pic: 8020 Chain Tensioning idea


This is my idea for our teams chain tensioning. the bearing block would be fairly loose in the channel, while the small piece that the bolt is in would be bolted through the 8020.

The hole in the tensioning piece is just a 1/4-20 hole, while the one in the bearing block is threaded. When the chain needs tension you would just tighten the bolt and pull the bearing block closer. There would be on of these little assemblies on each side of the wheel and they would need to be kept even.

Anyone see any major flaws? or have any advice?

Unless those are both COTS pieces I am unaware of, modify most of the pieces substantially to make them easier to produce.

Ideally, make it so as few machining operations as possible are required, and no machine operations are done for no functional reason (the excessive amount of rounds).

Get rid of all the external rounds, especially the one on the bearing bore, that would interefere with it’s function,

I would switch both pieces to a solid block with a flange on the bottom for mounting.

Also, the mounting holes look like they would interfere with both the tensioning bolt, and the body of the block.

Your machinist will be happier, and your parts will get done faster with the same functional performance; win-win if you ask me.

Don’t take this negatively, your idea is still a good one, the implementation could just use some work.

Thanks for the criticism

I was just kinda messing around with fillets in solidworks, and as my team mate just said to me “dude theyd be so much cooler” I’ll probably end up removing them soon.

I’m not sure about the mounting holes, it’s something I noticed but I just kinda went through, I may raise up the tensioning bolt hole, I have enough space behind the bearing to do that. If I move it up too high I’ll be pulling up on the bearing block also though, right?

external rounds do look cool, but unless they are functional or easy to do (as in rounded corners on a plate part), I would leave them off as they are substantially more work.

In the end, well designed simple parts look pretty $@#$@#$@#$@# cool regardless of rounds.

As for the bolt, checking bolt clearances is one of the most important things you can do in design. let me say that again, CHECKING BOLT CLEARANCES IS ONE OF THE MOST IMPORTANT THINGS YOU CAN DO IN DESIGN. You do not want to be the guy everyone on your team is looking at when the multiple-op CNCd part doesn’t work because bolt heads collide, and all you had to do is move it 1/8" over initially.

As for moving the bolt up, you could also switch the mounting bolts to flat heads if the flange is thick enough (also check how far the bolt is from the body, the head looks to be hitting the body as well… I’m talking about the imaginary unpictured bolt right now). Yes, moving the bolt up would “pull the block up”, but I imagine that force is minimal compared to the clamping force of the bearing blocks mounting bolts.

Also, keep in mind that you don’t need that much throw, do the math based on what size chain you are using to see how much it would have to move to change one entire link. That’s all the throw you need.

I would be more worried about the tensioning bolt vibrating loose during game play, if your just going to tap the baring block hole and use a bolt that’s a real risk. I would look into making it a straight bore through and using a longer bolt with a nylock nut on the other side, it wont slip and you can tension it just using a nut driver. It would also allow you to keep a more or less even tensioning on all your wheels because you could just easily measure the amount of bolt that’s through the nut and make adjustments accordingly on the remaining wheels.

Back when 177 used 80/20, we used to tension axles by actually welding threaded rod onto a slider nut that our bearing block was mounted to and the sliding the whole thing down the slot. You then just put a washer and nut on the rod at the end of the extrusion and tighten down. It’s a similar concept to what you have here, but less complex, and you don’t depend on the friction of the slider nuts keep things from sliding. There are a few limitations by doing this. You can’t mount anything from the end to the block, and have to leave some space at the end of the extrusion for the rod to stick out (crucial for fitting in box when doing this on a drive base).

To reduce the chance of the tensioner vibrating loose, you could tighten the bearing block down onto the 80/20 after the chain is properly tensioned. This requires bolt clearances though.

That exact thought occured to me just before I fell asleep, I’ll probably end up modifying to do this.

Why not just skip over the bolts and T nuts and just machine a T slot right into piece using something like a woodruff cutter.

Edit: Nvm I think I miss-understood design.

I like the idea. Where you are runing the bolt into the bearing block is that aluminum? If so I would use some type of insert, probably a heli-coil. With this you are less apt to pull the threads out. You can also use locking heli-coils that will reduce the issue with vibration.

The main reason so many teams have trouble with 80/20 fasteners sliding loose is beucase… of… well its becuase they used the 80/20 fasteners…

Our team takes steel 1/4-20 bolts and grinds the heads so they can fit in the 80/20 grooves. Then we put a lock nut on the top.

If your’re too lazy to grind bolts you could also buy these.

One heck of a lot cheaper than 80/20, and it will never come loose.

Or you could just put lock-tite on the 80/20 bolts, but thats a pain.

I know something we’ve done simliar to this in the past, but we drilled out the channel so a standard 1/4-20 bolt head would fit in, then we slide it where we need it. This works fairly well with good planning, this is how we’re going to bolt down the bearing block.

The method of using a bolt to pull a bearing block to tension the chain has been done many times by many teams over many years.

In my past experience with it, if you just check between each match you’ll be fine, and it often took a lot more than just once match to radically adjust the tension.

Also, contrary to popular belief, but the resistance to sliding provided by bolting the bearing block down is very minimal compared to the resistance to sliding provided by the tension the tensioner bolt supplies. There is a HUGE mechanical advantage provided by the threads on the bolt.