Chain and sprocket Design 4

[Helepolis]

Hi all,

I'm learning to design chain and sprocket mechanisms and i need some help figuring some stuff out.

All of the questions refer to a simple mechanism of single driving and driven sprockets.

I've read that if the mechanism has an alternating running direction (for example pivoting a turret), it is unadvised to use chain tensioners. This sound to me a bit counter intuitive especially if the plain of rotation is horizontal.

And i couldn't find a way to calculate the tension force needed for proper functioning.

 

[BrianPetersen]

Lots of real world experience here with motorcycle drive chains and the occasional internal timing chain inside an engine.

The problem with tensioners isn't so much the direction of rotation, as the torque reversals. If you have a spring-loaded tensioner, and the mechanism applies torque in the direction such that the tension is applied through the section of chain in which the tensioner is installed, the chain pull may overcome the force applied by the tensioner, causing it to back out. If the tensioner is capable of backing out far enough to allow the chain to slip a tooth, bad things happen.

Of course it is possible to apply enough tension to the chain via the tensioner to exceed the foreseeable drive forces, but this is likely to apply unnecessary force on the chain, possibly overloading it and/or shortening its life.

A tensioner that is mechanically locked in position, as opposed to applying a constant spring force, is less likely to back out. And when this type of tensioner is used, or if the chain center distance is locked in position (e.g. motorcycle drive chains), it should be set so as to NOT have any constant tension - it should actually have a wee bit of slack. Motorcycle final drives generally have a specification on the amount that the chain can be moved freely up and down at the midpoint of the slack section - in other words, there is (almost, aside from that applied by gravity) no tension on it but rather a little bit of slack.

Motorcycle timing chain tensioners (between crankshaft and camshaft) are subject to high-RPM torque reversals with every crankshaft revolution. Those chains have full-length guides along the slack sections, the tensioners generally apply a small spring tension but have a mechanism to stop them from backing out.

 

[desertfox]

Hi

Try this link

http://www.renold.com/upload/renoldswitzerland/conveyor_chain_-_designer_guide.pdf

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[BrianPetersen]

And watch this video. (I would consider this chain to be overtightened in the static position - it's almost a straight run when sitting under no load.)

https://www.youtube.com/watch?v=u6ZsWwQ072E

 

[dvd]

Think about buying a copy of this book -

http://www.cemastore.com/shop/item.aspx?itemid=73

 

[SwinnyGG]

Brian that's an interesting video. Would not have expected so much movement on the slack side of the chain!

 

[Helepolis]

Thanks for the helpful posts!
The video took me by surprise, wasn't expecting that at all.

But for the sake of the argument, in case i have an horizontally orientated chain and sprocket mechanism with alternating running direction (running at very low RPM), and guide runners are not an option.
What can be done to ensure that the chain stays on the sprockets?

 

[dvd]

http://www.renold.com/products/sprockets-and-accessories/roll-ring/

 

[dvd]

http://www.snapidle.com/

 

[3DDave]

Roller chain is not a great choice for operating sideways. It puts asymmetric loads on the link plates and pins. I think the cases where it is used in that orientation require wide/stiff chain, unlike bicycle chain, and it is used over short unsupported spans. Sideways deflection can allow the sprocket teeth tips to strike the link plates, causing the chain to come off the sprocket. Belts are a better choice as they remain in continuous contact and don't have a state where they are unable to re-engage until the belt is very loose.

 

[Helepolis]

Yeah I've sone those, but in both cases the sprockets are of similar size.
I think the firs one (hollow sprocket) will fall out of the chain by rolling closer to the bigger sprocket and the clamp will slide tawards the smaller sprocket.

 

[3DDave]

The hollow sprocket can't change position and must always be larger than either sprocket to maintain tension.

 

[Helepolis]

But if I use the hollow sprocket in an od diameter mechanisem and I make it larger than the the other two then I will loose most of the chain "grip" from the now medium-sized one, and I might be wrong but I think it'll still move towards the medium-sized sprocket.

 

[3DDave]

Every time a roller on one side moves one way a roller on the other side moves the other. The average is they don't move.

I guess your lack of posting a diagram of what it is you are doing is complicating what would be a general discussion by hiding the other 99% of the problems you probably have. If you are losing 'most' of the wrap then you don't have a good candidate for a chain drive anyway.

 

[dvd]

You understand that the "hollow sprocket" is elastomeric?

catalog
video

 

[Helepolis]

Every time a roller on one side moves one way a roller on the other side moves the other. The average is they don't move.

I guess your lack of posting a diagram of what it is you are doing is complicating what would be a general discussion by hiding the other 99% of the problems you probably have. If you are losing 'most' of the wrap then you don't have a good candidate for a chain drive anyway.

We got a bit side tracked,
I'm not constructing anything, just trying to learn some basics in chain-sprocket mechanism design and wondered what to do about the chain tension in a specific case of two sprockets, with odd diameter that can rotate in both ways and the rotation plain is horizontal.

Then dvd suggested the two options (clamp like tensioner and hollow sprocket),I replied that I've seen those but figured that they can only work in an even diameter sprockets mechanism and gave my opinion why.

And i still think that the hollow sprocket will move (for any setup with uneven sprockets) as the tension of the chain on one side or the other is always a bit looser, which results in slightly uneven friction forces on both sides where the chain grips the hollow sprocket, but again this is irreverent for the moment as it doesn't provide a solution for the hypothetical problem that i didn't found an example for, so far.

 

[SwinnyGG]

And i still think that the hollow sprocket will move (for any setup with uneven sprockets) as the tension of the chain on one side or the other is always a bit looser, which results in slightly uneven friction forces on both sides where the chain grips the hollow sprocket, but again this is irreverent for the moment as it doesn't provide a solution for the hypothetical problem that i didn't found an example for, so far.

The hollow sprocket cannot move- it has teeth.

There's only two ways for the sprocket to move- either it would have to have slip between its OD and the chain (which could only happen if the sprocket skips teeth) or the two pieces of chain moving across it would have to be moving at different linear velocities. This of course isn't possible with a single length of chain.

 

[hydtools]

This post poses a question similar to one I have been looking for an answer to. My camper trailer has expanding slides operated by 3/8 inch pitch roller chain drive pulling cables. How much tension, if any, should be on the slack side. Double sprocket on one drive shaft, two chains pulling opposed directions, 13-tooth sprockets, about 30 rpm,cables attached to essentially a box moving horizontally. The plane of the chains is horizontal, drive shaft axis is vertical.
My idea is that the slack side tension should be enough so that there is no chain sag due to gravity, i.e. chain, cable and chain-to-cable attachment fitting weight. Does this sound reasonable?

Ted

 

[Nescius]

The hollow sprocket thing isn't new. For example, it's called a "ghost chainring" when used on a bicycle. Bikes have different size driving and driven sprockets. Of course, the floating sprocket doesn't move.

 

[dvd]

[ol 1]
[li]Buy the chain book.[/li]

[li]A center drive may be good for reversing applications.[/li]

[li]Any take-up for a reversing drive needs to be able to withstand the tight-side forces.[/li]

[li]Tail shafts on reversing drives can be exposed to higher forces than for the head shaft.[/li]

[li]US Tsubaki has a nice free book.[/li]
[/ol]