Quote:
Originally Posted by cxcad
I meant something like this, but I think you answered my question. I don't quite understand how the tensioners work; can you post an image of them in action? Also why not exact center to center belt? |
Quote:
Originally Posted by highlander
I would also be very interested in learning how these tensioners work. I saw the picture you guys have, but how do you make sure that both sides of the shaft are aligned so that the wheel is straight? |
This is a pretty good picture of how our tensioner blocks work. The block in the upper left is positioned such that the side facing up would be bolted to the sheet metal.
The inner block slides relative to the outer block, the bolt comes from the front and provides a pull tensioner. The outer block has a lip that combined with the sheet metal fully captures the block. The springs return the block to neutral when you de-tension the wheel.
When the wheel is fully de-tensioned, the dead axle lines up with a hole in the frame. This allows us to push out the dead axle. Our wheels then drop out as one module (there should also be pictures of our wheels in the picassa albums). Once the wheels are tensioned, the dead axle no longer lines up with the hole and is captured by the frame rails. We find this to make wheel changes very easy. Also, with belts, it is critical that you are able to replace a broken belt quickly, and this does a good job of enabling this.
We align the wheel angle by eye. It turns out that the human eye is pretty good at judging this, we typically sight if the wheel is parallel to the lightening hole. We have never had an issue of having a miss-aligned wheel. We ended up going to a separated tensioner design because we really wanted a pull tensioner for various reasons (2012 was push and had lots of problems), and we couldn't come up with a way that was compact enough while maintaining accurate wheel angle.
We don't do exact center to center because it is hard to install them this way given our frame, but more importantly because you do not get the full strength performance of the belts if they are not tensioned properly. Our experience is that it is not reasonable to do this reliably with FRC tollerances. The drive belts are particularly important to get every last bit of performance out of because they are running out of spec. We do exact center to center in a lot of other mechanisms to save design and manufacturing time when the loads are low (IE the belt tying our two intake wheel sets together and the first stage motor reduction on our tusks).
Quote:
Originally Posted by highlander
I would also be very interested in learning how these tensioners work. I saw the picture you guys have, but how do you make sure that both sides of the shaft are aligned so that the wheel is straight?
I have a couple more questions:
How hard is it to replace the timing belt with this setup?
Sheet-metal wise, what tolerance does your manufacturer provide?
What thickness and alloy of sheet metal do you use?
Do you guys press in bearings into the sheetmetal? If not, what else do you add to keep them in there, and if so have you had any problems with them falling out? |
-Replacing timing belts is pretty easy. The center wheel is on a dead axle also, so it can drop out without us having to disassemble any of our transmission (the problem of belts in a traditional WCD).
-We always find that our sponsors are better at the tolerances than they are willing to commit to. They will give us their standard bend tolerance of /pm .015", but our experience is that they are much better than this. I think we spaced a .010" gap between anything in the drive train that was a bent part that needed to fit inside of another bent part. We didn't have any fit problems in assembly.
-We use mostly 060 on the robot, however the drivebase was half 060 and half 090. Any sheet metal that could be probably impacted by other robots (bellypan, outer rail, and maybe one or two others that I can't remember) were 090. The rest was 060 to save weight. This year was really brutal and both chassis still looks like they are new (probably helps that the bumpers also held up very well).
-We do press a lot of bearings in, however we are re-evaluating that for the future based on some bad experiences this year. We were disappointed to find that the vex pro bearings have a very large undercut by the flange that is about as thick as a piece of 090. Their bearings (especially the hex bearings) have a very poor press in sheet metal. We will likely be sourcing different bearings for next season.