Box tubing inconsistenies for WCDs
 

I am curious as to how other teams deal with the inconsistencies in box tubing width when making a West Coast Drive. Since the snap ring groves must be CADed exactly in place it is not very tolerant of the variations in the box tubing's width.

Re: Box tubing inconsistenies for WCDs
 

download the cad in my sig.

our bearing block design isn't affected by tubing thickness.

for other designs, extra length must be added to handle the tolerance stackup.

Re: Box tubing inconsistenies for WCDs
 

You need to accommodate manufacturing tolerances in all of the parts involved (both extruded tubing, as well as any parts you make yourself). As such, lengthening the shaft some to accommodate tubing width variations, as well as bearing housing manufacturing tolerances is necessary and not really a big deal. This is pretty common. If you want to get really picky, you can add precision shims later to make up for the difference, but again, this is rarely ever worth the effort. Making things "line to line" never works well in the real world as you have already figured out.

Re: Box tubing inconsistenies for WCDs
 

For a more thorough answer, we also have to account for tolerance stackup... we just have one less item to add. this really isn't a huge difference.

Re: Box tubing inconsistenies for WCDs
 

The most I've seen box tubing run over in the 2" dimension is around .003", and about .002" for the 1" dimension. (Inside dimensions are another story.) That's pretty high precision, I doubt most students could get similar precision on a manual lathe between two of those snap ring grooves.

While the box tubing per se is fairly high tolerance, once you start adding in other parts on the shaft, the tolerances really do start to stack up. As Travis said, line to line with something like snap ring grooves tends not to work very well. Instead, it's a good idea to add in a bit of wiggle room in the form of a small adder on the groove-groove dimension.

If you don't need to do two snap ring grooves, my team's used a slightly different solution which soaks up tolerances on the shaft stackup quite well. We put one snap ring on the shaft (this year, it was on the inside) and on the other end of the shaft, we tap a 10-32 hole. A screw in this hole with a few washers retains the wheel, and nicely soaks up any inaccuracies in the box tubing, bearing blocks, etc.

Re: Box tubing inconsistenies for WCDs
 

Quite honestly, until you're chasing after the last few available percentage points of efficiency in your drive system, tolerances down to the 3rd decimal place digits are ridiculous to make a fuss about. Precision to the hundredths of an inch or order of 0.005" at best would a more realistic goal for most teams when aiming to line up their snap ring grooves and drive components. It's better to have things exact, but at the same time it is not entirely critical to the drive system working.

While tolerances do stack up, adding on a 0.003" MAXIMUM variation (based on what I have seen and heard from others) is not going to have a profound influence when you aim to line up components over the length of your robot to within 0.020". If that 0.003" is the last straw, then you have another 0.017" somewhere in your system that you likely have more control over and would be more easily addressed. Go for the lowest hanging fruit first, then work toward a more elegant and optimized solution later as you chase excellence.

Re: Box tubing inconsistenies for WCDs
 

We typically guestimate to account for powder coat thickness, then add another 5 thou planned allowance (slop), then add another 20 thou or so per side with the anticipation of using a shimming washer between sprocket and bearing and between wheel and bearing. Also, it won't hurt anything if there's a little extra space between the grooves. If you want to get it just perfect, you can custom machine spacers to exact length to go between the wheels and bearings.

Re: Box tubing inconsistenies for WCDs
 

Easiest way we have found to account for this in our shaft design without having to think about it at all in advance is to use a snap ring on one side, and a screw + washer on the other (the wheel side).

Re: Box tubing inconsistenies for WCDs
 

We've seen deviation of +/-.005" easy on tubing, possibly more.

The issue with this is it will compress the bearings and add friction. The bolt will also loosen over time.

Re: Box tubing inconsistenies for WCDs
 

Would a wave spring rectify this issue?

Re: Box tubing inconsistenies for WCDs
 

If you're careful when you do it, it's really negligible. Just backdrive the shaft before by hand to get a feel for it, line everything up, red loctite the bolt, and tighten down carefully until it's just snug. In our case, it helps that our Colson hubs have a ridge on the edge such that the part of the wheel contacting the bearing just touches the inner race. Get it just tight, then turn by hand and be sure it feels the same resistance wise. It may add a little friction, I guess, but subjectively it hasn't been much.

Adam's right in that it does loosen - check it before every day of competition, more often if you don't use red loctite.

Re: Box tubing inconsistenies for WCDs
 

No. By the time you get any reasonable amount of preload to keep the bolt from loosening, you're clamping the bearing. Even if you have a spring, the spring will transmit this same force.

A wavespring could be used to take up slop in a bolted system for where the shaft is too long, but not when the shaft is too short as discussed.

The bolt also takes more time to install and remove than other methods.

Re: Box tubing inconsistenies for WCDs
 

While we're on the topic of retaining wheels, does anyone have a favorite brand/model of snap ring pliers? Ours all slip out of the hole in the ring too easily.

Re: Box tubing inconsistenies for WCDs
 

McMaster 5449A65

Re: Box tubing inconsistenies for WCDs
 

Not to hijack the thread, but what actual brand is that plier?

Re: Box tubing inconsistenies for WCDs
 

David,

The brand should be Knipex, we own the same set of snap ring pliers. Also got the 90 degree ones.

They were recommended to us by the cows (1538) and we are very very happy with the quality of the pliers.

-RC

Re: Box tubing inconsistenies for WCDs
 

I was about to suggest Knipex too. :D

Re: Box tubing inconsistenies for WCDs
 

I've been playing with those blocks, and just wondering how the ones in the slots are kept in place. Do they just float in the middle somewhere held by chain tension? That seems really weird. I don't see how you lock them down in your CAD.

EDIT: Now that I look at it, its only 3/8ths of wiggle. So it might not be as big as an issue as I think.

Re: Box tubing inconsistenies for WCDs
 

When the blocks are loose, a chain tensioner pulls them against a fully taught chain.

This position is locked in when the faceplates are tightened.

If a WCD drive bearing block doesn't clamp the frame, it will not work well. This rigidity is key

Re: Box tubing inconsistenies for WCDs
 

Im not entirely sure what you mean by the first bit. But it sounds like you'd have them loose when you're putting the chains in and slide them around to get your tension, then tighten them down. Your first sentence made it sound like there is another tensioner somewhere, though.

Re: Box tubing inconsistenies for WCDs
 

This other tensioner is most often a cam mounted inside the tubing that can be rotated to adjust the position of the bearing block along the length of the slot, and thereby properly tension the chain when the bearing blocks are loose. Then the bearing blocks are tightened in place to help maintain their position until they need to be adjusted again.

Re: Box tubing inconsistenies for WCDs
 

If memory serves, the tensioners are missing from some of 973's CAD files. Download the most recent one from Hurricane and you'll see the tensioners.