pic: 3 CIM WCD Ball Shifter CAD

[Bryce2471]

Originally Posted by Jared

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.

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.

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

As far as I can tell, the top CIM on the WCP 3 CIM DS gearbox is mounted in the same configuration.

[Jared]

Originally Posted by Bryce2471

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.

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.

As far as I can tell, the top CIM on the WCP 3 CIM DS gearbox is mounted in the same configuration.

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.

[Bryce2471]

Originally Posted by nathannfm

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.

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.

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.

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

[Arpan]

Originally Posted by Ronnie314

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.

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.

[Jared]

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

[Bryce2471]

Originally Posted by Jared

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

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

EDIT:

stress analysis on a couple of our gearbox plates in SolidWorks

Sorry, I missed that.

[Chris is me]

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

[Jared]

Originally Posted by Bryce2471

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

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.

Originally Posted by Chris is me

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.

More on pocketing in this great video: https://www.youtube.com/watch?v=wGmsnD0KQMs

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

[Chris is me]

Originally Posted by Jared

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

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.

[asid61]

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.

[AdamHeard]

Originally Posted by asid61

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.

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.

[Bryce2471]

Originally Posted by AdamHeard

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.

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.

[asid61]

Originally Posted by Bryce2471

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.

+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?

[Bryce2471]

Originally Posted by asid61

+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?

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

[asid61]

Originally Posted by Bryce2471

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

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.