Shimming of Metal Assemblies 7

[koodsg]

Hello Guys,
I am looking for the help on choosing the shim material for the Aluminium (mainly 7000 and 2000 series).
Looked back on the previous programs 2000 aluminium sheets are used with the T3 heat treated.
Thinking over using GFRP shim on this assemblies probably strength and the Galvanic corrosion will not be there.
If any one give some suggestion of this topic, thanks.

 

[Sparweb]

Are you waving the red flag on purpose?
Why introduce GFRP into a joint you just said was made of aluminum?
Why make a shim cost 50 times more than necessary, when a strip of AL can be cut to fit in minutes?

STF

 

[berkshire]

The most expensive material you want here is/are peelable shim assemblies. Otherwise Stacks of material cut from varying thicknesses of material to suit the assemblers needs is more than sufficient , unless you need tapered shims.
B.E.

You are judged not by what you know, but by what you can do.

 

[koodsg]

Sparweb thanks for your feed back, I didn't know about the red flag was mistake.
thanks berkshire.

 

[RPstress]

Peelable shims usually need multiple layers to match the shim thickness required. This in turn tends to make them a source of maintenance trouble in years to come due to corrosion, and sometimes primes have a sufficient dislike of them to ban them. If the shim is quite thick then milling a single layer for it is more reliable and doesn't usually cost much. While glass or carbon can be used as shims with quite high compression stresses and the shim won't corrode (of course, beware of using carbon if there's Al about), as SparWeb suggests they're likely to be quite expensive. Using some quite hard aluminium (perhaps not T3? Though it won't matter in practice) is usually the simplest way to go. And provided it's acceptable the peelable option is often simple and quick even if it does give a slightly greater chance of future problems.

 

[WKTaylor]

Koodsg...

There are 5- general-types of structural shim materials [with variations due to metal alloy]. Each is intended to rigidize the joint by eliminating gapping between members which then ensures some-what-predictable fastener load transfer/sharing between parts. In general, a gap that can be closed [0.002—0.005 remaining is usually OK] between parts with ‘finger-pressure’ will fasten-together “OK”: However a gap that CANNOT be closed between parts with ‘finger-pressure’ should be shimmed to prevent permanent gaps being built into the structure.

1. Solid metal, custom made. These are blocks, wedges, etc, machined from solid metal to fill an obvious/predicted structural gap due the natural fit of parts. These are usually designed before-hand and are manufactured from specified metals to a specified shape with a corrosion protective finish applied [IF/as required]. These usually developed by a designer and have a drawing-specified Part Number.

2. Solid sheet metal, standard sheet metal gages. These are normally made on-the spot to fill relatively uniform gaps between parts [usually expected], which may occur with wide gap variation for min-to max. These are generally trimmed to fit a gap-profile and may have slight taper. They usually have a corrosion protective finish applied after trimming/tapering/drilling has been accomplished.

3. Peel-ply-laminated metal shim. Foil or thicker individual sheets are bonded together. The planform may be trimmed to fit a joint shape; while laminates are individually trimmed to fill an odd off-size gap with ‘solid metal’. A sharp knife will generally score one laminate at a time so that each can be broken off until a it conforms with a gap shape. Once trimmed, drilled (or reamed)-to-fit, they are usually brush-primed for increased metals isolation [and to seal the trimmed bonded-laminate stack from moisture intrusion]. Typical: MIL-DTL-22499/*

4. NOTES.

4.1 Solid metal shims can be made from virtually any metal alloy.

4.2 Structural peel-ply laminated shims are generally made from aluminum, CRES or titanium foil or sheet alloys. Mechanical peel-ply shims are often made from steel and brass foil or sheet.

4.3 Current good practice demands that these metal shims be installed/fastened ‘wet’ with sealant or primer to seal moisture/fluid-drain pathways.

5. Prepreg fiberglass-epoxy shim, cured on installation. The soft multi-ply material is trimmed to fit a joint planform; and then is caught/crushed between the members causing localized thickening and thinning of the malleable material. This material is then hard-cured while on installation.

6. Liquid shim made from epoxy paste adhesives filled with powdered metals for improved crush resistance. The liquid shim materials is mixed to catalyze, then buttered onto the mating parts, which are pulled together tightly with temporary fasteners. As soon as the parts are settled together as-tightly-as-possible, then the excess liquid shim [squeeze-out] is generally wiped-away.

7. NOTES.

7.1 Typical non-metallic shim and liquid shim specs, see AMS3726/*

7.2 General good practice dictates that these non-metal shims be over-coated with epoxy primer or/and sealant to 100% seal them from contact with any moisture or random petroleum/synthetic fluids.

7.3 Non-metal shim materials are rarely ever used if exposed-to, or may-be exposed-to, fuel, moisture/water/hydraulic oil, etc.

7.4 CAUTION. Glass fibers absorb moisture and fluids along the fibers, often leading to corrosion or delaminations/disbonds. Other type fibers may also have this nasty characteristic.

7.5 Graphite [carbon, etc] fibers are highly dissimilar to aluminum, steel, etc which can generate rampant corrosion in the presence of moisture/oxygen/metals. They also have substantially lower thermal expansion/contraction rates than most metals… and are substantially stiffer than most metals: hence may become disbonded from the metals due to thermal cycling [many problems]; or worse-case, may adhere tightly to the metals and ‘pick-up-unintended loading from the metals’ due to stiffness mismatch.

7.5 My favorite liquid shim materials are made by Devcon... aluminum putty, titanium putty, stainless steel putty, etc: very useful stuff [made me a hero a few times].

http://www.devcon.com/

8. CAUTION. Shims appear to be minor structural elements. DO NOT BE FOOLED: they can be very critical. Suggest taking a little engineering-time to study/understand how they function; the affects of the various materials and finishes; and long-term implications for structural durability.

8.1 Airframe structural Design, Michael CY Niu

Analysis and Design of Flight Vehicle Structures, EF Bruhn [I think]


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[berkshire]

Anecdotal story,
During my time at GD Convair , we were producing MD11 fuselages for Douglas. Somebody red-flagged the number of shims going into the spaces between the formers and the skins. This produced a major brainstorm as to what to do about it. Several bad tools were identified, as were parts produced to the top or bottom of the allowable tolerances. A major campaign was started about making parts to nominal instead of to tolerance. (Machinists had a tendency to go to the high side of the tolerance to avoid making parts undersized), and supervisors were instructed to pay attention to parts where assemblers were having trouble making things fit, and call engineering down straight away. Within 3 months the company had shaved 300 pounds off the empty weight of the aircraft.
B.E.

You are judged not by what you know, but by what you can do.

 

[tbuelna]

In my personal experience with mechanically fastened components that required shimming at installation, it was usually cres peel shim or liquid shim.

Cres was used for peel shims since the material does not require corrosion protection, and the edges of the shim plies usually get scraped/scuffed when trying to separate them. Galvanic mismatch was not normally an issue with the cres shim and aluminum parts, since the aluminum parts were usually anodized or primered. The weight of a cres peel shim is also not usually a concern since the volume of shim material is typically quite small.

I have used liquid shim where uniform contact was required between uneven surfaces to distribute a bearing load, rather than filling a gap/space between mating faces with no contact.

 

[blakmax]

Whatever material you use with metals susceptible to stress corrosion cracking MAKE SURE YOUR SHIMS ACTUALLY FIT. I have spent large amounts of time repairing stress corrosion cracks in 7075-T6 on P3 Orion and C-130 Hercules (prior to J model) mainly because of bad shimming. Incorrect shimming resulted in bending of the wing plank risers causing stress corrosion cracking of the risers. For the P3 repairs, we undid the riser fasteners and used an epoxy as a liquid shim injected through specially made injection fittings which injected the resin at exactly the correct location in the joint. Once the epoxy had set, we installed a bonded carbon/epoxy patch over the cracks and then installed the original fasteners and tightened them up, without of course the residual stresses induced by bad fit-up.

Regards

Blakmax

 

[tbuelna]

The point about making sure the shim matches the gap is excellent. Laminated shims work fine as long as the gap faces are close to being parallel. If not, then measurements should be taken of the gap profile and a matching shim should be machined to fit the space. I don't think this is a problem you would see with original assembly of aircraft structures, since there should have been manufacturing/QA procedures that ensured any shims used had a proper fit. But I can certainly imagine a situation where a joint on an older airframe is dis-assembled for repair, and the gap at re-assembly is too large/misaligned to permit re-use of the original shim. In this case it would be good practice to make a new shim.

Modern production processes used for metal aircraft structures take advantage of things like accurate CNC machines, laser alignment equipment, and determinant positioning techniques that minimize the need for shimming at assembly. So you don't see a whole lot of shims being used on new metal aircraft structures anymore.

 

[blakmax]

Hi tbuelna

The examples I mentioned were from original manufacture but they were from 1960-1980 manufacture. I also forgot another example from F-111 where the crew module floor trusses suffered stress corrosion cracking due to bad shimming. In that case, x-ray stress assessment found that errors in shimming as small as 0.003 inches (less than 0.1 mm) resulted in sufficient residual stress to cause stress corrosion cracking.

Hopefully in these days things are better, but unless manufacturers and maintenance personnel recognise the risks of just tightening the fasteners I suspect the problem will continue.

Regards

Blakmax

 

[koodsg]

Thank you all, giving an brief understanding

 

[WKTaylor]

koodsg... All... You might find the following document enlightening, this/similar subjects... in it's own unique-way...

NAVWEPS 00-25-559 Tips on Fatigue

http://www.dtic.mil/dtic/tr/fulltext/u2/a397117.pdf

The embedded humor makes the subjetc digestible and adds so value in learning. I Try really hard to get this document out-there so a lot of these issues that have been around decades can gain traction in peoples brains.

NOTE. some of the figures, cartoons, sketches, etc in this document have shown-up in other/similar documents [especiallly for training purposes].

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[Sparweb]

Excellent info, Wil, as always.
Do you think Bill McCombs had something to add on the subject?
I don't have my copy of his Supplement right now, to check for myself.


STF

 

[rb1957]

Steve,

looking for a copy ? or you'd don't have it at hand ?

another day in paradise, or is paradise one day closer ?

 

[tbuelna]

I have a copy of that document. While it was published long before I started working in the aerospace industry, the subject matter is still relevant. It should be required reading for all A&P techs.

The most interesting thing I noted about this document is what it currently costs to obtain a copy versus what it cost when originally published. Today you can obtain a copy for free, but 52 years ago when it was originally published it would have cost you 70 cents!

 

[rb1957]

I was thinking of McCombs supplement to Bruhn ... but I think you're onto something else ?

another day in paradise, or is paradise one day closer ?

 

[Sparweb]

Thanks for the offer RB. My copy is at the office, of course. Like a pro chef, I keep the sharpest knives in the kitchen.
I am intrigued by your reference, too, Tbuelna.
I did mean the Supplement to Bruhn, by Bill McCombs.
I've perused my copy at work and it is quiet about the subject of shims (it really is more a detailed study of buckling conditions in thin metallic structures).
The chapter in Bruhn where shimming (and joggling, and fit-up stresses, and so much more) was written by McCombs, which is probably why I remembered his name on the subject.

STF

 

[tbuelna]

I was referring to the document linked by wktaylor, "Tips on Fatigue". This 134 page document presents some fairly complex subject matter regarding fatigue of metal structures in a way that aircraft technicians can understand. As some noted above, structural repair/rework procedures only work if they are followed. I've found that the most effective approach to getting technicians to follow work procedures is by taking the time to educate them on the importance of each step in the process, and the potential consequences of not following the process.