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Guest (visitor) (OP)
8 Jan 02 10:02
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?
butelja (Mechanical)
8 Jan 02 10:27
I believe that they are stronger and cheaper than blind rivets.
Helpful Member!  Guest (visitor) (OP)
9 Jan 02 5:55
Well, we can always choose comparable or even stonger blind rivets. Cherrymax typically have a steel shank with an Al sleeve, and are stronger than solid Al rivets.
butelja (Mechanical)
9 Jan 02 9:03
If they contain a steel shank, wouldn't weight become an issue as compared to a solid aluminum rivet?
brin (Aerospace)
9 Jan 02 20:14
I know there are stronger versions of blind rivets out there, but MIL-HDBK-5H lists higher joint strength for solid rivets (tbl 8.1.2.2(f)) than blind rivets (8.1.3.2.2(q)).

This is an interesting topic though, lets get some more discussion...

I was told that blind rivets did not survive well in a heavy vibration environment, but I believe this was an issue with older style blind rivets.  comments?
Helpful Member!(5)  wktaylor (Aeronautics)
10 Jan 02 13:56
Folks... Be VERY carefull when working with rivets... I love "blinds" but have learned respect for their limitations. First a few facts.

1.  Driven rivets come in a variety of alloys [dead soft aluminum to inconel and CRES]. They are far more economical to install due to looser hole tolerances and installation equipment. They are installed by a "forging-like" operation that swells the shank and "squashes" the tails. In general the hole/countersink-filling properties [and other cold-working benefits], produce a very durable fastener installation with yield strength relatively close to ultimate strength. Since they typically "fill" a hole tightly, most driven rivets are fluid tight and very corrosion resistant... or can be esily made so by installing them "wet" with very thin sealant or epoxy primer. Note: driven rivets can be successfully installed in slightly oversized/elongated holes!!!

2.  Blind rivets and bolts also come in a wide variety of alloys, similar to driven rivets. The primary benefit of blind fasteners is easy one-sided assembly. HOWEVER they are much more expensive to manufacure and install, IE: they are made from multiple precision pieces, and have to be installed in precision holes using specialized tools. They are installed by various mechanisms... usually the pin-core forms a bulb on the end of the sleeve on the opposite side of the assy. Certail specialized blind fasteners have shanks that swell to fill holes... but most of the common types do not have shanks that swell... and the heads remain rigid". There is usually a wide difference between the ultimate and yield strength of most BRs due to the poor hole filling and "multi-piece" design[pin +sleeve]. In general vibration performance of blind fasteners is inferior to equivalent driven fasteners due to their low yield strengths. Also, since these fasteners do not fit as tightly into the holes [and countersinks] they are much more difficult seal and may loosen and leak after repeted/vibration loading. If designed into the joint properly, and if quality parts are used [some mfgrs are poorer than others in QC aspects], then "blinds" can perform beautifully. NOTE: oversizes are absolutely required when installing in oversized/elongated holes.

3.   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 usually requires increasing diameter of the Blind rivet... and/or increasing the numbers of blind-rivets in proportion to the yield strength differences. I am also concerned regarding installation procedures [hole/csk tolerances] and am careful to specify sealing for corrosion protection. I do not recommend BRs for use in fuel wet areas [except under very specific circumstances].

4.   Each type of fastener REQUIRES very serious consideration and should be designed into the product from the start.

Regards, Wil Taylor
Guest (visitor) (OP)
12 Jan 02 6:34
Great answer, Wil! Thanks.
But I don't understand one point which you made. You said that the hole-filling characteristics of the solid makes the yield and ultimate strengths almost similar. Why should this be so?  
wktaylor (Aeronautics)
14 Jan 02 9:36
Hi...

Driven rivets are "one-piece solid-bodies" forged into the holes [radial compressive pre-stress] with an integral head and tail [also affected by driving conditions]. In most cases the driving action "work-hardens" the material to a higher strength level and higher yield values. Mechanical-strength values for these fasteners approach the "MIL-HDBK-5 values" for the alloy [tnsn, cmprssn, shear, strain-to-fail, etc].

TYPICAL mechanical-locked blind fasteners are a MINIMUM of (3) pieces of differing alloys [and/or tempers] that require internal clearances to "mechanically operate properly". Pieces include a core-pin, sleeve [hollow-shank and head] and a mechanical locking component [internal ring or swagged tail collar]. To "function" the internal pin must pull [or screw] thru the sleeve to deform the tail or pull the swagged-tail collar into position. Forming forces are very high within the fastener and internal lubricants are required to facilitate friction reduction. The multiple pieces, internal clearances, lubricants and lack of radial swelling... as well as the mixture of "metals"... makes a TYPICAL blind fastener yield [deform] at lower values than driven fasteners... even when the "ultimate" [single shear-event] may be higher than the equivalent driven rivet.

NOTE: I use the word TYPICAL to encompass about 90% of all blind fasteners... there are exceptions to these rules. Two Blind fasteners that pull into interference and retain higher yield and ultimate values in shear are the NAS1398 and NAS1399 BR's and the Monogram Radial-Lok Blind Bolts. Both of these fasteners induce significant shank swelling to fill holes and perform much better in a vibration/cyclic loading scenario than most other fasteners. HOWEVER... they also have some limitations/problems in useage: (a) the noted NAS BRs lack substantial tension capability [exceptionally small formed tail]; (b) and the Radial-Loks are extremely long-shanked Jo-Bolts that pull-hard into, and swell, outer-sleeves... installation/removal is virtually impossible without specialized tooling!!!

NOTE: The Boeing OSI fastener is similar to the Radial-Lok Blind-bolt.. just slightly different construction and performance requirements. Also there are [gagggg] tapered Jo-Bolts... but don't go there.

Regards, Wil Taylor
andrija (Electrical)
6 Mar 02 11:27
Hi folks.
 Just to add to this disscussion. I have seen
 successfull ultralight aircraft that uses
 ONLY blind rivets. It's called Wild Thing,
 manufacturer and designer is Kurtz Reiner
 from Hassfurt, Germany. It has been produced
 (2000 data) in more then 50 pieces.

 For ultralight construction manufacturability
 is very important. This aircraft can be assembled
 garage. This is all aluminum construction.

 Bye,
 Ekmedzic


Helpful Member!(2)  Kenneth (Aerospace)
14 Mar 02 3:10
The following Safety Recommendation issued by the NTSB following a Model B117 Eurocopter crash investigation may interest you: http://www.ntsb.gov/Recs/letters/2000/A00_46_50.pdf.

The whole thing is only seven pages long and worth reading, but for the less patient, the part concerning blind rivet substitution (CR3243-4 CherryMax https://etaf.textron.com/etaf/GetFile/ProductDocs.jsp/stdpage/cmax/CR3243.pdf in this particular case) for solid rivets picks up near the bottom of page 4 at "Manufacturer and Federal Aviation Administration Instruction for Rivet Use".  Note that these particular shear-ring, "bulb" type blind rivets fall into the 90% Wil talks about above; they do not provide the degree "hole-fill" or installed “interference” afforded by the NAS1388 (example: https://etaf.textron.com/etaf/GetFile/ProductDocs.jsp/stdpage/clock/CR2163.pdf) or NAS 1399 "wiredraw" type blind rivets Wil mentions.  It is also worth noting that many manufacturer’s don’t recommend mixing “solid” and “blind” rivets in a “pattern.”  The differences in hole-filling characteristics and relative fastener stiffness/compliance can result in uneven fastener loading, providing higher loading on a relatively fewer fasteners, and therefore a greater potential for failure.

Among the statements in the report:

"Postaccident testing conducted by Textron Aerospace Fasteners (TAF) and earlier testing conducted by Eurocopter consistently demonstrated that the joint fatigue life of materials fastened with blind rivets is less than the joint fatigue life of the same materials fastened with solid rivets.

The Safety Board is concerned that other maintenance personnel may install blind rivets in applications where solid rivets are required, thereby reducing the structural fatigue life of an airframe. Therefore, the Safety Board believes that the FAA should issue a maintenance alert to
all certificated airframe mechanics and inspectors to notify them of the circumstances of this accident and to inform them of the hazards associated with the installation of blind rivets."

For those interested, the additional related FAA Advisory Circulars (ACs) and Airworthiness Directives (ADs) referenced in the NTSB can generally be found on the FAAs website.  
  

To quote Wil: "Folks... Be VERY careful when working with rivets" ... "Each type of fastener REQUIRES very serious consideration and should be designed into the product from the start." Good advice indeed.
Gerryy (Aerospace)
26 Mar 02 10:58
Blind rivets usually require tighter hole tolerance and as such 1 of every 5 holes requires inspection. This drives up the processing cost, however the blind rivet only reuires one operator as opposed to two men to buck a solid rivet (providing solid rivet is NOT squeezed).
Guest (visitor) (OP)
2 Apr 02 7:35

you could have more informations about it on: www.emhart.com
good luck
Yann
Helpful Member!(2)  AJSalemi (Aeronautics)
10 Apr 02 7:42
Just for background, I am the lead structures engineer for the F-14 Tomcat at the Naval Air Depot (NADEP) in Jacksonville Florida.  Almost all of our emphasis is on repair versus manufacture.

There are several reasons for using solids over blinds, besides the strength and durability issues.  First is cost.  Looking for typical rivet examples, I chose 5/32" diameter, 1/2" long grip aluminum rivet in the solid and blind categories.  For an MS20426E5-8 (solid), the price is $10.62/lb.  Doing some simplified calculations, I figured there are about 1000 of these rivets in a pound for a unit price of 1.06 cents.  For a NAS1921B05-08 (blind), the unit price is $1.56.  Considering the 10's of thousands of rivets in an aircraft, the differences are huge.

Weight is also a factor.  I computed the weight of 1000 MS20426E5-8 rivets to be .959 lbs.  1000 NAS1921B05-08's (as installed) are 1.383 lbs.  Again, considering the number of rivets, that is an enourmous difference.  All aircraft in development have weight problems, and designers will shave weight off anywhere they possibly can.  I have seen small skin panels chem-milled just to eliminate a fraction of an ounce, at an enormous increase in cost.  Complicating an assembly step to use solids and save weight is not an issue.

For manufacture, the design and order of assembly operations is designed around using solid rivets wherever possible.  In the repair field, things get a little murkier.  Because of the usually small number of fasteners used in a repair, weight becomes less of an issue.  Accessability to the backside is often a problem, however, making installation of solids difficult.  Sometimes it is even difficult to access the front side.  You have to be able to get the rivet gun straight on to the rivet to properly install it.

The mechanic's ability is also a factor.  Not all mechanics are created equal, and there are some that are downright bad. Flush rivets are easy to install, because you use a flat headed rivet set.  If it wanders a bit off the head, it will usually not damage anything.  But the sets for button heads are concave.  If this set slides off, not only will it damage the rivet, it can chew up the structure.  I have seen bulkheads severely damaged by trying to install button head solids.  The risk of additional damage must be considered.

Another time we use blinds instead of solids is around bonded structure.  Original equipment manufacturers (OEM's) have specialized rivet installation squeezers.  We do not, and have to rely on rivet guns for most installs.  Near bonded structure, the vibration can cause severe disbonding.  Repair of bonded structures, especially near the edges, requires removal from the aircraft, and is time consuming and costly.

All engineering is a balance of strength, durability, risk, cost, weight, and schedule.  An inexperienced engineer only considers stenghth and durability.  A mature engineer also considers everything else.  These other issues are also the hardest to teach.
wktaylor (Aeronautics)
10 Apr 02 8:42
AJsalemi... Great contribution... Please join Eng-Tips.. I think Your comments will be very valuable!!

NOTE: The fighters I've worked on rely heavily on blind fastener installations, purely because of accessibility restrictions... even during manufacture. These fasteners were designed into the acft from the beginning, so the poorer performace was accounted for in design/testing...

The F-5, T-38 use blind-bolts to fasten the upper wing skins in position.
The F-16 uses blind bolts extensively on EVERY major assembly.
The F-15 uses blind rivets for exterior skins adjcacent to inlets; and screws or Jo-Bolts thru the upper wing-skin to substructure.
 ... these areas are all "thin/confined areas" where other fastener types simply cannot be installed.

EVERY acft I've ever worked on uses the most economical fasteners available to accomplish "the requirements"...  for precisely the reasons cited by AJSalemi.

BE VERY CAREFUL when defining fasteners for a job... it really is a complex task.
NOTE: we haven't even begun to discuss other fastener-critical subjects such as: corrosion protection [dissimilar matls], sealing [environmental/fluid], FOD-potential, hole-drilling/reaming tolerances, countersink/dimple limitations, fit-up/alignment [gaps/angularity] and structural-shimming, automated assembly, etc....

Regards, Wil Taylor

AhChoo (Mechanical)
10 Apr 02 11:41
Speaking of FOD potential, are there any unique ideas in which the blind rivet head can be removed from highly inaccessible areas without creating FOD?
(such as behind firewall)
The blind rivet head would just fall off into the inaccessible area when the shank is been punched to remove it.
We had the practice of tapping to check for any remnant FOD.
Extreme measures we have is using boroscope to find.
Even then, it is limited in retrieval.
 
wktaylor (Aeronautics)
10 Apr 02 13:10
AhChoo...

FOD removal can be a Catch-22 situation.

Sometimes Light/non-damaging FOD can be left in a cavity IF the cavity is sealed and does not have any other purpose, such as for fuel storage, air-flow/bleed, mechanical or electrical components stowage or movement, etc... If this is the case it can be allowed to "rattle around"... or can be entrapped by sealant or CPC [especially if people are "nervous" hearing "FOD stuff" "rattle around"].

CAUTION: heavy or sharp-edged FOD can rattle-around the interior and damage paint, scratch/score surfaces/edges, or "stuff-up" moisture drains and vents... so be careful! Aircraft vibrations and unusual attitudes can also vary causing FOD to "fly-around" inside the cavity... possibly lodging into corners or leaving thru a undetected hole/gap!

NOTE: FOD "punched" into honeycomb can be allowed to remain in position if it is sealed/adhesive-bonded in-position....and there are no "structural issues".

IF FOD retrieval is essential then You HAVE to get creative!

At times I have deliberately cut a hole(s) and patched the structure, simply to allow retrieval of all the FOD [fastener tails, drill/router-bit shavings, etc].

At other times I have placed tape-backed putty over the fastener tails... drilling and gently punching the debris into the putty; then removing the FOD by stripping-off the tape/putty.

We have also drilled/punched fastener tails or drill shavings with a powerfull vacuum-source sucking-away the debris.

Small assemblies may be turned up-side-down and "shaken and/or vacuumed clean".

ETC....

FYI:

Strictly speaking, some people use the term "FOD" to mean Foreign Object Damage; and the term FO to mean Foreign Object(s). However in practice "FOD" still seems to be used indiscrimanently to represent FO & FOD.

"Others" tend to use the term FOD to mean "Foreign Object Debris"... and follow-up by saying FOD damage if actual damage is possible or has occured.

Regards, Wil Taylor

AJSalemi (Aeronautics)
11 Apr 02 7:50
wktaylor, Thanks for the tip, I just signed up.

Because of the design, when we replace rivets in the Tomcat's wing, we almost never get the FOD out.  This is a wet wing design, but the fuel pickups have screens to catch this stuff.  We just let it rattle around until it gets to a screen, then the sailors can get it when they clean them.

 
Sometimes we'll squirt MIL-S-81733 sealant into the rivet hole so it drips down and seals the bucktail in place.  We have also made little rubber sheet seals to seal around wire bundles.

To get FOD out, you can try tape wads on the end of broom sticks.  Or, sometimes the best way, find a double-jointed mechanic with really skinny arms!
chopin (Chemical)
11 Apr 02 11:18

Wktaylor, let's move on to the fastener-critical subjects then. It's a "belief" at my workplace that blinds should not go to the inlet areas. The worry is that the inferior  vibratory tolerance of blinds means that they loosen relatively easily, which leads to an even bigger problem. Do you share this "belief"?

I have however never ever seen a blind loosen and dislodge from its position -- yet. Which is why I do not hesitate to prescribe blinds for inlets if the time to gain access is unacceptable from a schedule point of view. As AJSalemi said, everything is strength, durability, schedule, cost, weight, risk.

AhChoo, always ascertain that the FO cannot travel before you make the decision to leave it there. Even then, always try to inject adhesive to trap it. Besides borescope, you can use x-ray to ascertain position of the FO.
AJSalemi (Aeronautics)
15 Apr 02 9:51
We use blinds all the time in the inlets.  Always NAS1921 or MS90353 types here.

I always thought it was a bit weird how a corporate mentality about certain fasteners develops.  When in the Air Force working on B-52G's, we used CherryMax and CherryLock in the inlets all the time.  Here at the depot, CherryMax are regarded as only slightly better than sheet-metal screws!
 
I think that if you have a reliable type rivet with a positive locking mechanism, you will be alright most of the time.  

As a mechanic, I liked CherryMax - they pulled in anything, even with the bulb in the middle of a fitting, and made a pretty head.  As an engineer, I hate CherryMax - even when the bulb pulls bad, the head looks good.  Differing perpectives!

Be aware of the tricks available to artisans.  Some rivets (like CherryMax) can easily disguise a bad hole.  (another trick with the CherryMax - using a pair of diagonal cutters, cut almost through the shank about half way down the rivet.  This will swell it more and allow it to fill an even larger bad hole)  Some rivets can pull short and the heads can be shaved to disguise it.

Finally, yes, I do believe that blinds loosen more easily than solids.  But, I do not recall one coming out of the hole.  I have seen many more solid rivets with popped heads than blinds.  The biggest concern with loose rivets is not the FOD, but the loss of fatigue strength.
wktaylor (Aeronautics)
18 Apr 02 10:44
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 (Mechanical)
30 Apr 02 21:36
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 (Aerospace)
2 May 02 11:38
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 (Aerospace)
2 Jun 02 13:07
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.
Guest (visitor) (OP)
18 Jul 02 10:53
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 (Aerospace)
18 Jul 02 19:44
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"), 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 (Mechanical)
19 Jul 02 12:48
Kenneth, i was lost searching the catalog (slow connection)is it on?

thanks
Kenneth (Aerospace)
19 Jul 02 15:59
The  RT200-(  ) drawing starts on page 72 and is four pages long...
RLindustrial (Aerospace)
6 Aug 02 5:16
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 (Aerospace)
22 Sep 03 17:17
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 (Mechanical)
4 Oct 03 2:11
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 (Aeronautics)
15 Oct 03 2:01
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.
Helpful Member!  AeroE (Aerospace)
11 Nov 03 22:22
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 (Aerospace)
24 Nov 03 11:01
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 (Aerospace)
10 Jan 04 2:13
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 (Aeronautics)
29 Feb 04 18:09
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 (Aerospace)
29 Jun 04 5:54
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)

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