[ATM] PLOP Assumptions

[artbianconi@blast.net]

> Art Bianconi wrote:
 
> > I don't use PLOP.  I have reservations about the accuracy
> > with which any mechanical device can be built that responds
> > a uniform and consistent manner to changes in force vectors.

> > In a car engine perhaps where things are relatively crude.
> > but in a mirror mount?
> 
> It's your personal choice, of course - and any program of this
> character necessarily builds a model that is a simplification
> of reality. Still, you can use it in the sense it was made, to
> get a (probably decent) idea of what different alternatives can
> be expected to give - at least of the right order of magnitude.
> 
> The big question: If you don't use PLOP, what would you use to
> design a mirror cell?

Pro Engineer or Solid Works for the design of the cell, the 
mirror and to attribute the real world mass and material 
properties.

COSMOS to do the FEA and actually animate the model from 
horizontal to vertical and see how the loads change . 
> 
> > In examining the few multi-point cells made available to me, 
> > I am puzzled by the assumptions some are making in defining
> > the size and material used in the pressure pads.
> >
> > It would appear that many ATM's are assuming that the pad is
> > distributing the force equally over it's entire area and the
> > coincident surface of the mirror. Is that reasonable?
> 
> Most likely not. Why should any sensible person use large pads? 


I saw pivoting triangles on one years ago. I guess it must have 
had points. I don't recall. Still even a small surface, instead  
of a point, would avoid high concentrations or "point loads" and 
spread the pressure over a broader area as we are prone to do 
when trying to prevent structual failure. Furthermore, point 
loads are more likely to exceed the point at which the material 
becomes plastic. Hard as it is, glass is still plastic. I thought 
the whole idea was to reduce high concentrations of forces.

>From my own understanding of mechanics, an 18 point cell should 
reduce the point pressures applied over that of say a 9. Yet some 
here were surprised to discover that 6 points are better than a 9 
or an 18 (?). Frankly, I was too!

> > Is PLOP assuming that the pressure pad is NOT bending under
> > the load and that the distributed loads are uniform?

> AFAIK PLOP assumes point loads.

> Nils Olof

Thanks Nils. I am still a little puzzled though

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