I have been working on a basic drivetrain for CAD practice, this is the 3rd iteration, the first to use parts from the new VexPro line. It uses 4in. VersaWheels, and the three-stage 60:24 ball shifter, with a 15:22 sprocket reduction. Weight is currently 41lbs.
The problem is I need to find another way to support the shifters. I’m not sure if the angle bracket would be rigid enough to hold the shifter in place without bending. I thought about using a ball bearing to support the end of the output shaft, but if the weight of the shifter was putting a vertical load on the bearing wouldn’t that cause it to be less efficient? Any thoughts or comments would be much appreciated!
The bearing load should be fine. Teams normally put the whole weight of their robot on 8 bearings, which is 15lbs per bearing. Way more than the weight of the gearbox.
A better solution may be to brace the two gearboxes together however.
Take a piece of heavy gauge sheet aluminum. Form a C channel over the top of the two gearboxes… fold the ends down so that you can use the upper bolts on the gearboxes to bolt the C channel down to each gearbox.
Now the gearboxes support each other and you also have a rigid support running right across the chassis in the middle of your robot. If you don’t need the flat surface for mounting things, you can speed hole it.
It’s the way we used to do it in the pre-Toughbox KOP days. (But not in the drill motor days. :rolleyes: Where is the smiley with grey hair?)
Jason
P.S. One more word of advice… if you use the gearbox to directly drive one of your wheels then you’ll never lose mobility due to a chain coming off. I know… if you do your chains right, they won’t come off. But I’ll take direct drive any day.
You might use something like this – http://www.andymark.com/product-p/am-0756.htm – to pick up the formed hexes on the back side of the transmission and span the gap between them, not unlike how the transmissions on the Kitbot are braced together.
I agree with all the above posts. For OCCRA which is a small Michigan thing for robotics, we just bungee corded the two gearboxes together and that was beautiful. I don’t think you can use something like that in FIRST (I should look this up) but in general the point is just have something to brace the two gearboxes together and you’ll be fine.
If the output shafts are supported at the far end, a lot of the bending moment around the angle bracket would disappear. And of course a bar or plate across the top, from one edge to the other, including the tops of the gearboxes, would add considerable rigidity.
There’s an engineering problem there, what kind of plate or bar would be optimal (stiffness vs. weight)? Note that it does not need to be of constant cross-section, but beware of creating an “slender column” in compression. I’ll leave that as an exercise for you.
AndyMark makes a part http://www.andymark.com/product-p/am-0443.htm that secures the top to the Transmission and the outer lower rail. Can you weld gussets in the corners of the current mount?
I’m not sure of your resources , so I’ll comment as if a younger version of my team had designed this. Next time, it would help us to know if you have a full CNC shop at your disposal or two students with a multitool.
I’ve come to really dislike multi-level base frames, because they often add a lot of weight without a lot of functionality. There’s really no reason why you need a second level of frame here, especially made out of 80-20, which is quite heavy. If you want the flexibility in design that 80-20 provides, you can connect sections of it directly to the AM base, by bolting through holes in the c-channel to biscuits in the bottom of the 80-20. If you’re worrying about strength, the AM base is quite strong, especially with a bellypan, so there’s no need to strengthen it a lot.
I hope you’ve considered the benefits (and costs) of going dead axles. I’m a huge fan of live, sliding axles, but I do accept that some teams like to go dead. Don’t just jump to the conclusion that one or the other is better, think about the advantages and disadvantages in the context of your team.
In regards to bracing the transmissions, those two lower holes on the gearboxes look ripe to attach to two of these, like others have suggested.
Using the formed hexes on the back plates as mounts for a brace was my first choice, as it would have been the simplest for us. But according to the CAD the face-face length is .45" and i can only find .375" or .5" hex stock.
While we couldn’t weld anything, due to not having access to a welder, using a brace like this is an option. While I couldn’t use the same one from AndyMark, I could easily make one with the same hole pattern as the Ball Shifter.
I believe my design could be an example of the comment (in bold). The brace is made out of .125" aluminum so it could have the ability to bend up or down. Although I think it would be strong enough for it’s aplication.
We have access to a water jet through one of our sponsors and possible a laser cutter soon. As far as having a second tier made out of 80-20, we used a frame similar to this last year and were really happy with the flexibility it gave us. It proved to be very strong, and it’s makes it easier to attach the superstructure and make iterations as the season goes on. I agree that it’s heavy and a little unnecessary as far as strength, but I think it’s advantages are worth the extra wieght.
One thing to consider is that running your numbers through JVN’s calculator, your going to be pulling about 6.2 fps in high gear, which is definitely on the slow side.
I’ve measured (in the CAD model) and I found that it the front of the gearbox it has a two stepped hex 7/16" to capture a nut and then 0.5 for the stand offs to fit in for the third stage design. The back of the gearbox is just 7/16" for the nuts, unless we are talking about different parts of the gearbox.
I wonder if VEX will sell the brackets separately that way we could just buy a couple extra, bolt them on top and then put a piece of plate across that. It wouldn’t be the lightest option but it would be dead simple for teams to implement.
