Hi Lok Installation and cold working 1

[bohrer1]

I have recently been in a discussion with one of my esteemed colleagues concerning the value of installing hi-loks in interference fit holes. According to what I can ascertain from the hi-shear site and our own process specifications, hi-loks are supposed to be installed in transition fit or clearance fit holes. It is my understanding that hi-shear developed the hi-tigue fastener with it's rounded transition from shank to thread that allows installation in interference fit holes therefore burnishing and cold working the hole upon installation. It is my contention that driving hi-loks into interference holes does not cold work the surrounding area (at least in a positive way as to increase fatigue life) but rather damages the hole and decreases fatigue life. Simply put,...does a hi lok "cold work" a the material when driven in to an interference fit hole and increase fatigue life of the material. Your feedback is greatly appreciated

 

[Fatstress]

bohrer,

take a look into thread31-121526..
wk-taylor peovided quite an extensive explanation there..
Basically I have the same doubt as you are.
To answer your question, the best thing is for you to perform coupon test with different interference level. But you have to be carefull, which type of specimen to be used, a low load or a high load transfer with high secondary bending? And variation of specimen thicknesses and installation procedure (automatic or manual) will complicate you results even more..

 

[OldME]

Not sure of the reasons why but as a generalization Bombardier's preferance is an interferance fit, Boeing a transitional fit and Cessna a clearance fit and there's a lot of aeroplanes out there that have been built to their respective specifications.

 

[tbuelna]

Hi-Loks are primarily shear fasteners, so the closer the fit, the better.

A hi-lok body does not expand to fill a hole like a rivet does.

Cold working of holes is done with a "coining" process, prior to fastener installation.

 

[WhiteRabbit]

Some companies use interference fit holes for Hi-Loks and they are driven into the hole or pulled through using a temporary collar. They use approx a .0015/.0045 diameter interference for a pure aluminum stackup.

They open up the holes through the steel/titanium parts only with larger hole diameters in an attempt to match the spring rates of the base material for strain if there is an Aluminum and Steel/Titanium stackup.

 

[EVdave13]

Cold working a hole by definition involves plastic deformation. Therefore whether or not an interference fit fastener cold works the hole depends on the amount of interference and whether the surrounding material is stressed into the plastic region.

What is not in doubt is that fatigue life improves with an interference fit fastener. This is seen in the SSF analysis method used by Niu. The fit factors for an 0.001" interference pin exceed those for a driven rivet. See his Section 7.7 of his Airframe Structural Design. There's a small paragraph that states this explicitly, and his beta factors in Fig. 7.7.29 indicate that the best among the listed fasteners is a taper-lok (which is a high interference fit).

Coining or split sleeve cold working also improve fatigue life, but that is because of the way they prepare the hole, introducing a band of compressive stresses around the edge. FTI (the company that produces the split sleeve stuff) specifically shows in their test data that adding an interference fit fastener on top of the cold working produces a further increase in fatigue life (though whether it is worth the effort of doing both is debatable). Frequently, the interference fit and cold working are used only when you need that last extra bit of fatigue life to show the part life good.

Hope this helps.

 

[SkyPunk]

Little off subject, but having seen it in a design exactly what benefits could be obtained cold working holes, and then installing NAS9301 cherry max rivets in them? Personal opinion is due to the lower yield properties of the 5056 sleeve it is a waste of effort.

Thanks,

Skypunk

 

[bohrer1]

I have seen some instances where the design called for fasteners that were weaker than the surrounding structure but never with a cherry max.