I am going to start this thread cause I can’t believe this… We have been planning on making a WCD drive for the upcoming season but had no clue on how to do it. This just simplified and explained the proccess to us amazingly. Wow great job. If any other teams are planning on making a WCD drive and were stumped like us check this out might give you a few ideas on how to start.
Sorry, I don’t think I was very clear in my last post. I apologize for not communicating myself well.
My question was more along the lines of “why didn’t you mention the possibility of using a CNC to make the c-c distance exact for teams that have access to one?” My question was there just in case a team had a CNC, but didn’t really know that you could take advantage of c-c distance, and remove the need of a tensioner.
One again, apologies for not clearly asking my question.
The question that begs for an answer is the necessity for a chain tensioner.
This year’s kit chassis AM14U is belt driven and has no belt tensioner.
From what I read timing belt usually does not stretch and theoretically if you have the optimum C-C then you just set it and you’re done. But what is the optimum C-C distance? Is it the distance that the WCP calculator spits out using the pulley size and belt size?? I felt that there is more to that. When I build the prototype the belt tension has a significant influence on binding and the ability to rapidly go backward and forward without slop. Too tight and things bind, too loose and there is slop.
Right now we are using the Cam belt tensioner to adjust the tension until we have a smooth drive train that is also responsive to rapid change in direction.
My guess is once I find the right tension than I can leave it alone but what is the best way to find the optimum C-C distance for the optimum belt tension without using a CAM tensioner. Does anybody have a good formula for the optimum C-C distance?
From what I’ve read, you should put in tensioners regardless because of the tolerances required for belt engagement. Adding a cam tensioner is less than 0.1lbs anyway, so there’s no reason not to have one.
There is a reason not to have one, in that they cost money and can be safely skipped provided you have some other way of keeping the block from sliding.
The easiest way to do this is to simply assemble the thing without the cam, pull the versa-block until the belt is properly-tensioned, and then match-drill from one of the holes in the versa-block through the framing member and bolt it in place. You will then never have to worry about belt-tension again, and can save money on the cams. We did this last year at 449, and it worked perfectly. If your pulley is too close to the framing member to fit a nut on the other side of the versa-block, you can even tap one of the holes and just screw it into the versa-block itself.
Not to say it couldn’t be done with chain. 846 ran without issue through Buckeye, Silicon Valley, Champs, Chezy, and CalGames without any provision for changing chain tension. We nailed the C-C right the first time and never looked back.
Just my own experience. I would be comfortable running chain with the versablocks locked in place after the right chain tension was found. Just make sure your gearbox pilots into the frame.
I know that this system uses two 3-CIM transmissions but with only 2 motors each. Do you think that with the back-and-forth, high impact motion that a typical FIRST drive train will go through, the 9mm belts might not be strong enough? Would 15mm belts be a better choice for a drive train, especially for those who want to add on that third motor to each side?
If you put in tensioners, you could mess it up. If you don’t, you can’t. A very minor tradeoff, I know, but it’s worth noting. It’s surprisingly easy to overtension a belt.
You absolutely definitely do not need tensioners for a belt driven WCD, provided you can machine bearing holes in your drive tube with reasonably good accuracy. Our past three years of robots will attest to that.
We have had poor experiences with 9mm wide belts in the drivetrain using pulleys about this small. Last year several teams had belt failures doing this, though manufacturing issues at Vex may have contributed in addition to undersizing.
To me there’s basically no benefit to not going 15mm, and the reliability is much higher. Even if it does work, why would you want to gamble on the edge of a safety factor with your drivetrain? All of our 15mm setups have been rock solid.
By the way, a good calculator for belt center distance is here. http://www.wcproducts.net/how-to-belts/ We have had good results with zero center distance added. Your mileage may vary based on your tolerances.
449 ran 9mm belts last year without any issue on their WCD.
Looking at the belt specs (which are conservative - you can get more than the rated torque out of them given how short the lifespan is in FRC compared to industrial applications), if I were running an 8WD I’d probably be happier with 15mm between the two lower wheels. For a 6WD, where the vast majority of the robot’s weight is on the center wheel to begin with, I don’t think it’s a likely problem.
As for why bother with 9mm, it allows one to use the WCP gearboxes with pulleys-in-gearbox just as one would do with chain. Personally, I’m not a huge fan of that setup - if a belt does break (admittedly unlikely but it’s better not to tempt Murphy if you can avoid it) it’s a royal pain to fix - but even in other setups it’s nice and much more compact to be able to fit 2 9mm belts on one pulley than have two pulleys.