This morning we added a mechanical structure to the pipe linking the PR tower to the SPRB minitower with the aim to make it more stiff and possibly increase the frequency of its first modes.
The structure consists of a 'strong' leg bolted to the PR tower base and to the link pipe itself, and it mainly acts in the vertical direction (see attachements 1,2 and 3).
It can be removed when necessary. Note that the sand bag added on Sept 5th was left unchanged.
We then performed a tapping of the pipe structure (ITF unlocked) to check the mechanical modes.
Tapping along the radial directions (X and Y) excite mostly frequencies above 300Hz, while tapping along the beam direction (Z) excites mainly one mode around 200Hz (see attachements 4, 5 and 6).
Overall the machanical response of the pipe seems improved with respect to before: the tests done on May 23 showed radial mechanical modes of the pipe around 55Hz and 60Hz which also showed up in Z.
Presently the first modes look shifted up in frequencies and better dumped. The quiet spectra (last attachement) show a peak at 60Hz that moved to about 70Hz with reduced amplitude (X) and a peak at about 55Hz that moved to about 80Hz also with reduced amplitude (Y and Z).
Note that this comparison also includes the sand bag, more analysis to be done to disentangle the effect. Effects on Hrec and B4 beam to be checked.
Attached:
- picture
- picture
- tapping along X (radial horizontal direction: tapping on the pipe between the leg and PR tower)
- tapping along Y (radial vertical direction: tapping location as above)
- tapping along Z (beam direction: tapping acting on the flange)
- quiet spectra compared before and after the leg (plus the sand bag)
- drawings
