Preload additive to flight loads?

[Benbarca7]

Hi everybody,

I have a question regarding the preload, for Nacelle latches for example.
We usually preload the latches by compressing two surfaces against each other.
The flight loads have to overcome the preload and then the load in the latch increases.
the new fatigue spectrum would be preload to maxflight load instead of zero to max flight load.
The preload is not added to the max flight load.

It also depends on the stiffness of the compressed stack-up, if it is less stiffer than the hook/pin, the flight load may be added to the preload.

Please correct me if I am wrong until there.

Now my question is:
can a latch (or a system) be preloaded without having any compressed surfaces?
is that possible? or does it have to compress something? would the preload added to the flight load?
To give you an example, imagine a half cowl, with an upper and a lower latch preloaded, attached to a clamped pin at the top and the bottom.

I cannot test my assembly to know if the preload would be additive.

Thanks for your advice,

Ben

Ben
Nacelle Stress Engineer (repair on Civil Aircraft)

 

[SWComposites]

can a latch (or a system) be preloaded without having any compressed surfaces? is that possible?
> don't see how this can be done

or does it have to compress something?
> yes

would the preload added to the flight load?
>no

what is a "clamped pin"?

 

[rb1957]

given what you say, your latch is like a preloaded bolt ... the two surfaces are clamped together by the latch, and are being pulled apart by the aero suction. that said, your assumed fatigue cycle (preload > max flight) is typical of simplistic bolt analysis (ie the load in the bolt = preload until the joint gaps and then it = the external load). relatity is more complicated, there is a blend rather than a sharp kink ... load in the bolt increases as load is applied, but only by a small amount, so that the joint actually gaps at a load higher than the preload.

that said, i'd've thought that your preload would exceed the limit flight load ('cause gapping is something you want to Really avoid, no?). so if the joint doesn't gap, then the fatigue cycle would be preload > (preload + k*applied load), k < 1

 

[GrandpaDave]

Ben,
You might want to track down NASA JSC document NSTS 08307, Criteria for Preloaded Bolts... It was used for the Space Shuttle program for major bolt analysis.
The folks at NASA like to do science projects since they have too many centers and too many people spending tax payer's money.
Just search the web for "NASA Bolt Analysis" and up it will pop.
Good Luck --- Grandpa

 

[rb1957]

i wnet looking for your report ...

found it on the JSC site ... but the link is broken (yes, i asked someone to look into it)
found it on scribd ... but they want a subscription before you can download ... which raised a question in my mind ... should a site be allowed to charge to download something developed by NASA ? does a site (like Scribd) have the rights to post a NASA document ? i appreciate that NASA developed data is available freely to US citizens (how can you limit that on the internet ?) so i figured they said, "ok, we'll make our reports available on our website" but i'm surprised that a 3rd party can charge for access (well, not really surprised).

 

[GrandpaDave]

Check out this MIT site: snebulos.mit.edu/projects/reference/NASA.../NSTS_08307_RevA.pdf
All the meat is back in Section 3.0 and so forth... Good Luck. --- Grandpa

PS: Using keywords "NSTS 08307 or NSTS08307" should bring numerous sites including NASA sites.

 

[rb1957]

yeah, i started there, got lots of hits, mostly either finding the doc as a reference for a later work, or some sort of "scribd" type download site (and several to eng-tips !).

anyways, got it now, thanks ... for others, i copied the site, got error 404 (sigh) but followed the link on that error page, it's under the "general" references, "nasa generic", and you'll see the report number close to the end of the list.

a handy reference for preload scatters, etc.

 

[rerig]

I didn't see a restriction.
Here you go.