Re: Timing Pulleys - Chain & sprockets?

Hello,

The single biggest "problem" with chain drive is "chording". Here's
what that means:

Say you are gonna drive a gantry router using #25 roller chain. Not
too long ago, microstepping was out of reach financially; so you had
400 steps/rev from the motor(using halfstepping). Now the motion of a
chain drive is like a cable or belt drive in that the circumference
of the final drive wheel determines a few things, resolution and
travel per rev. being two. So you want to use as small a final
sprocket as possible, to get the best resolution.

Let's pretend you can find a 4T sprocket. Can you see that as the
chain (or belt, or cable) "wraps" around this, it is no longer
circular, but square? This means that the radius of the 4T sprocket
is going to vary as it turns, which means that the distance and speed
traveled will not be constant, but rather "jumpy" or "jerky".

(You can also think watching the end of the cord when you wind up an
extension cord around your hand and elbow-a 2 "tooth" sprocket- you
pull a bunch, then a little, then a bunch, then a little...)

Now most of the REAL routers which use chain drive have an output
sprocket of 12T, so the "square" becomes a 12 sided polygon. Much
closer to a circle, but still not as good as a belt, or cable, or
screw drive. But it DOES work.

Finally, most of these machines also used a 4:1 reduction before the
final output sprocket. So we have a pitch diameter of something
like .966 (comes from 12 times .25, for #25 chain,divided by PI-
3.14159; with the chording taken into account). And .966 times PI
equals 3.034, and we're gonna have 1600 steps to travel this distance
if we use a half step drive. Which means we get a resolution of
about .002 (about 525 steps/inch).

For a wood router/plastic fab/sign machine this is just fine, and as
I said, hundreds have been sold over the years. Nowadays you can use
a microstepping drive to improve things a great deal.

So, No, it's not nuts... A chain drive machine with Gecko drives is
an impressive combination. And cheap.

BTW, a properly designed chain drive has no more backlash than a
cable or belt drive. And it tolerates the shop environment rather
well. Some will speak of chain stretch, but again proper setup,
initiation/break-in and design reduce this to a non-issue for the
types of machines where this drive makes sense..

Hope this helps,

Ballendo

P.S. What you DO get in exchange for these tradeoffs is... Speed.
Even a cheap 200 RPM unipolar drive will get you moving at 180
inches/minute!
When John Kleinbauer starts offering plans based on this, remember
you saw it here first!<G>

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "rawen2" <rawen2@y...> wrote:
> Mark,
> I've wondered the same thing.
> What's the downside, if any, to using small chain & sprockets vs.
> using toothed belts and pulleys? Backlash? Cost? I'm sure chains
> would make more noise.
> I saw a picture of a router table a while back that drove the
gantry
> with an open-ended stationary roller chain looped between two idler
> sprockets and the drive sprocket. All three sprockets were mounted
> on the gantry and the chain was stretched along the length of the
> table.
> Would appreciate hearing from anyone who's used a setup like this.
> I've never heard of or seen the X & Y motors on a mill driving the
> leadscrews with chain. I suppose that should give me a clue but I
> still wonder....
> Ralph Wenzl
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, Markwayne <markwayne@m...>
> wrote:
> > Would it be nuts to use roller chain and sprockets in small
sizes,
> like
> > 10 or 20.
> >
> > Mark
> >