Here is an update on the fastening of the hinges for the STM-8 leaf spring vertical seis: Epoxying the hinges to the frames is not reliable in the long run. The seis here went bonk in the night and I assumed that it was simply out of the capture range of the feedback because of the seasonal temperature changes. But the remote zero did nothing, and investigation showed that two of the flexure strips on one side of the hinge had become unglued. In the initial ideas and design of the instrument, I thought that gluing the flexures would be a reliable simplification of the design, and it has survived for several years even on the original prototype. The epoxy surface preparation was limited to sanding with 400 grit paper. But any failure in something that has to run unattended for many years is not acceptable, so epoxying the hinge flexures is probably not a good idea in the long run; it still might be if proper surface preparations are made (McMaster has pages of adhesive primers). And indeed, the flexures that remained glued had to be forcibly peeled from the frame members. But one of the major VBB sensor companies has also had problems with adhesives peeling loose after several years that jammed the mass. So the ultimate long term reliability seems to favor clamped flexures, even though this involves many additional parts with the increased contribution of micropositioning noise from thermal and mechanical stress. It is possible that using a slow set acrylic adhesive or even Locktite under the clamps might reduce these effects. To replace the flexures on the STM vertical (as shown in the web site drawing) I used the same 0.005" by 3/4" wide phosphor bronze strapping, but with the full 3/4" width on each side of the 3/8" center section that is the actual hinge area (the epoxied flexures were simply straight 3/8" wide strips). I used a standard paper punch to make 1/4" large clearance holes for the 6-32 fastening screws; a good punch will make a clean, minimally stressed cut, and is also used to shape the 3/8" wide center flexing section. The aluminum brackets (1 1/2" x 1 1/2" x 1/8" thick angles 4 1/2" long as in the drawing) are drilled and tapped (with light oil) for the 6-32 screws which are 3/8" and 1" back from the hinge corner. The clamping bars are made from 1/2" wide by 1/16" thick brass bar (ACE hardware) strips cut 1 1/4" long, and filed to a shallow angle at the inside end. The 3/16" screw holes are 5/16" and 15/16" back from the inside end. After cutting and drilling, these are carefully deburred and rounded at the edges to provide a uniform clamping surface. To assemble the hinge, the two brackets are clamped in the X shape using an L bracket in the center; the hinge corners are spaced 0.005" apart each way using the hinge stock under each side of the L bracket and crossing through the center of the assembly (where the hinges themselves cross, the bracket corners are flattened by 1/8" so that the actual flexing area is untouched). The 4 flexures, 8 clamp bars, 16 SS screws and lockwashers are all loosely assembled and squared up before being gradually tightened so as not to torque the flexures. The inside angled edge of the brass clamps is aligned with the outside edge of the flattened section of the bracket corner in the flexure area, with the slightly longer side against the flexure strip. The two flexures should not touch. This should all be clarified by a drawing of the details. And a picture is worth 10^3 words next time I play with the camera and the scanner. Actually, I am inclined to redesign the whole seismometer to about 1/2 the size using the soldered box flexures and the bought parts coil/magnet assembly I have previously described. regards, Sean-Thomas ____________________ ____________________ \ / ----- ----- ____________________/ \____________________ __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>