**MINUTES OF MEETING ON Pixel-to-SCT INTERFACE AND LOAD CASES**

**held at EPFL-Lausanne on the 20^th^ September 2001**

## Presents: J.Cugnoni, M.Olcese, E.Perrin, G.Tappern, 

1)  [Discussion on load cases and results of simulation]{.underline}

The load cases corresponding to the current support conditions of the
Pixel Support Tube (PST) and calculated impact on SCT structure were
discussed again.

Assumed support conditions are the following:

-   PST-to-SCT:

-   2 points (same side), fixed x,y,z

-   2 points (other side), fixed x,y, with some stiffness in z direction
    (flexures)

-   end of PST-to-cryostat: fixed x,y, with some stiffness in z
    direction (flexures)

The following load cases have been considered:

a). Radial (x) displacement of 15 microns at the 4 mount point PST to
SCT. This displacement is due to the potential CTE mismatch PST-to-SCT
layer 3 support cylinder (2 ppm/°C) combined with a temperature gradient
of 30°C. This is in the very conservative case of an infinitely rigid
PST

b). Internal axial (z) force of 100 N on each of the 4 mount points of
the PST to SCT. This load is due to the axial stiffness of 2 mount
flexures and is induced by the potential CTE mismatch of SCT wrt to PST
(2 ppm/°C) combined with a temperature gradient of 30°C.

c). Axial action of 35 N on the two PST mount points to SCT fixed in z.

This load is due to the axial stiffness of the mount flexures at the end
of PST combined with a potential CTE mismatch PST to SCT and to
asymmetric position of the PST end supports wrt barrel axial constraint.

d). Radial offset of 2 mm of the supports at the end of PST. Different
combinations of the offset directions have been studies:

-   vertical offset of the two supports at one end

-   horizontal offset of two supports at one end

-   vertical offset of all four supports

-   horizontal offset all four supports

The results of the analysis carried out by Joel in terms of max induced
forces and displacements on SCT structure are summarized in the
following table:

  ------ -------------------- --------- ------- ------- -------------- ------- -------
  Load                        Max force                 Max                    
  case                        on one                    displacement           
                              SCT                       (μm)                   
                              support                                          
                              (N)                                              

                              Fx        Fy      Fz      ux             uy      uz

  A      ∆x=15 μm             63                                               

  B      Fz=100 N (internal)  TBD                       TBD                    

  C      Fz=35 N (external)                                                    60

  D1     ∆y~1~=2 mm                     360                            75      

  D2     ∆x~1~=2 mm           462                       65                     

  D3     ∆y~1~= ∆y~2~=2 mm              141                            29      

  D4     ∆x~1~= ∆x~2~=2 mm    325               588     67                     
  ------ -------------------- --------- ------- ------- -------------- ------- -------

The analysis takes into account the current stiffness of the PST.

E.Perrin makes the following comments on the results of the simulation:

1.  radial loads on SCT barrel due to load case A are considered
    acceptable

2.  impact of internal forces applied on SCT by load case B have to be
    evaluated, however provisions should be introduced to reduce such
    loads, which are considered too high

3.  axial displacement induced by load case C should be reduced to a max
    of 20 μm (this could be accomplished by reducing at least by a
    factor of 3 the current axial stiffness of the end fixtures of the
    PST)

4.  forces and displacements induced by load case D are not acceptable.
    The most severe scenario occurs when the offset were along the
    horizontal direction due to the higher stiffness of the SCT

To reduce the impact on the SCT barrel of load case D there are two
possible options:

a)  disconnect the end supports of the PST after the pixel insertion. In
    this configuration, during operation, the two forward parts of the
    PST are cantilevered by the barrel ID. There are two drawbacks to
    this solution:

-   the full load of the pixel detector included the forward services is
    almost entirely supported off the barrel SCT

-   any action coming from services going out radially from the end of
    PST would be transmitted to and absorbed by the barrel SCT.

b)  Decrease the bending stiffness of the forward PST.

This second option seems to be more the most attracting.

The following proposal was discussed and agreed:

-   Reduce the stiffness of the forward PST up to a given value, which
    would imply a max sag of 2 mm at the end of PST when the pixel is in
    place and the end supports of the PST are disconnected. 2 mm sag are
    still well within the envelope (4 mm), which should account for
    PST-end cap SCT misalignment during insertion. On the basis of the
    calculated bending performances of current PST combined with barrel
    SCT deflections this would mean a reduction of stiffness by a factor
    of 4-5. Corresponding loads and deflections on SCT would be reduced
    by the same factor, leading to acceptable forces and displacements

-   Modify the support conditions PST-to-SCT releasing one of the two
    axial constraints. This would significantly reduce the axial force
    associated with load case 4C. The support condition PST-to-SCT would
    then be:

-   1 support fixed x,y,z

-   3 supports fixed x,y

J.Cugnoni is requested to run (by the end of next week) an additional
simulation, determining the free sag of the PST at the two ends, in the
following conditions:

-   not supported at the ends

-   with the full load of the pixel detector inside (35 kg for barrel
    concentrated on the 4 supports, 20 kg for each forward part and
    uniformly distributed along the length)

The simulation will have to be repeated for two different values of the
stiffness of the forward tube: the current one and a factor of 4 less
(note that the stiffness of the barrel PST remains unchanged.

2)  [new proposal for PST-to-SCT supports]{.underline}

M.Olcese showed a proposal for a different concept of the PST-to-SCT
barrel fixations.

This basically consists of:

-   an independent thermal barrier for the SCT barrel. This should allow
    to carry out the X-ray testing without installing a temporary
    thermal barrier. The thermal barrier could be made out of a single
    CFRP skin with heaters on its inner bore. This thermal barrier would
    fit within radius 255 and 260 mm

-   SCT interlinks lasting inside the SCT thermal barrier

-   Four interconnecting blocks penetrating through the SCT thermal
    barrier with a similar sealing feature like at the SCT barrel outer
    radius. These blocks could have a similar design as those initially
    proposed by E.Perrin, with same connections to the interlinks.

-   The PST is connected to the SCT blocks by means of four noses
    integrated with the barrel PST flanges.

-   V grooves will allow a relative axial motion of PST wrt the SCT
    interlinks

-   Adjustments in x and y of the order of 3 mm max are in principle
    possible. All the adjustment features are outside the SCT thermal
    barrier.

-   SCT blocks would have an inner envelope around R240: need to check
    whether the X-ray system could be made to fit. If not, the four
    blocks have to be removable with the thermal barrier in place.

3)  [Next steps]{.underline}

M.Olcese will discuss with his colleagues at LBNL responsible of the
design of the PST about the above proposals and next iteration is
foreseen at next ID Engineering meeting at CERN on September the 8^th^.

The aim is to converge on a solid baseline for the fixation scheme
PST-to-barrel SCT, which is an important input for the preparation of
the Conceptual Design Review of PST (Oct. the 17^th^).

M.Olcese and E.Perrin, with the support of LBNL will prepare and
circulate a draft version of the new Pixel-to-barrel SCT interface
document some days before next ID Engineering meeting.