Dob altitude bearing question...

I'm sure plenty of you know the answer to this-- and I think I do, too, though
I want some comfirmation since it's late and my mind's not working too well.
The question, regarding Dob altitude bearings... what effect on the force
necessary to move the Dob in altitude would increasing the spacing between the
two teflon pads on each bearing have? Thanks in advance.

"Bettrel" wrote in message
...
I'm sure plenty of you know the answer to this-- and I think I do, too,
though
I want some comfirmation since it's late and my mind's not working too
well.
The question, regarding Dob altitude bearings... what effect on the force
necessary to move the Dob in altitude would increasing the spacing between
the
two teflon pads on each bearing have? Thanks in advance.


Closer spacing equals less force.


--
Martin
Remove "ilikestars" from email address

"Martin R. Howell" wrote in message link.net...
"Bettrel" wrote in message
...
I'm sure plenty of you know the answer to this-- and I think I do, too,
though
I want some comfirmation since it's late and my mind's not working too
well.
The question, regarding Dob altitude bearings... what effect on the force
necessary to move the Dob in altitude would increasing the spacing between
the
two teflon pads on each bearing have? Thanks in advance.


Closer spacing equals less force.

....and less stability, which can mean tipping/rocking, vibration, and
long settling time. You could also get the same effect by leaving the
pads at the same distance and just increasing trunnion size.

-Mike

...and less stability, which can mean tipping/rocking, vibration, and
long settling time. You could also get the same effect by leaving the
pads at the same distance and just increasing trunnion size.

Changing the trunnion size is a pretty big job compared to moving the pads on
the mount....

I think the important factor in the smoothness of a DOB bearing is not the
force required to move it but rather ratio of the static friction to the
kinetic friction, if it takes significantly more to get it started than it
does to keep it moving, then the motion will be jerky..

For some reason the spring loaded devices on the Asian DOBs seem to help with
the motion, never have quite figured out the mechanism...

jon

(Jon Isaacs) wrote in message ...

...and less stability, which can mean tipping/rocking, vibration, and
long settling time. You could also get the same effect by leaving the
pads at the same distance and just increasing trunnion size.

Changing the trunnion size is a pretty big job compared to moving the pads on
the mount....

This assumes we're modifying rather than designing...

I think the important factor in the smoothness of a DOB bearing is not the
force required to move it but rather ratio of the static friction to the
kinetic friction, if it takes significantly more to get it started than it
does to keep it moving, then the motion will be jerky..

This is absolutely true, but both static an dynamic friction are
proportional to the normal force. The normal force goes down as you
decrease the arc between the bearings. A good, "non-sticky" system
would feel pretty sticky if the bearings were set at 150 degrees.

-Mike

This is absolutely true, but both static an dynamic friction are
proportional to the normal force. The normal force goes down as you
decrease the arc between the bearings. A good, "non-sticky" system
would feel pretty sticky if the bearings were set at 150 degrees.

-Mike


Since the kinetic friction and the static friction are indeed proportational to
the normal force, the ratio between them is a constant.

This ratio is what makes the motions smooth or jerky. The magnitude of the
force required to initiate the motion is not as important as what happens ones
everything starts moving. If the force drops then the scope will be jerky,
regardless of how much force is required to initiate motion.

Jon

(Jon Isaacs) wrote in message ...
Since the kinetic friction and the static friction are indeed proportational to
the normal force, the ratio between them is a constant.

This ratio is what makes the motions smooth or jerky. The magnitude of the
force required to initiate the motion is not as important as what happens ones
everything starts moving. If the force drops then the scope will be jerky,
regardless of how much force is required to initiate motion.

Jon

I'm with you here, but my point is that while the ratio remains
constant, the resultant force requirements between static and dynamic
are not constant. For example, a ratio (in lbs.) of 1.1:1 is
preferable to 110:100. That's an extreme example, but it makes my
point.

I'm probably not being as clear as you. Does the above make any sense?

-Mike