Gearbox Design Feedback

[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 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]

Originally Posted by 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 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]

Originally Posted by T^2

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]

Originally Posted by 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.

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]

Originally Posted by 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.

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



-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?

[Bryce Paputa]

Originally Posted by SerpentEagle

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]

Originally Posted by 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.

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

[SerpentEagle]

Originally Posted by 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.

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

[SerpentEagle]

Originally Posted by Ari423

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.