hi-locks with interference

[jphm]

Hi,

For strucural parts, what is the best mounting ? Hi-locks with interference or hi-locks with clearance ? I know of course that using of hi-locks with interference is better concerning fatigue performance but for assembly and process matters which is better ?

 

[GrahamBig]

Know of one aircraft company that used interference holes in titanium pylon structure with dry film lubricated c/sk Hi-Lites. They were finding the heads were continually proud. Turns out the installation in the hard material was scraping off the dry film lube which bunched up under the head and did not allow the head to seat properly. They changed the fit to a "transition fit".

 

[kmerk]

Obviously interference fit gives a better hole filling factor, but it should be noted that Hi-loks are not preferred for interference fit/fatique situations. Note that radius-lead-in type fasteners are preferred for interference and fatigue (see Hi-shear fasteners like Hi-lite's). If I remember correctly, hi-lites have a rounded edge at end of shank near the threads (to reduce scoring the inside surface of the hole) and a slightly different radius between shank and head (to reduce the chance of fatigue which could possiblty causes heads to break-off). They have a slight variation in the collars, also, when compared to hi-loks.
It is true that interference fit in titanium is not a good application. Some of the big companies prefer close tolerance fit (which is zero clearance to a low clearance) to transition fit.

 

[rb1957]

i'd install Hi-Loks to the process that Hi-Shear specify. By their nature, Hi-Loks develop some beneficial interference, but nothing i'd take into account in analysis (unless i had to !). If you're looking to maximise fatigue performance, Hi-Tigues (i think, gosh knows where that name came from!!) are better (and probably more expensive).

 

[tbuelna]

I'm not a structures specialist, but from what I've seen, holes for high performance shear fasteners (like Hi-Loks) are drilled, reamed and "coined", in order to maximize fatigue properties. If the hole is coined, I don't really see that an interference fit of the fastener would provide additional benefit. At least in regards to the increased difficulties it would present at assembly.

Of course with rivets, this is not an issue, because the rivet body swells to fill the hole when it is "bucked".

As a side note, most (shear type) Hi-Lok's used on commercial aircraft are titanium bodies (with DFL) and aluminum collars (with a cetyl alchohol lube). The cost break for most commercial aircraft is over $300/lb, so even titanium fasteners at $50/lb are cost effective.