01-12-2014 10:06

Arpan

Re: pic: 3 CIM WCD Ball Shifter CAD

Those ratios are just about perfect.

Have you considered trying to make the Cims or shafts removable to facilitate easy gearbox removal?

Also, you may want more than just the two holes in the WCD block to mount the gearbox to the tube. Two more holes higher up on the gearbox would add a lot of strength.

01-12-2014 10:40

jagoldman

Re: pic: 3 CIM WCD Ball Shifter CAD

That is a great looking gearbox!


I have a few questions though...

1) How much does it weigh?
2) What are the actual gears that are being used? The tooth count would be good, no need for product numbers.
3) How do you plan on mounting the gearbox?

01-12-2014 10:57

JesseK

Re: pic: 3 CIM WCD Ball Shifter CAD

It is as if Bryce's and R.C.'s two minds are like one. All that's missing are wave washers and an adjustable CIM mount for multiple CIM pinion options. The standoffs are slightly nested into the side plates for extra alignment support, but that's probably optional if there are other supports.

This makes me wonder if we can simply replace the output shaft and output gears on the WCP gearbox with their ball-shifting counterparts to get something that works with minimal modifications.

01-12-2014 11:14

Arpan

Re: pic: 3 CIM WCD Ball Shifter CAD

Why would you do that? What advantages do ballshifters have?

I seem to recall that ballshifters wear out faster ,but I could be wrong there.

01-12-2014 11:23

JesseK

Re: pic: 3 CIM WCD Ball Shifter CAD

Faster shift-on-the-fly. Yet I agree there are tradeoffs, and I don't me to imply I'm entertaining executing the idea, but rather mulling over its possibility.

01-12-2014 11:51

Chris is me

Re: pic: 3 CIM WCD Ball Shifter CAD

I think the lightening on the plates is just a bit too aggressive. Reducing weight is nice and all but you still want to keep your gearbox rigid and strong. I would thicken the webs and probably add another horizontal web(s) between the edge and the bearing for the first reduction. Just seems like not a lot of support to me. If you're that desperate to save a couple of ounces, lighten the gears before you lighten the sideplates that aggressively.

I would also be concerned about how little you are supporting the top CIM on this gearbox.

01-12-2014 13:16

AdamHeard

Re: pic: 3 CIM WCD Ball Shifter CAD

I have my doubts about the press fit into the end of the ballshifter shaft in such a configuration.

01-12-2014 13:40

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

The Cims are already removable. (although not very easily) I've considered making the gearbox removable, but I haven't come up with anything slick yet.
It weighs 1.9 lbs without motors and 10.5 with them according to inventor.
First stage is 12 to 54, second stage low is 24 to 60, and second stage high is 40 to 44.
It's mounted with the standard bearing block and the two bolts the are out to the sides.
The VEXpro ballshifter has a small aluminum shifting rod that tends to wear out. In this design, we would make out own out of steel for longevity.
That's just how I roll. If we're already going to be milling out the plates, why not optimize for the lowest weight reasonable?
This uses a simple one piece output shaft. Machinable on a manual mill and lathe.

Thanks for all the great responses! Keep it up!

01-12-2014 13:42

AdamHeard

Re: pic: 3 CIM WCD Ball Shifter CAD

Can you post a cross section of the shifting then?

Curious how that assembles if you've replaced their setup.

01-12-2014 13:54

Chris is me

Re: pic: 3 CIM WCD Ball Shifter CAD

What I am saying is that your weight right now is potentially lower than reasonable. As you remove so much material, the gearbox becomes less rigid, and stuff will deflect away from each other under load. The CIM mounting is especially concerning as the moment applied by the mass of the motor will tend to bend the pinions away from the cluster gear, weakening both of them.

There's no reason to lighten a gearbox so aggressively when you have all that extra material in your gears that serves so much less function than the material you've removed from the gearbox. You could still have a heavily lightened gearbox, with just more material and better motor support. You would probably even have a net weight loss if you faced the gears and drilled some holes in them well below the root of the teeth in addition to adding material to the gearbox.

01-12-2014 14:09

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

I don't see how the top motor is mounted less securely than the top motor of the WCP 3 CIM DS gearbox. In this design the plate is thinner, but the upper standoffs are closer to the CIM.

If I was going to lighten the gears, I would lighten them as much as reasonable too, but that is a different story.

01-12-2014 15:02

Jared

Re: pic: 3 CIM WCD Ball Shifter CAD

I agree with others who say that the top CIM isn't mounted well enough. Your gearbox might survive, but adding that extra support for 0.05 lbs gives you a lot more gearbox strength. The heavy CIM motor hanging off the end of the plate will cause the plate to deform, especially when your robot accelerates/decelerates really quickly during a collision.

If you look at many teams' gearbox plates, you'll notice that the plate outlines tend to be completely convex, rather than having little things sticking out. The outline is entirely made up of straight segments, and segments of circles that have their centers inside of the outline of the plate. This also adds a ton of strength, and requires the same size stock to machine from.

The top one is mounted in a weaker configuration because of the orientation of the mounting screws.

01-12-2014 15:50

RonnieS

Re: pic: 3 CIM WCD Ball Shifter CAD

The two lower bolts on the gear box should be able to go through the tubing used on drive rail. You would drill a clearance whole on the outer wall and the correct size on the interior wall.

01-12-2014 15:56

nathannfm

Re: pic: 3 CIM WCD Ball Shifter CAD

It looks like one CIM mounting bolt on each CIM is blocked by the gears, and that this problem could be solved by slightly rotating the CIMs but this might just be becuase of the angle of the render.

In yours there is much less webbing at the edges of the CIMs. Particularly the top one where there it is best to have webbing near the bottom of said CIM as this will prevent the CIM from sagging. I would recommend adding a web between the top standoffs on the back plate which would cross over the lower face of the top CIM.

01-12-2014 16:05

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

I've looked at these gearbox designs before, and I wasn't able to see how it benefited the strength of the gearbox at all. If you could be more specific about how that improves the gearbox strength, that would be really helpful.
As far as I can tell, the top CIM on the WCP 3 CIM DS gearbox is mounted in the same configuration.

01-12-2014 16:21

Jared

Re: pic: 3 CIM WCD Ball Shifter CAD

Imagine trying to twist off the little round parts that the standoff screws go through with a pair of pliers on your gearbox design, then imagine trying to do the same thing but with a 0.15" thick outline around the entire plate, so it's a convex shape. Right now, it's like a little tab that can twist off without causing the rest of the plate to deform, but if you had the convex shape, you'd have to push a lot harder and make much more of the plate deform before you break off the mount. It also makes the gearbox more rigid and ties everything back to the standoffs.

On the lower two CIMs, it appears that the mounting screws are close to vertical. When gravity tries to pull down the end of the CIM motor, it tries to pull the upper mounting screw out of the CIM.

On the upper CIM, your mounting screws are horizontal. Instead of trying to pull out the screw, gravity will deform the gearbox plate and swing the motor down. Last year, I did a simple stress analysis on a couple of our gearbox plates in SolidWorks that were really revealing with motor placement/plate design. I'll see if I can post some screenshots from those to show you what I'm talking about.

01-12-2014 16:33

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

You would have to use a ball end hex wrench to remove it currently. This could also be made easier by swapping the low gears and high gears.
I think the WCP gearbox only has one web that goes vertically to the bottom of the CIM. That is what this configuration has as well. Here is a close up of the CIM mount:
https://scontent-a.xx.fbcdn.net/hpho...e9&oe=54D168FA

01-12-2014 16:33

Arpan

Re: pic: 3 CIM WCD Ball Shifter CAD

Might just be the angle of the render , but it to me looks like those 4 bolts are in a horizontal line. This won't help; the weight of the gearbox and CIMs would be cantilevered on those 4 bolts. Adding 2 bolts above or below those 4 will add much more strength.

01-12-2014 17:09

Jared

Re: pic: 3 CIM WCD Ball Shifter CAD

I modified my gearbox plate to look more like yours and did a simple factor of safety analysis. A lower factor of safety corresponds to more load on the material, and a greater deformation of the part.
You could likely get away with what you have drawn up, but some simple improvements will yield huge strength increases. The biggest change you should make is using the 0.75" boss on the front of the CIM to help locate the CIM. It'll increase the positional accuracy of the CIM by quite a bit, and it'll make the part even stronger.

All tests were done with the the mounting holes as fixed geometry, and the only load was gravity acting on the 2.82 pound CIM motor. You can roughly double the strength of the part while adding about 0.03 pounds.

You can also see why having a convex outline helps too.

http://imgur.com/a/xTIpW

This is what we came up with last year for a 3 CIM configuration:
http://imgur.com/DSjie1M

01-12-2014 18:06

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

Thanks you! This is useful information.
I will consider these images when modifying the design.
What software did you use for this?

EDIT:
Sorry, I missed that.

01-12-2014 18:13

Chris is me

Re: pic: 3 CIM WCD Ball Shifter CAD

Honestly I think it's best practice to design a gearbox as if it was made from solid plate, then lighten after the fact. So the outline of the gearbox is defined by the edge of the CIMs, etc - then remove material. Right now it looks like you're making webs and connecting them, and it's a lot easier to mess that up than it is to go the other way. Correct me if I'm wrong though.

For those reading who want to learn, more on pocketing in this great video: https://www.youtube.com/watch?v=wGmsnD0KQMs

01-12-2014 18:33

Jared

Re: pic: 3 CIM WCD Ball Shifter CAD

I used SolidWorks with a Simulation plugin. It's a great piece of software for comparing designs and comes with a "simulation adviser" plugin that explains how to use everything.

Here's how I usually do it:
I start with a layout sketch of center to center distances, and use it to make a second sketch with outlines for bolt holes and bearings. Next, I draw in the webs as just single lines, usually between the center of two circles. I use the offset tool to create the outline of the webs. In the same sketch, I also draw the outline of the gearbox and use the offset tool again to give it thickness. Finally, I fillet and mirror everything. How else are people doing it? I really like the method described in the video.

Here's an example of my normal approach:
http://imgur.com/a/gXYzS

01-12-2014 18:43

Chris is me

Re: pic: 3 CIM WCD Ball Shifter CAD

I've pretty much always done it in a way similar to the video, though I was way worse at it before 973 RAMP came out (thanks Adam!). To me, pocketing is an extra. You make the robot unpocketed and then when you're done, you reduce weight if you have extra design and machining time. If we desperately needed to pocket to make weight, we made a robot that was too complex or we "brute forced" our way through a design problem by throwing way too much material at it. We also sometimes build parts on our practice robots unpocketed and pocket for our competition robot (though this is not really a good idea and has major flaws as a design methodology).

The best part about pocketing last is that if you do it and then you indeed do have to change something, you don't have to practically start the whole design all over again. You just suppress / delete the pocketing, make your changes to the original layout sketch, then redo the pocketing in a new feature. Sometimes you don't even have to redo it.

01-12-2014 19:05

asid61

Re: pic: 3 CIM WCD Ball Shifter CAD

On lightening:
I've tried a few different styles and I use something similar to Bryce's method nowadays. However, I start with a huge plate, cut all the holes, and then I draw lines connecting the holes and offset them. Then I cut away all the parts that aren't connecting beams or clearances around the holes. I fillet at the end.
I would love to see your ballshifting shaft. It might be harder to make than you realize if it's in one piece.

01-12-2014 19:37

AdamHeard

Re: pic: 3 CIM WCD Ball Shifter CAD

I started this way, and now prefer the method in the video that Chris linked.

We try to go with thinner plates, and skip pocketing alltogether, so the style lends itself well to that as well.

I hate parts blowing up on changes, so the connect the dots method is a no go for me.

Granted. I'm a grumpy jerk and hate tedious/manual CAD methods. I don't have the energy to brute force with dumb methods like I used to when I was younger.

01-12-2014 22:35

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

I prefer thicker plates because they support the bearing better.
I figure, if we're going to use a CNC machine to cut the plates, we may as well pocket them.

I've found that if you use formulas for as many dimensions as possible, use constraints to eliminate as many dimensions as possible, and use rule fillets instead of manual ones, then most changes won't take nearly as much effort.

01-12-2014 23:00

asid61

Re: pic: 3 CIM WCD Ball Shifter CAD

+1 on the thicker plates; I like to conterbore the bearings so they're more secure and easier to press, and they allow for flush mounting with the 2x1 easily.

I wasn't sure if you saw, but do you have a an isometric view of the ballshifter shaft?

01-12-2014 23:35

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

Here's a pic of it:
https://fbcdn-sphotos-f-a.akamaihd.n...d4841aad532276

01-12-2014 23:56

asid61

Re: pic: 3 CIM WCD Ball Shifter CAD

Ok. That doesn't look too hard to machine, but the machinist will need a collet block or indexing head... if you have only a 3-axis CNC, you will need to acquire one of those. Very nice design; it's possibly the simplest shifting shaft I've seen to date.

02-12-2014 00:57

nathannfm

Re: pic: 3 CIM WCD Ball Shifter CAD

Ah, this clears it up, most of that webbing was not visible in the original render and that makes me much happier about the support of the CIM.

This sounds like a good idea but I would caution that you should know the tolerances of that boss and the tolerances of the equipment used to cut the hole in the plate. We made our first waterjet part ever a few weeks ago with a 0.750" hole for the boss and suprise suprise it comes back and the CIM won't fit.

Would this be all steel? I know the ball capture area is steel on the VexPro model presumably for wear reasons. Also, I know it would be pretty difficult to machine but I would recommend a fillet at the hex to round transition as that looks like its a pretty big stress riser.
Related to this component, if you make the hex shaft attach to the ball shifter with a pin or through bolt instead of making it one part it would make removal of the gearbox pretty simple. Just pull the pin, retract the shaft toward the outside of the robot and pull the gearbox either out the top or bottom.

Also, just to comment on that video I prefer the "sketch fillet" tool to the 3D fillet tool as it crashes much less, trims your lines for you, and contains all your pocketing features in one "Cut-Extrude". Also, if it crashes you are already in sketch mode to fix it.

02-12-2014 01:07

asid61

Re: pic: 3 CIM WCD Ball Shifter CAD

+1. Use the boss. If you just use screws, then the cim can wobble more than a couple thousandths and lower efficiency. The boss should be used.
We used 0.752" without problems last year, allthough it was sort of by accident- I plunged with a 0.75" end mill in place of a 5/8" and thought I screwed up. But it fit perfectly!
So plunging in with a good 0.75" endmill produces an almost-perfect hole, although boring or even better over-sized reaming would be preferred if you can manage it. They have 0.751" reamers on the market for a really good fit.

02-12-2014 01:46

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

Thanks for the heads up.
The vex pro website says that ball shifter shafts are 7075-T6 aluminum.
I considered this, but I didn't want to cut a negative hex into the end of the shaft.

02-12-2014 01:48

R.C.

Re: pic: 3 CIM WCD Ball Shifter CAD

The pressed in output hex shaft is. But the actual ball shaft is steel.

02-12-2014 01:56

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

Interesting. Is the small shaft that engages the balls steel as well then? I find it hard to believe that they would use one steel and the other aluminum.
Also, here is a picture of an updated version that uses some of the suggestions posted in this thread:
https://scontent-b-sjc.xx.fbcdn.net/...e4&oe=551455E2
Thanks for everything so far. Questions and comments are still welcome of course!

02-12-2014 01:59

R.C.

Re: pic: 3 CIM WCD Ball Shifter CAD

The small "plunger"/shifter shaft is currently AL, might see an upgrade in the Future?

02-12-2014 02:16

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

Lol
Do you know if vex did any tests that suggest the ball carrier needs to be steel?

My team built a ball shifter for our shooter last year. (I'll admit it's a different situation) We experienced that the ball carrier had the least wear out of the three shifter components.

02-12-2014 03:02

Mike Marandola

Re: pic: 3 CIM WCD Ball Shifter CAD

Some teams saw extreme wear on their plungers this year.

02-12-2014 03:16

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

I know. I addressed this issue earlier.
I was not referring to that part in this instance.
the part I am talking about currently is what surrounds the plunger.

02-12-2014 03:33

Mike Marandola

Re: pic: 3 CIM WCD Ball Shifter CAD

Ah. I just realized that.

02-12-2014 10:47

Chris is me

Re: pic: 3 CIM WCD Ball Shifter CAD

I sometimes (often) fillet in 2D, but the advantage to this that I've noticed (especially if you save right before you attempt 3d fillets) is that you can suppress / delete your fillets, adjust your original design, and your pocketing is more likely to still work after the fact than if you trim and fillet in the sketch layer. It also takes a bit more work / time.

Seeing the support of the CIM in more detail makes me more comfortable in it - I guess it's just my personal preference to have a member follow the outer edge of the CIM to best support the face. You're probably good to go with what you have.

02-12-2014 12:59

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

I'm sure that adding a member would increase strength, but after the revisions I posted, I think it would be unnecessary in this case.

02-12-2014 14:13

Cory

Re: pic: 3 CIM WCD Ball Shifter CAD

To each their own, but I would consider this very poor practice. It makes the part much less transparent to anyone else who may need to work on your design and is far less flexible, as mentioned by Chris.

If you need to fix fillets, like Adam displayed in his video, you can use the fillet tool to do face fillets that will bridge the intersection of faces where two fillets intersect. Then you don't need to sketch anything.

02-12-2014 16:26

Jared

Re: pic: 3 CIM WCD Ball Shifter CAD

Excellent point. I made all our gearbox plates last year to somewhere between 0.750 and 0.751, and only one CIM motor we had fit. We found it interesting that our CIM motors that were 8-10 years old fit perfectly in the 0.750" plate, but the news ones all seemed to be a touch bigger.

03-12-2014 00:08

JorgeReyes

Re: pic: 3 CIM WCD Ball Shifter CAD

What gear sizes did you end up using on this gearbox?

03-12-2014 00:18

JorgeReyes

Re: pic: 3 CIM WCD Ball Shifter CAD

wait never mind i just realized you already said this

03-12-2014 00:20

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

The current gears are 12 to 50 for the cluster, 40 to 44 for high gear, and 24 to 60 for low gear.

Hope that makes sense.

03-12-2014 00:42

asid61

Re: pic: 3 CIM WCD Ball Shifter CAD

Why can't you use the 34t ball shifter and 54t ballshifter gears? Do they not fit?

03-12-2014 01:23

Bryce2471

Re: pic: 3 CIM WCD Ball Shifter CAD

I thought about doing this, but the bottom of the cluster gear runs into the large bearing. I don't like the resulting gear ratios as much anyway.

03-12-2014 14:45

Paul Copioli

Re: pic: 3 CIM WCD Ball Shifter CAD

^This. In my organization, sketch fillets are extremely discouraged. 3D fillets are more robust and can be suppressed in order to simplify the model in large assemblies.

The options in SolidWorks for 3D fillets makes it so almost all situations can be handled with a 3D fillet. Also, the hidden line selection option makes it so selecting a bunch of short line segments in, let's say, a sheet metal part with lightening holes for filleting very easy.

Paul