Thank you for your advice and warnings, Eric. Although I haven't had to
design anything more than simple assembly aids before, I know enough to
treat the issue of stiffness with respect. I have done some vibration
testing before, and know the results of having a jig that's not stiff
enough, or has too many parts to it.
I note what you say about mag alloy - if memory serves, it's not such nice
stuff to machine as aluminium. However, I've spoken to our jig designer,
and he's confident that he can produce another jig that's stiff enough for
the job, and will still meet my low weight requriement. His current effort
weighs in at over 20 kg, and I need one that doesn't exceed 14kg, so he's
got quite a lot of shaving to do. He, too, came up with carbon fibre as an
alternative material, but winced visibly at the thought of what it would
cost.
Good test houses here in Singapore are about as rare as rocking horse pooh,
which is why we're having to try and do so much ourselves to get what we
want when we want it - though things don't go to plan sometimes.
Thanks again for your help. Interesting that we have very similar e-mail
addresses, but are from totally different companies.
Best regards
Peter
ST Aerospace
Singapore
Eric Christison <[log in to unmask]> 11/07/2002 05:32 PM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to Eric
Christison
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: Re: [TN] Light-weight Jig Materials
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
>
> [This e-mail is confidential and may also be privileged. If you are not
the
> intended recipient, please delete it and notify us immediately; you
should
> not copy or use it for any purpose, nor disclose its contents to any
other
> person. Thank you.]
>
>
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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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