Blind fastener hole tolerance. 2

[Rob130]

I have read the great thread on blind fasteners but need to ask this question.

My SRM and the Cherry/Textron Cherrymax manual call for using a hole tolerance of .160-.164 for a nominal -5 diameter rivet. M7885/( )-5

The SRM calls for a #20 finish drill. The Cherry/Textron specs will not allow for a #21.

Knowing what we do about the lack of hole filling ability and vibration wouldn't a hole closer to the nominal diameter of .157 be preferred?

Nominal rivet diameter = .157
#20 drill = .161
#21 drill = .159

It seems to me the #21 is preferred, and what I have used for the past 20 years, but now I instruct sheet metal technicians and can't, of my own accord, tell them something contrary to the SRM.

I would like to offer an explanation to them on why this seeming contradiction on hole size is in their manual.

Can any body think of one?

TIA, Rob

 

[tbuelna]

Rob-

Actually, pull rivets (or blind rivets) expand and fill their hole pretty well because they are tubular, as opposed to solid bodied.

Your QA guys won't sign off on deviation from established M&P procedures, nor should they. Of course, from my experience, QA does not approve each individual rivet installation. Instead, they approve a rivet installation "process" and rely on mechanics that are "qualified" to perform that particular process using "calibrated" tools.

Finally, hand or match drilling close tolerance holes is rather "iffy". For hand drilled holes, I would normally specify a tolerance of +.004/-.002 on a 5/32 drill.

My advice to you is: Don't deviate from approved practice.

 

[WKTaylor]

Rob130...

A practical issue of tolerances is also a "big" problem here...

The nominal 0.157" dia [for NAS9301-5-x rivet = M7885/2] has a shank tolerance of +0.003/-0.001 [inches]. For this rivet,

The rivet shank diameter min/max, per spec, is 0.156--0.160. The hole min/max is specified as 0.160--0.164.

Per NAS907 [standard drill bits], the standard 2-flute bit has a +0.0000/-0.0007 tolerance in this diameter range. So...

#21 nominal diameter [0.1590] is actually 0.1583--0.1590

#20 nominal diameter [0.1610] is actually 0.1603--0.1610

[Note: drilled holes are actually drilled more like -0.0010/+0.0030 tolerance... if You are lucky].

Obviously the #21 drill-bit is on the small-side! When combined with a rivet on the high-side there is a high interference-fit ... and I guarantee You cannot install BRs & BBs in this condition, without severe damage.

The #20 drill-bit, even on the small-side, combined with a rivet on the high-side would result in a small clearance fit.

Note: manufacturers have fatigue-tested extreme "fit" circumstances. When the rivet/hole is a tight fit (such as the preceding paragraph) there is fairly good fatigue/sonic performance. On-the other-hand, where the fastener shank is on the low-side and the hole is maximum, then fatigue durability performance decreases dramatically [goes-into-the-toilet]. Per rivet and hole specs, there is a potential 0.008 max clearance... which is a lot. This wide-tolerance issues [and resulting gross differences in fatigue performance] are the primary reason why many airframe companies severely limit use of blind rivets in fatigue/sonic environments.

Note: blind rivets/bolts are designed to be installed in a clearance fit for another important reason. The internal mechanisms [core-pin or threaded pin] MUST have certain minimum internal clearances to allow the mechanism to work. If fitted in interference, the hollow fastener shank will obviously contract slightly and the internal mechanisms may see too-high friction forces and fail to "set" the fastener tail/collar as required, resulting in a faulty installation. NOTE: there are blind bolts [Monogram Radial-Loks or Boeing OSIs] that are designed to have very large shank expansion (~0.006 average) and will result in predictable interference fits… which provides good fatigue durability and shear stiffness/strength. Also the NAS1398/1399 have fairly decent shank swelling along a majority of the length… but very little bulb for “pull-thru” resistance. Caution: I worked for an OEM that machined fittings from 7075-T6. A clever mechanic figured-out how to NAS1398/1399 CRES BRs in nominal diameter holes, resulting in very high pre-stresses. We ended-up replacing a lot of fittings after stress corrosion cracking [SCC] split-thru fastener rows aligned grain-wise, within MONTHS of delivery.

Note: I HAVE installed MS90353/90534 blind bolts in holes reamed for a "net-fit" based on hand-selected fastener shank diameters. They could be forced-in using sealant or primer as a lubricant... and they functioned [set-mechanically] "OK"... since these steel fasteners are very stiff and the holes weren't too tight. Aluminum blinds would not be capable of this procedure. These were installed in SCC tolerant alloys.


Regards, Wil Taylor

 

[Kenneth]

Wil:

Is the Boeing OSI you mention the Monogram OSI (

http://www.monogramaerospace.com/products/osi/osiintro.html)?

If so, describing it as a "shank expansion" fastener (in the context of other blind fasteners that are inserted in net/clearance fit holes where the interfernce fit is accomplished by expansion of the shank during the installation to effect an interferenc fit) may be confusing to some. While the Monogram OSI will not fail to install after the body has been driven ito an interference fit hole that would cause installation failure of other blind fasteners, the shank does not become larger in diameter.

For those interested in the Radial-Lok, it accomplishes an interference fit by forcing the fastener body into a thin, ductile sleeve that is an integral part of the blind bolt assembly during the installation process (See

http://www.monogramaerospace.com/products/radial/install.html).

 

[WKTaylor]

Kenneth... Ok Ok Ok..

I over-simplified the description/mechanism of Radial Loks & Boeing OSIs due to an act of “brevity”. They are obviously very similar for good reason: Boeing worked with Monogram to develop OSI fasteners based on the Radial-Lok... but with Boeing needs/design philosophies in-mind. Radial-Loks and Boeing OSIs look/smell the same with minor variations in construction and installation.

I used Ti Radial-Loks for a special repair in the mid-90's.... that was actually fatigue-tested with other repair concepts and came out way-ahead of it's competition... and I fell in love with them. They are truly worthy of the title "fatigue-resistant/durable" in SCC resistant materials.

Radial-Lok fasteners are designed to be installed in a clearance-fit hole… and then shank swelling REALLY [no kidding] occurs to generate true interference. A description of the fastener and installation process are ROUGHLY as follows.

Radial Loks are SFL coated JO-Bolts with an extremely strong [tension] mushroom-collar design that allows for high "pull-up" [tension] loads. The secret is that they also are fitted with a sleeve [as you noted] that has a "head-flange" [protruding or flush] that is pressed partially onto the end of the JO-bolt shank [sleeve ID is actually ~0.006 smaller than the shank]. This pre-assembled combo then slides into appropriately sized holes until the sleeve-head seats in-place [surface or CSK]. This is where the fun begins. JO-bolt installation tools, but with a MUCH longer toque cycle, then start pulling the collar in-place like a typical JO-Bolt. However, when the collar begins to grip the opposite surface, it simultaneously begins pulling the core JO-bolt into the sleeve which then swells the sleeve and generates very high radial compression forces. Installation is complete when the core JO-bolt seat fully into the sleeve and the collar is fully deformed. Break the pin and You are good-to-go. NOTES:

Actual installation interference can be fairly tightly controlled by hole-size relative to anticipated sleeve swelling. IE: for high interference, install fastener in a hole on the small side of the installation range. For lower interference, ream the hole to the high side of the installation range. I presume there are similar “rules” for the Boeing OSI.

Liberal use of SFL on sliding surfaces, and a strong admonition to NOT stop in the middle of the installation process allow the fastener to mechanism to function smoothly. The small INITIAL interference between the core-bolt and partially-installed sleeve allow the pull-up process to overcome start-up friction and stabilized sliding friction loads. HOWEVER stopping in the middle of the process will make re-start virtually impossible, since start-up friction forces would likely overcome tool-drive-in-forces… essentially the installation seizes and the core-pin would deform/break…. forcing drill-out… which is a nightmare for these HS fasteners [thank God we didn’t have to remove any Radial-Locks for any reason]!!!!!!!!

OH Yeah… I almost forgot… Monogram engineers supplied us with “free” sample fasteners and tools for our test repairs… the actual costs for these items were “steep” [beyond our budgets].


Regards, Wil Taylor