25 Nov
2008
25 Nov
'08
6:41 p.m.
For small pulleys, check out a hobby shop that deals in radio controlled model airplanes
and cars. For small gears, try www.nwsl.com. They also have small drills, bolts, and metal
rods (e.g., 1.5mm x 6"). In England, try http://www.ultrascale.co.uk/ for small
gears.
Bob
25 Nov
25 Nov
9:49 p.m.
Hi
When we ground the pulley down, we checked the rate as
we ground it down. What I'd like to know is how does one
pedict the effective diameter when the pulley is crowned,
as in the SA800?
Dwight
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10:16 p.m.
On 25 Nov 2008 at 21:49, dwight elvey wrote:
When we ground the pulley down, we checked the rate
as
we ground it down. What I'd like to know is how does one
pedict the effective diameter when the pulley is crowned,
as in the SA800?
I measured the pulley across its widest part (i.e. top of the crown).
My (perhaps flawed) logic was that it was the highest part of the
crown that determined the speed. Were it otherwise, the speed would
be related to the width of the belt.
Cheers,
Chuck
26 Nov
26 Nov
12:48 p.m.
=20
Hi
When we ground the pulley down, we checked the rate as
we ground it down. What I'd like to know is how does one
pedict the effective diameter when the pulley is crowned,
as in the SA800?
My guess is that, since the blet runs on the maximum diameter part of the
pulley (the point of crowning being to keep the belt on), then taking the
maximum diamaeter should be OK for this.
Thinking about it, should the ratio of diamaters of the spindle flywheel
and the motor pulley be the same for all positions across the width of
the belt? In other words should the (larger diameter) flywheel have a
steeper crowing angle than the motor pulley?
-tony
10:03 p.m.
From: ard at p850ug1.demon.co.uk
Hi
The flywheel is flat. Only the motor pulley is the only part that
has a crown. When the belt runs on the pulley, the center of the
crown stretches the belt a little. This is what makes the crown
keep the belt centered. The stetching means that the effective
diameter of the pulley is slightly less than the crown. I
would guess that one would need to know how much the belt
stretches at the crown in order to know how much to compensate
for the diameters.
The belts on the SA800s are quite stiff so I suspect that using
the crown would get one close.
Dwight
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=20
Hi
When we ground the pulley down, we checked the rate as
we ground it down. What I'd like to know is how does one
pedict the effective diameter when the pulley is crowned,
as in the SA800?
My guess is that, since the blet runs on the maximum diameter part of the
pulley (the point of crowning being to keep the belt on), then taking the
maximum diamaeter should be OK for this.
Thinking about it, should the ratio of diamaters of the spindle flywheel
and the motor pulley be the same for all positions across the width of
the belt? In other words should the (larger diameter) flywheel have a
steeper crowing angle than the motor pulley?
-tony 
27 Nov
27 Nov
2:20 p.m.
Tony Duell wrote:
Only one of the pulleys should be crowned. If they're both flat, or
both crowned, it will throw the belt. No, I don't know why.
Gordon
=20
Hi
When we ground the pulley down, we checked the rate as
we ground it down. What I'd like to know is how does one
pedict the effective diameter when the pulley is crowned,
as in the SA800?
My guess is that, since the blet runs on the maximum diameter part of the
pulley (the point of crowning being to keep the belt on), then taking the
maximum diamaeter should be OK for this.
Thinking about it, should the ratio of diamaters of the spindle flywheel
and the motor pulley be the same for all positions across the width of
the belt? In other words should the (larger diameter) flywheel have a
steeper crowing angle than the motor pulley?
3:19 p.m.
On 27 Nov 2008 at 22:20, Gordon JC Pearce MM3YEQ wrote:
Only one of the pulleys should be crowned. If
they're both flat, or
both crowned, it will throw the belt. No, I don't know why.
Just checked a Siemens FDD-200 here--only the motor pulley is
crowned. I maintain that the speed is determined by the maximum
diameter. The purpose of crowning a drive pulley is to keep the belt
from wandering off the pulley; the lower parts of the
crown actually
play little part in furnishing drive.
See, for example:
http://www.lhup.edu/~dsimanek/scenario/crowning.htm
Cheers,
Chuck
10:32 p.m.
From: cclist at sydex.com
On 27 Nov 2008 at 22:20, Gordon JC Pearce MM3YEQ wrote:
Hi
I agree that the bottom part usually doesn't even touch the
belt but the part of the belt that is not stretched over the crown
is closer to the same stretch as would be on the other pulley.
I would tend to think the closest to the shaft that is just
touching the pulley is closer to the speed of the belt
than the stretched part over the crown. Depending on
how elastic the belt was, this could be quite different
than the top of the crown.
Still, the belt would need to have a small amount of stretch
length wise to force it over the crown. This would mean
that the effective diameter was closer to the diameter
where the belt was just touching and not the crown.
As for why it stays on, that is simple. First, as anyone
who has tried to put a belt on a moving pulley will
tell you, the belt will no walk up a tappered pully.
To get the crown to work, one needs to get some amount
over the top. It will then center.
Lets look at what happens as the belt is off center.
The side that has the wider distance from the crown
will be stertched less and ride lower on the pulley.
The narrower side will ride higher. Because it rides
higher, it will have more tension than the wider
side. This will pull it back to the center.
If it gets pulled too far uneven, there won't be
enough tension on the narrow side to pull it back
and the belt will slip off. This effect can also
happen if the load is too large.
These effects can be demonstrated on disk drives.
Dwight
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Only one of the pulleys should be crowned. If
they're both flat, or
both crowned, it will throw the belt. No, I don't know why.
Just checked a Siemens FDD-200 here--only the motor pulley is
crowned. I maintain that the speed is determined by the maximum
diameter. The purpose of crowning a drive pulley is to keep the belt
from wandering off the pulley; the lower parts of the crown actually
play little part in furnishing drive.
See, for example:
http://www.lhup.edu/~dsimanek/scenario/crowning.htm
Cheers,
Chuck
28 Nov
28 Nov
9:14 a.m.
On 27 Nov 2008 at 22:32, dwight elvey wrote:
If it gets pulled too far uneven, there won't be
enough tension on the narrow side to pull it back
and the belt will slip off. This effect can also
happen if the load is too large.
These effects can be demonstrated on disk drives.
But crowned pulleys work pretty much with any kind of belt. A belt
operating on a cone-shaped pulley will always position itself to the
largest possible diameter on the drive pulley, even if the belt's a
wide flat stiff one, such as leather.
While the belt may hug the smaller-diameter parts of the pulley due
to elasticity in the lateral axes, most rubber belts are made to be
relatively inelastic in the longitudinal axis. A belt with
significant longitudinal elasticity can be unstable in a driving
situation, which is why rubber bands work poorly as drive belts.
We may not settle this one for certain. The sure-fire way is to
measure what's going on, but I suspect the net difference between
your theory and mine will be exceeding small. Still, it's fun to jaw
about it.
Cheers,
Chuck
8:40 p.m.
From: cclist at sydex.com
---snip---> >
But crowned pulleys work pretty much with any kind of belt. A belt > operating on a
cone-shaped pulley will always position itself to the > largest possible diameter on
the drive pulley, even if the belt's a > wide flat stiff one, such as leather. >
---snip---
Hi
I've read this a couple times but practical experience
says this is not true. If it were true, I'd be able to mount
a belt on a crowned pulley by just holding it to the pulley
and turn the pulley.
I've never been able to have this happen. In fact I see
just the opposite. The belt keeps pushing off the pulley,
as one would expect. It just doesn't want to ride up
onto the pulley and over the crown. I always have to
get a small amount over the top of the crown, by force,
before it will pull itself on.
Dwight
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ard＠p850ug1.demon.co.uk -
cclist＠sydex.com -
dkelvey＠hotmail.com -
feldman.r＠comcast.net -
gordonjcp＠gjcp.net