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Fatigue life enhancement with Hi-lite

Fatigue life enhancement with Hi-lite

Fatigue life enhancement with Hi-lite

Dear All,

Maybe somebody can explain what is the advantage of Hi-lite over Hi-lok regarding fatigue ? I have visited Hi-shear site already, but not so much of an explaination that I can get from there regarding the above subject. What I am looking for is actually why can we install Hi-lite with higher interference compared to Hi-lok and how much. But I have recieved an answer from Hi-shear already and according to them the higher interference that can be achieved with Hi-lite is due to the different lead-in radius design. The remaining question would then be how much interference and fatigue life enhancement that can be achieved of joints using Hi-lite compared to Hi-lok.  Thanks Sparweb, for your suggestion to start a new thread.


RE: Fatigue life enhancement with Hi-lite

Hi-Lok's have the feature of limiting the torque that can be applied to the collar when they are installed.  By not over-torquing, they are not significantly stretched, so the diameter of the fastener does not decrease, either.  There is some torque, though, so the Hi-Lok is okay for shearing loads.

Hi-Lites are a little different.  They have the features of the Hi-Lok that I described above, but the threading is changed.  There is an improvement in how the collar threads onto the pin that increases fatigue life of the fastener, meaning the collars for Hi-Lok and Hi-Lite are not interchangeable.  The different threading also reduces the size of the collars, cutting down their weight (hence the name).

There are also Hi-Tigue bolts, that have a slight bulb in their shank.  Apparently, as you tap them into an undersize hole, the bulb cold-works the hole surface.  By burnishing it, a residual stress remains that improves fatigue life.  From reading about it, I expect that Hi-Tigues are much more expensive, too.

If sealing the joint is important, note that the sealed collars may not be available for all types of fastener.

Steven Fahey, CET

RE: Fatigue life enhancement with Hi-lite

Thanks Sparweb,

But can we also obtain some slight cold work effect on the hole wall with Hi-Lite and hence increase the fatigue life of the joint ?



RE: Fatigue life enhancement with Hi-lite

I'm afraid I don't know the answer to that one.  I can't see how a Hi-Lite would be any different than a Hi-Lok in that regard... where's Mr. W.K. Taylor when you need him?

Steven Fahey, CET

RE: Fatigue life enhancement with Hi-lite

Fatstress... and you-too SparWeb...

In a nutshell…

Hi-Loks [HL] were the first generation fastener of it's type. Designed for installation in close-tolerance clearance and transition-fit holes [I favor the term “net-fit”… where hole diameter ~= pin diameter ~+/-0.0010%]. Although precision made, HL pins have a relatively sharp transition edge between threads and shank… and have threads that are only slightly smaller than the pin shank… which hinders installation in true interference-fit holes. HLs can only be pressed-into light interference fit holes [-0.0010% to -0.0020%] in aluminum by CAREFULLY pressing-in wet with primer or sealant (which act as a “lubricant” to prevent damaging the hole walls). HLs can ONLY be installed in medium interference-fit holes [-0.0020% to 0.0030% if shrunk to minimum diameter in LN2… and are installed RAPIDLY but CAREFULLY into holes pre-coated with primer of sealant (and God-help-You are slow to drive the pins into position and allow the fasteners to re-expand before fully installed!!!!). HL collars reflect the first generation nature of these fasteners (match the HL pins and are moderately weight conscious).  Essentially HL Pins and Collars are permanent light-weight bolt/nut installations!!!

Hi-Tigues [HLT] were designed explicitly for installation in interference-fit holes [fatigue reduction]. The transition between the shank and threads is a polished-smooth radius with a very slight bulge. Although the radius was advertised as a feature for “burnishing” holes during fastener installation, it doesn’t seem to be of value for that purpose. HOWEVER, what is important is that HLTs have relatively low drive-in force and can be installed in heavy interference in aluminum structure… and light/medium interference [frozen] in titanium structure, without damaging the holes. HLT threads are recessed substantially below the shank diameters to minimize potential for thread-scoring the holes in high-interference-fit conditions. HLTs collars accommodate the thread profile changes and the radiused/bulged transition between the shank/threads and are moderately weight conscious.

Hi-Lites [HST] are a “next generation Hi-Tigue” style fasteners. They retained the “benefits” of HLTs but were re-designed with slightly reduced features such as shank diameters, lowered head profiles, reduced thread diameters and lengths, reduced size collars, etc… for absolute minimum installed weight VS performance (trying to approach the minimum installation weight-to-performance of interference-fit swaged-collar lock-bolts… and the good interference characteristics of HLTs).

Note: All three fastener types benefit from hole cold working... although benefits of heavy interference are as apparent in CX'd holes

The esteemed [late] Dr Lincoln [fatigue guru for the USAF] cautioned me about the "benefits" of high performance fastener installations: they are only as good as the WORST possible random combination of hole quality, fastener fit and installation abuse... and the chance that these "worst-case" discrepancies will occur [undetected] at the worst possible structural locations.

Regards, Wil Taylor

RE: Fatigue life enhancement with Hi-lite

Hello wktaylor,

Thanks a lot...Quite a thorough explaination..
I appreciate it..
Just a simple question. Which is then better in term of fatigue ? Hi-Lite or Hi-Tigue ?


RE: Fatigue life enhancement with Hi-lite

Hi-Lites are Titanium Hi-Loks, and more expensive.

Hi-Tigues (i think) have a slightly different thread form, and so would be slightly better in a tension loading situation.

i would have thought that they (Hi-Loks, Hi-Lites, Hi-Tigues) were reasonably equivalent for shear loads; tho' ,cynically, i expect that the Hi-Shear salemen can show that the more expensive one is "better".  <watch out, in-coming>

i would prefer to use an AN bolt (or something like it) for tension loading so i can control the pre-load better ... by that i mean Hi-Lok collars shear off at a predetermined value which is probably very good at quality control on the preload, but doesn't allow me to specify a high preload for a particular tension loading situation.

RE: Fatigue life enhancement with Hi-lite


I suspect that Hi-Tigue and Hi-Lite [pin/collar] fasteners would perform about the same in a one-on-one testing [assuming properly set with similar hole fit].

The advantage to Hi-Lites would be a weight savings of ~4%-to-8% PER FASTENER installation... depending on whether grip-length was properly sized for the stack-up. Individually this is a very small savings… but multiplied by several thousand [hundred thousand] fasteners this savings eliminates several hundred pounds [kilos] of dead weight.

Regards, Wil Taylor

RE: Fatigue life enhancement with Hi-lite



There is an improvement in how the collar threads onto the pin that increases fatigue life of the fastener.
Wil’s correct.  Take a look at the fastener minimum lot acceptance tension-tension fatigue lives (and loads) in the respective Hi-Lok/Hi-Tigue (http://www.hi-shear.com/specifications/hs342.pdf) and Hi-Lite (http://www.hi-shear.com/specifications/hs380.pdf) Procurement Specifications.  I think you’ll find they’re the same.

As noted, Hi-Lites (HSTs) -- and Veri-Lites (VLs) http://www.alcoa.com/global/en/products/product.asp?prod_id=540&amp;Business=&Product=&Region=eri and Aero-Lites (ALs)  http://www.alcoa.com/global/en/products/product.asp?prod_id=1133&Business=&;Product=&amp;Region= and Air Industries Light Weight (LW) Pins (http://www.air-industries.com/products.htm) -- all have a radiused shank lead-in designed to ease installation in interference fit installations.  Installing fasteners in interference fits (as noted by Wil and modified by his very neat and pertinent caution) can increase structural fatigue lives.   


Hi-Loks … have threads that are only slightly smaller than the pin shank… which hinders installation in true interference-fit holes


HLT threads are recessed substantially below the shank diameters to minimize potential for thread-scoring the holes in high-interference-fit conditions.


Hi-Lites … were re-designed with slightly reduced features such as shank diameters, lowered head profiles, reduced thread diameters…
The shank and thread diameters (and the differentials between these diameters) of standard configuration Hi-Lok (http://www.hi-shear.com/fastener_hl_stds.htm), Hi-Lite  (http://www.hi-shear.com/fastener_hst_stds.htm) and Hi-Tigue (http://www.hi-shear.com/fastener_hlt_stds.htm) Pins  are the same.  

Take a look for example at comparable, standard configuration, protruding shear head, 6Al- 4V titanium, HL, HLT, and HST pins:

HL10 (http://www.hi-shear.com/standards/hl10.pdf)
HLT450 (http://www.hi-shear.com/standards/hlt450.pdf)
HST10 (http://www.hi-shear.com/standards/hst10.pdf)

Taking the ¼ inch diameter as an example, note that all the above have a shank diameter of 0.2495/0.2490 and a thread major diameter of 0.2440/0.2410.  A look at the head dimensions/profiles will also show them to be the same as well.

There are a small number of “non-standard” special configuration Hi-Tigue Pins with increased shank diameters. (Example: HLT10 http://www.hi-shear.com/standards/hlt10.pdf) They are however, the exception.

Hi-Lites achieve their weight reduction in a number of ways.  For standard (non-oversize) diameter HST pins the maximum length of the transition from the “full” diameter shank to “full” thread minor diameter is one and one half pitch, shorter than the two pitch maximum of Hi-Lok Pins and the 2.5 pitch maximum length of the typical Hi-Tigue Pin.  This reduction in the pin thread-to-shank transition length permitted a small reduction in the depth of counterbore (and therefore overall height) in the design of the mating threaded collar.  This counterbore must be deep enough to avoid “shanking” in the minimum grip condition, and is therefore a direct function of the maximum thread-to-shank runout length.  These differences in runout length and counterbore depth are among the reasons why you must not mix components (pins and collars) from these systems.  There are a few exceptions to this prohibition, including a few alloy steel Hi-Tigue pins that were assigned "HL" pin number before the existence of the "HLT" designation.

While http://www.hi-shear.com/fastener_hst.htm details some of the above, and goes on to state that additional pin thread length reductions were made possible by collar “redesign” and strength increases, it leaves unsaid that the most significant reduction in the collar height and pin thread length (and therefore system weight) of the lightweight systems (HST, VL, AL, LW) was made possible by the use of 7075-T73 as the material for the mating aluminum threaded collars (example: http://www.hi-shear.com/standards/hst79.pdf;  http://www.alcoa.com/fairchild_fasteners/catalog/pdf/Voi-Shan-Pin_Systems-Veri-Lite.pdf) in lieu of the 2024-T6 aluminum used for the earlier Hi-Lok (example: http://www.hi-shear.com/standards/hl79.pdf)  and then Hi-Tigue example: http://www.hi-shear.com/standards/hlt77.pdf) designs.  

The minimum thread lengths required in the design of the pins in these systems are largely driven by the tensile requirements of the aluminum collar.  In general, if the pin threads are long enough to meet the system tensile requirements for the aluminum collar, the rest of the collar material options (CRES, etc.) will meet their respective tensile requirements.  The higher shear strength of the 7075-T73 collar (compared to 2024-T6) permitted a decrease in the collar and pin thread lengths while still meeting the aluminum collar “tensile” (thread shear) strengths of the heritage “baseline” Hi-Lok and Hi-Tigue Systems they were trying to replace.  

While 7075 in the overaged T73 temper generally has an acceptable stress corrosion resistance threshold, a number of Hi-Lite aluminum collar stress corrosion failures (of collars properly heat treated per the applicable heat treatment specification) a while back forced Hi-Shear to add additional qualification (30 day alternate immersion per ASTM G44 of installed assemblies as well as 60 days exposure to 80°C and 95% relative humidity), and more to the point, lot acceptance (a combination of both conductivity -- %IACS -- in conjunction with and yield strength or hardness, see page 19 at http://www.hi-shear.com/specifications/hs381.pdf ) criteria for aluminum Hi-Lite collars to their Hi-Lite collar procurement specification.  When these collar cracking /splitting problems originally surfaced, there were more than a few people in the industry patting themselves somewhat smugly on the back for sticking with the heavier Hi-Lok/Hi-Tigue systems and not making the switch to a “lightweight” system with its 7075 collar (even in the T73 condition).


The transition between the shank and threads is a polished-smooth radius with a very slight bulge.
The Hi-Tigue thread-to-shank transition incorporating the bulge (or “bead”) transition is one of several used on various Hi-Tigue Pins (not all Hi-Tigue Pins incorporate the “bulge” or “bead” transition).

RE: Fatigue life enhancement with Hi-lite

obviously kenneth is well informed about Hi-Shear fasteners ... there's actually a neat table in the collar spec (table IV in spec 345 in the Hi-Shear catalogue) that defines minimum preload.  this comes out to be about 50% of the 95ksi steel fastener tension allowable ... higher than i'd have thought.

still if you've a cyclic tension loading, possibly an AN bolt would allow higher preloads (which would be better).

RE: Fatigue life enhancement with Hi-lite

Kenneth Good Catches... You earned a (*) from me. Thanks for setting this reply "straight".

Guys, this was an obvious "aw-shoot" (**) on my part for sloppy work on "details". Teaches me to refer to old data  ... and just a "quick-pass review" of data.... before responding.

NOTE: In my experience, I ONLY used interference fit Hi-Tigues with ~0.0010+ larger shank diameters such as HLT10, etc..

(**) A Red Star = Atta-boy. HOWEVER, a thousand atta-boys, can easily be canceled by one "aw-shoot".

Regards, Wil Taylor

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