Why HiLoks when Solid Rivets would Suffice .. ? 3

[edmeister]

After repairing Several Boeing aircraft(767, 747)- I am mystified why 95% of the airframe (non-pressure skin)construction consists of HiLok Fasteners. I would assume that Solid fasteners (D, DD, or E rivets)would be more then adequate. I acknowledge the requirement for hi-strength fasteners in fittings, etc. - but why are they used in thin metal structural components where hi-strength requirements are not required - such as componet mounting brackets, braces, gussets, etc.
Solid fasteners would have the following advantages
1/ faster to install
2/ superior fatique resistance qualities
3/ weight saving
4/ less disimiliar metal corrosion
5/ less cost (cheaper fasteners)

What is the Boeing (& Airbus) design philosophy that maintains such a high usage of Hilok fasteners in structures where solid fasteners would be substitutable ?

 

[WKTaylor]

edmeister...

You pose a challenging question: Why install Hi-Loks instead of driven rivets when it appears that rivets would suffice (for reasons You've noted)????

As a liaison engineer, understanding the rational for fastener selection/usage is a VERY critical aspect of Your repair work. The answers are not always obvious... but are almost always based on solid engineering requirements and/or manufacturing realities/issues, as follows.

Driven rivet instl process: drill a clearance-fit hole, CSK A/R, deburr, place rivet... then drive tail by gun-and-bar or "squeeze" [for automated/semi-automated instl]. Automated rivet installations can set all sizes of high-strength driven rivets with repeatable precision and low stress. Rivets can tolerate sloppy holes, yet create excellent joints if set properly. Hand installations [especially high strength rivets larger than 3/16D] require a lot of individual skill and "raw hard sweat". Accessibility and set-up for rivet installation [bucking or squeezing] is also a serious factor that requires significant OPERATOR SKILL and FORTITUDE]. NOTE: Rivet installations in pressurized or fuel zones are actually very precision installations. Boeing and Airbus use PRECISION rivets [tight dimensional tolerances] installed in REAMED [precision-tolerance] holes installed by automated equipment [hole sizes measured, squeeze-loads monitored, etc].

Swagged collar Lock-Bolt [LBs, also known as pin/collar rivets] instl process: drill/ream a precision-fit hole, CSK A/R, deburr, place pin-fastener... then place collar and drive by gun-and-bar or "squeeze" [for automated/semi-automated instl]. Automated LB installations can set all sizes of high-strength LBs with repeatable precision and low stress. Hand installations [especially high strength LBs larger than 3/16D] require a lot of individual skill and "raw hard sweat" to set the collars. Accessibility and set-up for LB installation [bucking or squeezing] is also a serious factor that requires significant OPERATOR SKILL and FORTITUDE]. NOTE: a theme and variation of the pin/collar LB is a Bi-metallic pin-washer “Rivet” combination The “hard” titanium pin has a soft titanium-columbium alloy welded to the tip that is “crushed” to form a tail over the washer [automated instl, ONLY].

Hi-Loks: drill/ream a precision-fit hole, CSK A/R, deburr, place HL pin, thread collar [or nut] onto HL pin and slowly torque collar/nut to shear-off or preset value using hand wrench or powered tooling. Automated HL installations can set all sizes of high-strength HL assys [pins/collars] with repeatable precision and low stress. Hi-Loks can be installed RELIABLY and CONSISTENTLY by hand methods, up to a limiting size of about 1/2"D, when all other PERMANENT fasteners become difficult to install by hand.

A few possible reasons for the design selections that “bug You”...

Driven rivets and LBs require direct access to both sides to install. Obstructions can create substantial set-up penalties… or be impossible to overcome without specially designed set-ups. Hi-Loks can be installed in areas of obstructions with low-profile hand or robotic tools that “pin” the hex-drive tip and thread the collar/nut to break-off/final torque.

Hand driven [bucked] rivet and LB installations can induce tremendous vibration stress on unsupported members that can crack or deform parts immediately; or pre-stress [and/or cold-work harden] areas that could eventually experience premature fatigue failures.

Driven rivets will not clamp structure together: structure MUST be clamped [cleco’ed, etc] tightly together before driving [gun or squeeze]. If a small gap exists when the driving process starts, then the parts will remain gapped and rivets “shanked” for the life of the structure. Shanked rivets, though tight in holes, are essentially “loose rivets” because the fastened parts are free to flex independently of each other. Some LB [pin-pull type] and MOST HL installations have the ability to pull structural parts together during the install process... and maintain a beneficial clamp-up pre-load for the life of the structure.

Driven rivets are only rated for the typical tension loads associated with lap-shear. This is an especially important concept with hand driven rivets... since the consistency of bucked-tails may vary a LOT. Out-of-shear-plane loading mandates a fastener that can carry cyclic tension/shear such as a Hi-Lok [yes, even shear-head HLs have tension allowables].

Typical driven rivets [aluminum/titanium] have much lower shear and tension allowables than HLs.

Mixing fastener types is generally unacceptable for both structural and practical [assy/instl] purposes. If an HL or bolt [or structural screw] is required, then the surrounding fasteners must be of similar stiffness and strength... driven rivets cannot match even low strength bolts [some exceptions for exotic alloys]. Also, automated assembly tooling MANDATES that fastener variety [types, diameters, lengths, etc] be minimized for maximum installation efficiency/speed. A mix of a-few-of-this-and-a-few-of-that drive-up over-all production, inspection, stockage, etc cost and thru-put times tremendously.

There are several other factors affecting fastener choices that I have NOT mentioned… including bulk purchasing, material stack-up trends, sealing reliability, etc.. that I do not have the time to address. I recommend You carefully study the installations that bother You, relative to my comments... You will likely see these and many more aspects.
Regards, Wil Taylor

 

edmeister -

I spent a few years at Boeing Commercial working as a stress analyst and can honestly say that we specified solid rivets whenever possible. As you pointed out, they are less expensive to install than hi-loks, lighter and offer good fatigue resistance. As I recall, the bulk of the fasteners used in the 747 fuselage minimum gage areas were rivets.

I'd be guessing as to why you're seeing hi-loks used where you feel rivets would be justified. Perhaps in some cases it's due to access limitations and in other cases its due to secondary tension loading on the fasteners, like that resulting from IDT field effects. In general, rivets are intended for shear only applications and require greater access to install than hi-loks, so it's possible one of these effects resulted in the selection of hi-loks over rivets.

If you're a Boeing employee, the best way to get an answer is to call the project group. They should be able to answer your question and they can touch bases with the cognizant stress analyst as required.

It is worth noting that solid rivet installations do not always offer superior fatigue resistance to hi-lok installations. This depends on several factors, but most notably, hole clearance. When installed in a positive interference fit hole, bolts (hi-loks/hi-tigues ... et al) provide much greater fatigue resistance than solid rivets. Hole filling can effectively lower the local stress concentration factor associated with an open hole. A positive interference fit goes one step further in that it can significantly reduce the local operating stress range. You can find information on these effects in Bruhn, Fuchs, Mike Nui's books, Boeing's "Book 2" ... et al.