Peter,
Be careful about producing a lightweight jig. If it isn't stiff enough
you'll end up wasting a lot of time and money.
The tables on vibration machines are normally made from magnesium alloy
which I believe has a slightly better stiffness/weight ratio than
aluminium. However the real reason for using mag alloy is that is has a
lower loss tangent than aluminium.
I think you have three options:
1. Decide your current fixture overdesigned and either redesign it or
start cutting chunks off it.
2. Decide it isn't overdesigned but redesign it in carbon fibre - you
won't achieve a great increase in stiffness/weight ratio by moving to
another metal.
3. Take the easy option and start phoning test houses....
Regards,
[log in to unmask] wrote:
>
> Hi T-Nagers,
>
> I know that this is a largely Mechanical question, but I need your
> recommendations for light-weight materials that I could use in place of
> aluminium to make a vibration test jig. I have been landed with designing
> an ESS jig to take a 14.5kg (32.5lbs) box that will be attached in three
> places - by a wedge-shaped tongue into a groove arrangement along the rear
> bottom edge, and by two ARINC fasters at the bottom of the front panel
> below all the interface connectors.
>
> The ESS Vibration test is 10 minutes Random vibration in one axis.
> Frequencies range between 20Hz and 2000Hz, with the vibration profile
> ramping up between 20Hz and 80 Hz at 3dB/octave to an acceleration spectral
> density of 0.04g^2/Hz, then ramping down again between 350Hz and 2000Hz at
> -3dB/octave. The vibration table we have will take a maximum theoretical
> weight of 82 lbs at a spectral density of 0.04g^2/Hz before it self-aborts,
> so allowing for some oscillation around the max s.d. figure of the profile
> and a margin of safety for the weight of the Unit to be tested, I reckon I
> have a maximum weight allowance for the vibration test jig of 37.5 lbs.
>
> The jig we have, designed by someone else without a lot of science or math
> involved, weighs in at around 26kg (58lbs), so it has to lose a lot of
> weight before we can use our own vibration table.
>
> If I can't put the jig on an effective diet, I'm faced with relying on
> renting expensive time at an outside facility that's already pretty
> popular, so my windows of opportunity are unlikely to match the times when
> I need to use the vibrator without a lot of dead waiting time.
>
> Hope someone out here knows what I'm talking about and can help. I've never
> known this forum to be be completely stumped for an answer to anything yet.
>
> Peter
>
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--
Eric Christison
Mechanical Engineer
STMicroelectronics
33 Pinkhill
Edinburgh
EH12 7BF
tel: +44 (0)131 336 6165
fax: +44 (0)131 336 6001
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