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&Business=&Product=&Region=eri
and Aero-Lites (ALs) http://www.alcoa.com/global/en/products/product.asp?prod_id=1133&Business=&Product=&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:
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.
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
) 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
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).