In a message dated 30/10/00, sean@........... writes:

>  Epoxying the hinges to the frames is not reliable in the long run.
>  The seis here went bonk in the night 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; 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. 

Dear Sean,

    No adhesive is perfect, but epoxies can give excellent bonds if you allow 
for their limitations. They a) have a relatively low peel strength b) do not 
like water one little bit and c) need to be fully cured.

    You can usually design joints to increase the resistance to peel forces. 
This could be done for the hinges by sticking the foil onto the 1.5" L beam 
and then sticking a cover plate twice the foil width over the top, to give an 
overlapping sandwich construction. 

    It is not generally realised how seriously epoxy joints are weakened by 
damp surfaces. The adhesion may be markedly improved by heating the materials 
to be joined in an electric oven at 150 C for half an hour, which removes 
most of the surface adsorbed water. You remove the material from the oven and 
put adhesive on while it is still hot. It may then be rapidly contact cooled 
and the joint fully assembled while still warm, but before the glue has set. 
Water contamination is especially noticeable when gluing ceramics and glass. 
You can also use an electric blower / paint stripper, but the temperature 
control may be more difficult.

    Many epoxies will develop stronger characteristics if they are cured at a 
raised temperature (maybe 60 C?) after they have 'set' at room temperature. 
Do check the specification sheets. Select a low curing temperature from the 
range recommended to minimise cold joint stresses.

    I hope that these observations may be of some help in turning the 50% 
success rate into 100%!

    Regards,

    Chris Chapman
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