Minutes of the EM Testing Meeting
December 1, 2003
Present: Pisa, Bari, Hytec, UCSC, SLAC
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Erik said that the torque of 2.8 N-m could be doubled for the bolt that we are using between flexure and vibration fixture, but we need also to make sure that the helicoil can take the torque. He was skeptical about reliability of trying to evaluate the friction in the joint.
The issue discussed was whether the shear of the shoulder joint can be relied upon to support the tower in vibration. Sandro reported that the shoulder is ¼ inch minus 3 mils, while the hole in the flexure is ¼ inch plus 1 mil. He doesn’t yet know how precisely the shoulder fits into the hole in the fixture, but they will measure that tomorrow. The shoulder extends a few mm into the fixture.
Sandro said that the 4 mil slop is necessary for assembly of the tower to the grid. They were barely able to fit it together as it was. They had to use the following sequence:
4 screw installed in the corners. Then 4 screws with holes in the side flexures, then 4 screws with slots in the side flexures.
This was the only order possible. Tighter tolerance would make it very hard or impossible to mount tower.
The assembly tends to have all of the flexures pushing on one side or another of a shoulder bolt. Therefore, ideally friction between the titanium flexure and aluminum vibration fixture has to keep the flexure from slipping from one edge of the shoulder to the other during vibration. Hytec did not see a problem with this in the static testing (Erik, please re-evaluate this and check what torque was used in the static testing).
Sandro said that the z axis sine sweep test initially gave the right horizontal frequencies (Sylvia: peak at 182 Hz on accel #3 on the low-level signature z axis sweep), but it seems that locking on the shoulders was lost by the time of the x vibration. Erik noted that non-linearities could explain a drop in resonant frequency between the low-level sweep and high-level random vibration. In fact, based on the static test results of that tray, which was softer than the tray presently at Hytec, he did not expect the x-axis frequency to be as high as 180 Hz at high levels. What did the z-axis signature sweep show after the random vibration? Erik will look in detail at the data.
Martin agreed to review the design with Tom and Erik. Erik will review the data before assessing what analysis to do. Dick will get John Ku also engaged in the analysis, perhaps working with Steve Spencer. however, Erik announced that Steve will not be available any more after this week. He will talk to Vince at Hytec about getting new analysis support in place.
Erik will look into traveling to SLAC asap to meet with engineers there. Data from the test are all on the Bari web site. Sylvia will make them available to the world tonight. Sandro will post photos on the Pisa web site.
There was some more discussion of the interface. Sandro expressed skepticism about the steel shoulder interfacing into the aluminum grid. Maybe a steel insert is needed? Tom said that the screw threads were intentionally left slightly loose, so that the screw would center on the shoulder (hence we need to know how tightly the shoulder fit into the fixture). Sandro suggested using a ¼ inch thread with a tight fit and no shoulder. Erik did not think that the precision on threads would be good enough, even if we went to class-3 threads. He suggested possibly going to ¼ inch threads and a larger shoulder (which would require redrilling the flexures already mounted on the tray).
Sandro noted that abrasion of the aluminum by the flexure was primarily only around the screw locations, suggesting a limited region of friction contact.
Erik asked what is the finish on the inside of the vibration fixture. Tom will talk to the machine shop. It looks like an EDM cut and is kind of rough. What should be required there?
We discussed whether we could do the T/V test first, before completing the vibration test. There is an issue of thermocouples between trays. A thermocouple of 1mm height is probably too much, as that would reduce the distance allowed for tray movement. There is also a risk of too much assembly and disassembly before doing the vibration. Sandro is especially worried about the cables, with the fragile Nanonics connectors. There seemed to be agreement that it would not be wise to do the T/V first.
Nanda said that Alenia has a classified customer doing vibration this week.
Nicola spoke with Alenia today. They have lots of work this week and perhaps also next week. We can send a request for a window for activity when we know what we want to do.
Sandro said that they will remove the fixture from the tower tomorrow and measure it. He will also check the torque on the other sidewalls. Robert asked about checking the other 3 sides of the bottom tray. He cautioned that we need to be careful not to make the same mistake as 1.5 years ago. When sidewall screws backed out in that test we fixed that problem and went into a second test without knowledge that a crack had already developed in the bottom tray. Sandro will think about the risk of checking the other sides of the bottom tray. Taking off all sidewalls presents a risk of damage to the sidewalls. Tom suggested that maybe the bottom tray could be removed entirely with just 1 sidewall removed (two allow the two cables to disconnect from the bottom tray first).
Sandro was at Plyform today and related a new issue: they inspected all of the honeycomb cores. At a very close view they observed that it is quite common to have some small damage near the edges of the light honeycomb, with slight bending over of the walls near the top or bottom. He will send samples of various levels of damage to SLAC. Also he will make non-flight panels with a variety of samples and measure them with ESPI to evaluate whether they have any impact on performance. Robert asked if the damage was right at the edge, which would then not be significant, since all the support of the facesheets at the edges is provided by the closeouts. Sandro said that it actually extends inward somewhat. The theory is that the effect comes during the cutting process. Tom said that there are Mil-Spec standards for the level of damage allowed. He will collect these specs and send them to Sandro.
Sandro reported that making inserts flush with the closeouts will require redoing the tooling, as well as remaking the inserts, to make them longer. This would require delaying the start of tray production into February, instead of December 9. He asked if the screw head could be made larger, with a 120 degree head. Tom will look into that. It would be completely non-standard but maybe possible.
Erik said that there is possibly an issue with the compression data from Plyform, which suggest a significant reduction in the B allowable. He is worried that the margin from analysis will no longer be positive. He will write a note detailing this.
Tom will set up a teleconference for Wednesday morning. The next meeting will be at the Tuesday engineering meeting.