RE: [ATM] Use of bearings in (static) mirror cells

["Jerry" <wa4guu@verizon.net>]

Glue and Edge Supports could co-exist in the same cell. It doesn't have to
be either or. As I understand it some like to glue so as not to need
"Richard Schwartz Mirror Clips" hooking in front of the mirror surface.
(Don't for get to tighten them down real tight.) They might have edge
supports that don't retain the mirror from falling out when looking at
spiders on the ground. And then I guess there may be some who glue to avoid
needing any edge support.



Assuming a alt-azimuth mount where when pointed to less than 90 degree
altitude the same edge supports are supporting the mirror:

Referring to Fig. 1A  at
http://home.hetnet.nl/~artm/atm/articles/deformation.html

Rotate the figure 30 degrees clockwise so that the support bar at the top is
horizontal. Instead of having that pair of support points on a cantilever
fix them and then only the lower two pairs are on cantilevers.

Now both the edge supports and the two points that are no longer on a
cantilever are solidly connected to the cell and are glued to the mirror BUT
AT THAT GLUED JOINT IS A FLEX BEARING. Proper cell design would require only
enough flex to compensate for the differential expansion of cell components
and what really should be minimal mechanical flexures in those parts, and
whatever lateral micro-movement is allowed so as not to pinch the mirror. 

You might call this a 4 by 4 cell. Four immovable points the "top two back
and bottom two edge supports are solid and glued but with flex bearings. And
there are four floatation points on two cantilevers.

Now if there is any residual "hanging" from the glue joints as altitude is
reduced due to lateral flexure in the edge supports this would be an
appropriate place for a counterweight to act against the lateral flex. I
think these could be made rigid enough not to need counterweights.        





Now forgetting the 4 by 4 cell above and going back to the design in fig. 1A
and 1B

With the cell oriented as above (rotated 30 degrees clock wise from fig. 1A)
There would be none of levered hanging on the cantilever as that one is
horizontal now. You might have hanging but it is not coupled through the
cantilever.

Referring to Fig 1B at
http://home.hetnet.nl/~artm/atm/articles/deformation.html

The lower pair of cantilevers now are 30 degrees from vertical reducing the
vertical component between the support points on both from 75mm to about
65mm separation. Would this not be an improvement? If so it is an
improvement attained just by cell orientation.


Jerry



----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Arjan te Marvelde
Sent: Friday, April 22, 2005 10:12 AM
To: atm@atmlist.net
Subject: RE: [ATM] Use of bearings in (static) mirror cells

Hello Jerry,

In my opinion, flexures are OK as are the RTV flexures, as long as you don't
glue them to the mirror.
No one seems to care though.

Stiction is no big problem if you choose the proper materials.
As an example, PTFE (Teflon) on steel has a coefficient of kinetic friction
of only 0.05 or so, and the static cof is only slightly more, probably 0.06
or so. Consider a lever in a cell of 75mm long: the force you have to excert
to overcome stiction is in the order of 3.6 gr, assuming a bearing load of
600gr and a working diameter of 7.5mm  (which I consider pretty large).
Smaller bearings will have smaller effect.

These numbers can be compared with an analysis of deformation of a mirror
glued to its cell: http://home.hetnet.nl/~artm/atm/articles/deformation.html
As you will see, the force couple caused by glueing the mirror to the cell
is way more than this 3.6 gr caused by stiction:
120gr when pointing at horizon,
60gr when pointing at 60deg elevation,
0gr when pointing at zenith.

My conclusion is that you will only see the effects of stiction when the
mirror slides on its support points. You do need a side support, but the
deformation caused by such (properly built) side support is close to none.

Cheers,
Arjan

> I have noticed Don's zeal urging consideration of the compliant
> hinges and/or flex bearings for various telescope parts. This discussion
has
> been helpful for me to understand his zeal. I'd bet his would be a very
good
> mirror cell. I'm surprised there is so much resistance to his suggestions.
> But then I am not an enginier.

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