Looking at a link provided here about steel belts for driving the
encoder shaft eventually led me to look a several web sites of
companies that produce stainless steel belts, or drive bands. It seems
that all the applications where a timing element, such as a toothed
pulley, are not used, belt creep occurs. This happens because the belt
is only moving at the same speed as the pulley for a small part of
rotation. On one side of the pulley the belt is stretched. On the
other it's compressed. A miniscule amount. But .0001" is also
miniscule. This is probably where the "backlash" comes from that I'm
seeing. Or, maybe belt creep and belt stretch. If I can get a little
free time I can find out what is really going on. I'll keep the group
posted.
Eric

See above repaired subject line

On Thu, 06 Oct 2005 15:55:13 -0700, Eric R Snow
wrote:

Hey Eric,

There's another new one on me. What the heck is a "sand belt" ?

Brian Lawson,
Bothwell, Ontario.

On Thu, 06 Oct 2005 15:55:13 -0700, Eric R Snow
wrote:

Looking at a link provided here about steel belts for driving the
encoder shaft eventually led me to look a several web sites of
companies that produce stainless steel belts, or drive bands. It seems
that all the applications where a timing element, such as a toothed
pulley, are not used, belt creep occurs. This happens because the belt
is only moving at the same speed as the pulley for a small part of
rotation. On one side of the pulley the belt is stretched. On the
other it's compressed. A miniscule amount. But .0001" is also
miniscule. This is probably where the "backlash" comes from that I'm
seeing. Or, maybe belt creep and belt stretch. If I can get a little
free time I can find out what is really going on. I'll keep the group
posted.
Eric

Virtually ALL machine tool spindle encoders use rubber type timing
belts, including C axis lathes. The OmniTurn cnc Lathe uses a .30
width rubber belt with "steps" about .08 apart between teeth.

This allows threading into a feature. The spindle repeatability on the
C axis spindle positioning is .04 degree. with its larger ladder type
spindle drive belt.

Gunner

"Pax Americana is a philosophy. Hardly an empire.
Making sure other people play nice and dont kill each other (and us)
off in job lots is hardly empire building, particularly when you give
them self determination under "play nice" rules.

Think of it as having your older brother knock the **** out of you
for torturing the cat." Gunner

On Fri, 07 Oct 2005 01:30:03 -0400, Brian Lawson
wrote:

On Thu, 06 Oct 2005 15:55:13 -0700, Eric R Snow
wrote:

Hey Eric,

There's another new one on me. What the heck is a "sand belt" ?

Brian Lawson,
Bothwell, Ontario.
The sand belt is of course that part of town that lies along a sandy
beach or the edge of the Mojave. In some places that would be the rock
belt or cliff belt. Sorry about that. A sand belt is what you use in a
belt sander. Oh wait. That's not it either. I know. It's a typo. The
subject should have read "Encoders and belt creep". But you knew that.
Eric

Eric R Snow wrote:
... where a timing element, such as a toothed
pulley, are not used, belt creep occurs. ...
This is probably where the "backlash" comes from that I'm
seeing. ...

Are you saying that you do NOT have toothed pulleys? How hard would it
be to cut teeth?

Bob
BTW - why isn't is "pullies"?

Gunner wrote:
Virtually ALL machine tool spindle encoders use rubber type timing
belts, including C axis lathes. The OmniTurn cnc Lathe uses a .30
width rubber belt with "steps" about .08 apart between teeth.
....

Do they use toothed pulleys? Bob

Belt creep doesn't produce backlash but rather repeatibility errors.
Due to belt creep, there needs to be some indexing mecahnism to the
transmission medium so that the mechanism doesn't creep. You can do a
smooth belt if you repeatedly go back to a "start" calibration position more
often than the expected creep ould produce but if you're running something
like a mill or lathe, that really isn't practible, especially if the machine
isn't motor driven and able to slew to the reset position.

--
Why do penguins walk so far to get to their nesting grounds?

On Fri, 07 Oct 2005 13:21:44 -0400, Bob Engelhardt
wrote:

Eric R Snow wrote:
... where a timing element, such as a toothed
pulley, are not used, belt creep occurs. ...
This is probably where the "backlash" comes from that I'm
seeing. ...

Are you saying that you do NOT have toothed pulleys? How hard would it
be to cut teeth?

Bob
BTW - why isn't is "pullies"?
Bob,
I wasn't using toothed belt pulleys for two reasons. First, getting
the diameter exactly 1.0000 in circunference would be tough. And they
are not available in this size. I need this because the encoder, for
now at least, is direct drive and the encoder max pulse output is
10,000 pulses per rev. which is my target resolution. Second, even
though all my CNC machines use toothed belt drives the setup is
different. The belt drives a screw with a pitch of .200. This means
that an encoder mounted on the driven screw can have 1/5 the
resolution for the same 10,000 pulses per inch. This screw business
also means that any rotational error from the motor is reduced to
1/5. If the encoder is on the screw then drive belt backlash also
doesn't matter as much. Finally, the CNC control has a backlash
parameter which can be changed any time. So any backlash in the
positioning system is accounted for by the software whenever an axis
is reversed. The counting display I have has no such compensation. It
does have a scale multiplication parameter that can be used to
compensate for slight errors. If I wanted lower resolution then the
display could compensate for larger errors.
Eric

On Fri, 07 Oct 2005 13:23:00 -0400, Bob Engelhardt
wrote:

Gunner wrote:
Virtually ALL machine tool spindle encoders use rubber type timing
belts, including C axis lathes. The OmniTurn cnc Lathe uses a .30
width rubber belt with "steps" about .08 apart between teeth.
...

Do they use toothed pulleys? Bob

Of course. Hence the "teeth" in the belts

Gunner

Confronting Liberals with the facts of reality is very much akin to
clubbing baby seals. It gets boring after a while, but because Liberals are
so stupid it is easy work." Steven M. Barry