Flat belts like this are a little bit of a mystery of physics.
They magically manage to stay tracking without
any sort of flanges on the pulleys.
And, even if I push this belt to the side a little bit,
like that, now if I turn it,
it just magically comes back on center.
So, I'll show you why it does this,
with a little demo piece I built.
So, here I've got a Leggo belt transmission
with a exaggerated crown pulley, here.
And, these two wheels, kind of form a flat pulley.
And, I've got a belt on here, and so let's move
this off to the side,
and so now we've got it offline. And...
make sure it's off here as well.
And now I'm just gonna turn this.
And, look what happens, it just magically wanders
back onto the center.
So, how the heck does it do this?
The key to what makes a flat belt
track on center is how the belt stretches,
so I'm using this piece of rubber band, to show you.
So, if I'm gonna pull this rubber band,
just on the one edge, like this,
it forms an arc, like you can see in the middle there,
even where I'm not touching it,
it's formed a curve, and if I pull on the other side,
forms another curve. And, this curvature
when I stretch this band asymmetrically,
is key to what makes the belt come back on track.
So, I've put the belt on here
to be off-center. And, because this part
of the pulley is higher than this part,
it pulls more on this side of the pulley.
And, not only will it do this on the one side,
cause the effect doesn't need to be very strong.
So, you can not quite see it, but the belt actually curves.
And, now if I should turn it,
and it's on center. like magic.
Even though this belt here, is actually not entirely
straight, cause it's just a piece of rubber band
that was attached to some broccoli I got from
the grocery store.
Most machinery nowadays is just powered by a V-belt,
because they're much easier to deal with.
You don't need very much tension on them, and they
can deal with a lot of misalignment.
But, they're not very efficient and they can't be run
at very high speed, so in this application,
this fan here is powered by a motor running 3500 RPM
and the belt steps it up to 11,000 RPM,
which is much more than a V-belt could
and it's also much more efficient. Let me show you
by turning it on.
And, as you watch it spin down, you can see it
just keeps running for a long time, because
the flat belt is very efficient.
has very little friction to it.
The principal that keeps a flat belt tracking on a crown pulley
is also exploited by a band saw.
The band saw wheels also have a slight crown on them,
although it's kinda hard to see under the blade right now,
and that's what keeps the blade, which acts as a belt
centered on the two wheels.
Now, on a band saw, there's always a little
adjustment knob, which allows you to tilt the top wheel,
just in case your belt
has a tendency to wander to either side
so you can, sort of, bring it back in or adjust how
the blade tracks. Let me demonstrate
how that works, with this band saw, so let me just
turn it on. And, we can see the blade is
tracking on here, now if I adjust this
the tilt on the top wheel, that will throw it
off a little bit.
Now, see that the blade has moved forward,
and now, if I put the top pulley straight again,
it'll immediately go right on top again.
So, that's how it does it on the band saw.
I've actually seen a band saw mill
that had no blade guides and just relied on the tracking
of the crown pulleys, to keep the blade aligned.
In fact, the blade had teeth on either edge.
So, this tracking can actually hold quite a lot of force.