Preload additive to flight loads?
Preload additive to flight loads?
(OP)
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
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)





RE: Preload additive to flight loads?
> 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"?
RE: Preload additive to flight loads?
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
RE: Preload additive to flight loads?
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
RE: Preload additive to flight loads?
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).
RE: Preload additive to flight loads?
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.
RE: Preload additive to flight loads?
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.
RE: Preload additive to flight loads?
Here you go.