Can any one tell me if there are different clearances for holes in different materials i.e alum and titanium. Where can you find approved data on this?
Check your SRM Chap 51 for fastener hole sizes. Typically in aluminum structure you install Hi-Loks in transition fit holes and in Ti or Steel or Ti/alum, steel/alum combinations you would use close ream tolerances.
Hi-Loks are installed in interference fits also. Generally, you want to install nominal sized Hi-Loks in light interference to prevent the threads from damaging the hole. For oversized Hi-Loks, tighter fits will work as the thread major diameter is still the same as the nominal fastener.
If you need high interference fits with nominal dimensions, suggest using Radius Lead-In bolts which are designed for intereference fits.
From my understanding, it is irregardless of material used. It all depends on the fatigue enhancement you want. However, something to keep in mind is that you are more likely going to damage a Titanium bolt if you are installing it in a tight interference fit in a Titanium or steel plate.
So, to summarize, if the bolt is not much harder than the material you are installing it in, go with a close ream or net fit. If you need the fatigue enhancement, perform an operation like coldworking or shot peening rather than a tight interference fit.
from the technician perspective ..
1/ in light stainless skin panels if a hole is reamed to the same size as aluminum .. the panel will be distorted (damaged - if not cracked) by the time the fastener is installed.
2/ in difficult ares - thru heavy members (+.25 inch) the holes are usually larger because the fastener head will be damaged from installation impact by the time it is installed. .. also if direct impact on the head is not possible due to tight confines .. it will not be possible to install the fastener - the fastener has to be installed sufficiently so that several threads are protruding - otherwise the fastener cannot be drawn thru. holes in castings are usually reamed on the larger limit of the tolerance.
3/ Aluminum Hiloks in aluminum thin skins are usually tight because they almost always "spin" otherwise.
There are typically (4) types of structural fastener installations... I'll submit the following definitions for these catagories...
close-reamed = .0005--0.0015 clearance [hole shall be slightly larger than max true* diameter of a bolt, but still defined by very tight tolerance]
transition fit (-)0.0008-to-(+)0.0008 [nominal hole size equal to true* fastener diameter, +/- a small tolerance]
light interfernce fit = (-)0.0005-to-(-)0.0015+ [hole size guarenteed to generate interference with fstnr shank]
high interfernce fit = (-)0.0015-to-(-)0.0045 [hole size guarenteed to generate significant interference with shank, and require significant effort to drive-in]
Here are rules of thumb I have learned the hard way...
Use clearance or transition fit for applications as follows:
A) where ANY of the base material(s) have an SCC [stress corrosion cracking] threshold below 25KSI in the ST orientation.
B) material stack-ups of aluminum thicker/deeper than +2Dia thickness [simply to assure high quality fastener installation W/O having to hammer/press fasteners into tight holes in deep structure [can damage holes and fastener heads!!]
C) for any stack-up that includes (any) layer(s) of steel, CRES or titanium.
Use light interference for most jobs in SCC resistant aluminum materials for trade-off on installation of good quality fastening and fatigue performance. Note: use of Hi-Loks is probably Ok in light interference... but Hi-Tigue style HL would generally be easier and less damaging to install. SOMETIMES this stack-up can include titanium... but I don't recommend it without careful study of installation forces!
For heavy interference only use Hi-Tigue style Hi-Loks (variation on Hi-Lok design for smooth fit during interference installations) in aluminum structure ONLY... after considerable study of potential fatigue-durability benefits VS risks of installation damage (may require extreme force to drive into position)... and potential for SCC in [usually] alloys/tempers.
Regards, Wil Taylor
* true fastener diameter as defined by the fastener [or mnfctrs] specification
I am not aware of any single Approved source for the data you want. Go to the SRM for the aircraft you are working on or at least that particular manufacturers guidelines. If SRM it may be Approved or Accepted data depending on local. I am partial to Boeings numbers as I am experienced with them, and that they stand up for fit, fatigue, and ease of use. The are not 100% percent consistent with other manufacturers requirements.
Usually the protcetive coating is damaged to some extent on installation and you can argue in some cases it is redundant for the normal life span. I do believe most manufacturers install Hi-Loks wet with their sealant of choice to provide some environmental isolation.
