pic: 2x1 gearbox

[Chak]

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[Cothron Theiss]

Chak is reminding me that there are gearbox designs I'm supposed to finish and haven't.

As for the packaging, it looks great! I assume that the gear is exposed more on the bottom of the rail than the top because of the center-dropped axle, so would it be worth making this gearbox direct drive the back wheel instead of the center? That way you can flip the gear cut-outs and have the top side be more exposed. I honestly have no idea if that's worth it; I remember we had issues with the large driven gears eating into the carpet on the 2015 Vex chassis, but that was with gears almost as large as the 4" omnis we had at the time.

[KohKohPuffs]

After swearing to myself that I would never build another gearbox after designing like 30 of them, I'm finding myself breaking that pledge

Might be my tendency to have reasonable doubt when it comes to structural integrity, but my concern is if 1/16" thick boxtubing will be strong enough, especially to support the CIMs.

[MattC9]

This is a good start, when I was designing gearboxes one thing I would make sure to account for is how it would be mounted. I see two empty holes on either side of the output shaft, are these going to function as your mounting points for a face to face mount? If they do you will have trouble getting a nut or screw into the tubing because you will have a gear behind it. A good alternative might be to extend the ends of the tubing a bit and put mounting holes on the outside edge of the CIM motor. My last thought for this IF you are planning on a face mount is to use smaller screws and place them on the inside of the tubing. This will allow you to have a flat face mount without having to drill clearance holes in the chassis, except for the bearing.

[Richard Wallace]

I am becoming a fan of tube stock transmissions. Our robot included two of them this year, one for shooting and one for intake. Both used 775pro motors.

Thanks for posting this example using CIMs. We may try something similar, perhaps using 3x1. I also agree with those who favor 1/8" wall. Thinner walls make me worry about stress at the bearing seats and motor faces.

[hrench]

Looks very simple and that's good. Actually it looks very Cimple, as in similar to the Andymark box.

[adciv]

Originally Posted by KohKohPuffs

After swearing to myself that I would never build another gearbox after designing like 30 of them, I'm finding myself breaking that pledge

Might be my tendency to have reasonable doubt when it comes to structural integrity, but my concern is if 1/16" thick boxtubing will be strong enough, especially to support the CIMs.

Do you have any sources you'd recommend for designing gearboxes? I'm looking for information on design theory for the structure and so on. And please no gear ratio basics from the peanut gallery -_-

[Chris is me]

1/16" wall tubing is not a great idea for a gearbox like this, particularly because the bearings are only supported by a thin amount of material and also because you have to make an aggressive pocket to leave clearance for the gear. I've never seen bearings directly mounted in 1/16" tubing that support loads as large as a drive wheel work out well.

You really want to use 12T (or 11T) pinions on an enclosed gearbox like this because you are giving up the ability to use the CIM boss to pilot the gearbox when using a 14T gear, since your hole has to be big enough to clear the gear

[ASD20]

Originally Posted by adciv

Do you have any sources you'd recommend for designing gearboxes? I'm looking for information on design theory for the structure and so on. And please no gear ratio basics from the peanut gallery -_-

FRC Designs has some examples (https://frcdesigns.com/designs-gearbox/ ), but for theory, I would start by searching CD for old white papers.

[Chak]

Originally Posted by Cothron Theiss

As for the packaging, it looks great! I assume that the gear is exposed more on the bottom of the rail than the top because of the center-dropped axle, so would it be worth making this gearbox direct drive the back wheel instead of the center? That way you can flip the gear cut-outs and have the top side be more exposed. I honestly have no idea if that's worth it; I remember we had issues with the large driven gears eating into the carpet on the 2015 Vex chassis, but that was with gears almost as large as the 4" omnis we had at the time.

If the wheel is directly driven, the wheel would be concentric to the gear and it doesn't really matter which way the gear faces. Currently, the 64t gear has a outer diameter of 3.3", so a 4" wheel should be fine. I actually wanted to use a 3" wheel for 9.71 ft/s, but later realized it wouldn't work.
In the 3x1 version, the 66t gear has a outer diameter of 3.4", which should be fine also.

Originally Posted by Richard Wallace

I am becoming a fan of tube stock transmissions. Our robot included two of them this year, one for shooting and one for intake. Both used 775pro motors.

Thanks for posting this example using CIMs. We may try something similar, perhaps using 3x1. I also agree with those who favor 1/8" wall. Thinner walls make me worry about stress at the bearing seats and motor faces.

Done. http://imgur.com/a/RNfht

Btw, both versions of this gearbox has a SRX mag encoder on the back. Using magnets means that I can loosen a shaft collar and take the output shaft out easily without dealing with the encoder. That makes me a little more willing to have holes blocked by the big gear. Still, as MattC9 said, it's better to have mounting holes that are not blocked at all, so I added it for the 3x1 version.

At this point, this is just a WCP Single Speed Single Reduction gearbox with one less motors and less parts.

[Lil' Lavery]

How close are those top CIM mounting screws to the upper wall of the tubing? Are you having to remove wall material to thread those screws in? Would you if you increased to 1/8" wall tubing?

What's the clearance between those same CIM mounting screws and the cluster gear?

What retains the output shaft from shifting?

Originally Posted by Chris is me

You really want to use 12T (or 11T) pinions on an enclosed gearbox like this because you are giving up the ability to use the CIM boss to pilot the gearbox when using a 14T gear, since your hole has to be big enough to clear the gear

It appears to me the hole on the CIM side of the tubing is sized to pilot the boss, and the 14T pinions are to be installed via the hole on the opposite side of the tubing once the CIMs are already screwed in.

[Chak]

Originally Posted by Lil' Lavery

How close are those top CIM mounting screws to the upper wall of the tubing? Are you having to remove wall material to thread those screws in? Would you if you increased to 1/8" wall tubing?

What's the clearance between those same CIM mounting screws and the cluster gear?

There's 2 sets of answers, for the 2x1 gearbox and the newer 3x1 gearbox. For the 3x1 gearbox, the clearances are huge (more than the diameter of the screw) and there is definitely no problem. For the 2x1 gearbox, it's a lot tighter. The CIM mounting screws are .01" from the wall, and in theory no wall material will be removed. The CIM mounting screws are .0125" from the gear. The tight spaces are another reason to use 3x1.
If I use 2x1x1/8, the CIM mounting holes and the output bearing hole will cut the wall there down to 1/16 anyways, so I figured there wasn't much of a difference. Using a 3x1 is much safer all around anyways.

It appears to me the hole on the CIM side of the tubing is sized to pilot the boss, and the 14T pinions are to be installed via the hole on the opposite side of the tubing once the CIMs are already screwed in.

Correct, except that I am using 11t pinions, so there is no problem anyhow.

What retains the output shaft from shifting?

The output shaft is turned down to a circle on the back end to prevent it from moving backwards. The circle extends past the bearing and a shaft collar holds it on the other side. It's shown in the render in the link in my previous post. This setup allows me to remove the output shaft by removing only a shaft collar.

[Andy Baker]

Thomas,

You've got a nice, simple design here. One more tip to add to the comments already provided in the thread:

You will need more space machined in the top or the bottom of the tube to allow for inserting the 64 gear into the tube. Unless you have some magical conjuring skills, that gear can't currently be inserted into the pocket you have designed.

Sincerely,
Andy B.

[bkahl]

Originally Posted by Andy Baker

Thomas,

You've got a nice, simple design here. One more tip to add to the comments already provided in the thread:

You will need more space machined in the top or the bottom of the tube to allow for inserting the 64 gear into the tube. Unless you have some magical conjuring skills, that gear can't currently be inserted into the pocket you have designed.

Sincerely,
Andy B.

It might fit in through a larger pocket on the bottom on the tube?

Just an assumption though- this definetly applies if there is an identical pocket on the bottom as there is on the top.

[GeeTwo]

I haven't designed or built a custom gearbox myself, but a few comments based on basic physics:

Originally Posted by Chak

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This can be a separate gearbox, or it could be the drivetrain rail itself.

I like the simplicity, though I'm a bit leery about a gear ratio of less than 6:1, especially if your robots tend to barely make weight. I see this as much more likely built in to the chassis rail, especially as I try to figure out how to mount this segment of tubing to a separate chassis.

Originally Posted by Cothron Theiss

I assume that the gear is exposed more on the bottom of the rail than the top because of the center-dropped axle, so would it be worth making this gearbox direct drive the back wheel instead of the center? That way you can flip the gear cut-outs and have the top side be more exposed.

Originally Posted by KohKohPuffs

Might be my tendency to have reasonable doubt when it comes to structural integrity, but my concern is if 1/16" thick boxtubing will be strong enough, especially to support the CIMs.

I agree with KohKohPuffs (and others) that 1/16" seems rather light for most FRC purposes, but not because of CIM support. A CIM weighs under three pounds, and it's only cantilevered out a few inches. If you were worried about it, you could move the CIM back a bit farther with some spacers and put a bearing in the far wall to provide more support than you should ever need for the CIM, unless you were expecting someone to stand on the motor. The drive shaft, on the other hand, can easily support half the weight of the robot as it rocks from the front to rear wheels (or vice versa). For this reason, I endorse keeping the bearing mount low, putting more of the tubing structure above the bearing hole. (Rule 1 that we figured out about drive chassis design is to first consider the stress of keeping the robot off the ground; supporting motors and wheels is minor next to that.)


The lower pocket is clearly larger than the top based on the number of teeth exposed. As others have noted, It must be large enough to allow the gear to be inserted, unless you're planning to machine the tubing and gear from a solid block of aluminum .