Hello,
Because of manufacturing issues last season, this season, for the first time ever, our team is opting out of the kit base. Without knowing the game, we are looking at running a WCD made from the Rev Ion Extrusion (which we will experiment with in the next month). However, I have a few questions.
How do you determine the drop height of the drop-center wheels, do most WCD still use one?
How do you accomplish the drop with bearing blocks if they are all the same size (the kit bot has the hole o?
Is there an advantage of using chain over belts?
Is there much advantage to using 3 motors per side over the traditional two?
We are looking at using this PDF as a model but would love to see other build guides.
I am not the best at mechanical design, so any help would be appreciated.
Yes you will need a drop (baring Omni in the corners), 1/8-1/4 is typical.
Vex bearing blocks include the drop. We make our own without a drop and use oversized wheels in the center, but that requires custom wheels or treads.
We use chains so that they can go into the tubes, but others probably have more opinions on this.
More Power= More better! To be honest if your using brushless you can hit the friction limit with 2 motors depending on the wheels and weight, so 3 motors really don’t matter.
Thanks, that is excellent info. and really helpful.
Do you have difficulty tensioning the chain in the tubes?
Do you see noticeable stretch throughout the season?
My team has been using wcd since 2015 at least and we machine those ourselves so i won’t be able to answer your questions about the rev ion kit. For the chains vs belts, the chains are more reliable if you plan to do more than one competition but they are also heavier so depending on the robot weight it might be an issue. Also, chains won’t normaly skips if you put down more power or if you are trying to fight off defense.
For the motorisation, the more power you have the faster you should be able to go but it really goes down to what acceleration you’re able to get. Most fields won’t need your robot to go 20 feet/s to be competitive. Finally, having 6 motors on your drivetrain can be thé best option if for exemple you have a PTO in order to reuse them for the climb. This particular method is more complex but super efficient.
These AndyMark Blocks are my favorite ones. They are narrow enough to accommodate leading edge wheels (a favorite feature), and the drop is only 0.04" which I’ve found to be enough to prevent scrub but not so much there’s notable rocking. Also the clamping force is hella strong, we did not need the cam to keep the blocks in place.
If you use belts I strongly recommend 15mm wide ones. We use #35 chain because we were able to break #25 in testing, and I didn’t want to have to keep thinking about it.
We ran 3 motors per side in 2022 because there wasn’t much of a reason not to, and it would have been easy to remove if we needed to (for weight, or to use the motors elsewhere).
Edit to add: Even with corner omnis I recommend a small drop (personally I like a raised end over center drop, but I doubt it matters). We tried to do flat/no drop in 2019 and it was mildly sad when we discovered that 6" colson wheels were actually around 5.995" and 6" omni wheels were actually around 6.005". We didn’t do anything about it and it turned out fine but I was still sad I overlooked that possibility, I’d have rather had slightly more normal force on the higher traction wheels.
Drop-center height - 1/8" is pretty typical. Some teams choose to run omni-wheels on either the front or back axles to eliminate the need for a center-drop and the associated rocking of the base that you will get. Omnis on both front and back is also possible, but further limits traction.
With the VEX bearing blocks, the holes are offset by 1/16" from center. You install the center one flipped compared to the outboard ones to net a 1/8" center drop.
It’s more compact to use chain if you are trying to get it inside the drive tubes. Belts inside tubes might be possible with big enough tubes. Belts need larger pulleys to get enough teeth engaged to handle the torque.
If you are using the newer brushless motors, you’ll probably find that two motors per side will give you sufficient top-end speed and not be traction-limited.
I’m not in a place right now to link any drivetrain calculators, but assuming that someone else does not post links to the most common ones, I’ll try to remember to do it later.
We hand tension the blocks by pulling on the blocks as we tighten them. This is not the easiest method, and by not securing the blocks other than the clamping force we have to re-tension several times a comp. Tensioning isn’t harder due to chain in tube, but tensioning should be considered. When we used the vex blocks we hated their tensioners, we used 1/4-20s to pull the blocks at one point, but this allowed us to massively over tension the chains and skew the bearing alignment on shafts.
We used them and hated them. You can’t apply too much force with them without stripping the aluminum hex. They should only be used to hold the blocks in place once you tension by hand.
The main thing I don’t like about vex blocks is that both halves are the same and have 2 threaded and 2 unthreaded holes. This requires you to install bolts from both the inside and outside. We use custom blocks so we can access all screws by removing the wheel. Looks like the AM ones could also be used with all screws from the outside.
Sometimes we have built tensioners into the tubes, other times we’ve left it alone at nominal center center with no major issues. This assumes relatively coarse auto mode requirements, tight auto performance requires tight drivetrain build quality.
Note that you should use a relatively small center adder, something like 0.012", vs exposed inboard chain where you could run 0.018".
The chains or belts in the tubes make it annoying to attach additional things to the drive rails. Your students will be fighting interferences that they can’t see. I wouldn’t do it first year.
We have settled on inboard chain with clear plastic covers over them to keep wires out while preserving visibility, instead of chain in tube.
Hi! Yes, this was a conscious design choice with our bearing blocks to allow the fasteners to all be accessible from one side. It was also a conscious choice to utilize nut pockets with nylock nuts for the threaded feature, instead of tapping the block. We wanted these to never come loose, and be reusable many many times.
@Mr.R_2
If you have any particular questions I’d be happy to answer anything I can, and we have 7 different WCD robots around at the moment using custom and COTs components. I’d be happy to grab pictures or jump on a call to discuss our experiences.
Will also add that we’ve used the cams for the last several years and loved em. As a team with no easy way to machine accurate center-centers, it makes building a WCD super easy with nothing more than a saw and drill (or drill press if we’re so lucky).
We tighten our bearing blocks so they just gently hug the tubing, and then turn the cam with a wrench until our desired tension is achieved. From there we tighten up the bearing block bolts a bit more.
It seems most people are replying with information about VersaChassis and not Rev ION. We built a Rev ION chassis over the past month and it functions in a similar way as VersaChassis but it is different. I do not know of a way to do a drop center so you will need to use Omni wheels on one end if you have 3 wheels per side or in each corner if you have 4 wheels per side.
If you want to run chain/belt inside the MaxTube you are going to need to use plates so that the bearing doesn’t take up the space that the sprockets/pulleys need. We were able to build it with chain inside the tube using REV-21-2012 sprockets.
We have never run chain in tube before but this makes it so easy that we are probably going to do it this year especially since the MaxSpline spacing makes it so you don’t need to tension the chain.
I’d imagine that the AM/Vex clamping tube blocks still work on the ion system. I haven’t looked at CAD yet, but I’d imagine you could use the predrilled max spline holes as the hole the axle goes through (not centered). That would remove most machining, even if the only machining required was a drill.