Custom 3D printed 30t drive pulleys for 15mm pulleys (i am concerned about the size of the pulleys, we do not have experience with smaller pulleys at high torque. this team said they had success with 24t pulleys in tube Belts for drive train. We like how the belts last all season without being changed.)
It looks like you’re double loading the belts by the gearbox here, don’t do that. Moving the gearbox to the middle position would resolve this. I really like the packaging of the second reduction in tube but I don’t see why you’re doing shifitng
Hexagonal Tubing, 1/8" Wall Thickness, 3/4" Wide
You can use this to reinforce the bore of the 3dp pulleys, but I still wouldn’t really trust them.
This is a pretty neat design. Regardless of the merits of the design itself I think it might be best to stick with a more orthodox design like chain-in-tube or even chain outside tube. There’s a lot that could go wrong here, but I don’t think it necessarily will. First of all this looks like a huge hassle to assemble. It looks like you have to slide the belts, pulleys, and the second stage reduction all into the tube at the same time. Second, the pulleys themselves are unlikely to hold up to the rigors of competition on your drivetrain. The gearbox is pretty neat but shifting doesn’t really seem worth it for the complexity with the prevalence of brushless motors today.
I am not sure what you mean by double loading the pulleys? You can hide the piece of tubing to see the pulley setup inside. the 1.5 stage is a two speed gearbox I have attached the calculations in the post above. I do like the inserts but the strength of the belts are my concern less the pulleys. We would do testing to ensure we trust 3d printing (we use a stratasys printer that gives us great results but we may look at using a material such as onyx or similar).
The belt between the wheel closest to the gearbox and the middle wheel sees twice as much load as the other belt because it’s effectively transmitting torque to two different wheels simultaneously. You can avoid this issue by moving the gearbox to near the middle wheel or by having a belt run the entire length of the drivetrain from the gearbox to the far wheel adjacent to the other belt run to the middle wheel.
We may consider chain in tube but we have no experience with it. We understand how to use the tensioning systems for chain but we are concerned about the wear and not sure how much tensioning is required throughout the season and whether links will need to be removed. Info on this would be appreciated.
chain stretches by a max of 1/8" over a season, you don’t need to replace links. It’s possible to not run tensioners but that’s a little risky imo.
Due to the nature of chain-in-tube drivetrains, the walls of the commonly used 2x1 extrusion constrain the chain so there’s no slipping. For best performance a bit if center add, iirc somewhere around 0.01" helps with tensioning.
More specifically, the shifter shaft has a plunger inside it that pushes ball bearings in or out of their holes. When each set of balls is engaged, the shaft rotates along with that gear.
This gearbox is a pretty clever shifter design. When the larger gear is engaged, no additional reduction is applied. When the smaller gear is engaged, the larger gear rotates the smaller gearset on top, eventually spinning that smaller gear and transmitting torque through the shaft.
Of to note, you cannot assemble the last reduction stage you have on your gearbox. You should look into increasing the cutout size in the extrusion to facilitate the assembly of the last driven gear.
So if the piston is retracted, the left gear would be engaged, but the entire shaft will spin? If the right gear is engaged, the left will not because it is to the left?
What the shifter is doing is changing which gear is connected to the shaft and which one is dead axled. When the piston shifts to the left, the left gear is powered by the motor pinions sending power to the shaft. When the right gear is engaged, the motor pinion drives the left gear (but it isn’t engaged, it’s just dead axled over the shaft), which drives another auxiliary gear which ends up driving the right gear. Since the right gear is engaged, the shaft spins.
We ran a belt-in-tube drivetrain in 2018 and 2019, but with a 2x2" tube. I would suggest putting lots of access holes near the wheels so you can poke around to line things up for assembly.
I suspect the 3d printed pulleys will not hold up unless you have a really lightweight robot and drive carefully. We used vexpro pulleys. For the center wheel, we took two pulleys and faced off one flange on each to get them to fit in the tube. Using aluminum pulley stock to make your own pulley would be better.
The belts in this drivetrain configuration see the same maximum load as belts in the usual WCD configuration. I wouldn’t be concerned about it.
The biggest difficulty with belt in tube is assembly and maintenance. You will want access holes on the top of the tube above the pulleys. Even still, getting the shafts to line up with the pulleys and bearings requires some finesse. We are planning to move to Thunderhex which will hopefully help. You can check out the CAD for our 2019 drivetrain here. Our access holes have tabs that we bend up slightly to provide a smooth edge for the belt.
Once you have the belts inside the tubes, your entire team needs to be disciplined about keeping debris out. If metal shavings, rivet heads, or other things get in, your belt will get shredded. Make some polycarb covers for the access holes. We let our guard down this year, and a shaft collar ended up inside which shredded a belt. Fortunately this happened during practice at our shop.
I don’t have any experience with 3D-printed drive pulleys, so I can’t say how well they would hold up. The failure mode would likely be rounding of the hex bore. 30 tooth pulleys will be fine. We use 24 tooth, but have not tried it with anything larger that 6 inch wheels. We buy aluminum pulley stock from BB Manufacturing, which we machine and hex broach. You can find our pulley CAD here.
You could look at the load ratings for the belting and see if anything is under-rated. Heck you might be able to change the further belt to a smaller width to save space/weight.
If you are concerned about the fact that you’ve doubled your failure points (i.e. if you snap the rear belt, both front wheels are out of comission) you could run two lengths of belt- one from rear to center, one from rear to front.
