Blind vs Solid Rivets 15

Aircraft are conventionally built using solid rivets. Blind rivets are only used in areas which are, well, "blind". What advantages does solid rivets offer over blind ones? Why does conventional wisdom still dictate that solid rivets should be used wherever possible?

 

[WKTaylor]

1. I almost choked on my tongue when I read AJSalemi’s last statement about techs “crimping blind fasteners to improve fit"!!! As a production Liaison engineer and as a field engineer I have see “similar attempts to make fasteners work by shit-house engineers! Altering fasteners not an artisan’s “trick-of-the-trade”… it is non-standard [unacceptable] practice that provides a feel-good measure to believe that a fastener works, when it does NOT... and is a guaranteed way to fail parts in-service under light operational loads. In the case of blind-fasteners this can definitely interfere with full installation [proper formation of the tail] due to increased friction [since tail is blind... You may never see how irregularly they were installed].

WARNING: NEVER ALTER ANY FASTENERS without intimate familiarity with their function! I just happened to have spent a LOT of time discussing fastener installation, removal and performance with many OEM [manufacturer’s] fastener engineers*... and discovered [obviously] that fasteners perform PREDICTABLY only when installation and usage variables are rigidly controlled by the TECHNICIANS. I wish I could attach a cartoon to make this point. Sub-title: “Who really determines the strength of a fastener??" Shows an engineer, a metallurgist, a processor [for fastener fabrication] and a BIG-LUG “man with a wrench” [mechanic]. [courtesy of SPS]

*** MIL-HDBK-5 Committee engrs for Cherry, Huck, Fairchild [VSI], SPS, Hi-Shear, Monogram, Allfast, JO King, Boeing, McDonnell-Douglas, Grumman, etc…

One thing I discovered Years ago was the truth of: “In-God-We-Trust... All Others Will Be Investigated”

Aircraft quality workmanship means accomplishing work to the engineered standards that the specific aircraft was built to... and to all other converging standards [maintenance, repair, etc]. If tech data requires “X”, then “X” or “X+” must exist, otherwise the work does not meet aircraft standards. Integrity is everything and is the basis of aviation. As AJSalemi indicated, this can be violated by deceitful actions, up-front and pass inspection... but the evidence of poor engineering or workmanship is guaranteed to surface in loose fasteners, cracks, corrosion, etc... up-to and including catastrophic failure.

2. Regarding blind fasteners in inlets… and other critical applications: I emphasize that blind fasteners should be substituted for other type fasteners only on a very thoughtful basis... not only can the engineering strength and DURABILITY of the joint be compromised, but if failure occur, and FOD can damage or destroy an engine . It is amazing what a small/hard piece of metal can do to a $2M engine in a blink of an eye [especially titanium].

2.1 General Practice has been to match existing fastener installations in FOD critical structure, such as engine inlets, for (2) reasons: fastener performance [strength/durability] is predictable and designed into the assy; and the fasteners are “mechanically locked so that” potential for fastener disintegration is minimized. If blinds were originally used [typical many years ago], “no sweat”... just apply good engineering sense and analysis for replacement parts... and be FOD aware! Otherwise be very careful!!!

2.2 WARNING: engine inlets are subjected to severe vibration loading [long-term fatigue], severe loading due to compressor stall/stagnation conditions and environmental abuse [erosion/corrosion]. I have witnessed OBVIOUS fastener failures and cracking that caused significant engine damage [turbine & piston]! In other cases I have seen enough damage within inlets that had the potential to generate FOD. I am a believer!!!

2.3 I have used blinds in engine inlets, under very strict controls as follows:

a. NEVER EVER use aluminum-shank blinds rivets and (ANY) JO-BOLTS in inlets, unless specified in the repair manuals.
b. Always use Steel, CRES, Monel or Titanium OVERSIZE pull-type blind-rivets or bolts [plated or coated, wet with sealant or primer]. I prefer MONEL or CRES Cherry-Max [NAS9307-thru-9313] and "MS" style blind-bolts [MS21140/21141/90353/90354] due to their very predictable installation and in-service performance. BE CAREFUL: ALL BLIND FASTENERS MADE TO THE SAME SPEC BY DIFFERENT VENDORS MAY NOT PERFORM THE SAME DUE TO MANUFACTURING & TESTING VARIATIONS!!!!!
c. Holes MUST be within specified tolerances or the installation is doomed to fail [pull/tear-thru, loosening, cracking, fastener-head failure, etc]. NOTE: Anytime structure is disassembled and a new fastener must go into an “old hole”, the old hole MUST be oversized and have a matching fastener installed. Driven rivet-holes can be enlarged significantly [+0.008” typ]… to meet this intent… and the same diameter/type rivet installed by slightly harder driving to swell a rivet to fit [within specific limitations]. ALL other repair requirements MANDATE drilling/reaming hole(s) to match selected O/S fastener(s). PERIOD. NOTE: almost every blind fastener comes in an oversize equivalent… use it!!!

d. Inlet skin and substructure must fit tightly against each-other, IE: NO VISIBLE GAPPING [could be the reason WHY you are patching the skin… or replacing fasteners prematurely!!!

Regarding blind fasteners use, in-general.

3. I worked for the USAF and NOW for a major manufacturer… and have noted distinct differences in philosophy regarding fasteners. These differences are most notable in a field environment and at the depot for the B-52/KC-135/E-3 acft.

a. Acft T.O.s and T.O. 1-1A-8 consider blind fasteners like “another tool in the maintenance tool-box.” The technology is clearly illustrated and is “well-known to all”. Use is encouraged and “wide-open” to any specified fastener”… especially when expedience is required. It must be noted that the expectation is that these field repairs will be reviewed by experts during depot maintenance, every 4—7 years… which allows some room for poor repair jobs to “survive long-enough to get aircraft to depot”.

b. The OEM [aircraft manufacturer] has an aversion to using blind fasteners, due to durability issues. This influences [prejudices] depot practices… forcing them to lean towards “driven solids”. ALSO a lot of structural rivets on these jets were “ice-box [DD] rivets… which are very high strength [Fty & Ftu]. Most “aluminum blinds” compare favorably to the [Fty] of “AD” rivets… which have substantially lower strength than “DDs” [~80% of DD]. A lot of “aluminum blinds” have been improperly substituted for DD rivets by untrained techs, especially in critical structure. In these T.O.s there is a specific ratio defined for installing “aluminum blinds” in-lieu-of “DD solids”; it varies from (5)-BR for-every (4) DD, up to (6) BR for-every (4) DD. For a 1 : 1 replacement, the structure is already compromised.. even with 1/64-O/S BRs. NOTE: “weakness” in blind rivet usage is NOT well understood [or even known] by most techs… so problems persist. In some highly loaded areas vibration loads are critical… blinds are NOT APPROVED what-so-ever since they have failed prematurely.

Regarding blind fastener installation.

HINT: Regarding in-service inspections for fastener, and “other”, damage: inspect aircraft when they have been flown a lot, but have NOT been washed for “awhile”. Loose fasteners, cracked spot-welds, fretting, fluid leaks, etc… will be VERY obvious due to smudging and streaking [“smoking”]. Unwashed airframes will “tell-all-sins”: while washed airframes remove the “tell-tale traces of problems”, making inspections much less accurate and reliable!
Regards, Wil Taylor

 

[jesterdan]

wktaylor, Thank you for your integrity and steadfastness, it was required. Some compromises are not worth the result.
From a heavy transport catagory operation perspective, blind fasteners are used in a no option condition. Then as soon as we can, we replace. Schedules are schedules. The problem I have run into are that the blinds actually enlarge the hole and require a second oversize solid, at least to replace. ED sometimes becomes an issue. Being from the older type aircraft, the regs come into play on the repair criteria, if blinds are installed, rework or replacement is required, within a certain amount of cycles. Long term blind fasteners just don't work in that envirnoment. I said, heavy, so I think everyone should realize that a popped fastener through the core engine could cost the minimal of the earlier cost plus revenue loss, ferry, engine position considerations, the logistics of the loss. This doesn't mean just an engine. Airspeed and impact cost the same. Take a cherry max, from a temp fuselage repair, putting a hole in a boundary layer, and on some tranistion check, a line guy sees it. The night just got a whole lot longer. What gets out today may haunt you tommorrow.

Again, it needed to be said, Thank you

 

[liaisoneng]

Bravo, wktaylor. Outstanding posts. I also spent some time as an Air Force engineer in fighters and tankers. Presently work on -135 variants in the liaison/PDM role. I wholeheartedly agree with your comments on blinds in inlets - solids would most certainly be preferable, but often the lips and shrouds give no reverse side access. The hole filling nature of the solids is paramount, especially in pressurized skin applications with cyclic loading. A word on installing rivets wet with sealant - Boeing has stated in their tech data for the -135 series that solid rivets are installed wet with primer. Their stated concern is that hydraulic resistance of the sealant might inhibit the rivet swelling and keep it from filling the hole. Question to AJSalemi - standard AF practice is to use MIL-S-8802 sealant in fuel applications not the 81733. Have y'all had any problems with leakage?

 

[johnfalky]

Wow! this is a very dynamic area! All set off by blind fasteners! I strongly concur with Mr. Taylor. I have seen some severe engine damage resulting from the use of blind fasteners in inlets, caused by "technicians" and "shit-house" engineers. It is my viewpoint that when such components need Blind fasteners, they have reached a point where they are unfit for service. However, remember that under FAR 145, technicians have more power and authority than Engineers! I found some of this "artisan" work going on and found myself abruptly unemployed 2 years ago. However, as Engineers, or Technicians, the standards we discuss in this forums must be maintained. As for blind fasteners and oil-canning in control surfaces, stop! If the fasteners were originally solids, how where they removed? I assume by drilling and punching out the tails. More than likely, the sub-structure is now deformed and as the blind fasteners are pulling the structural parts together, the oil-canning you see is actually the "twist" or the misalignment that was not corrected after the structure was opened up.

 

I am attempting to remove a front nose hinge off of a UH-60 Blackhawk. It is cracked and the engineers sent us a diagram what they want us to do. They call for drilling 3/8 in diam. hole thru the fiberglass rim of the nose door, to expose the lower two hi lok rivets holding on the hinge. Then to remove the four existing hi lok rivets, replace the hinge, and place a patch over the holes I drilled to expose the lower two hi loks on the hinge. My question, it is tight in there and the manufactures head of the hi lok pin rivet is only facing out on the top two locations which are already exposed, and I was wondering if I could drill right thru those, pop the head off and tap out the hi lok? Very tight space, that is why I am not chiseling off the nut on the back, and how would I remove the lower two, when I only have a 3/8 hole to work with. thanks for your help metal man in Boise

 

[Kenneth]

I am (personally) reluctant to recommend "free hand" drilling of Hi-Lok heads for removal, except as a last resort. It is all-to-easy to damage the structure, or wander off-center, even assuming you can produce a relative deep, and accurately centered, drill "start". It would be far preferable to unthread the nut (or threaded "collar&quot, if at all possible.

Based on your 3/8 inch diameter hole and typical Hi-Lok pin head and collar base diameters, I am assuming the Hi-Loks in question are 3/16 (-6) or 5/32 (-5) inch diameter parts. Are the "nuts" used really "nuts" (i.e. "wrenchable" hex or 12-point, non-counterbored, nuts) or are they frangible, threaded Hi-Lok "collars"? Is the access on the upper two, in which you apparently have at least some degree of access to the nut/collar side, adequate to use a "collet" type Hi-Lok collar removal tool? [see:

http://www.fairchildfasteners.com/pdf/Fairchild_Aerospace_Tools.pdf

and search for "RT200- ( )"]

If I understand you correctly, the 3/8 inch holes you are drilling through the fiberglass for the lower two fasteners are to gain access to the nut/collar side. If you can't talk your engineers into a hole diameter large enough to accommodate a collet-type removal tool (preferable), the drilled and broached hex recess in the end of the pin may help center your drill bit. I don't recommend even attempting to drill out the nut/collar side unless you can prevent the pin/collar assembly from rotating while you drill (perhaps by gripping the O.D. of the pin head -- hopefully a protruding head -- on the other side, etc.). I don't have to tell you to be careful, and only drill as deep as you need to remove the collar...

I won't be surprised if some on this site comes up with a better idea than what I've listed. Of course, run anything you intend do by your engineering folk...

Hope this is helpful.

 

[boo1]

Kenneth, i was lost searching the catalog (slow connection)is it on?

thanks

 

[Kenneth]

The RT200-( ) drawing starts on page 72 and is four pages long...

 

[RLindustrial]

Main reason is cost reduction. Blind rivets were invented for blind areas as one of two reasons. The other is ease of assembly and less manpower. Most specs show the same shear strength as a solid rivet of the same size.

Ray

 

[taoofeng]

This was hinted at before, but a blind fastener does not create the tight fit that a rivet does. At Boeing they take a 20% reduction in allowable fatigue stress for the blind vs riveted hole. So, unless you're dealing with a non-fatigue critical component, you should try and avoid them.

 

[pennpoint]

Not being an aerophile you, I have a question as to the sealing capabilities of the blind rivet vs the solid type. Is'nt the outer skin the containment vessel? Is the leak rate equal for both?
I've enjoyed all the previous posts re. this subject. Great exchanges.

pennpoint

 

[Nagler]

Brief notes for repairing structures:
1. Maintain original strength. Avoid using up any of the original MS.
2. When possible and practicable avoid using blind fasteners
3. Always replace solids with solids unless access is against you.
4. Consider installing access panel to permit installing solids making sure the net effect is not negative.
5. If you have to use blind fasteners, make sure that manufacturers' installation requirements are followed and that fastener allowables comply with Mil-HDBK-5.
6. Replace Jo-bolts with hi-loks due superior clamping.
7. If available use guidance such as Boeing's Structural Engineering HDBK and their airline engineers structural repair course notes or Airbus equivalent.

 

[AeroE]

Hello - it's been a couple of years since I looked around here. I am a senior strength engineer at a major manufacturer of fighters and I have a few comments and suggestions.

Removing hi-loks - if conventional collars are installed, grap the collar with a pair of vice grips and an allen wrench in the bolt, then just turn the collar off. Unless there is a pile of sealant or severe corrosion, the collar will turn off easily, there is very little torque on the collar. If a nut is used, use a box or open end wrench.

Trapping FO - sometimes the blind side of a fastener that must be drilled out falls into a blind inaccessible cavity, particularly on flying airplanes that can't go into a roll over fixture. The last resort after vacuuming and so on is to trap the FO is to pour some sealant into the cavity to encapsulate the pieces and prevent them from migrating elsewhere.

"Loose" Fit Blind Fasteners - There is a common misconception that NAS1398 and NAS 1399 rivets produce a clearance fit when installed. Wrong; I believed this, too at one time. These rivets, or similar rivets procured to NAS1400 must expand (in free air) to a diameter significantly larger than the nomimal hole size they are installed in. For example, a 5/32 inch diameter rivet of this type must expand to 0.171 inch diameter - it's typically installed in a 0.160 to 0.164 diameter hole. In fact, when steel rivets are used, the interference fit is sufficient to cause stress corrosion cracking in some aluminum alloys and are prohibited at our plant.

Hole filling solid rivets - if the hole is drilled to the correct diameter, a solid rivet will fill the hole. There are at least a couple of engineering advantages to hole filling and interference fit fasteners - 1) the fastener "props" the hole, reducing the stress concentration, and 2) each fastener in a joint can be relied upon to pick up load (not necessarily equal load, that is a topic for calculating the distribution of the fastner loads).

Vibration resistance of blind fasteners - NAS1398/1399 rivets originally produced by Olympic have a better lock ring than the Cherry design and demonstrate better resistance to loosening in vibration environments - we have proven this by testing (albeit severe testing). In fact we approve of the use of the Olympic Lok for field repair in inlets; the problem with this is the customer probably has Cherry Lock fasteners and tools on board.

Fatigue life of joints with blind rivets - the culprit is poor clamp up of the joint compared to hi-loks or bolts, hence higher peaked bearing stresses due to the greater deflection of the joint.

Substitution of rivets - Don't do it unless you understand the difference between a bearing critical and a shear critical joint. Don't install aluminum rivets in joints with steel or titanium details, no matter how small you think the loads might be. 1100 alloy rivets do not work harden to the same strength as AD rivets, contrary to the beliefs of some pikers.

Rivet strength - know what the hell you are doing, don't guess. If you look at MIL-5 or design manuals from a manufacturer, pay close attention to all the details accompanying the allowables, and if you don't recognize a fastener designation and can't verify beyond a shadow of a doubt what it is, don't guess. Solid and blind rivets come in all sorts of materials and they are hardly ever equal strength. Verify the fastener you are removing from the joint - there are many airplanes still flying with ice box rivets and you cannot blindly substitute AD rivets in the joint. The good news is a 7050 alloy rivet (K or KE abbreviation) that can be substituted.

Vibration driving of fasteners - This is a practice guaranteed to damage composite panels with delaminations, and should be avoided (read this to mean prohibited) in the flanges and webs of machined parts, particularly the large expensive parts that can't be R&R'd - you will crack the part.

Remember, you're not building or repairing cars - it's a long scary trip to the ground when something goes wrong.

 

[JimCulliton]

The FAA benchmark can be found in Chapter 4 of AC 43.13-1B. While this is acceptable data, as opposed to "approved data" it is s a good description of both types of fasteners. However, if a manufacturuer issues a specific repair, using a specific rivet, then that would over ride the acceptable data of AC 43.13.

Olympic fasteners are also used but the equipment for pulled rivets can be quite expensive in the way of off-set heads, etc.

I also second the motion that the pulled rivets are too expensive for a rivet by rivet exchange. Also, if a pulled rivet loses the little lock ring, then FOD is a huge problem. UH-1 and OH-6 aircraft have a plemum chamber that is a natural harbor for such FOD.

 

[edbgtr]

Hi Folks
I have been monitoring this Thread since its inception, and it can't be bettered!!
Copy, by whatever means you have available(PDF995 works for me), this contribution and paste it into your favourite text book or structures manual. The information in here cannot be obtained from any text book. It is priceless.
Ed.

 

[Stache]

To up-date the FAA position on blind rivets. The FAA now requires a field approval to install blind rivets in type certificated aircraft during repairs. However if the manufacture calls out a blind rivet a field approval is not required. We at the FAA see allot of blind rivets being install in small General Aviation aircraft and it is causing some serious problems with cracking, corrosion, and elongated holes, which has caused failures.

Denny

 

[koopas]

Hi all,

I've just stumbled upon this wonderful post, and some things were unclear...

Wktaylor alluded that even though the Fsu of the blind rivet (BR) may be higher than the solid rivet's, the BR is still not as durable in terms of vibration resistance. It was mentioned that the driving action of the solid rivet "work-hardens" the material and augments its yield strength...a benefit not found in BR's due to their poor hole-filling properties and multi-piece design. Wktaylor further mentioned that the vibration "resistance" is proportional to yield strength. Since the BR displays a lower yield strength than its driven solid counterpart, it is not as durable in a vibratory environment.

Q1: I am a bit ignorant about the subject of vibrations...why is the vibration resistance proportional to yield strength and not ultimate strength? A broader question would be "why is the vibration resistance of a fastener related to its material strength?"

Q2: Can we conclude that even though the blind rivet is statically stronger than the solid rivet (based on Fsu), other modes of failures such as vibration render the blind rivet inferior?

Q3: When the term "yield strength" is used, is that referring to the yield shear strength or yield tensile strength? Since we're talking about rivets, I would assume the former would apply, since they're rather poor performers in tension.

Q4: Wktaylor wrote that "In general I use blind rivets to substitute for driven rivets ONLY when I can prove the BR YIELD STRENGTH is equal to, or higher than, the solid rivet yield strength". This is related to the previous question. I am a little confused. Where do you find yield shear strength? I can only find the Fsu allowable, the shear ultimate strength. Could you please present an example substantiating the substitution of a solid rivet with a BR?

Q5: In terms of fatigue, reading all the posts, I gather that the BR is not as durable because of two main factors: poor hole filling and poor clamp-up. Now, clamp-up for the BR was previously compared to a hi-lok or bolt, and I can see that the clamp up for the BR is indeed inferior. Is the clamp-up for the BR truely less than the solid rivet?

Q6: The NAS1898 and 1899 BR keep on coming up...are these better than the Cherry Max BR's? We only use Cherry Max at my airline as I've been told they were the best. Is that so?

Q7: To sum up, is it reasonable to qualify the BR, when compared to the solid rivet, as:

-equivalent statically, based on Fsu
-inferior in fatigue
-inferior in vibratory environments
-more expensive
-heavier
-harder to install due to hole tolerances
-poorer sealing

that said, would it be acceptable to use of BR's on a part with difficult backside access with daily repetitive inspection and replacement of the BR's with solid rivets per B/P at the next check? I understand that it depends on many factors, but I am wondering if counting on repetitive inspections is an acceptable and prudent action to mitigate the risk of undetected BR failure.

Thanks for reading,
Alex (always learning)