pic: Two-speed low profile gearbox + PTO

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After seeing the other inverted low profile gearbox, I decided to look into the general design, as well as take my first step into PTO options on gearboxes.

Specs:
High speed: 17.69fps
Low speed: 6.49fps (ha…)
Weight: 11.49 lbs

I didn’t calculate anything for the PTO. Basically, the gearbox was to get familiar with PTO mounting so when I make future designs, at least I know how to do it.

If anyone wants to look at the model, it can be found on the grabCAD:
https://workbench.grabcad.com/workbench/projects/gcOhfXOehYETdalk4HNJ_3A0HceTxrt6Dv4ytxedJppYkO#/space/gcy-RpuR7fueMhPxpys8KUh7yF1NwcTEWAlStxFm6c3ypj

Oh wow that’s a sleek profile, good job! 11.5 lbs, a PTO, 3 cims, a 2 speed and it looks like it can rest in the chassis. Are the plates symmetrical or do you have to build the gearbox in reverse to put it on the other side?

I might have to make a gearbox in reverse because I have configurations for countersunk screws for the standoffs.

If you hand countersink or countersink on the mill you could just make two configurations of the plate for counterinks/machining purposes only.

Looks great! The one thing that I would consider adding is an easy place to mount an encoder.

Keep up the good work.

If you hand countersink or countersink on the mill you could just make two configurations of the plate for counterinks/machining purposes only.

Ehh… more like 4 configurations because the plates are not symmetrical

Looks great! The one thing that I would consider adding is an easy place to mount an encoder.

Keep up the good work.

Thanks!
As for the encoder, I do believe I need to add a gear and a shaft for that. At that point, wound’t it be easier to mount encoders to the wheel shafts?

If the front and back plates are not the same, why not get rid of all the extra holes in the front plate? and maybe lighten it a little more (haven’t decided if thats a good idea or not though)

But why cant they be identical plates? If they can be identical, we can add the countersink with the mill and just cut 2 identical copies for the front and back plate. Much easier

As for the encoder, I do believe I need to add a gear and a shaft for that. At that point, wound’t it be easier to mount encoders to the wheel shafts?

Perhaps you can add small gear on the output shaft. sdp-si has a ton of gears for your choosing. Broach a small one and you can fit it on the output hex shaft where you have a spacer. you might have to make a small plate to mount the encoder, else you cant mount the gearbox directly to the 2x1 (which is what i am assuming you are doing)

Or gear it directly to the CIM gear. Looking at your CAD, there seems to be space right next to the CIM. It would solve the problem of needing a dedicated plate just to mount a encoder, but it would make life difficult for your programmers. :stuck_out_tongue:

If the front and back plates are not the same, why not get rid of all the extra holes in the front plate? and maybe lighten it a little more (haven’t decided if thats a good idea or not though)

But why cant they be identical plates? If they can be identical, we can add the countersink with the mill and just cut 2 identical copies for the front and back plate. Much easier

They are identical. It’s just that I added configurations for countersink holes. Probably a slight misunderstanding there.

As for removing extra holes on the front plate that aren’t being used, it wouldn’t make a big difference since doing so would mean replacing those with something else for support. Even so, not a huge weight difference once again.

Perhaps you can add small gear on the output shaft. sdp-si has a ton of gears for your choosing. Broach a small one and you can fit it on the output hex shaft where you have a spacer. you might have to make a small plate to mount the encoder, else you cant mount the gearbox directly to the 2x1 (which is what i am assuming you are doing)

Or gear it directly to the CIM gear. Looking at your CAD, there seems to be space right next to the CIM. It would solve the problem of needing a dedicated plate just to mount a encoder, but it would make life difficult for your programmers.

I think I see what you are talking about: those shafts that have the gear directly driven by the CIMs.

If encoders are going there, then there would be some math to calculate the rotations on the output shaft, but hopefully that shouldn’t be much of a challenge. In addition, the code would require a value to store the shifting piston state, because high gear and low gear have different speeds.

The real challenge would be its mounting position: say that we put this design with the encoder mounted to the front plate on the shaft directly driven by the gear for the 2016 game. Because of defense, I’m sure that another robot might accidentally break the encoder with some manipulator that goes outside its frame perimeter. And it doesn’t really look like I have room on the reverse side to mount the encoder…

I think I see what you are talking about: those shafts that have the gear directly driven by the CIMs.

hmmm. This is gonna be kind of hard to describe without a cad model in front of both of us, but there is a spacer right in front of the 60t(?) dog gear. You can flip the placement of the larger and smaller dog gears, put a thin gear from sdp-si on the hex output where the spacer currently is, and have it be geared 1-1 to an encoder on the inside of the plate. That way it cant ever be destroyed, and it will always be 1-1 with the output, since it is driven by the output shaft.

But that would require a lot of repositioning of gears, as well as requiring a small gearbox plate dedicated to mounting an encoder. Perhaps too much work for something that can be mounted elsewhere with relative ease?

just my two cents.

also, is that a versahub that is on the dog gear for the PTO? the part says vex, but searching up the ID gives a gearbox

Ok then if the plates are the same, don’t bother making another configuration. Not worth the effort.

Also, is there an advantage to putting the encoder on the gearbox itself? I feel like putting it on a wheel shaft yields more accurate information for coders to determine stuff like distance traveled, etc. since its reading how much the wheel has turned, which directly determines how far you are moving. no math involved(which leads to slight inconsistencies with whats expected vs. whats actually happening). Thoughts?

The only issue with mounting it on the output shaft is that its often more difficult to get a secure mounting that wont wobble or come off (ie look at our 2015 robots drivetrain encoder). Of course there has been some talk of using a follower wheel so that it accounts for slipping, but not much research has been done into that.

What has been done, however, is mounting a small 3d printed gear on the back side of the output shaft, between the piston and the gearbox plate. Taking a look at the CAD, you seem to have enough room on the shaft itself, but depending on the mounting, the encoder might end up very close to the ground.

The debate on whether encoders should be placed in the gearbox or on the wheel came up recently in another thread. In the end I took away that placing it in the gearbox is both safer and gives less backlash (thus is easier to apply controls to).
You could put something like an S4 encoder on the end of nearly any shaft in the gearbox it seems. Just put a hole in the end of a shaft and add a set screw.

Mmmm… not really because of the aluminum box tube that the gearbox is mounted on. Also, I still don’t like the notion of placing the encoder on the front plate, where… wait, we have bumpers this year, so hopefully those might provide protection from manipulators from other robots.

But don’t forget the shifter pistons on the other side; there are two of them since this is dual speed and PTO. I could perhaps do the idea where we use small gears to connect the output shaft to the encoder. Perhaps use a shoulder bolt as a shaft for the driven gear.