Measuring clamping forces

[jbarbito]

I have a quick question concerning measuring clamping force and the use of temporary fasteners and was hoping someone could provide some helpful tips.

Quick synopsis:

I have a contoured composite skin that is installed onto 6 metal frames. Before permanent installation, the skin must be mated with the frames and gaps measured for the possibility of liquid shimming. Currently, we always shim. This process takes a whole day and I would like to remove it if possible. Per a spec, I am allowed a particular amount of clamping force every 8 inches. If I could somehow figure out how to measure the force when using Clecos to hold the skin onto the frames, I could potentially minimize all gaps so much that we do not have to shim.

My research:

SO far I've thought of and have been recommennded a couple options:

1) Use sand bags (I don't like this because it's inaccurate and shoddy)

2) Use force washers w/ Clecos (I've looked into this but since the skin is contoured I do not think the washers would work properly, not to mention the process of using them would take a considerable amount of time. I'd need about 50 washers to hold the entire skin onto the assembly)

Anyone have any suggestions for a new process engineer in the aerospace industry?

 

[Sparweb]

The only thought I have is to cleco everything, then remove one cleco, measure the gap that springs back (if any) and measure the force required to push the surfaces back together. Measure that force with a supporting surface holding the skin and put pressure on the metal tab/flange it's being attached to. To measure the force use a scale like this:

(just a random picture search selection, not a product recommendation)


Are you guys riveting composite skins to metal frames? The designers aren't ready for the big leap to plastic?
Have you not noticed that one of the major advantages of composite external surfaces, ie. smoothness, has just been throw out by riveting the structure?


STF

 

[rb1957]

whilst it might be "shoddy" i think sand bags should be acceptable, say 10 lbs bags on top of supports. other than that i'd use "light manual pressure" from the installors ... if they can get clecos on without putting a shoulder onto it (!?) that should be ok.

mind you now the OML will conform to the frames whereas currently i think you're tooling the OML (like Piaggio do for their Avanti turboprop), so this will affect the outer surface (mostly the appearance, little impact on drag).

Quando Omni Flunkus Moritati

 

[blakmax]

A word of caution. I am aware of one case where during the assembly of composite panels on a fast jet the assembler tried to eliminate the time for installing a faying surface liquid shim. The result was that a very significant number of panels exhibited delaminations when the aircraft was first inspected in service. At first it was thought that the technician had over-torqued the fasteners, causing the damage, but he assured the investigation that they didn't need a torque wrench because they knew the fasteners were correctly torqued when they heard six "clicks", which were of course the initiation of the delaminations.

I would be very wary of eliminating the sealant step. Your suggestion of measuring loads and skipping the step may work for most cases but Murphy says that there will be some cases where your measurements and the relevant geometries of panel and frame do not align. The cost of repair (or replacement as was the case I discussed above) far exceeded the savings gained.

Regards

Blakmax

 

[PBPouw]

Why don't you use this type :

Then find a spring which bottoms out out your desired maximum force and place this between the body and the thumbscrew.

If you can pull the panel without the springs bottoming out you know you are within the limit. You can even measure the force with a feeler gauge between 2 spring coils

 

[dbaird]

Would force measurement such as

http://www.futek.com/files/pdf/ApplicationDrawings/102.pdf

be of any benefit on your application?

David Baird
Sr Controls Designer
EET degree.

http://www.afctool.com

 

[jbarbito]

I received some help from a fellow Boeing friend and am going to be using a torque controlled gun with a specific type of clecos for the clamping. Before I was unsure if the installation gun could be calibrated to such a low force, but apparently it can. Thanks for all the helpful tips and suggestions!