Gearbox Design Feedback

[SerpentEagle]

Hi, I've been working on a generic drivetrain gearbox design and would appreciate any feedback.

https://d2t1xqejof9utc.cloudfront.net/screenshots/pics/dd4ccd3b32dbe95d1ed15d6257b3d12b/medium.JPG

Link to model: https://grabcad.com/library/drivebox-1-1#

[Michael Corsetto]

Reminds me of my favorite kids book...

https://img1.etsystatic.com/029/1/7211752/il_340x270.586998129_gb5u.jpg

-Mike

[billbo911]

This is a great start. Granted this appears to be a single speed generic gear box, it is still a great learning step when it comes to designing gear boxes.

To Michael's point, pockets in the plates do have advantages, but there is cost associated with them.

The two most obvious advantages are: Weight saving, and they look cool.
That's about it.

Here are some of the costs associated with them: Extra design time, extra machine time, not as robust as flat plates (this point is arguable), time to clean up and de-burr.

Here is a really excellent YouTube Channel (https://www.youtube.com/channel/UCX0Y091TMRKi8PqNfYoImiw) put up by 973. Adam has invested a lot of time in creating content about robot design. This includes a couple videos on transmission design and even on pocketing. One excellent point he makes is to NOT invest time into pocketing until you are absolutely certain your design is right. Trust me on this, it is excellent advice!

All that said, I also want to acknowledge your work. That trans looks really nice!

[Knufire]

Hi, I've been working on a generic drivetrain gearbox design and would appreciate any feedback.

Link to model: https://grabcad.com/library/drivebox-1-1#

I generally try to have a solid ring of material around the outside of the gearbox, so the bolts aren't cantilevered just on the ribs. You also don't want a snap ring groove right on the output axle after the gearbox like that, it'll create a stress concentration and greatly reduce the side load required to snap the output shaft.

[s_forbes]

2 points of input:

Gear clearance - You have the spacing between adjacent shafts set to the exact pitch diameter of the gears, which in a perfect world would work. If you add up tolerance issues with the gears, shafts, and machining precision of the plates, you could end up with gears that are "too close" and bind. Its suggested to add a little bit of extra space between shafts to compensate for that and get a smoother running gearbox. We add 0.003" between meshing gears in our transmissions.
Retaining ring on the output shaft - Retaining ring grooves are stress concentrators and reduce how much torque you can put through the shaft. You should try to avoid putting them in a section of the shaft that transmits power. I'd suggest looking for a different way to retain the shaft, or perhaps just move the clip all the way to the end of the shaft, past the wheel. (I'm not sure that you'd even be able to install it as pictured, since the rings don't open far enough to go over a hex shaft. An E-style clip (http://www.mcmaster.com/#e-style-retaining-rings/=yhrasu) could be installed though.)

Structurally it looks like it is strong enough for FRC applications, though I'd prefer to use larger diameter standoffs.

[SerpentEagle]

Reminds me of my favorite kids book...

https://img1.etsystatic.com/029/1/7211752/il_340x270.586998129_gb5u.jpg

-Mike

XD

You're probably right, I could definitely simplify the structure a bit more.

[SerpentEagle]

This is a great start. Granted this appears to be a single speed generic gear box, it is still a great learning step when it comes to designing gear boxes.

To Michael's point, pockets in the plates do have advantages, but there is cost associated with them.

The two most obvious advantages are: Weight saving, and they look cool.
That's about it.

Here are some of the costs associated with them: Extra design time, extra machine time, not as robust as flat plates (this point is arguable), time to clean up and de-burr.

Here is a really excellent YouTube Channel (https://www.youtube.com/channel/UCX0Y091TMRKi8PqNfYoImiw) put up by 973. Adam has invested a lot of time in creating content about robot design. This includes a couple videos on transmission design and even on pocketing. One excellent point he makes is to NOT invest time into pocketing until you are absolutely certain your design is right. Trust me on this, it is excellent advice!

All that said, I also want to acknowledge your work. That trans looks really nice!

Thanks for the feedback. When I was designing the plates, I actually did the "pocketing" in one sweep. All I did was lay out the holes for bearings, motors and standoffs, and then basically connected the dots. As for time, I probably would have saved an extra 15 minutes, and assuming this is water jetted, an extra 5 minutes max. I don't think 20 minutes is too much to lose.

I've actually watched many of Adam's tutorials and they were extremely helpful. You're right, I should have kept the plates generic until I am absolutely set on the design; it would be a pain in the butt to go back and make changes to the spacing without redoing all the linkages.

Thanks for your advice!

[SerpentEagle]

I generally try to have a solid ring of material around the outside of the gearbox, so the bolts aren't cantilevered just on the ribs. You also don't want a snap ring groove right on the output axle after the gearbox like that, it'll create a stress concentration and greatly reduce the side load required to snap the output shaft.

Yeah, thats how I usually do things, but I thought I would try something different this time. Your probably right though.

Also thanks for the comment on the snap rings, I have made change accordingly.

[SerpentEagle]

2 points of input:

Gear clearance - You have the spacing between adjacent shafts set to the exact pitch diameter of the gears, which in a perfect world would work. If you add up tolerance issues with the gears, shafts, and machining precision of the plates, you could end up with gears that are "too close" and bind. Its suggested to add a little bit of extra space between shafts to compensate for that and get a smoother running gearbox. We add 0.003" between meshing gears in our transmissions.
Retaining ring on the output shaft - Retaining ring grooves are stress concentrators and reduce how much torque you can put through the shaft. You should try to avoid putting them in a section of the shaft that transmits power. I'd suggest looking for a different way to retain the shaft, or perhaps just move the clip all the way to the end of the shaft, past the wheel. (I'm not sure that you'd even be able to install it as pictured, since the rings don't open far enough to go over a hex shaft. An E-style clip (http://www.mcmaster.com/#e-style-retaining-rings/=yhrasu) could be installed though.)

Structurally it looks like it is strong enough for FRC applications, though I'd prefer to use larger diameter standoffs.

I never thought about giving some tolerance spacing for the gears. I'll definitely add some on in the future.

I'm probably going to do what you said for the snap rings, just have it on the outside of the wheel. I've made changes accordingly. We've been using standard snap rings as I've done in the model without problems, but i'll defiantly look into the E-style clips.

Also the standoffs are now 1/2" opposed to 3/8". Thanks for your advice.

[T^2]

You're gonna have a hell of a time taking those CIMs off without disassembling the entire gearbox.

Standoffs do not need to be 1/2". You have too many of them, in any case; the ones on the far right and left are completely unnecessary.

[Ari423]

You're gonna have a hell of a time taking those CIMs off without disassembling the entire gearbox.

From what I can see, the two screws holding in the CIMs are accessible from the outside. One screw looks to be the top standoff screw, and there is a hole (in line with top and bottom standoffs, below CIM output shaft) to unscrew the other screw. Then it should be as simple as popping off the CIM gear and sliding it out of the enclosure. I could be wrong, but I imagine this was SerpentEagle's intent when designing it.

This is a nice, compact gearbox. How much does it weigh? Also, did you think about the abundance of possible pinch points where someone could get a finger stuck? I imagine it shouldn't be that hard to cover with some clear plastic (which will also help keep out scruff) but maybe you have a better idea.

[hectorcastillo]

The design looks really cool! One thing that I thought of is you could add two or three #8 tapped holes in a circle around the front bearing and use some button head screws to hold your bearing in place instead of a retaining ring.

[GeeTwo]

The design looks really cool! One thing that I thought of is you could add two or three #8 tapped holes in a circle around the front bearing and use some button head screws to hold your bearing in place instead of a retaining ring.


Is that a flanged bearing? If so, you wouldn't need a retainer on this side of the gearbox if you inserted the bearing from the inside of the box.

[hectorcastillo]

It looks like the flange is on the outside, but yes, flipping it around would be an easier solution.

[Richard Wallace]

My team has been retaining things on hex shafts by means of 1/4-20 button head cap screws and standard washers at each end. Thunderhex shaft stock makes this method very easy, because its center hole is just right for the tap. We often drive the shaft using a 1/2 inch speed wrench while holding the tap stationary. Of course this method requires us to cut shafts very close the proper length to hold whatever components are riding on them -- we use a scribe to mark them and a small lathe to face them for that.

[asid61]

The way your standoffs are supported, they're almost cantilevered off the main body of the gearbox for the left and right ones. You are also using 6 standoffs versus the traditional 4, any particular reason for that? Removing the leftmost and rightmost standoffs would simplify it a little.
I think you have 2-4 too many pockets, but proper build season planning can offset the machine time in the case of a drive gearbox.

[SerpentEagle]

You're gonna have a hell of a time taking those CIMs off without disassembling the entire gearbox.

Standoffs do not need to be 1/2". You have too many of them, in any case; the ones on the far right and left are completely unnecessary.

Not really. The bottom two screws are externally accessible, and only the top two are combined with standoffs.

Im a bit confused with the standoffs. I've never had experience with these kinds of gearboxes. I see respected teams such as 254 use some pretty large standoffs, and some teams use smaller ones. Would there be any issues with rocking if the standoffs are too small?

And I agree the left and right standoffs are unnecessary, I've made changes accordingly.

[SerpentEagle]

From what I can see, the two screws holding in the CIMs are accessible from the outside. One screw looks to be the top standoff screw, and there is a hole (in line with top and bottom standoffs, below CIM output shaft) to unscrew the other screw. Then it should be as simple as popping off the CIM gear and sliding it out of the enclosure. I could be wrong, but I imagine this was SerpentEagle's intent when designing it.

This is a nice, compact gearbox. How much does it weigh? Also, did you think about the abundance of possible pinch points where someone could get a finger stuck? I imagine it shouldn't be that hard to cover with some clear plastic (which will also help keep out scruff) but maybe you have a better idea.

No your right, that was the intent.

According to solidworks it weights approx. 6.5 lbs, so ~7 lbs.

And no I didn't think about pinch points, good thing you brought it up. I'm thinking that a 3d printed shell with screw holes would work nicely.

[SerpentEagle]

The design looks really cool! One thing that I thought of is you could add two or three #8 tapped holes in a circle around the front bearing and use some button head screws to hold your bearing in place instead of a retaining ring.

That's a really interesting idea. I ended up just reversing the bearing to eliminate the problem altogether.

[SerpentEagle]

My team has been retaining things on hex shafts by means of 1/4-20 button head cap screws and standard washers at each end. Thunderhex shaft stock makes this method very easy, because its center hole is just right for the tap. We often drive the shaft using a 1/2 inch speed wrench while holding the tap stationary. Of course this method requires us to cut shafts very close the proper length to hold whatever components are riding on them -- we use a scribe to mark them and a small lathe to face them for that.

We do that too but just for backup in case the snap rings fail. Its definitely a neat technique.

[SerpentEagle]

The way your standoffs are supported, they're almost cantilevered off the main body of the gearbox for the left and right ones. You are also using 6 standoffs versus the traditional 4, any particular reason for that? Removing the leftmost and rightmost standoffs would simplify it a little.
I think you have 2-4 too many pockets, but proper build season planning can offset the machine time in the case of a drive gearbox.

Yeah I agree. Next time I'll work on keeping things closer to the bulk of the body, or just put a ring around everything. I put these extra standoffs as support for the cims, but I realize that a whole another set of standoffs is unnecessary.

I personally don't think that pockets are a time problem for machining, as a water jet or CNC mill would zip right through them in a matter of minutes. But I will definitely work on simplifying the structure a bit more.

[SerpentEagle]

Thanks to all the helpful pointers you guys gave me, I have come up with a v2.

Link to model: https://grabcad.com/library/drivebox-2-1

https://d2t1xqejof9utc.cloudfront.net/screenshots/pics/76397766abe00b23fa16e7a2d3ab99bb/large.JPG

-4 Standoffs instead of 6
-Front output shaft bearing is flipped to eliminate need of retaining the bearing
-Simpler structure?

I plan to mount this in an unorthodox way. The bottom two standoff bolts are combined as mounting bolts an the bottom CIM mounting holes on the front plate is tapped to seve as another set of mounting holes. Good idea or a dreadful mistake?

https://d2t1xqejof9utc.cloudfront.net/screenshots/pics/119d312bafac02cf8b7e132d574ff87f/large.png

[Bryce Paputa]

I plan to mount this in an unorthodox way. The bottom two standoff bolts are combined as mounting bolts an the bottom CIM mounting holes on the front plate is tapped to seve as another set of mounting holes. Good idea or a dreadful mistake?



This is very similar to what we did this year, and we had an issue or two where the small bolt loosened, fell out and got stuck in the chain. I would recommend making that bolt go all the way through the gearbox with a standoff so that even if it loosens up it won't be able to fall out, but that might make it harder to assemble.

[SerpentEagle]

This is very similar to what we did this year, and we had an issue or two where the small bolt loosened, fell out and got stuck in the chain. I would recommend making that bolt go all the way through the gearbox with a standoff so that even if it loosens up it won't be able to fall out, but that might make it harder to assemble.

Would loctite solve this problem? But yeah I could probably just run the bolt all the way through to the CIM.

[Chak]

From what I can see, the two screws holding in the CIMs are accessible from the outside. One screw looks to be the top standoff screw, and there is a hole (in line with top and bottom standoffs, below CIM output shaft) to unscrew the other screw. Then it should be as simple as popping off the CIM gear and sliding it out of the enclosure. I could be wrong, but I imagine this was SerpentEagle's intent when designing it.

Would loctite solve this problem? But yeah I could probably just run the bolt all the way through to the CIM.

Wouldn't that ruin the accessibility of the CIM in the original design?
So Loctite would be the better choice.

[SerpentEagle]

Wouldn't that ruin the accessibility of the CIM in the original design?
So Loctite would be the better choice.

Not really. There would be an access hole in the chassis and you would just loosen it up, but I would definitely prefer the screws to be separate for the CIM an mounting.

[BenGuy]

Just so you know Shashank - Superb is making this - I would have to spend the entire build season on the mill to make this... :D :D But a cool looking design for sure!

[SerpentEagle]

Just so you know Shashank - Superb is making this - I would have to spend the entire build season on the mill to make this... :D :D But a cool looking design for sure!

Aw man! Looks like I'm gonna have to find someone else up for the challenge lol. Thanks.

[BenGuy]

Aw man! Looks like I'm gonna have to find someone else up for the challenge lol. Thanks.

Yeah... because anyone else on the team would do this... Superb would have a hard time with this!!! :D

[Dunngeon]

Have you given any thought to how this would be machined?

[SerpentEagle]

Yeah... because anyone else on the team would do this... Superb would have a hard time with this!!! :D

Actually superb can't cut the plates out since the max thickness for laser cuts is 1/8".

EDIT: On second thought, they might be able to since their site says they can cut up to 1/2" thick steel. Ill have to contact them on this.

[SerpentEagle]

Have you given any thought to how this would be machined?

Yeah, water jetting or CNC milling.

[Munchskull]

Question about filleting. How do you determine the largest end mill you can use in a fillet?

[asid61]

Question about filleting. How do you determine the largest end mill you can use in a fillet?

Depends on what you're machining. For a 1/4" plate I would not go below a 3/16" end mill if I could avoid it, a 1/4" if at all possible due to its greater rigidity.
A fillet should be at least the radius of the end mill. So if you choose a 1/4" end mill, then you need at minimum a 1/8" fillet on corners. However, it's not good to go into a corner and move out of it so suddenly with an end mill, so it's better to go a bit larger (0.135"? not sure).

EDIT: End mills are referred to by diameter, and fillets by radius.