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#
Reminds me of my favorite kids book…
-Mike
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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 https://www.youtube.com/watch?v=(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!
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.
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 could be installed though.)
Structurally it looks like it is strong enough for FRC applications, though I’d prefer to use larger diameter standoffs.
XD
You’re probably right, I could definitely simplify the structure a bit more.
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.
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.
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.
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.
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.
It looks like the flange is on the outside, but yes, flipping it around would be an easier solution.
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.
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.
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.
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.
That’s a really interesting idea. I ended up just reversing the bearing to eliminate the problem altogether.
We do that too but just for backup in case the snap rings fail. Its definitely a neat technique.