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Re: [ATM] Use of bearings in (static) mirror cells
Hello Arjan,
This is interesting. I put some comments in-line:
----- Original Message -----
From: Arjan te Marvelde
> 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.
This is, if you are able to balance your support system in such a way that
id won't add distortion on itself (gravity, mirror aimed anywhere but the
zenit)...
With Dale's cell design, I guess this could be achieved if the flex hinges
are immobilized and the triangles suspended exactly at their (3D) centre of
gravity, right?
> 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.
If you plan using a non-altaz mount for your scope, choosing appropriate
side support points is far from trivial, and is probably the main reason for
gluing the mirror to the cell (a 2nd valid reason is to avoid the primary
dropping from the cell at close-to-horizontal scope position).
Best regards,
Olivier
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