2x8.4m TELESCOPE
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LBT PROJECT 2x8.4m TELESCOPE |
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ALUMINIZING BELL JAR STRUCTURE DESIGN CRITERIA -
MEETING IN ARCETRI on 03/01/97 -
First point defined was the max. allowed vertical obstruction of the bell jar, (3800 mm) including cryo-pumps assembled externally to the bell jar, having 1300 mm diameter and 1300 mm height. The flanges for the above pumps, have the same dimensions of the MMT ones, have been fixed in n. 6, all positioned on the same diameter of ~5600 mm, equally spaced of 60 ° each starting from the vertical axis, so that to have the possibility to assemble, as alternative to the externally flanged cryo-pumps, three rectangular strips of cryo -panels inside the bell jar using the flanges for supporting the Nitrogen inlet and outlet connections and all the other electrical auxiliaries.
The center flange, also if having the aperture partially reduced by the structural reinforcement beams, will be dedicated only to the connection/entrance of all the other electrical utilities.
The structural reinforcement are to be dimensioned only for the strength and stability of the bell jar shell under vacuum, so to give the max. free area for the frame supporting the aluminizing crucibles with their baffles and electronic. Auxiliary flanges (n. 6) for safety vacuum valves (n.2) and control auxiliary devices, shall be placed immediately over the flange in the cylindrical area, immediately at the side of the flanges for the attachments of the electro-mechanical locking devices that shall connect the bell-jar to the M1 cell upper flange, exerting a force of 6÷7 ton each, that means preloading the two flanges with interposed rubber sealing with about 400 KN.
Two auxiliary supports at the top of the same shall easier, being the bell jar suspended at the two maintenance crane hooks, the positioning and fixation of the two pieces before starting the pumps of the vacuum system (thus in a position to be verified at the end of the Ohio design).
On the enclosed drawings are indicated the internal area at disposal, referred to the M1 cell upper flange, area that cannot be augmented because of the limit of 3800 mm total depth to comply with the hatch dimension.
Starting from this geometry, we continue verifying the FEA model to understand which will be the deformed shape of the flange when it shall be positioned with the crane in front of the top one of the M1 cell, and the max. stresses and deformations that shall occur under vacuum. The structural analysis shall be completed with the buckling verification of the unit submitted to the external pressure of 1 atm.
The bell-jar model consists of:
| item | el. type | th. [mm] | weight [daN] |
| - Interface flange | shell | 40; 15 | 2052.4 |
| - bell-jar shell | shell | 12 | 8652.3 |
| - 6 vacuum flanges | shell | 10; 15; 20 | 1882.8 |
| - bell-jar ribs | shell | 10; 12 | 1224.8 |
| - handling interfaces | beam | 282.8 | |
| | 14095 | ||
| - cryo pumps | / | / | 3 x 700 |
| - aluminizing equipment | / | / | 2 x 2500 |
| | 7100 | ||
| Overall Weight: | 21195 |
The dead masses are subdivided and applied to the ribs where approximately the real items should be fixed on.
The center of gravity coordinates are:
The material properties (Steel) are:
Only at the intersections of the ribs with the flange the nodes are prevented to move in the plane, allowing only the rotational degrees of freedom. These restricted x, y z, degrees of freedom simulate the six locking devices.
This boundary condition does not consider the presence of the vacuum sealing; this was intentionally done in order to be conservative in terms of bell-jar displacements and stresses.
Five different load cases were considered, each one representing a separate step of the handling and vacuum operations:
For each case the gravity was -Y pointing, according to the actual operations.
A further load condition regards the bell-jar under vacuum test condition (bell-jar axis along Z, gravity along -Z axis); in this condition a buckling analysis was done to determine the elastic buckling load eigenvector.
x The results of the specified load conditions are:
| Load | Interface Flange | Von Mises | |||||||
| case | Out of plane displ. [mm] | Radial displ. [mm] | max stress | ||||||
| A | B | C | D | A | B | C | D | [daN/mm2] | |
| 1) | -1.3 | -1.3 | 0.6 | 0.3 | -1.3 | -1.3 | 1.9 | 0.8 | 2.2 |
| 2) | -1.5 | -1.5 | 0.5 | / | -1.26 | -1.26 | 1.9 | 0.7 | 2.2 |
| 3) | 0.2 | 0.2 | / | / | 0.11 | 0.11 | / | -0.4 | 2.0 |
| 4) | 6.6 | 6.6 | -1.2 | 7 | 5.5 | 5.5 | / | -9.9 | 23.8 (*) |
| 5) | / | / | / | / | / | / | 0.9 | 0.9 | 15.5 |
(*) The stress is so high because the bell-jar is supported only in two nodes.
Scheme of the measurement points
| D | |||
| A | B | gravity | |
| C |
A buckling analysis was done to investigate the bell-jar capability to withstand the depression load; the eigenvector for the elastic buckling is equal to 8.16, which means the bell-jar undergoes elastic instability at 8.16 atm depression applied.
Of course, this is an upper limit which shifts downward because of the imperfection of the structure, the material et so on.
Note that the yield stress for the bell-jar is caused by a pressure far lower than the buckling one.
An enhanced scheme of the bell-jar with thicker portion of shell around the six openings for the external pumps was modeled, basing ourselves on the details of the MMT bell jar received during the January Tucson meeting.
It differs from the previous version for the presence of an annular disk around each flange; the dimensions are:
With this model only the load case regarding the vacuum operation and the buckling analysis were repeated; the presence of the reinforcements around the flanges is meaningless in terms of (displacements -measured at the interface flange- referred to the previous model.
The maximum Von Mises stress for the gravity and vacuum condition is 11.88 daN/mm2, whereas the eigenvector for the elastic buckling is equal to 10.6.
The note before still holds here.
Next follow plots of the stressed and deformed models.
This page last modified Wednesday, February 17, 1999 by J. M. Hill.