team, and lets us use 3.25" wheels because it has more ground clearance.

I don't want to run 3.25" wheel that wears down on the same shaft as a 3.1" OD gear. Also, this design is cheaper, and meets the needs of our team better.


The goal is for the transmission to be cheap, to have two speeds, and to be maintainable. These two transmissions would cost our team $321.71. Two WCP DS transmissions cost $520, and require that we spend $50 more on wheels. That is a $500 savings on two robots. It is also significantly easier to remove than a WCP transmission, and weighs almost the same.

I like your design, but I'm not a fan of cantilevered gears, especially with questionably fitting hex bearings. Last year, we had cantilevered gears, and we saw accelerated wear and eventually failure. Also, it seems that your design would be slightly thicker, as you do not have the cluster gear over the shifting shaft.

Yep. If I make the cluster gear bigger, it hits the dog. I've played with the gear ratios a lot, and it just isn't possible with vex gears.

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We use 5500 as our CIM free speed. Last year we ran 9FPS as our low (4 CIMs) and found this to be fine (no blown breakers, and I am pretty sure we had a couple pushing matches in there), although we didn't do a current analysis like you have done.

I personally would go with the 7.3 low, and 15.8 high given the options you have listed (assuming you are using a similar free speed to what we use), but then again we don't like slow robots. I should add that faster robots require more driver practice and good drive code to get the full advantage of them, so I would recommend picking speeds that fit your strategy/resources best.

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How a cantilevered gear right next to the plate frame any worse than the cantilever of the pinion on your CIM output shaft? Also the cluster gear fits over the area where the pneumatic cylinder is, so no space is lost. You are right that flipping the CIMs takes out CIM area, but you are forgetting that the pneumatic cylinder is there and you still need a cutout in your bellypan large enough to service with. I think you'll find that in the end you're not going to be saving as much room as you think you will be.

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Good point about the pneumatic cylinder. I didn't even notice it on yours. For space, I think it all comes down to what you're putting in the middle of the robot. For us last year, the pneumatic cylinder was low enough that it didn't get in the way of the ball, but the year before we wouldn't have been able to fit our climber in if we went with that design.

Since my first post on your gearbox thread (where I shared that cantilevered gears worked well for us), we had multiple failures of these gears.

From my experience with hex bearings and hex shafts, there is a small amount of play between the shaft and bearing. This isn't really noticeable when the gear is in between the bearings, but it becomes significant once you have the gear hanging off the side. This makes it so that the gear is always slightly in the wrong spot, and you end up with a wear pattern like the one on the remaining teeth in this gear (http://www.chiefdelphi.com/forums/at...9&d=1395019667).

The cantilevered CIM shafts are steel as opposed to aluminum, and can bend under heavy use.

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Where is the other end of that CIM output shaft? Right, securely in the motor, at the other end of the motor, where it's held securely so it just about can't move around and flex, just rotate.

If you really wanted to, you could model this as a statics problem. Draw a long beam, with a support at one end, a support near but not at the other end, and then a load at the end without the support. Now draw a short beam with one support, and the same load at the end. Apply all relevant beam equations. For bonus points, translate those loads into actual shaft loadings and see which shaft needs to be beefier to prevent bending (and uneven wear, etc.). I'd be willing to bet that the cantilever shaft supports need to be stronger, and it's likely to need a tougher shaft even though it's shorter. (general-case solution, obviously dependent on specifics of design)


Please also note carefully that this team did use cantilever last year and "saw accelerated wear and eventually failure". That statement right there indicates that for whatever reason, a cantilevered gear (presumably right next to a frame plate) is not holding up the same as a CIM pinion gear.

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Could you provide pictures of your setup? What you seem to be experiencing is very different than what I have experienced.

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Seems similar to what I was toying around with a few months ago.

Any reason why you appear to have the WCP cams with the milled-slot bearing blocks? I can't see how those work together.

Is there any support for those gearboxes other than the two visible bolts with the spacers on them? If those are it, I'd be worried about that. Not having the plate in contact with the framing member removes a lot of rigidity.

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The cams were designed to be used with the milled slot blocks.

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They were? I was under the impression they were designed for the clamp-style versa-blocks. We used the "milled-slot" blocks on 449 in a previous offseason project, and they couldn't really slide at all. Do you oversize the bolt holes to give them wiggle room?

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The design was part of the original west coast drive design originated by teams 60 and 254 around 2004ish, when the milled slot block was used as well. Milled slot + cam tensioning has been used for years up until recently when VEX developed the new VersaBlock. Of course you add a little bit of wiggle room tolerance to allow for things to slide, and use .196"-.201" slots for the bolts.

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Keep in mind traction-limited versus power-limited. IIRC, 8-10 fps is the "traction limited" range (there's a calculator on West Coast Products for this). Your high gear should be at 15+ fps and your low gear should be close to the maximum traction-limited range.

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10 fps is almost certainly not going to be traction-limited for a 4CIM drive unless you have a really light robot or really slippery wheels.

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Oh, 4 cim. I didn't notice that. :o

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It needs encoders. Encoders are usually a trivial thing to do, but on this particular design it looks like an idler sprocket, gear or wheel would have to be used?

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West Coast Drives typically put the encoder on one of the outer wheel shafts.

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So? WCD's usually do not drill/tap the ends of the shafts either, and that's the #1 thing which complicates adding an encoder to the outer wheel shaft. What would you do to mount an encoder?

For example, could one of the button head bolts could be replace with a hex head bolt, and that hex head bolt be drilled out to the diameter of a grayhill encoder shaft? What happens to the encoder if the bolt works loose?

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I don't know about other teams, but I know that we always drill the outer shaft for a 1/4" encoder shaft.

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We have a bunch of Grayhill encoders, so my plan was to put one on the back of the gearbox, and use a piece of surgical tubing as a shaft coupler between the gearbox and the encoder shaft.

254 puts one of the US digital encoders (the S4, IIRC) with ball bearings on the end of their outer wheel shaft, and just zip ties it in place.

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I'm going to guess that you drill out a hole in the axle then press fit a smaller shaft into that hole for the encoder to attach or do you guys just lathe down the end of the axle and use a collar or roll pin to keep the sprocket attached?
-Ronnie

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For encoders,

A handful of teams (254/1538/973/1323 etc..) have just drilled a hole at the end of the shaft, shove an encoder in there, and zip tie the wiring down.

http://puu.sh/bSXmC/4694259cf2.jpg

I would atleast snap ring the sprocket side and use a bolt/washer on the other side. If you don't have the resources you could always just buy the VersaChassis hex shaft:

http://content.vexrobotics.com/vexpr...awing-Rev2.PDF

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What's your experience with the 0.251 ream on the encoder bore? I've wondered how close of a fit we'll want, and we haven't tested anything yet. How easily can you insert and remove the encoder, and how well does it stay engaged with the shaft?

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Aren,

We've done a .2505 - .2510 ream for the past few years. We have never had an issue, most of the time we've been able to slide in and out with little force. We used to just drill the hole but switched to a drill + reamer to get a nicer fit.

We use the S4 with a ball bearing so there is very little drag.

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We've also had success drilling the hole out with a regular 1/4" drill

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Aloha!


Been away from the CD scene for a little bit. Turns out this thread is what our team has been working on for the off season.. So I thought it would be a good time to share it.

Transmission:
http://www.chiefdelphi.com/media/img...2202479c_l.jpg

Drivetrain:
http://www.chiefdelphi.com/media/img...40fec228_l.jpg

Here is our Tiki Tech Transmission and prototype drive frame.

The transmission plate is based on the awesome stuff from West Coast Products and Vex Pro.

This transmission is set up to run two motors on either side of the frame rail. We have designed this to use on our 2015 prototype drivetrain. It has enough clearance to go over our newly designed 1.5x3” chain and sprocket in tube frame. We have changed the mounting points to a much wider and easily accessible for quicker removal.

The system uses basically everything you get in a WCP 2 speed (DS) transmission. Plus one idler gear. Which is a duplicate gear of one that is already in the kit.

Great thing about it is that it uses any of the dog gear sets from WCP. Mix and match pinions as well, so you can get that speed ratio that suits your approach best. It is nice and compact at less than 3” (2.3 to be exact). With the pneumatics and encoder there is still lots of room in the center for use.. Battery centered is easily done!

It has an encoder mount as well. Which easily adapts to your preferred encoder.

Since this transmission plate uses WCP gears it is quite easy to setup. Just purchase their 2 speed transmission (or just the gears) and all that has to be done is turn down the input shafts to desired length (depending on if you are running the chain/belt in frame or not) and purchase the extra idler gear, input shaft and bearings. In fact when we turned down the input shafts, we just made one longer and let the one protrude past the bearing into the encode. That is how we discovered where to put the encoder mount it.. It can be done very easily with limited tools!

This new plate can be made as a solid plate or milled (as shown). Depending on your machining capabilities.

The second prototype has grooving along the outer edge for Lexan covers to keep fingers and debris out.

Questions or comments let me know, we love to hear what you think.
Enjoy!

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Looks like an awful lot of weight with that 4-gearbox setup - any particular motivation for that rather than a single 3-CIM gearbox?

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Here's an updated version with HSMXpress toolpaths: https://drive.google.com/file/d/0Bzf...ew?usp=sharing

Pictures:
https://drive.google.com/folderview?...&usp=sha ring

Free speeds - 15.81 high, 7.38 low
JVN 81% speed loss - 12.81 high, 5.98 low
3.25" versa wheel

The cost for everything (without CIM motors or the pneumatic cylinder) comes out to $730, assuming big aluminum pieces for the frame come for a reasonably priced local place.

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Those are some really good speeds for 4 cim. Hope it works out for you if you build it.

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Are you saying that 254/1538/973/1323 don't have any kind of retaining system on the interior axle shaft besides the encoder? I've always thought the shafts should be at least snapring or e-clip retained. The picture shows some kind of piece between the encoder and shaft, but I'm not sure what it is.

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Sorry in the CAD model there is a groove for a snap ring but no snap ring (I get lazy sometimes!). But yes you need a snap ring on both sides of the shaft or snap ring + bolt.