This is the first gearbox I have designed in Solidworks. I normally use Inventor, but I am required to take a Solidworks course for my engineering program, so this is my final project.
This design took some inspiration from Chak’s gearbox found here.
It is geared for 9.6fps free speed in low, and 21.4fps free speed in high, on a 3.5" wheel.
This design is entirely reversible. Meaning it can be installed facing the other way with no modifications to the parts.
It doesn’t look like your 775pros have flux rings( the little sheetmetal ring around it which is about 1mm thick). You might want to get a different CAD model, as 775pros with flux rings probably won’t be able to fit through the holes in the gearbox plates.
Two reasons: (1) they contribute to the motor’s performance by getting more magnetic flux to the rotor, and (2) removing them is an illegal modification per FRC rules – or I should say it has been for as long as I have been a robot inspector. I don’t forsee that rule changing.
Back on topic: this is a really neat design. The outboard 32DP gear meshes will need shielding. What C-C distance did you use between the cluster and the output shaft? Like a ball-shifter, or a DS, or an EVO, or something custom?
Very well done, Bryce, I really like it. I’d wager it’ll be tough to get a 4 motor shifting 775pro gearbox much better than this (incoming 7 people saying that can easily make it better). I’d maybe consider adding some sort of bearing plate on the other side of the tube that connects back to the gearbox for a little more support, but either way it’s still a great design. Do you have an approximate weight on this thing? The pocketing is really solid all around and it looks comparable to most 2 CIM shifting transmissions in terms of how heavy it is.
What a clever take on a 775pro gearbox! Honestly, I can’t find much to critique except maybe the non-COTS 85t gear (unless that’s a modified AM one?) Does it use a COTS shifter shaft?
This gearbox is well thought out. Everyone else has shared the same compliments. Upon first glance, it almost reminded me of an inverted style gearbox as you show with Chak’s, but I love how it isn’t and looks that good.
Great work, thanks for sharing!
Side question: why have most approaches to a 775pro gearbox tended to opt for 4 motors in them? I know that 2451 originally planned for 5, then stepped down to 4, and others who used them this season went for 4 motors as well. I would think that 3 would bring you plenty of performance, but I must be missing something. Could someone please explain to me? Thanks!
It’s much easier to burn out 6 775pros than it is 8. Less current per motor means better efficiency and less heat in each one.
Plus, the symmetry works out really well.
I think the biggest worry is durability of 775pros in a drivetrain. People have had problems in the past of 775pros burning out in their drive train. My best guess is that people find the best way to approach that issue while still getting the speeds they want is to add more power so you are traction limited. In other words instead of your motors stalling when pushing another robot or running into a wall, your wheels just spin on the carpet. Bad things happen quickly to 775pros when they are stalled. So 4 motors in a gearbox instead of the usual 2-3 cims or mini cims (which are generally harder to burn) help ease the stalling probability of the 775pros.
Oops. I’ll have to fix that. Hopefully it won’t affect too much else.
Back on topic: this is a really neat design. The outboard 32DP gear meshes will need shielding. What C-C distance did you use between the cluster and the output shaft? Like a ball-shifter, or a DS, or an EVO, or something custom?
The final stage has a C-C distance of 1.452". I’m not sure if any of the gearboxes you listed use this or not.
Thanks! I have used something just like what you are describing on gearboxes in the past, and it worked great. However, this year the team ended up not using any exterior support on a three CIM gearbox, with no problems, so I’m no longer convinced that it’s necessary.
Do you have an approximate weight on this thing? The pocketing is really solid all around and it looks comparable to most 2 CIM shifting transmissions in terms of how heavy it is.
Solidworks says 4.97lbs. That seems a little optimistic to me, but I guess I’m not used to working with drive motors that are only 0.8lbs.
Thanks! The 85t gear is CADed as a modified AM, but it would probably remain unmodified if this was ever built in real life. The shifter shaft is a simple custom design, much like what 2471 has used in the past.
Thats incredible work, and I would love to make one. I do have a question fo everyone though. Why the push for 775s in the drivetrain? If weight and power come out to be really close or the same but you need twice as many controllers, wires, and fuses, wheres the advantage? Is there something I am missing about 775s on the drive?
8 775pro have the same about the same power as 6 CIMs if each is lowered to 10V max. At 12V each, they have ~145% more power (but they’re more likely to burn out if over-stressed).
EDIT: I’ll also echo the other comments in saying that this looks like a really nice gearbox. It would be great if you could post the CAD so we can get a closer look at it.
The weight savings from 775pros is actually quite significant, having a drive motor that weighs 0.8lbs instead of the 2.8lb CIM is an interesting game changer. There quite a lot of power in that small package that is very attractive to people.
Although a downside would be from using up more PDP slots, this can be minimized by effective usage of pneumatics… I’m personally interested in seeing potential robots in the future that completely standardize to using entirely 775pros and pneumatics for their mechanisms.
As for the gearbox, most of everyone else’s compliments have echoed my first impressions, it looks amazing and I’m sure many of us would love to look at that CAD more in detail.
I guess I will have to do the math but I still don’t see any significant advantage if any at all. I recognize small advantages and disadvantages. For example weight. I can see that, but its not a 1:1 ratio since you need more than one motor in addition to an additional controller, wire, and fuse. Add that up and the weight advantage is slight, but still an advantage at the expense of volume or room. So it may be dependent on design.
Combine this small weight advantage with the extra volume trade off and the lower durability if stalled or run below optimal level and the overall advantage becomes elusive? Perhaps its me. I am willing to switch If I felt the advantage was significant. We burned up half a dozen 775 during recycle rush because we didn’t use them correctly. They work best when they actually run fast. As soon as they get loaded or get below 75 % free speed they really build heat fast.
I can’t help but chuckle when I think of all the negative comments we got when we posted the first CAD of this type of gearbox late last year.
Bryce, this looks fantastic, I hope you build some soon and see how well they really work.
Seems like your material properties in SW is correct, we were at 5.6lbs with 4X 775Pros on each gearbox, including all fasteners and about 8" of 10ga wire and connectors on each motor.
Think of an entire gearbox with the power of a 6-CIM gearbox for the weight of two CIM motors.
Would you mind finding the link to this discussion? I’d love to see how much CD has evolved on this design. And thanks to your team for once again being the vanguard on new designs. You are always about three years ahead of the rest of us.
I’ve never made a drivetrain that ran on 775s before, so I can’t say for certain that one way is superior to the other. However, the it seems that the strength to weight ratio gains might be pretty significant.
Lets do some back of the napkin math on how much the weight savings is. I think its fair to compare the weight of this gearbox to a 3 CIM equivalent, and lets assume that the gearboxes weigh the same. A six CIM drive has 6 motors that weigh 2.8lbs each. That’s 16.8lbs in motor weight. An eight 775pro drive has 8 motors that weigh 0.8lbs each. That’s 6.4lbs in motor weight. The 775pro drive now needs 2 more motor controllers, if they are Talon SRXs, that adds 0.52lbs. That means that the total weight savings is somewhere on the order of 9.9lbs. There are not a lot of opportunities on these robots to change to something more powerful that saves almost ten lbs of weight.
Combine this small weight advantage with the extra volume trade off and the lower durability if stalled or run below optimal level and the overall advantage becomes elusive?
The extra volume in my mind is non existent. If you compare this gearbox to any three CIM gearbox, the total volume taken up will be significantly lower. I think it is even a significant volume savings when compared to most two CIM shifting options. The lower durability is definitely a concern, and it is an unknown quantity to me right now. But it sounds like burning motors can be avoided with good gear ratios and software.
That’s awesome! Although it’s almost creepy how similar it looks.
What speeds and ratios are you going for in that design? Or are you going to make a separate thread for it?
Here is a link to the CAD. Let me know if you have trouble finding something.
