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?

 

[butelja]

I believe that they are stronger and cheaper than blind rivets.

 

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]

If they contain a steel shank, wouldn't weight become an issue as compared to a solid aluminum rivet?

 

[brin]

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?

 

[WKTaylor]

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

 

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]

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]

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

 

[Kenneth]

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]

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).

 

you could have more informations about it on:

http://www.emhart.com

good luck
Yann

 

[WKTaylor]

Folks...

Great sites for aerospace-quality fasteners, and fastener info [instl, specs, etc] are as follows:

http://www.taf.textron.com/

http://www.fairchildfasteners.com/

http://hi-shear.com/main.htm

http://www.huck.com/

http://www.monogramaerospace.com/

http://www.spstech.com/

http://www.allfastinc.com/

http://www.fastenergroup.com/

addtnl fastener web-sites:

http://www.fastdim.com/Pages/product.html

http://www.dzus.com/

http://www.aircraftfast.com/

Regards, Wil Taylor

 

[AJSalemi]

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]

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]

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]

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]

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]

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]

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.