I realized I never attached the CAD, so it is in the original description.
Quote:
Originally Posted by Bryce2471
Agreed. This is a common break point on many gearboxes. However, I think that if a 4140 steel gear from vex pro is used it should hold up well. (Guessing it's a 14 tooth)
To OP: Why did you put the CIM motor mounts so high relative to the rest of the gearbox? It seems like you could have lowered the COG and made the gearbox smaller by lowering the CIMs to just above the pneumatic cylinder.
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Good idea on the steel 14t gear. I'll make that change. I didn't really consider the placement of the CIMs very much. I could push them closer together if I need a thin gearbox or pull them down if I want to lower CoG. It shouldn't affect the design much.
Quote:
Originally Posted by Andrew_L
- 2.0 lbs - compared to 3.1 lbs for comparable COTS gearbox - Good target weight, though I think the reason your transmission is a pound lighter than the COTS alternative is more due to some cut corners than it is to a more weight-friendly design. Make sure your plates cover the circumference of your CIMs almost completely, otherwise there's room for bending and awkward loading. Also, make sure all of your material and mass properties are assigned correctly. It's easy to realize later that some non-custom parts, like the motors, shifting shaft, and shifting cylinder, were vastly underweighted in CAD. The spacing also looks a bit off on your shifting shaft, so unless that's custom, I'd double check with the WCP DS CAD model to see if you're using it right.
- Will the 32dp gears hold up? (They should according to the Lewis Formula) - We have done 32dp gears for our initial stage on a gearbox before on 1323, though I wasn't on the team at the time (maybe RC can comment on it). I do know that we don't actively do it anymore, so I'm guessing we stopped for a reason. Where most of the problem is going to arise is when you're in high gear, and the 32dp gears are the weak link in the chain of power transmission. Those pinions look pretty thin as they are, and I'm not sure how well they'd hold up, even if they were steel. You can achieve a lightweight 2 stage gearbox for an 8" wheel using 20dp gears, you just need to get creative. 
- Am I doing dog shifting correctly? (This is my first dog shifter) - Like I said earlier, double check with the WCP DS model. Your shifter shaft looks a bit off to me, and it looks like you have an extra shifting cylinder mounting plate attached to your gearbox plate (you only need one, and none on the plate itself). Also you shouldn't need a shaft collar to hold your bearings in. Look into either geometrically retaining them (aka the model's geometry physically prevents the bearings from coming out), or by using the head of a #10 button head cap screw to retain the bearing flange (as seen on the WCP DS). |
The reason for the big weight difference is that all-20dp COTS gearboxes need 3 stages for the same reduction, and the extra stage adds a lot of weight. I will consider adding more plate to support the CIMs from bending. The shifting shaft is custom because the dog gear is from AndyMark while the rest of the shifting mechanism is Vex. The AM dog gear is 1/2" hex and the Vex one is 5/8" hex, so I would have to use Vex's output shaft to use Vex's dog gear.
I'd be interested in hearing why you stopped using 32dp first stages. I don't have much experience with actually making custom gearboxes so I appreciate any real-world experiences I can get.
The pneumatic cylinder only has one plate, which screws into the rear plate of the gearbox. I tried to constrain the bearings on the output shaft by squeezing the flanges in between the plates. The dog gear means I can't put spacers between the gears, so the bearings could slide inward. I would use a screw and washer to constrain them, but in the front there is a shaft sticking out (i.e. it's not the end of the shaft) and in the back the shifter shaft goes through the center so I can't put a screw there. If there's something obvious I'm missing, please hit me on the side of the head with knowledge.