Questions and comment are welcome.
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.
That is a great looking gearbox!
I have a few questions though…
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.
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.
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.
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.
I have my doubts about the press fit into the end of the ballshifter shaft in such a configuration.
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!
Can you post a cross section of the shifting then?
Curious how that assembles if you’ve replaced their setup.
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.
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.
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.
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.
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.
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.
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.
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/hphotos-xap1/v/t1.0-9/988970_856191711099097_7489374310229813017_n.jpg?oh=6c5cac63ab55bc23cdfb75257e141be9&oe=54D168FA
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.
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.
This is what we came up with last year for a 3 CIM configuration:
