Re: Sprockets as Chain Tensioners?
 

You need current and speed, or current and voltage, or voltage and speed.

Ideally one of the two parameters is held fixed to make it easier to compare.

If you can measure all three then the extra data can be used as a cross-check of the test's integrity.

Re: Sprockets as Chain Tensioners?
 

On our first failed ball control system, we used a free-floating sprocket as a chain tensioner, just as you are describing. It worked great, but did fall off once or twice with a hard hit. I think it was a 24 tooth 25-chain sprocket that was in the middle of two 12 tooth sprockets spaced about 6 inches apart. So it was a small system, but it did work very well.

Re: Sprockets as Chain Tensioners?
 

Chain "stretches" (actually, it is wearing, not stretching, but the end result is the same) and gets longer with use. A system that is initially designed with perfect spacing will end up with loose chain as the system is used.

Re: Sprockets as Chain Tensioners?
 

With short chain runs, the stretching is negligible and is often not an issue. We have used this numerous times over the past few years with no issues. On the longer chain runs, tensioning is much more necessary.

Re: Sprockets as Chain Tensioners?
 

I doubt that a static tensioner has a negligible effect on efficiency.

If you've got a two stage spur gear reduction and a single chain run from the gearbox you're already down to 83% efficiency or so. If you then shove a block of plastic into the chain to tension it, it would seem you would be liable to lower efficiency quite a bit.

We have no data, but empirical evidence has suggested to us that our overall drivetrain design is noticeably more efficient than the average drive similar to ours in terms of same number and type of motors, similar gear reductions, top speeds, and wheel type/friction.

Re: Sprockets as Chain Tensioners?
 

I'm not saying you're wrong, but without any data how do you draw this conclusion? What "noticeable" observations did you make? Not criticizing, just very curious.

Re: Sprockets as Chain Tensioners?
 

I think the difference is evident in a few ways.

First is that engineering judgement would tell you anything and everything you add into the chain circuit is going to increase friction and reduce efficiency.

Secondy, the robot performance on the field confirms the statement. 254's robots always move faster and are more durable, even compared to other robots that are using a very similar drive.

Anyone who has seen their robots up close, or competed with them on the field, would agree.

Re: Sprockets as Chain Tensioners?
 

If I may, I have a couple of questions, Cory.

I infer that you do not use static tensioners, is that correct? If so, do you use rolling tensioners, floating-sprocket tensioners, or no tensioners at all?

When you are comparing the performance of your drivetrain design to other drivetrains similar to yours, is "similar" limited to designs which use static tensioners (but are similar in all the other ways you mentioned), or does it also include designs that use the same tensioner design as yours?

Re: Sprockets as Chain Tensioners?
 

254 has a "mobile" sprocket system, where the end bearing blocks get pulled outwards/inwards to tension the chain. The only things on the chain are the gearbox sprockets and the wheel sprockets (it's a live-axle system). I don't know how they compare, but there aren't a lot of people who use the bearing-block tensioning system that I know about. So it seems reasonable that they look at the basic drive system without regard to the tensioners.

Re: Sprockets as Chain Tensioners?
 

My questions for Cory were an attempt to discern from the anecdotal evidence given whether 254's superior drivetrain performance is due exclusively to tensioner design, or whether other design issues or craftsmanship play a larger role. This is an important distinction which has practical ramifications for other teams trying to emulate their success.

Re: Sprockets as Chain Tensioners?
 

The design has an emphasis on efficiency throughout. All ball bearings, a minimization of parts & gear reductions whenever possible, and no tensioners touch the chain (sliding bearing blocks as already said).

Each item is a small percentage, but I imagine they certainly add up. We run a very similar drive, and teams often comment on how quiet it is.

Re: Sprockets as Chain Tensioners?
 

I will buy that static tensioners are inefficient (seems obvious). But what about idler sprockets? Worst case you are adding in the friction of an extra ball bearing, plus the angular momentum of the sprocket itself. The losses from such a system should certainly be dwarfed by factors like alignment, lubrication, and gearing.

Re: Sprockets as Chain Tensioners?
 

Well considering every link in a chain is essentially a steel pin in a steel bushing, the more you make those move or twist (bends in the chain path), the less efficient it will be.

Re: Sprockets as Chain Tensioners?
 

my team used some larger zipties to tighten up our chain and they worked very well.

Re: Sprockets as Chain Tensioners?
 

My gut instinct is that the friction at the interface of the idler sprocket teeth with the chain would be a bigger factor (albeit still small) than the idler's ball bearing friction.

That's my guess too. Which is why it would be most instructive to see some real apples-to-apples data comparing a well-designed system to itself: with and without a tensioner touching the chain.